Insulin

Background

Insulin is a hormone that regulates the amount of glucose (sugar) in the blood and is required for the body to function normally. Insulin is produced by cells in the pancreas, called the islets of Langerhans. These cells continuously release a small amount of insulin into the body, but they release surges of the hormone in response to a rise in the blood glucose level.

Certain cells in the body change the food ingested into energy, or blood glucose, that cells can use. Every time a person eats, the blood glucose rises. Raised blood glucose triggers the cells in the islets of Langerhans to release the necessary amount of insulin. Insulin allows the blood glucose to be transported from the blood into the cells. Cells have an outer wall, called a membrane, that controls what enters and exits the cell. Researchers do not yet know exactly how insulin works, but they do know insulin binds to receptors on the cell's membrane. This activates a set of transport molecules so that glucose and proteins can enter the cell. The cells can then use the glucose as energy to carry out its functions. Once transported into the cell, the blood glucose level is returned to normal within hours.

Without insulin, the blood glucose builds up in the blood and the cells are starved of their energy source. Some of the symptoms that may occur include fatigue, constant infections, blurred eye sight, numbness, tingling in the hands or legs, increased thirst, and slowed healing of bruises or cuts. The cells will begin to use fat, the energy source stored for emergencies. When this happens for too long a time the body produces ketones, chemicals produced by the liver. Ketones can poison and kill cells if they build up in the body over an extended period of time. This can lead to serious illness and coma.

People who do not produce the necessary amount of insulin have diabetes. There are two general types of diabetes. The most severe type, known as Type I or juvenile-onset diabetes, is when the body does not produce any insulin. Type I diabetics usually inject themselves with different types of insulin three to four times daily. Dosage is taken based on the person's blood glucose reading, taken from a glucose meter. Type II diabetics produce some insulin, but it is either not enough or their cells do not respond normally to insulin. This usually occurs in obese or middle aged and older people. Type II diabetics do not necessarily need to take insulin, but they may inject insulin once or twice a day.

There are four main types of insulin manufactured based upon how soon the insulin starts working, when it peaks, and how long it lasts in the body. According to the American Diabetes Association, rapid-acting insulin reaches the blood within 15 minutes, peaks at 30-90 minutes, and may last five hours. Short-acting insulin reaches the blood within 30 minutes, it peaks about two to four hours later and stays in the blood for four to eight hours. Intermediate-acting insulin reaches the blood two to six hours after injection, peaks four to 14 hours later, and can last in the blood for 14-20 hours. And long-acting insulin takes six to 14 hours to start working, it has a small peak soon after, and stays in the blood for 20-24 hours. Diabetics each have different responses to and needs for insulin so there is no one type that works best for everyone. Some insulin is sold with two of the types mixed together in one bottle.

History

If the body does not produce any or enough insulin, people need to take a manufactured version of it. The major use of producing insulin is for diabetics who do not make enough or any insulin naturally.

Before researchers discovered how to produce insulin, people who suffered from Type I diabetes had no chance for a healthy life. Then in 1921, Canadian scientists Frederick G. Banting and Charles H. Best successfully purified insulin from a dog's pancreas. Over the years scientists made continual improvements in producing insulin. In 1936, researchers found a way to make insulin with a slower release in the blood. They added a protein found in fish sperm, protamine, which the body breaks down slowly. One injection lasted 36 hours. Another breakthrough came in 1950 when researchers produced a type of insulin that acted slightly faster and does not remain in the bloodstream as long. In the 1970s, researchers began to try and produce an insulin that more mimicked how the body's natural insulin worked: releasing a small amount of insulin all day with surges occurring at mealtimes.

Researchers continued to improve insulin but the basic production method remained the same for decades. Insulin was extracted from the pancreas of cattle and pigs and purified. The chemical structure of insulin in these animals is only slightly different than human insulin, which is why it functions so well in the human body. (Although some people had negative immune system or allergic reactions.) Then in the early 1980s biotechnology revolutionized insulin synthesis. Researchers had already decoded the chemical structure of insulin in the mid1950s. They soon determined the exact location of the insulin gene at the top of chromosome 11. By 1977, a research team had spliced a rat insulin gene into a bacterium that then produced insulin.

In 1891, Frederick Banting was born in Alliston, Ontario. He graduated in 1916 from the University of Toronto medical school. After Medical Corps service in World War I, Banting became interested in diabetes and studied the disease at the University of Western Ontario.

In 1919, Moses Barron, a researcher at the University of Minnesota, showed blockage of the duct connecting the two major parts of the pancreas caused shriveling of a second cell type, the acinar. Banting believed that by tying off the pancreatic duct to destroy the acinar cells, he could preserve the hormone and extract it from islet cells. Banting proposed this to the head of the University of Toronto's Physiology Department, John Macleod. Macleod rejected Banting's proposal, but supplied laboratory space, 10 dogs, and a medical student, Charles Best

Begining in May 1921, Banting and Best tied off pancreatic ducts in dogs so the acinar cells would atrophy, then removed the pancreases to extract fluid from islet cells. Meanwhile, they removed pancreases from other dogs to cause diabetes, then injected the islet cell fluid. In January 1922, 14 year-old Leonard Thompson became the first human to be successfully treat-ed for diabetes using insulin.

Best received his medical degree in 1925. Banting insisted Best also be credited, and almost turned down his Nobel Prize because Best was not included. Best became head of the University of Toronto's physiology department in 1929 and director of the university's Banting and Best Department of Medical Research after Banting's death in 1941.

In the 1980s, researchers used genetic engineering to manufacture a human insulin. In 1982, the Eli Lilly Corporation produced a human insulin that became the first approved genetically engineered pharmaceutical product. Without needing to depend on animals, researchers could produce genetically engineered insulin in unlimited supplies. It also did not contain any of the animal contaminants. Using human insulin also took away any concerns about transferring any potential animal diseases into the insulin. While companies still sell a small amount of insulin produced from animals—mostly porcine—from the 1980s onwards, insulin users increasingly moved to a form of human insulin created through recombinant DNA technology. According to the Eli Lilly Corporation, in 2001 95% of insulin users in most parts of the world take some form of human insulin. Some companies have stopped producing animal insulin completely. Companies are focusing on synthesizing human insulin and insulin analogs, a modification of the insulin molecule in some way.

Raw Materials

Human insulin is grown in the lab inside common bacteria. Escherichia coli is by far the most widely used type of bacterium, but yeast is also used.

Researchers need the human protein that produces insulin. Manufacturers get this through an amino-acid sequencing machine that synthesizes the DNA. Manufacturers know the exact order of insulin's amino acids (the nitrogen-based molecules that line up to make up proteins). There are 20 common amino acids. Manufacturers input insulin's amino acids, and the sequencing machine connects the amino acids together. Also necessary to synthesize insulin are large tanks to grow the bacteria, and nutrients are needed for the bacteria to grow. Several instruments are necessary to separate and purify the DNA such as a centrifuge, along with various chromatography and x-ray crystallography instruments.

The Manufacturing
Process

Synthesizing human insulin is a multi-step biochemical process that depends on basic recombinant DNA techniques and an understanding of the insulin gene. DNA carries the instructions for how the body works and one small segment of the DNA, the insulin gene, codes for the protein insulin. Manufacturers manipulate the biological precursor to insulin so that it grows inside simple bacteria. While manufacturers each have their own variations, there are two basic methods to manufacture human insulin.

Working with human insulin

• 1 The insulin gene is a protein consisting of two separate chains of amino acids, an A above a B chain, that are held together with bonds. Amino acids are the basic units that build all proteins. The insulin A chain consists of 21 amino acids and the B chain has 30.
• 2 Before becoming an active insulin protein, insulin is first produced as preproinsulin. This is one single long protein chain with the A and B chains not yet separated, a section in the middle linking the chains together and a signal sequence at one end telling the protein when to start secreting outside the cell. After preproinsulin, the chain evolves into proinsulin, still a single chain but without the signaling sequence. Then comes the active protein insulin, the protein without the section linking the A and B chains. At each step, the protein needs specific enzymes (proteins that carry out chemical reactions) to produce the next form of insulin.

STARTING WITH A AND B

• 3 One method of manufacturing insulin is to grow the two insulin chains separately. This will avoid manufacturing each of the specific enzymes needed. Manufacturers need the two mini-genes: one that produces the A chain and one for the B chain. Since the exact DNA sequence of each chain is known, they synthesize each mini-gene's DNA in an amino acid sequencing machine.
• 4 These two DNA molecules are then inserted into plasmids, small circular pieces of DNA that are more readily taken up by the host's DNA.
• 5 Manufacturers first insert the plasmids into a non-harmful type of the bacterium E. coli. They insert it next to the lacZ gene. LacZ encodes for 8-galactosidase, a gene widely used in recombinant DNA procedures because it is easy to find and cut, allowing the insulin to be readily removed so that it does not get lost in the bacterium's DNA. Next to this gene is the amino acid methionine, which starts the protein formation.
• 6 The recombinant, newly formed, plasmids are mixed up with the bacterial cells. Plasmids enter the bacteria in a process called transfection. Manufacturers can add to the cells DNA ligase, an enzyme that acts like glue to help the plasmid stick to the bacterium's DNA.
• 7 The bacteria synthesizing the insulin then undergo a fermentation process. They are grown at optimal temperatures in large tanks in manufacturing plants. The millions of bacteria replicate roughly every 20 minutes through cell mitosis, and each expresses the insulin gene.
• 8 After multiplying, the cells are taken out of the tanks and broken open to extract the DNA. One common way this is done is by first adding a mixture of lysozome that digest the outer layer of the cell wall, then adding a detergent mixture that separates the fatty cell wall membrane. The bacterium's DNA is then treated with cyanogen bromide, a reagent that splits protein chains at the methionine residues. This separates the insulin chains from the rest of the DNA.
• 9 The two chains are then mixed together and joined by disulfide bonds through the reduction-reoxidation reaction. An oxidizing agent (a material that causes oxidization or the transfer of an electron) is added. The batch is then placed in a centrifuge, a mechanical device that spins quickly to separate cell components by size and density.
• 10 The DNA mixture is then purified so that only the insulin chains remain. Manufacturers can purify the mixture through several chromatography, or separation, techniques that exploit differences in the molecule's charge, size, and affinity to water. Procedures used include an ion-exchange column, reverse-phase high performance liquid chromatography, and a gel filtration chromatography column. Manufacturers can test insulin batches to ensure none of the bacteria's E. coli proteins are mixed in with the insulin. They use a marker protein that lets them detect E. coli DNA. They can then determine that the purification process removes the E. coli bacteria.

PROINSULIN PROCESS

• 11 Starting in 1986, manufacturers began to use another method to synthesize human insulin. They started with the direct precursor to the insulin gene, proinsulin. Many of the steps are the same as when producing insulin with the A and B chains, except in this method the amino acid machine synthesizes the proinsulin gene.
• 12 The sequence that codes for proinsulin is inserted into the non-pathogenic E. coli bacteria. The bacteria go through the fermentation process where it reproduces and produces proinsulin. Then the connecting sequence between the A and B chains is spliced away with an enzyme and the resulting insulin is purified.
• 13 At the end of the manufacturing process ingredients are added to insulin to prevent bacteria and help maintain a neutral balance between acids and bases. Ingredients are also added to intermediate and long-acting insulin to produce the desired duration type of insulin. This is the traditional method of producing longer-acting insulin. Manufacturers add ingredients to the purified insulin that prolong their actions, such as zinc oxide. These additives delay absorption in the body. Additives vary among different brands of the same type of insulin.

Analog insulin

In the mid 1990s, researchers began to improve the way human insulin works in the body by changing its amino acid sequence and creating an analog, a chemical substance that mimics another substance well enough that it fools the cell. Analog insulin clumps less and disperses more readily into the blood, allowing the insulin to start working in the body minutes after an injection. There are several different analog insulin. Humulin insulin does not have strong bonds with other insulin and thus, is absorbed quickly. Another insulin analog, called Glargine, changes the chemical structure of the protein to make it have a relatively constant release over 24 hours with no pronounced peaks.

Instead of synthesizing the exact DNA sequence for insulin, manufacturers synthesize an insulin gene where the sequence is slightly altered. The change causes the resulting proteins to repel each other, which causes less clumping. Using this changed DNA sequence, the manufacturing process is similar to the recombinant DNA process described.

Quality Control

After synthesizing the human insulin, the structure and purity of the insulin batches are tested through several different methods. High performance liquid chromatography is used to determine if there are any impurities in the insulin. Other separation techniques, such as X-ray crystallography, gel filtration, and amino acid sequencing, are also performed. Manufacturers also test the vial's packaging to ensure it is sealed properly.

Manufacturing for human insulin must comply with National Institutes of Health procedures for large-scale operations. The United States Food and Drug Administration must approve all manufactured insulin.

The Future

The future of insulin holds many possibilities. Since insulin was first synthesized, diabetics needed to regularly inject the liquid insulin with a syringe directly into their bloodstream. This allows the insulin to enter the blood immediately. For many years it was the only way known to move the intact insulin protein into the body. In the 1990s, researchers began to make inroads in synthesizing various devices and forms of insulin that diabetics can use in an alternate drug delivery system.

Manufacturers are currently producing several relatively new drug delivery devices. Insulin pens look like a writing pen. A cartridge holds the insulin and the tip is the needle. The user set a dose, inserts the needle into the skin, and presses a button to inject the insulin. With pens there is no need to use a vial of insulin. However, pens require inserting separate tips before each injection. Another downside is that the pen does not allow users to mix insulin types, and not all insulin is available.

For people who hate needles an alternate to the pen is the jet-injector. Looking similar to the pens, jet injectors use pressure to propel a tiny stream of insulin through the skin. These devices are not as widely used as the pen, and they can cause bruising at the input point.

The insulin pump allows a controlled release in the body. This is a computerized pump, about the size of a beeper, that diabetics can wear on their belt or in their pocket. The pump has a small flexible tube that is inserted just under the surface of the diabetic's skin. The diabetic sets the pump to deliver a steady, measured dose of insulin throughout the day, increasing the amount right before eating. This mimics the body's normal release of insulin. Manufacturers have produced insulin pumps since the 1980s but advances in the late 1990s and early twenty-first century have made them increasingly easier to use and more popular. Researchers are exploring the possibility of implantable insulin pumps. Diabetics would control these devices through an external remote control.

Researchers are exploring other drug-delivery options. Ingesting insulin through pills is one possibility. The challenge with edible insulin is that the stomach's high acidic environment destroys the protein before it can move into the blood. Researchers are working on coating insulin with plastic the width of a few human hairs. The coverings would protect the drugs from the stomach's acid.

In 2001 promising tests are occurring on inhaled insulin devices and manufacturers could begin producing the products within the next few years. Since insulin is a relatively large protein, it does not permeate into the lungs. Researchers of inhaled insulin are working to create insulin particles that are small enough to reach the deep lung. The particles can then pass into the bloodstream. Researchers are testing several inhalation devices much like that of an asthma inhaler.

Another form of aerosol device undergoing tests will administer insulin to the inner cheek. Known as buccal (cheek) insulin, diabetics will spray the insulin onto the inside of their cheek. It is then absorbed through the inner cheek wall.

Insulin patches are another drug delivery system in development. Patches would release insulin continuously into the bloodstream. Users would pull a tab on the patch to release more insulin before meals. The challenge is finding a way to have insulin pass through the skin. Ultrasound is one method researchers are investigating. These low frequency sound waves could change the skin's permeability and allow insulin to pass.

Other research has the potential to discontinue the need for manufacturers to synthesize insulin. Researchers are working on creating the cells that produce insulin in the laboratory. The thought is that physicians can someday replace the non-working pancreas cells with insulin-producing cells. Another hope for diabetics is gene therapy. Scientists are working on correcting the insulin gene's mutation so that diabetics would be able to produce insulin on their own.

Where to Learn More

Books

Clark, David P, and Lonnie D. Russell. Molecular Biology Made Simple and Fun. 2nd ed. Vienna, IL: Cache River Press, 2000.

Considine, Douglas M., ed. Van Nostrand's Scientific Encyclopedia. 8th ed. New York: International Thomson Publishing Inc., 1995.

Periodicals

Dinsmoor, Robert S. "Insulin: A Never-ending Evolution." Countdown (Spring 2001).

Other

Diabetes Digest Web Page. 15 November 2001. <http://www.diabetesdigest.com>.

Discovery of Insulin Web Page. 16 November 2001. <http://web.idirect.com/~discover>.

Eli Lilly Corporation. Humulin and Humalog Development. CD-ROM, 2001.

Eli Lilly Diabetes Web Page. 16 November 2001. <http://www.lillydiabetes.com>.

Novo Nordisk Diabetes Web Page. 15 November 2001. <http://www.novonet.co.nz>.

M.RaeNelson

Insulin

Definition

Insulin is a hormone secreted by specialized cells in the pancreas, a large gland lying behind the stomach. Together, insulin and glucagon, another pancreatic hormone, act to control blood sugar (glucose)

levels in the body. Synthetic (artificial) insulin is used to replace the body's insulin when insulin production is absent or defective.

Description

Glucose is a type of sugar that the cells of the body require for producing energy and promoting growth and repair. The body must break down the glucose it takes in from food, however, and transport it from the bloodstream into the cells. When the levels of glucose in the body rise after food is consumed, insulin—secreted by beta cells within the pancreas' islets of Langerhans cells—is released to stabilize the glucose level. When the levels of glucose fall (such as between meals), the pancreas secretes the hormone, glucagon. Thus, the action of both hormones is necessary to keep fasting blood glucose levels within the normal range of 70 to 100 milligrams of glucose per deciliter (mg/dL) of blood. After eating, a normal blood glucose level should be below 180 mg/dL.

In type 1 diabetes mellitus (diabetes), which often begins in childhood or adolescence, the pancreas either does not produce insulin or the amount produced is insufficient. When blood glucose levels rise above 180 mg/dL—a condition called hyperglycemia results. A person with type 1 diabetes must be injected with insulin under the skin (subcutaneously) every day to give the body the insulin it needs to survive. The insulin must be injected because the digestive enzymes in the gastrointestinal tract would break down any oral medication and render it ineffective.

Type 2 diabetes, which often begins in adulthood, is the most common form of diabetes. In type 2 diabetes, the pancreas either produces an insufficient

amount of insulin or the cells of the body do not respond properly to the insulin (called insulin resistance). Often oral medications are effective in treating this form of diabetes, but when drugs fail to control blood glucose levels, insulin injections may be necessary.

Preparations

Since insulin was first discovered in 1922 by Canadian scientists Frederick Banting and Charles Best, there have been many changes in the research and development of its synthetic preparation. The first insulin preparations were purified from pancreas tissue extracted from animals—either pigs or cattle but contained impurities and produced an allergic reaction in some people.

The manufacturing of beef insulin for human use in the United States was discontinued in 1998, and in 2006, pork insulin for human use was no longer manufactured or marketed in the United States. After 2005, individuals were allowed to import beef or pork insulin for their personal use from a foreign country only under strict US Food and Drug Association (FDA) and US Department of Agriculture (USDA) guidelines.

Since the 1980s, however, most insulin—referred to as “human insulin,” has been made synthetically by recombinant deoxyribonucleic acid (DNA) technology, or genetic engineering. This insulin, which is chemically identical to its human counterpart, is produced in a laboratory. The insulin-encoded gene is cut from a human chromosome and inserted into a rapidly reproducing weakened strain of the common bacterium, Escherichia coli (E. coli), which inhabits the human digestive tract. Thus, the bacterium is the factory that produces the genetically engineered insulin. Yeast cells are also used to manufacture insulin. In this way, large amounts of insulin can be created relatively quickly and cost efficiently. As of 2008, there were over 20 types of insulin products available in the United States obtainable in a variety of delivery systems, including vials, prefilled syringes, cartridges, pumps (computerized devices worn on the belt or in the pocket), jet injectors, and pens.

In 2006, the FDA approved the first inhaled insulin combination product for diabetes treatment. Exubera, a short-acting inhaled powder form of recombinant human insulin for the treatment of adults with type 1 or type 2 diabetes, was the first new insulin delivery option introduced since the discovery of insulin in the 1920s. But in 2007, the drug manufacturer withdrew the product from the market because the drug was found no more effective than the injectable forms of insulin and was more costly. Other drug companies continued their work on an inhaled product.

Duration of action

The terms used to describe the time it takes for different types of insulin to work inside the body are

• Onset, or the amount of time needed for the insulin to begin working
• Peak, or the amount of time needed for the insulin to reach maximum effectiveness
• Duration, or the amount of time that the insulin remains effective

Types of insulin

The four basic types of insulin and their onset, peak, and duration—and examples of each—are as follows:

• Rapid-acting—begins working after 15 minutes, peaks in 30 to 90 minutes, and has a duration of 3 to 4 hours (insulin aspart [Novolog], insulin lispro [Humalog], and insulin glulisine [Apidra])
• Short-acting—begins working in 30 to 60 minutes, peaks in 2 to 3 hours, and has a duration of 3 to 6 hours (regular [R] humulin or novolin)
• Intermediate-acting—begins working in 90 minutes to 6 hours, peaks in 4 to 14 hours, and has a duration of up to 24 hours (Neutral Protamine Hagedorn [NPH]
• Long-acting—begins working in 6 to 14 hours and remains effective for 24 to 36 hours (insulin glargine [Lantus], insulin detemir [Levemir])

Many people with diabetes use combinations of different types of insulin to better control and manage their condition. These mixtures include:

• Humalog Mix 75/25 (75% insulin lispro protamine and 25% insulin lispro)
• NovoLog Mix 70/30 (70% aspart protamine and 30% insulin aspart)
• For patients using human insulin, an alternate mixture is available consisting of 70% NPH and 30% regular insulin

Recommended dosage

The recommended dosage and frequency of insulin injection varies with the individual and depends on the type of diabetes diagnosed (type 1 or type 2), the type and brand of insulin prescribed, as well as a variety of additional factors, such as

• age
• allergies or sensitivities to medications
• concurrent medical conditions
• condition of injection sites
• daily routines
• duration and phase of diabetes
• eating habits
• exercise regimen
• lifestyle habits such as drinking or smoking
• results of blood glucose monitoring
• weight

Precautions

It is important for individuals on insulin therapy to follow their physician's instructions regarding diet, exercise , and lifestyle and to take insulin exactly as prescribed. Also, the type of insulin or the brand or model of syringe or needle should not be switched without consulting the prescribing physician. Because an individual's insulin needs may change over time—for example, older adults often adapt to higher blood glucose levels than younger people—dosages may require periodic adjustment.

Other issues to keep in mind when using insulin are listed below.

• Be sure that the insulin is exactly the kind and dosage the doctor ordered and that its expiration date has not passed.
• Do not use a vial that contains visible lumps in the insulin.
• Clean the site of the injection thoroughly with an antiseptic, such as rubbing alcohol, before injecting the insulin.
• Change the injection site daily.
• If a physician prescribes two types of insulin for better glucose control and recommends mixing the insulin into one syringe, always draw the regular insulin (clear) into the syringe first. Also, do not shake the bottle; tip it gently from side to side to mix.
• Some insulin mixtures take time to combine; others react quickly and require immediate injection, so consult a doctor or pharmacist before using any mixtures.
• When traveling, always carry an abundant supply of diabetic medications and supplies and, if possible, a prescription for insulin and syringes.
• Do not store insulin in a place that is extremely hot, like a the glove compartment of a car, or extremely cold, like a freezer.
• Check with a doctor or pharmacist before taking any over-the-counter medications, such as those for cough, cold, diet, allergy, asthma, or sinus conditions. These products may affect blood glucose levels.
• Insulin requirements may change in the presence of other illnesses, so consult a doctor when other conditions are diagnosed.
• Blood glucose levels should be monitored in both mother and child during pregnancy and while the mother is nursing.

Side effects

Like all prescription medications, there are some side effects to keep in mind when using insulin. Minor side effects include skin reactions such as redness, swelling, itching , or rash near the injection site. To avoid this reaction, it is important to change injection sites with each shot. Serious side effects are often related to allergic reactions (severe skin rash) or hypoglycemia.

Hypoglycemia

Hypoglycemia (low blood glucose) is the most common side effect that occurs when taking insulin, especially when too much insulin is taken. A severe insulin overdose may cause disorientation and coma , and immediate medical attention is required. Symptoms of hypoglycemia include

• blurred vision
• confusion
• drowsiness
• depression
• feeling cold
• feadache or lightheadedness
• feart palpitations
• irritability
• muscle weakness
• nausea
• sleep problems
• sweating
• tingling in the lips, hands, or feet
• tremors

If symptoms are mild, eating something or drinking a beverage that is high in sugar (for example, fruit juice) often improves hypoglycemia. Discussing the reaction with a physician to determine the source of the problem is advised.

Hyperglycemia

Too little insulin may cause symptoms of hyperglycemia (high blood glucose), including

• appetite loss
• confusion
• dry skin
• fatigue
• flushing
• frequent urination
• fruity breath odor
• rapid breathing

A doctor may modify the dosing schedule or change the insulin dosage if these symptoms occur.

Interactions

A wide variety of medications, foods, and supplements interact with insulin.

Increased insulin action

Some medications are known to increase the action of insulin and may lower blood glucose to a dangerous level. To prevent hypoglycemia when using these drugs, the insulin dosage may need to be reduced. Such drugs or substances include

• alcohol
• monoamine oxidase (MAO) inhibitors (phenelzine [Nardil])
• beta blockers (propranolol [Inderal])
• salicylates (aspirin or salsalate [Disalcid])
• anabolic steroids (methyltestosterone [Android])

Although other drugs may increase the blood glucose-lowering effect of insulin, they are less likely to interact with insulin or have less of an effect. Such drugs include

• tetracycline antibiotics (doxycycline [Vibramycin] or guanethidine [Ismelin])
• oral hypoglycemic drugs (glyburide [Diabeta])
• sulfa antibiotics (sulfadiazine)
• angiotensin-convering enzyme (ACE) inhibitors (captopril [Capoten])

In addition, alcohol may increase the action of insulin, leading to hypoglycemia, so people using insulin should avoid alcohol.

Decreased insulin action

Some drugs decrease the effect of insulin. Although interactions between these drugs and insulin are less likely or less serious than drugs that increase insulin action, these drugs include

• calcium channel blockers (diltiazem)
• corticosteroids (prednisone)
• epinephrine
• estrogens
• isoniazid
• niacin
• oral contraceptives
• thiazide diuretics (water pills) such as hydrochlorothiazide or furosemide
• thyroid medications like levothyroxine (Synthroid)

Smoking may also decrease insulin activity and increase the health problems associated with diabetes, so people with diabetes are cautioned to avoid smoking.

Dietary supplements and foods

Any food or dietary supplement—such as garlic , ginger, ginseng, hawthorn, licorice, fenugreek, or nettle—that affects blood glucose levels may alter the amount of insulin required by an individual with diabetes. Although chromium, biotin, vitamin E , or an herb called gymnema sylvestre may boost the body's ability to use glucose, these supplements may increase the risk of abnormally low blood glucose levels when taken in conjunction with insulin therapy. The hormone dehydroepiandrosterone (DHEA ) may also alter sensitivity to insulin. So it is important for a person with diabetes to consult a physician before taking any supplements—or before planning a diet.

Caregiver concerns

Care givers should be sure that, if possible, the person with diabetes

QUESTIONS TO ASK YOUR PHARMACIST

• Will any of my prescription medications, over-the-counter drugs, or supplements interact with insulin?
• What should I do if I miss a dose or have to take it later than the scheduled time?
• Where is the best place to give myself the shot?
• Will you always give me the same brand of insulin?
• Where should I store my insulin?
• How can I tell if I'm having an allergic reaction to insulin?
• Can I smoke or drink while taking insulin?

• Monitors their blood glucose levels throughout the day.
• Stores unopened insulin in a refrigerator between 36 and 46°F and never puts insulin in a freezer. Opened insulin vials may be kept at room temperature for up to a month.
• Injects insulin at the same times each day in relation to meals.
• Draws up their insulin carefully to ensure the correct dosage.
• Does not change insulin dosage, brand, or method of delivery without consulting a physician.
• Writes down the exact time and amount of every insulin shot.
• Learns how to balance food, insulin, and exercise, using information from their physician.
• Rotates the area of the body where shots are given—within an area (arm, leg, buttock), move to a different site for each shot. Each shot should be at least a thumb-width away from the last one given.
• Makes sure that friends and family are aware of the symptoms of an insulin reaction and know what to do if they observe any of the symptoms in the person with diabetes.
• Carries a card or wears a bracelet that identifies the person as a diabetic.
• Always has insulin and syringes handy.
• Eats on a regular schedule and avoids skipping meals.

KEY TERMS

Diabetes mellitus —Also known as “diabetes,” a group of metabolic diseases characterized by high blood sugar (glucose) levels, which result from defects in the production or action of insulin, or both.

Glucagon —A hormone produced by the pancreas that raises blood sugar levels and has the opposite action of insulin.

Glucose —Also known as dextrose, the main sugar the body makes from proteins, fats, and carbohydrates. Glucose is carried through the bloodstream to provide energy to all cells in the body with the help of insulin.

Hormone —A chemical substance produced in the body that controls and regulates the activity of certain cells or organs.

Hypoglycemia —Low blood sugar (glucose).

Islets of Langerhans —Pancreatic cell clusters that make and secrete hormones such as insulin.

Metabolism —The biochemical processes that occur within any living organism and involve the buildup and breakdown of substances such as food, and the transformation of these substances into energy.

Pancreas —Organ lying across the back of the abdomen and behind the stomach that contains specialized cells that secrete digestive enzymes and the hormones glucagon and insulin.

Resources

BOOKS

Scheiner, Gary. Think Like a Pancreas: A Practical Guide to Managing Diabetes With Insulin. Emeryville, CA: Marlowe & Company, 2004.

Walsh, John, and Ruth Roberts, Chandra B. Varma, and Timothy Bailey. Using Insulin. San Diego: Torrey Pines Press, 2003.

PERIODICALS

Allen, Scott. “Insulin in the brain: Researchers suspect that low levels might be linked to Alzheimer&s, schizophrenia.” The Boston Globe(January 30, 2006). Available at: http://www.boston.com/news/globe/health_science/articles/2006/01/30/insulin_in_the_brain (March 29, 2008).

Barry, Patrick. “New Route to Insulin-Making Cells.”Science News 173 (February 2, 2008): 78.

Schaffer, Amanda. “In Diabetes, a Complex of Causes.” New York Times(October 16, 2007). Available online at http://www.nytimes.com/2007/10/16/health/16diab.html?_r=1&oref=slogin March 15, 2008.

OTHER

von Wartburg, Linda. “Interesting Insulin Facts.” Diabetes Health. December 20, 2007. http://www.diabeteshealth.com/read/2007/12/20/5572.html(March 29, 2008).

“What Is Insulin?” http://www.endocrineweb.com/diabetes/2insulin.html (March 30, 2008).

ORGANIZATIONS

American Diabetes Association, 1701 North Beauregard St., Alexandria, VA, 22311, (800) 342-2383, http://www.diabetes.org.

BD, 1 Becton Drive, Franklin Lakes, NJ, 07417-1880, (201) 847-6800, (888) 232-2737, (201) 847-4856, http://www.bddiabetes.com.

International Diabetes Federation (IDF), Avenue Emile De Mot 19, Brussels, Belgium, B-1000, +32-2-5385511 l, +32-2-5385114, [email protected], http://www.idf.org.

Genevieve Slomski Ph.D.

insulin Glucose, dissolved in the blood (blood sugar), is one of the main sources of energy for the body. Different organs and tissues use other fuels to varying extents, but the brain uses glucose exclusively. To protect vital functions mammals have evolved a mechanism for keeping the concentration of glucose in the blood fairly constant — an example of homeostasis. This includes diverse hormonal responses that increase the blood glucose concentration. Yet, in what seems a remarkable oversight of nature, the body relies almost entirely upon just one hormone, the protein insulin, to bring about a decrease in blood glucose. Insulin facilitates the entry of glucose from the blood into the tissues of the body.

It has been known since 1899 that removal of the pancreas from dogs led to diabetes, with its characteristic persistent increase in blood glucose (hyperglycaemia) and presence of sugar in the urine (glycosuria). The fascinating saga of the eventual discovery of insulin by Banting and Best in 1921 is well known. They were able to show that injection of an extract from the pancreas of a healthy dog led consistently to a decrease in the amount of sugar in the blood and urine of diabetic dogs. They published their account, entitled ‘The Internal Secretion of the Pancreas’, in 1922. Their experiments, which were to prove life-saving, assured the insulin molecule a key place in medical history, and won a Nobel Prize for Banting and Macleod (in whose Canadian laboratory the work was done) as well as earning the grateful thanks of diabetic people in their millions around the world. It was some 30 years later that Frederick Sanger embarked on his painstaking but highly successful molecular dissection of insulin, which unravelled its precise amino acid sequence. This was a landmark achievement, representing as it did the first successful sequencing of any protein molecule, and it earned Sanger his first Nobel Prize. With subsequent establishment of its three-dimensional structure, insulin was revealed as a vital protein of classic polypeptide design. Despite 300 million years of divergent evolution, the molecular form and function of insulin has been remarkably well conserved across the entire zoological spectrum.

The dynamic glucose–insulin system on which the body's metabolism so critically depends is controlled and modulated by various factors impinging on the ‘b-cells’, which are found in the pancreas in cellular nodules, the Islets of Langerhans — named after the German pathologist who described them in 1869, long before their function was known. These ‘b-cells’ detect glucose in the blood and secrete insulin in appropriate amounts in response to the meal-induced tidal changes in blood glucose level.

Insulin is normally quite rapidly removed from the blood and survives in the circulation for only 5–15 min, thus placing a continuing demand on the b-cells for the release of more insulin in order to establish an effective feedback control of blood glucose concentration. This moment-by-moment process requires, in the b-cells, mechanisms for the manufacture, storage, and rapid release of insulin. To replenish its insulin supply, the genes of a pancreatic b-cell switch on their protein manufacturing machinery, producing a much larger single chain precursor molecule, called pro-insulin, which contains the amino acid sequence of insulin. Successive and controlled proteolysis (breakdown of this protein molecule) finally leaves the 51 amino acid sequence of insulin itself, and ensures its correct folding to create the three-dimensional shape of the molecule. Once formed in the b-cell, insulin is stored in granules as a symmetrical hexagonal array of 6 insulin molecules combined in a stable crystalline structure with 2 atoms of zinc. When released into the circulation at effective concentrations, insulin is transported as, and normally acts as, a single molecule.

To exert its effects on target cells — muscle, liver, or fat cells — the insulin molecule must first be recognized by specific insulin-receptor protein molecules in the cell membranes. Part of the insulin receptor spans the membrane, so that, when an insulin molecule binds to the external part of the receptor, a signal is transmitted across the membrane to other molecules, leading to a cascade of enzyme activity in the target cells.

Insulin resistance may occur when the blood glucose level is not well controlled, as in a type of diabetes which begins in adult life, where the pancreatic b-cells do not produce enough insulin. Not only does this lead to the appearance of the symptoms of diabetes, but the high level of glucose in the blood decreases the sensitivity of the target cell receptors for insulin and so makes the situation worse. It is possible to treat this type of diabetes by mouth with agents that boost the output of insulin from any viable b-cells that are present, or reduce the blood sugar by other means. If, on the other hand, pancreatic b-cells have all been destroyed, as in juvenile diabetes, then insulin must be injected daily as replacement therapy.

Unfortunately insulin cannot be given orally because it is a peptide and is therefore rapidly broken down by enzymes in the gut. Different preparations of insulin are available for injection, depending on the duration of action required. Insulin was originally extracted on a massive scale from the pancreas of animals (cows or pigs). It can now be obtained by genetic engineering of bacterial cells, causing them to express human insulin. It is noteworthy that insulin was the first protein to be commercially produced by such recombinant technology. Although this allows large scale production and isolation, pig pancreas remains the main source of insulin for human treatment: pig insulin differs from human insulin by only one amino acid.

Various insulin formulations may combine rapid-, medium-, or long-acting forms of crystalline insulin so that individual requirements for insulin can be matched to blood glucose levels following meals. Insulin has thus become firmly established in modern medicine as a remarkably effective therapeutic agent, but a whole life-time of constant injection is an unwelcome hazard for anyone suffering from juvenile diabetes. The design and production of a non-peptide, orally-active insulin analogue therefore remains a major goal of pharmaceutical research.

E. K. Matthews

Insulin

History

Types of diabetes

Insulin is a hormone secreted by the pancreas gland, one of the glands in the endocrine system. Insulin, working in harmony with other hormones, regulates the level of blood sugar (glucose). Endocrine glands are ductless glands; that is, they pour their products (hormones) directly into the bloodstream. The pancreas, a gland in the upper abdomen, has cells within it that secrete insulin directly into the bloodstream. An insufficient level of insulin secretion leads to high blood sugar, a disease called diabetes mellitus or, simply, diabetes. Specifically, diabetes is a metabolic disease caused by the body’s inability to use the hormone insulin to effectively convert carbohydrates into the simple sugar glucose that cells store and use to perform vital functions.

Without glucose to fuel their activity, the cells use fat instead, producing ketones as a waste product. Ketones build up in blood and disrupt brain functions. Common signs of diabetes are excessive thirst, urination, and fatigue. The disease can also cause vision loss, decreased blood supply to hands and feet, pain, and skin infections. If left untreated diabetes can induce coma and cause death. Diabetes often runs in families. In the United States about 10% of the Caucasian population suffers from diabetes, and it is even more common among African-American, Mexican-American, and certain Native American groups. The sixth leading cause of death in the United States, diabetes remains a major health problem. According to the American Diabetes Association, about 20.8 million children and adults (about 7% of the U.S. population), as of 2006, suffer from diabetes mellitus. About 14.6 million people have been diagnosed with diabetes. However, about 6.2 million people (about one-third) do not know that they have diabetes.

History

Prior to the twentieth century, diabetes was a fatal disease. Its cause was unknown, and the method of treating it had yet to be discovered. Not until 1921 did the research of Sir Frederick Grant Banting (1891–1941), a Canadian surgeon and medical scientist, and Charles Herbert Best (1899–1978), a Canadian physiologist and medical scientist, reveal that insulin is crucial in blood glucose regulation. The two scientists then isolated insulin, an achievement for which they were awarded the Nobel Prize. Their work was built on previous research by Paul Langerhans (1847–1888), a German pathologist and biologist who described the irregular, microscopic collections of cells scattered throughout the pancreas. These cells later were named the islets of Langerhans and were found to be the source of insulin secretion.

In 1952, British biochemist Frederick Sanger (1918–) analyzed insulin and discovered it was made up of two chains of amino acids. One chain, called the alpha chain, has 21 amino acids and the second chain, the beta chain, has 30 amino acids. The chains run parallel to each other and are connected by disulfide bonds (made up of two sulfuratoms). With the discovery of the chemical structure of insulin, efforts to synthesize it began.

Types of diabetes

Diabetes manifests in two types. Type I diabetes is also called childhood onset because it begins in early childhood. Adult onset diabetes or Type II affects adults.

Glucose is a source of energy to the muscles of the body. Normally, the glucose content of blood is determined by the demands made by the muscles; the secretion of insulin, which lowers blood glucose; the secretion of glucagon, also manufactured and secreted in the pancreas, which raises blood glucose; extraction by the liver of glucose from the blood to be converted to glycogen and stored; and other hormones secreted by the adrenal and pituitary glands. The secretion of glucagon in the pancreas is a function of the alpha cells of the islets of Langerhans, and the secretion of insulin is the function of the beta cells of the same islets.

If the beta cells fail to function properly or are genetically insufficient in number to provide the needed hormone, diabetes can result. Also, in adults, the damage to, or failure of the cells resulting in lowered insulin secretion can lead to diabetes mellitus.

No cure has been found for diabetes; that is, no way has been found to restore full function to the beta cells in the islets of Langerhans. Individuals who have diabetes must take insulin by injection or take pills that control carbohydratemetabolism.

The insulin used by diabetics used to be extracted from porcine (pig) and bovine (cattle) pancreas. However, with the genetic revolution, genetic engineering has allowed bacteria to be transformed to produce human insulin protein, which is purified from large industrial cultures. Thus, human insulin is obtained from bacteria that have human genes inserted into them. Human insulin technology has advanced to produce new, more effective forms of insulin. One of the problems with insulin therapy is that diabetic patients must inject themselves with insulin regularly– an unpleasant process for many people. The development of Humulin-L, Humulin-N, Humulin-R, and Humulin-U have allowed some patients to reduce the frequency of shots required. These new forms of human insulin (from which the name Humulin comes) have effects that last longer than ordinary insulin, making fewer injections possible. As a result, more diabetes patients are likely to stick to their insulin therapy.

Insulin

Insulin is a small peptide (protein) consisting of fifty-one amino acids synthesized and stored within the pancreas, an organ situated behind the stomach. The protein itself consists of two chains, denoted A and B, linked by disulfide (sulfur-sulfur) bridges between cysteine residues (see Figure 1).

Insulin is a hormone, a chemical transported in the blood that controls and regulates the activity of certain cells or organs in the body. When blood sugar levels rise following a meal, the pancreas is stimulated to release insulin into the bloodstream. In order for tissues to absorb glucose from the blood, they must first bind insulin. Glucose metabolism is necessary for cell growth and energy needs associated with cell function. When insulin binds to receptors on cell membranes, glucose transporter proteins are released from within the cell to the surface of the cell membrane. Once on the exterior surface of cells, glucose transporters can carry sugar from the blood into the tissue where it is metabolized. Without insulin, cells cannot absorb glucose and effectively starve.

A deficiency in insulin production results in a condition called diabetes mellitus. Approximately 6.2 percent of the population in the United States is affected with diabetes. Type 1 diabetics account for 10 percent of those individuals suffering from diabetes mellitus. It is also known as juvenile diabetes and generally develops in young people, typically between the ages of ten and fifteen years, as a result of an autoimmune disorder. Why the body's immune system turns on itself, attacking and destroying beta cells, the pancreatic cells in which insulin is synthesized, is not clear. The unfortunate consequence is insulin deficiency.

The majority of individuals afflicted with diabetes mellitus suffer from type 2 diabetes. The onset of type 2 diabetes occurs much later than for type 1 and typically in people over the age of fifty. The pancreas of type 2 diabetics continues to produce and release insulin. However, cells do not respond appropriately to insulin levels in the blood. This condition is known as insulin-resistance and is associated with obesity and high blood pressure. Children who are obese can also develop type 2 diabetes.

Gestational diabetes affects 3 to 5 percent of pregnant women. Onset generally occurs in the fifth or sixth month of pregnancy. In pregnant women, the placenta produces hormones to support the growing fetus. Some of these hormones (e.g., estrogen and cortisol) interfere with the actions of insulin. Insulin-resistance develops despite adequate blood insulin levels. Gestational diabetes does not last beyond pregnancy and the condition disappears after delivery.

Before the discovery of insulin, type 1 diabetics usually died within a few years of onset of the disease. During the early 1920s, a young Canadian physician, Frederick Grant Banting, working with John James Rickard Macleod, professor of physiology at the University of Toronto, and Charles Best, a medical student, discovered insulin while performing investigations on extracts acquired from dog pancreas. Soon after, Banting and Macleod were awarded the 1923 Nobel Prize in physiology or medicine for their discovery (Banting reportedly split his share of the prize money with Best).

Insulin was rapidly put into clinical use, chiefly through the efforts of August Krogh, a Danish scientist who cofounded the Nordic Insulin Laboratory in Copenhagen, Denmark, for the production of insulin. Large quantities of insulin were initially acquired from the pancreatic tissues of slaughtered animals, typically cows and pigs. Although bovine (cow) and porcine (pig) insulin are still the major components of commercially available insulin in the United States, the use of human insulin preparations is rapidly growing. Since the 1980s, recombinant DNA techniques have made human insulin available for clinical use. Genetically modified strains of Escherichia coli bacteria or Saccharomyces cerevisiae (baker's yeast) containing human genes coding for insulin have been developed for the mass production of human insulin.

Insulin is not a cure for diabetes. It does, however, allow diabetics, especially those with type 1 diabetes, to gain some control over their condition. Insulin is typically administered by subcutaneous (under the skin) injection. There are four principal types of insulin preparations commercially available. Insulin lispro (ultra-short-acting insulin) is rapidly absorbed into the blood and lasts only 3 to 4 hours. Regular insulin, on the other hand, takes 30 minutes to become effective, and lasts 5 to 7 hours. Lente and NPH (neutral protamine Hagedorn) insulin preparations are longeracting formulations typically administered every 12 hours. Finally, mixtures of insulin preparations are also commercially available. Typical mixtures consist of 70 percent NPH and 30 percent insulin lispro.

see also Genetic Engineering; Glycolysis.

Nanette M. Wachter

Bibliography

Dewitt, Dawn E. and Hirsh, Irl B. (2003). "Outpatient Insulin Therapy in Type 1 and Type 2 Diabetes Mellitus: Scientific Review." Journal of the American Medical Association 289(17):2254–2264.

Katzung, Bertram G. (1998). Basic & Clinical Pharmacology, 7th edition. Stamford, CT: Appleton & Lange.

Internet Resources

National Institute of Diabetes & Digestive & Kidney Diseases of the National Institutes of Health. Available from <http://www.niddk.nih.gov>.

Nobel Foundation E-Museum. "The Nobel Prize in Physiology or Medicine 1923." Available from <http://www.nobel.se>.