Insulin

Our Heroes…. Frederick Banting and Charles Best.

Are these guys Banting & Best?

The Early Years….

In 1920, Canadian surgeon Frederick Banting visited the University of Toronto to speak to the newly appointed head of the department of physiology, John J.R. Macleod. Macleod had studied glucose metabolism and diabetes, and Banting had a new idea on how to find not only the cause but a treatment for the so-called "sugar disease."

The Pancreas…..

Islet of Paul Langerhans• Late in the nineteenth century, scientists had realized

there was a connection between the pancreas and diabetes. The connection was further narrowed down to the islets of Langerhans, a part of the pancreas. From 1910 to 1920, Oscar Minkowski and others tried unsuccessfully to find and extract the active ingredient from the islets of Langerhans. While reading a paper on the subject in 1920, Banting had an inspiration. He realized that the pancreas' digestive juice was destroying the islets of Langerhans hormone before it could be isolated. If he could stop the pancreas from working, but keep the islets of Langerhans going, he should be able to find the stuff! He presented this idea to Macleod, who at first scoffed at it. Banting badgered him until finally Macleod gave him lab space, 10 experimental dogs, and a medical student assistant.

While MacLeod’s Away…

• In May, 1921, as Macleod took off for a holiday in his native Scotland, Banting and his assistant Charles Best began their experiments. By August they had the first conclusive results: when they gave the material extracted from the islets of Langerhans (called "insulin," from the Latin for "island") to diabetic dogs, their abnormally high blood sugars were lowered. Macleod, back from holiday, was still skeptical of the results and asked them to repeat the experiment several more times. They did, finding the results the same, but with problems due to the varying purity of their insulin extract.

• Macleod assigned chemist James Bertram Collip to the group to help with the purification. Within six weeks, he felt confident enough of the insulin he had isolated to try it on a human for the first time: a 14-year-old boy dying of diabetes. The injection indeed lowered his blood sugar and cleared his urine of sugars and other signs of the disease. Banting and Best published the first paper on their discovery a month later, in February, 1922. In 1923, the Nobel Prize was awarded to Banting and Macleod for the discovery, and each shared their portion of the prize money with the other researchers on the project.

As the principal discoverer of insulin, Dr. Banting was showered with awards, money and unending gratitude. In 1923, he and his fellow-researcher J.J.R. Macleod were awarded the Nobel Prize in physiology, an award that Banting chose to share with his partner Charles Best. In 1934, he was part of the last group of Canadians to be knighted by King George V. It was a fitting tribute to a discovery that had repercussions around the world.

Insulin: DNA Protein… a Changeling Journey

• Insulin is a small protein and is produced as part of a larger protein to ensure it folds properly. The 3-D geometry of proteins is critical to their proper function.

1. The importance of insulin is juxtaposed with that of glucose, our body's basic unit of fuel.

2. In order to regulate glucose metabolism, insulin circulates through blood vessels to deliver its message by means of a "handshake" with its cognate cell surface receptor.

3. For those with diabetes mellitus, the body is either unable to produce insulin, or unable to produce it in sufficient amounts, and is therefore powerless in maintaining proper levels of blood glucose.

4. Even though insulin is a life saver, it does not cure the disease. Insulin injections themselves are not without risk. Improvements in the treatment of diabetes will come from a better understanding of how insulin is made in the pancreas and released into the bloodstream, and how it promotes uptake of circulating glucose by tissues including muscle and fat.

• A hormone precursor, preproinsulin is synthesized first. This is an inactive protein.

• Preproinsulin contains an amino-terminal signal sequence that is required in order for the precursor hormone to pass through the membrane of the endoplasmic reticulum (ER) for post-translational processing.

• The post-translational processing clips away those portions not needed for the bioactive hormone. Upon entering the ER, the preproinsulin signal sequence, now useless, is proteolytically removed to form proinsulin. Once the post-translational formation of three vital disulfide bonds occurs, specific peptidases cleave proinsulin.

• The final product of the biosynthesis is mature and active insulin. Finally, insulin is packaged and stored in secretory granules, which accumulate in the cytoplasm, until release is triggered.

Insulin Secretion

Insulin Secretion Mechanism Explained…• The process by which insulin is released from beta cells, in

response to changes in blood glucose concentration, is a complex and interesting mechanism that illustrates the intricate nature of insulin regulation.

• Type 2 glucose transporters (GLUT2) mediate the entry of glucose into beta cells (see previous slide). As the raw fuel for glycolysis, the universal energy-producing pathway, glucose is phosphorylated by the rate-limiting enzyme glucokinase. This modified glucose becomes effectively trapped within the beta cells and is further metabolized to create ATP, the central energy molecule.

• The increased ATP:ADP ratio causes the ATP-gated potassium channels in the cellular membrane to close up, preventing potassium ions from being shunted across the cell membrane. The ensuing rise in positive charge inside the cell, due to the increased concentration of potassium ions, leads to depolarization of the cell.

Insulin Secretion Mechanism continued…• The net effect is the activation of voltage-gated calcium

channels, which transport calcium ions into the cell. The brisk increase in intracellular calcium concentrations triggers export of the insulin-storing granules by a process known as exocytosis. The ultimate result is the export of insulin from beta cells and its diffusion into nearby blood vessels. Extensive vascular capacity of surrounding pancreatic islets ensures the prompt diffusion of insulin (and glucose) between beta cells and blood vessels.

• Insulin release is a biphasic process. The initial amount of insulin released upon glucose absorption is dependent on the amounts available in storage. Once depleted, a second phase of insulin release is initiated. This latter release is prolonged since insulin has to be synthesized, processed, and secreted for the duration of the increase of blood glucose. Furthermore, beta cells also have to regenerate the stores of insulin initially depleted in the fast response phase.

How Insulin Works

• Insulin binding to the insulin receptor induces a signal transduction cascade which allows the glucose transporter (GLUT4) to transport glucose into the cell.

Insulin…Hypoglycemia

• As important as insulin is to preventing too high of a blood glucose level, it is just as important that there not be too much insulin and hypoglycemia.

• As one step in monitoring insulin levels, the enzyme insulinase (found in the liver and kidneys) breaks down blood-circulating insulin resulting in a half-life of about six minutes for the hormone.

• This degradative process ensures that levels of circulating insulin are modulated and that blood glucose levels do not get dangerously low.

What Goes Wrong In Diabetes?• The body''s response to blood sugar requires the coordination

of an array of mechanisms. Failure of any one component involved in insulin regulation, secretion, uptake or breakdown can lead to the build-up of glucose in the blood. Likewise, any damage to the beta cells, which produce insulin, will lead to increased levels of blood glucose.

• Diabetes mellitus, commonly known as diabetes, is a metabolic disease that is characterized by abnormally high levels of glucose in the blood. Whereas non-diabetics produce insulin to reduce elevated blood glucose levels (i.e. after a meal), the blood glucose levels of diabetics remain high. This can be due to insulin not being produced at all, or not in quantities sufficient to be able to reduce the blood glucose level.

• The most common forms of diabetes are Type 1 diabetes (juvenile onset, 5-10% of cases), which is an autoimmune disease that destroys beta cells, and Type 2 diabetes (adult onset, 90-95% of cases), which is associated with insufficient insulin. In either case, diabetes complications are severe and the disease can be fatal if left untreated.