2. T T he endocrine portion of the pancreas represents 1-2% of its total weight and it is consisted of 1- 2 million islets of Langerhans that are collections of at least 4 types of cells; A or α cells, B or β cells, D or δ cells and F cells. These cells secrete at least 4 types of hormones glucagone, insulin, somatostatin, and PP respectively.`

3. INSULIN HISTORY: In 1889 both Von Mering and Minkowski demonstrated that pancreatectomy produced diabetes. The link between the pancreatic islets and diabetes was suggested by De Mayer in 1909 and by Sharpy – Schaffer in 1917, however; it was Banting and Best who proved this association in 1921. These investigators used acid- ethanol to extract from the pancreatic tissue an “ islet cell factor” that had potent hypoglycemic activity. This factor was named insulin and within a year both bovine and porcine insulin was widely used for treatment of human diabetes.

4. Insulin was the first protein that:- proved to have hormonal bioactivity was crystallized was sequenced (1951) was synthesized biochemically (1964) was synthesized by recombinant DNA technology for commercial use.

5. CHEMICAL STRUCTURE:- Insulin is a polypeptide consisting of two chains (A and B) that are connected by interchain disulfide bridges that connect A7 to B7 and A20 to B19. A third intrachain disulfide bridge connects the residues 6 and 11 of the A chain. The A and B chains contain 21 and 30 amino acids respectively in most species. Porcine and bovine insulin were the standard therapy for diabetes before human insulin was prepared by DNA technology. Porcine insulin differs from human insulin in substituting alanin instead of threonin at B30. However, bovine insulin has this substitution in addition to 2 other substitutions at A8 and A10.

6. Synthesis and Release Insulin is first synthesized in the pancreatic islet B cells as [pre- prohormone] with a molecular Wt. of 11500 D. This molecule is directed by the hydrophobic 23 amino acid leader sequence into the cisternae of the endoplasmic reticulum and is then removed to make the 9000 D proinsulin molecule. Starting from the amino terminal this molecule is [ B chain- connecting peptide(C- peptide)- A chain].

7. The proinsulin molecule undergoes a series of site specific peptide cleavages that result in the formation of the mature insulin and C- peptide in eqimolar amounts. Proinsulin molecule varies in its length between 78- 86 a.a. due to the variation in the C- peptide.

8. Proinsulin and insulin have the same solubility and isoelectric point and both of them form hexamers with zinc crystals which are present in the islets in high concentrations. However, proinsulin has only 5% of insulin bioactivity and a small amount of it is normally secreted into the circulation together with insulin when the release of the later is stimulated and in larger amounts in pancreatic tumors. Proinsulin has a longer half – life than insulin and it reacts with insulin anti-sera in the RIA overestimating insulin bioactivity. C- peptide molecule has no known biological activity and with no distinct antigenic property and that’s why its measurement can distinguish endogenous insulin from that given exogenously as a therapy. The human insulin gene was identified to be located on the short arm of chromosome no. (11).

9. Regulation of Insulin Secretion The human pancreas secrets 40- 50 units of insulin daily. A number of mediators are implicated in insulin release including:1.Glucose: High plasma glucose concentration is the most important physiological regulator for insulin secretion. The threshold conc. of fasting plasma glucose for insulin release is 80- 100 mg/dl (while the max. response is at FPG of 300- 500). Two theories can explain the role of glucose ; Glucose receptors at the cell surface of B- cells. Rate of metabolites influx from different biological processes including pentose- phosphate shunt, citric acid cycle or glycolytic pathway.

10. 2.Hormonal factors: Alpha-adrenergic hormones (like ephedrine) inhibits insulin release whereas β – adrenergic hormones increase insulin release. Other hormones increase insulin secretion including chronic exposure to GH, cortisol, placental lactogen, estrogens and progestins (pregnancy?). 3. Drugs: Sulphonylurea compounds are used most frequently in the therapy of type 2 diabetes. Drugs like tolbutamide stimulate insulin secretion by a mechanism different from glucose.

11. INSULIN METABOLISM Insulin has no plasma carrier protein, thus its plasma half- life is very short (3-5 min). The major organs involved in insulin metabolism are liver, kidneys, and placenta. Two enzyme systems are involved in its metabolism; the 1st includes an “insulin specific protease” and the second one is “ hepatic glutathione- insulintranshydrogenase” which reduces the disulfide bridges resulting in insulin rapid degradation.

12. INSULIN ACTIONS Insulin has got a major role in the metabolism of CHO, protein, and lipid metabolism. These include: High Blood Glucose Insulin Decrease Glucagon Signals release Low Blood Glucose Signals release Increase

13. 1.Effects on the cell membrane transport: Insulin enhances the entry of glucose from the extracellular to the intracellular compartments using [carrier mediated facilitated diffusion] in almost all body tissues mainly adipose and muscle tissues when entry of glucose is followed by glucose phosphorylation and further metabolism. In the liver tissue insulin does not enhance facilitated diffusion, but it acts on enhancing phosphorylation of free glucose through inducing the activity of the enzyme ‘ hepatic glucokinase’ and in this way it enhances the glucose entry by simple diffusion (based on the conc. gradient of free glucose).

14. 2. Effects on glucose utilization: Insulin enhances the intracellular utilization of glucose where about 50% of glucose is converted to energy through the “ glycolytic pathway”, while the other 40% is converted to fat and about 10% is converted to glycogen. It stimulates several key enzymes in the glycolytic pathway including glucokinase, phosphofructokinase, and pyruvate kinase. Insulin stimulates the conversion of glucose to glucose- 6- phosphate by glucokinase and hexokinase II enzymes mainly in the liver and the muscle tissues, which is then converted to glucose-1- phosphate that will be incorporated in the structure of glycogen through the action of ‘glycogen synthase’ enzyme which is under direct stimulation by insulin.

15. 3. Effects on glucose production: Insulin inhibits the production of glucose from non- CHO sources where it decreases the amount of the key enzyme [PEPCK]; an effect that is usually opposed by glucagone, glucocorticosteroids, angiotesin II, and others. 4. Effects on Lipid Metabolism: Insulin is well –known to have a potent lipogenic activity, meanwhile, it is a potent inhibitor of lipolysis in the liver and the adipose tissue. It inhibits [hormone- sensitive lipase] enzyme activity. It thus decreases the amount of circulating free fatty acids and hence it affects the glucose metabolism as well as the free fatty acids inhibit glycolysis at many steps and stimulates gluconeogenesis.

17. acetyl Co ketoacidosis In patients with insulin deficiency, the hormone- sensitive lipase activity gets increased resulting in increased concentration of circulating free fatty acids together with increased glucagon release (which opposes most actions of insulin). The increased free fatty acids will be converted to acetyl Co A which enters the citric acid cycle and converted to CO2 and water. However, in this case the capacity of this cycle is usually exceeded resulting in the conversion of the excess acetyl Co A into ketone bodies (acetone, acetoacetic acid, and β- hydroxybutyric acid) in high concentration resulting in ketoacidosis.

18. 5. Effects on Protein Metabolism: Insulin is known to have an anabolic role in protein metabolism where it enhances the entry of neutral amino acids into the muscle cells and thus protein synthesis and it however prevents protein degradation.

19. Glucagon

20. Glucagon A cells It is a single- chain polypeptide hormone (M Wt ~ 3500 D) synthesizes mainly in the A cells of the pancreatic islets. It consists of 29 amino acids and it is also synthesized from a proglucagon precursor molecule of ~ 9000 D first. It shares some biological and biophysical properties with (enteroglucagon) which is released from the duodenal mucosal cells.

21. It also flows freely in the plasma without being associated to any carrier protein and thus its plasma half – life is only about 5 min. It is inactivated in the liver by removing the 1 st 2 amino acids from the amino terminal end. Secretion of glucagon is inhibited by glucose, an action that emphasizes the opposing metabolic roles of glucagon and insulin. However, many other substances including amino acids, fatty acids and ketones, GIT hormones and neurotransmitters affect glucagons secretion.

22. Glucagon Actions In general, the actions of glucagon oppose those of insulin. As insulin promotes energy storage by stimulating glycogenesis, lipogenesis, and protein synthesis, glucagons causes rapid mobilization of potential energy sources into glucose by stimulating glycogenolysis and into fatty acids by stimulating lipolysis. It is also the most potent gluconeogenic hormone.

23. The liver is the primary site of action for glucagon where it binds to specific cell surface receptors and its action is mediated through c- AMP mechanism. In the liver, it inhibits ‘glycogen synthetase’ enzyme and thus decreases glycogen synthesis, and meanwhile it promotes glycogenolysis. This action on glycogen metabolism is tissue specific i.e. it does not affect muscle glycogen.

24. It promotes the synthesis of more [PEPCK] enzyme and thus the conversion of more amino acids to glucose through the process of gluconeogenesis and this is the opposite action to insulin which decreases the gene transcription of PEPCK. In the adipose tissue it stimulates the[ hormone- sensitive lipase] enzyme resulting in the increased rate of lipolysis, this enzyme induction is through increasing adipose cell c- AMP level.

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