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• reduced receptor affinity for insulin;
• total/partial failure of insulin binding to initiate an intracellular response.
Diabetes mellitus is currently the fourth leading cause of death in most developed countries. The worldwide incidence of diabetes is increasing. In 1995, some 135 million people were affected. This figure is projected to increase to 300 million by 2025. The increasing incidence is due to a number of factors, including increasing world population, population ageing, unhealthy diets, sedentary lifestyles and obesity.
A second form of diabetes is also recognized: diabetes insipidus, which is caused by a deficiency of the pituitary hormone, vasopressin. Vasopressin promotes water reabsorption from the kidney, hence a deficiency also induces symptoms of excessive urination and thirst. A key diagnostic difference between the common diabetes mellitus and the rare diabetes insipidus, is the absence of glucose in the urine in the latter case. Until a few decades ago, a popular way to differentiate between the two diseases was to taste the patient’s urine to see if it was sweet.
The first proof that diabetes mellitus was caused by a factor produced by the pancreas was obtained in 1889, when two Strasburg doctors removed a dog’s pancreas and found that the dog promptly developed diabetes. By the early 1900s, doctors in the USA had illustrated that the islets of Langerhans of diabetics were completely/almost completely destroyed. In the Spring of 1921, two researchers at the University of Toronto (Frederick Banting and Charles Best) showed that injection of an extract from the islets of Langerhans could revive diabetic dogs who were close to death. The researchers then decided to try out their discovery on humans, initially checking its safety by injecting each other (they were lucky to have used low doses).
The first diabetic patient to receive insulin was a 14-year-old boy, Leonard Thompson. His recovery from near-death was speedy and he was discharged from hospital within a few weeks — although dependent on regular insulin injections.
Toxicity problems (due to impurities) associated with prolonged repeat insulin administration soon put this treatment in jeopardy. However, a biochemist, James Collip, devised an improved purification scheme entailing insulin crystallization, which overcame such toxicity.
The work culminated in Banting and John MacLeod (his Professor) receiving the Nobel prize, although many felt that the contribution of Best and Collip also deserved Nobel recognition.
Figure 8.1. Proteolytic processing of proinsulin, yielding mature insulin, as occurs within the coated secretory granules
precursor, preproinsulin. This 108 amino acid polypeptide contains a 23 amino acid signal sequence at its amino terminal end. This guides it through the endoplasmic reticulum membrane, where the signal sequence is removed by a specific peptidase.
Proinsulin-containing vesicles bud off from the endoplasmic reticulum and fuse with the Golgi apparatus. Subsequently, proinsulin-containing vesicles (clathrin-coated secretory vesicles), in turn, bud off from the Golgi. As they move away from the Golgi, they lose their clathrin coat, becoming non-coated secretory vesicles. These vesicles serve as a storage form of insulin in the b cell. Elevated levels of blood glucose, or other appropriate signals, cause the vesicles to fuse with the plasma membrane, thereby releasing their contents into the blood via the process of exocytosis.
Proinsulin is proteolytically processed in the coated secretory granules, yielding mature insulin and a 34 amino acid connecting peptide (C peptide, Figure 8.1). The C peptide is further proteolytically modified by removal of a dipeptide from each of its ends. The secretory granules thus contain low levels of proinsulin, C peptide and proteases, in addition to insulin itself. The insulin is stored in the form of a characteristic zinc-insulin hexamer, consisting of six molecules of insulin stabilized by two zinc atoms.
Mature insulin consists of two polypeptide chains connected by two interchain disulphide linkages. The A-chain contains 21 amino acids, whereas the larger B-chain is composed of 30 residues. Insulins from various species conforms to this basic structure, while varying slightly in their amino acid sequence. Porcine insulin (5777 Da) varies from the human form (5807 Da) by a single amino acid, whereas bovine insulin (5733 Da) differs by three residues.
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Although a high degree of homology is evident between insulins from various species, the same is not true for proinsulins, as the C peptide sequence can vary considerably. This has therapeutic implications, as the presence of proinsulin in animal-derived insulin preparations can potentially elicit an immune response in humans.
The insulin receptor and signal transduction
The insulin receptor is a tetrameric integral membrane glycoprotein consisting of two 735 amino acid a-chains and two 620 amino acid b-chains. These are held together by disulphide linkages (Figure 8.2). The a-chain resides entirely on the extracellular side of the plasma membrane and contains the cysteine-rich insulin-binding domain.