Insulin is synthesised and stored (bound to zinc) in granules in the p-islet cells of the pancreas. Daily secretion amounts to 30-40 units, which is about 25% of total pancreatic insulin content. The principal factor that evokes insulin secretion is a high blood glucose concentration.
Insulin is a polypeptide with two peptide chains (A chain, 21 amino acids and B chain, 30) linked by two disulphide bridges. The basic structure having metabolic activity is common to all mammalian species but there are minor species differences, which result in the development of antibodies in all patients treated with animal insulins, as well as to unavoidable impurities in the preparations, minimal though these now are.
• Bovine insulin differs from human insulin by three amino acids and is more antigenic to man than is
• Porcine insulin differs from human by only one amino acid
• Human insulin (1980) is made either by enzyme modification of porcine insulin, or by using recombinant DNA to synthesise the proinsulin, precursor molecule for insulin. This is done by artificially introducing the DNA into either Escherichia coli or yeast.
The three forms of human insulin have the same amino acid sequence, but are separately designated
2 Abbreviated from Lawrence R D 1961 King's College Hospital Gazette 40: 220. Transcript from a recorded after dinner talk to students' Historical Society.
as insulin emp (Enzyme Modified Porcine), prb (Proinsulin Recombinant in Bacteria) and pyr (Precursor insulin Yeast Recombinant). Although one of the incentives for introducing human insulin was avoidance of insulin antibody production, the allergies to older insulins were largely caused by impurities in the preparations, and are avoided equally well by using the highly purified, mono-component porcine and bovine insulins. Other preparations have been withdrawn. There is no systematic difference in activity between human and animal insulin, but any change in preparation prescribed to a patient should be monitored with care (see below).
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