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Insulin
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| Insulin is a two chain polypeptide hormone with a molecular weight of 6,000. It has a wide effect on metabolic processes. A unifying characteristic is that insulin promotes anabolic processes and inhibits catabolic ones in muscle, liver and adipose tissue. Specifially, insulin increases the rate of synthesis on glycogen, fatty acids and proteins and also stimulates glycolysis. The important action of the hormone is promoting the entry of glucose, some simple sugars and amino acids across the plasma membrane into muscle and fat cells. Where epinephrine and glucagon signal that glucose is scarce, insulin signals that glucose is abundant.
Work by Sanger and his coworkers (Sanger, F. & Thompson, E.O.P. & Kitai, R., Biochem J., 59, 521, 1955) elucidated the primary structure of insulin, and the X-ray crystallographic work of Hodgkin et al (Adams, M.J., et al, Nature, 224, 491, 1969 and Bentley, et al, Nature, 261, 166, 1976) has given us the secondary, tertiary and quaternary structures of the native two-zinc crystalline hormone. Bentley (1976) reported the X-ray structure of a second form of insulin, a slow-acting clinical preparation, which crystallizes with four zinc ions when the sodium and chloride ion concentration in the solution is 6% or more.
Insulin is isolated from the pancreas glands of bovine and is prepared by the procedure of Smith, Am. J. Med., 40, 662, 1966.
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