Insulin Insulinase

Internalization of protein hormones is a principal means of hormone degradation in lysosomes through a receptor-mediated pathway [70], There may also be membrane-associated degradative activities for insulin (insulinase) and other protein hormones, however for EGF, degradation occurs exclusively through an intracellular process [68,71]. 

Site of synthesis and secretion Enzyme that degrades insulin and its source Half-life of insulin Insulin is a polypeptide hormone produced by thep cells of the islets of Langerhans of the pancreas. Its synthesis involves two inactive precursors, preproinsulin and proinsulin, which are subsequently cleaved to form the active hormone. Insulin is stored in the cytosol in granules that are released by exocytosis Insulin is degraded by the enzyme insulinase produced primarily by the liver. Insulin has a plasma half-life of approximately six minutes. 

Substrates for hepatic metabolism include insulin, glucagon, and t-PAs [89,90]. For insulin, an acidic endopeptidase (termed endosomal acidic insulinase ) appears to mediate internalized insulin proteolysis at a number of sites [91]. Specifically, the endosomal activity results from cathepsin D, an aspartic acid protease [92]. Similarly, proteolysis of glucagon has also been attributed to membrane-bound forms ofcathepsins B and D [93]. 

Authier, F., G.M. Danielsen, M. Kouach, G. Briand, and G. Chauvet. 2001. Identification of insulin domains important for binding to and degradation by endosomal acidic insulinase. Endocrinology 142 276-289. 

Insulinase. An enzyme that hydrolyzes insulin and is prepd from hog pancreas Brink, Lewis, U.S. pet. 2,957, -809 (I960 to Merck Co.). May be obtained from commercial pancreatin or trypsin. Even the purified crystals contain large amounts of elastase. Review Thomas, Postgrad. Med. J. Suppl, 49, 940 (1973). 

Sapin, R., J-C. Ongagna, F. Gasser, and D. Grucker. 1998. Insulin measurements in hemolyzed serum Influence of insulinase inhibitors. Clinica ChimicaActa 274 111-117. 

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