Journal of the Japan Diabetes Society
Online ISSN : 1881-588X
Print ISSN : 0021-437X
ISSN-L : 0021-437X
Studies on Insulin Metabolism Insulin Degrading Enzyme and Its Biological Significance
Hideyo Sakai
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1977 Volume 20 Issue 5 Pages 624-635

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Abstract
A soluble muscle enzyme which preferentially degrades insulin has been isolated from the supernatant of pig skeletal muscle centrifuged at 100, 000×g. This enzyme was purified 3521-fold by fractionation with (NH4) 2SO4, Sephadex G-200 column chromatography and Ca3 (PO4) 2 gel adsorptionelution. The enzyme was shown to be located mainly in the soluble fraction of the cell. The molecular weight of the enzyme was estimated to be about 140, 000 by its behavior on the Sephadex G-200. The enzyme exhibited a relatively narrow pH range with the optimum activity occuring at pH 7.O. The Km for insulin degradation was estimated to be 112 nM. Marked specificity for insulin degradation was found, and the degradation of insulin was shown to be proteolytic. The enzyme activity was independent of dithiothreitol, whereas N-ethylmaleimide was shown to be a potent inhibitor. These data suggest that a sulfhydryl group in the enzyme molecule is necessary for its activity. Thus, the enzyme could be verified as different from glutathione-insulin transhydrogenase and insulin-specific protease by their, enzymatic characteristics, and from cathepsins by their enzymatic characteristics. Therefore, the enzyme seems to be established as a unique insulin degrading enzyme.
In order to clarify the correlation between the enzyme activity and blood levels of insulin, changes in the activities of the enzyme partially purified from rats under various dietary conditions were observed. The enzyme activity correlated positively only with IRI (r=0.631, p<0.01), and not with body weight and blood glucose. Changes in the enzyme activity were shown to be well correlated with changes in the IRI level, while the increase in the enzyme activity was shown to be inhibited by actinomycin D. These data suggest that the enzyme activity may be regulated with its induction by available insulin.
From the results of the studies on the insulin degrading enzyme and its correlation with blood levels of insulin, it may be concluded that the enzyme degrades insulin with marked specificity and, with its optimum pH in the physiological range, ensures this one enzyme to be the insulin degrading enzyme. Its biological significance is that this enzyme may be one of the factors which regulates the blood levels of insulin.
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