The nature of insulin-degrading activity in the human placenta has been examined. Two soluble enzyme activities, I and II, have been demonstrated, in addition to an activity in the cell debris fraction. Activity I is sulfhydryl dependent and can be partially resolved into two components on C-100 Sephadex chromatography. The minor component chromatographs with activity II in the void volume. The major portion is of smaller molecular weight, as evidenced by its greater elution volume. Activity I has a pH optimum of 7.2 to 7.4 and a broad range of substrate specificity. These properties, along with the demonstration of glutathione-insulin transhydrogenase activity in placental supernatant, raise the possibility that a substantial part of activity I is due to a glutathione-insulin transhydrogenase. Activity II is not sulfhydryl dependent but appears to be stimulated by sulfhydryl and inhibited by p-chloromercuribenzoate. It has a pH optimum of 6.3 to 6.6, shows greater specificity for insulin than does activity I and is less readily inactivated at 60° C. (t1/2, I = 25 min. II = 120 min.). It shows the same elution volume on G-150 Sephadex as rat muscle protease. The characteristics of activity II suggest that it is a sulfhydryl protease similar to that observed in rat skeletal muscle.

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