Protein phosphatase activities have recently been classified into two major groups of enzymes termed type 1 and type 2. In the present study, this classification scheme has been used to explore the types of protein phosphatase activities found in rat heart, liver, and skeletal muscle, and to determine the type of protein phosphatase activity affected by diabetes. Protein phosphatase activities have been measured under conditions designed to minimize the influence of effectors of these enzymes, and a thermostable protein phosphatase inhibitor, termed inhibitor-2, has been used as a probe to specifically inhibit type 1 protein phosphatase. The specific activity of protein phosphatase-1 in extracts of heart, liver, and skeletal muscle from control rats ranged between 0.34 and 0.44 U/mg protein. The specific activity of a type 2 enzyme, termed protein phosphatase-2A, was approximately the same as protein phosphatase-1 in the case of skeletal muscle extracts, but was about 50% higher than type 1 in extracts from liver and heart. The only significant effect of diabetes was on hepatic protein phosphatase-1 in which a 50% decrease in specific activity was noted. Therefore, the effect of diabetes appeared to be confined to protein phosphatase-1 and this effect was only seen in liver.

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