Subcellular Distribution and Characteristics of Hepatic Protein Phosphatases Acting on Glycogen Phosphorylase and on Glycogen Synthase

Abstract

An important part of the hepatic Phosphorylase phosphatase activity is associated in a largely latent form with the nuclear fraction.1 In contrast, the phosphatase activity that is responsible for the activation of glycogen synthase is located in the post-mitochondrial supernatant, which comprises the glycogen fraction, the microsomes, and the post-microsomal supernatant. In our laboratory the synthase-phosphatase activity is recovered partly in the supernatant and partly as an enzyme-glycogen complex.2 These two enzyme fractions have distinct regulatory properties.3 No significant synthase-phosphatase activity could be detected in the microsomal fraction.3 In contrast, Cohen’s groups4 recently reported the presence of synthase-phosphatase activity in the microsomal as well as in the glycogen fraction; they adduced evidence in favour of a single protein phosphatase (type-1) that also displays Phosphorylase phosphatase activity. In view of these discrepant data we have reassessed the characteristics of the protein phosphatases present in the glycogen fraction, the microsomal fraction, and the cytosol. The present report summarizes the results and offers some explanations.