Particulate‐associated Protein Phosphatases of Rat Hepatomas as Compared with the Enzymes of Rat Liver

In the course of investigating the neoplastic alterations of protein phosphatases, the particulate fractions of rat liver and AH‐13, a strain of rat ascites hepatoma, were chromatographed on DEAE‐cellulose and assayed for protein phosphatase using glycogen synthase D and phosphorylase a as substrates. The synthase phosphatase activity of rapidly growing AH‐13 was due almost entirely to a divalent cation‐inhibited protein phosphatase, tentatively designated phosphatase N, the level of which was elevated remarkably in the hepatoma as compared with liver. Other hepatomas including primary hepatoma induced with 3′‐methyl‐4‐dimethylaminoazobenzene also exhibited high levels of this phosphatase. Phosphatase N exhibited M(r)=49,000 (gel filtration) and has been partially purified with little alteration in properties. Partially purified phosphatase N was inhibited by divalent cations, rabbit skeletal muscle polypeptide inhibitor‐2 and heparin, and released the catalytic subunit of type‐1 protein phosphatase upon tryptic digestion. It is therefore apparent that phosphatase N is a type‐1 protein phosphatase. There is some evidence to suggest that the high levels of phosphatase N in neoplastic cells are due primarily to enhanced synthesis of its non‐catalytic (regulatory) subunit.