Subcellular distribution of inorganic pyrophosphatase activity in various normal and neoplastic cell types

Inorganic pyrophosphatase (PPiase) activity was measured in cell fractions of rat, mouse, and human erythrocytes; normal rat liver; Novikoff hepatoma; Morris 3924A hepatoma; and mouse Ehrlich and Sarcoma 37 ascites tumors. Despite high intracellular activities, when precautions were taken to maintain cell integrity, only negligible activities were found on the surface of intact erythrocytes, Novikoff ascites hepatoma, Ehrlich carcinoma, and Sarcoma 37 cells. Suspensions of intact Ehrlich and Sarcoma 37 cells exhibited low PPiase activity, but this was only about 1 to 2% of the intracellular activity and was completely accounted for by activity present in the suspension medium. It is considered to be due to extrusion of the intracellular enzyme. Systematic fractionation of subcellular components revealed that 92% of the total PPiase activity of rat liver and 97.5% of that of Hepatoma 3924A were in the cytosol. The soluble activity consisted of a major form, accompanied by very low activities of two minor forms, all of which migrate toward the anode on electrophoresis. About 4.5% of the liver activity was present in the mitochondria in two forms, one remaining at the origin and one migrating toward the cathode. The same cytosolic isozymes were present in Hepatoma 3924A, and the cathodic form was present in mitochondria but in much reduced amount. No evidence was obtained for specific isozymes in nuclei or microsomes. Only negligible PPiase activities were found in cell membranes isolated by sucrose gradient centrifugation. These results discount the occurrence of PPiase activity as an ectoenzyme or in the plasma membrane of these cells and point to the cytosol as the major and mitochondria as a minor locus of intracellular PPiase activity.