Evidence of acid phosphatase in the cytoplasm as a distinct entity

A study of subcellular acid phosphatase distribution in mammalian tissues shows that isozymes with specific functions are compartmentalized in the cells. The enzyme may be generalized into two types: type A and type B. They are shown by several means to be distinct entities. Type A is confined to the cytoplasm and is inhibited by Cu2+, HCHO, and the coupling agents (for enzyme staining) fast blue RR salt and fast Garnet GBC salt (newly discovered inhibitors), but is insensitive to fluoride and L-(+)-tartrate. Type B is localized in the organelles, presumably lysosomes, in both soluble form and membrane-bound form, with inhibitor sensitivity exactly opposite to that of type A enzyme. Types A and B consist of different sets of isozymes, with sensitivities to inhibitors resembling those observed with the crude extracts of subcellular fractions. Acid phosphatase that exhibits a phosphoryl transfer property was identified as type A enzyme. Type A enzyme has a slightly higher optimal pH and is inhibited by alloxan, whereas for type B, the addition of alloxan broadens the optimal pH to a higher range and elevates the activity of pH 7.4 from negligible to about 30-40% of that obtained under optimal conditions. The alloxan-mediated elevation of type B enzyme activity to this level at the physiological pH may be of considerable significance. Type B enzyme has a high affinity for metabolic intermediates and nucleotides, while type A has an extremely low affinity for these substrates. Cytoplasmic acid phosphatase (type A) is a significant enzyme population and its activity is not related to the lysosome density in the cells. Type A enzyme in the cytoplasm is thus shown to be an entity distinctly different from type B enzyme in the lysosomes. These findings suggest that the physiological functions of type A acid phosphatase, such as metabolic regulatory processes, merit further studies because of the phosphoryl transfer activity and cytoplasmic localization of the enzyme.