BIOCHEMICAL STUDIES ON PHOSPHOHYDROLASE ISOENZYMES

The widespread occurrence in animal tissues of enzymes that in vitro, hydrolyse phosphate esters at alkaline pH has been recognized for many years, and the results of numerous biochemical studies on these alkaline phosphatases have been reported. In spite of these investigations, several features of the biochemistry of the alkaline phosphatases have remained in doubt, notably the precise limits of their substrate specificity, the identity or dissimilarity of alkaline phosphatases from the several tissues of a species, and the number of such phosphatases to be found within a given tissue. Although active extracts of phosphatase-containing tissues can readily be obtained, preparation of highly purified alkaline phosphatases from human sources has proved more difficult, and at least some of the uncertainties concerning these enzymes can be attributed to the use of enzyme preparations of a low degree of purity. One human alkaline phosphatase-that from the placenta-has now been crystallized,' however, and the present paper describes studies on the phosphatases of human liver, bone and small intestine after purification of the order of 1000to 2OOO-f0ld.~.~ The results of these investigations are considered from the viewpoint of enzyme specificity and interand intra-tissue heterogeneity.