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A study was made on the total ATP-ase, alkaline phosphatase, and glucose-6-phosphatase activity in homogenates prepared from normal monkey kidney cells in tissue culture. A characterization of such activity under standardized assay conditions revealed that in comparable amounts of cell homogenate ATP-ase was 8—10 times as active as alkaline phosphatase and glucose-6-phosphatase. Substrate hydrolysis was brought about by the activity of each enzyme at a rate which was constant with respect to time. The amount of substrate hydrolyzed, in each case, was directly proportional to the concentration of homogenate used in the assay system. Within certain limits this was also true with respect to substrate concentration. However, with increasing amounts of substrate and a constant amount of cell homogenate a point was reached where the activity of each enzyme became independent of initial substrate concentration. Under one set of assay conditions the total ATP-ase in cell homogenates was shown to be optimally active at approximately p H  7 while under another the enzyme showed a nearly directly proportional increase in activity per unit time between p H  4 and p H  10. Using standardized assay conditions such ATP-ase activity was markedly enhanced over that found in similar systems lacking added calcium and magnesium ion. No further stimulation of the activity of this enzyme, or of alkaline phosphatase and glucose-6-phosphatase, resulted from their interaction with 2.4-dinitrophenol. The substrate hydrolysis produced by the activity of each enzyme in cell homogenate preparations resulted from irreversible reactions. Such homogenates were incapable of carrying out an exchange reaction of radioactive orthophosphate with the phosphorous from any of the substrates used in this study.

Phosphatase Activity in Homogenates from Normal and Poliovirus Infected Tissue Cultures: I. Characterization of the Normal Activity