A reinterpretation of phosphatase-N

In order to determine the substrate specificity of the alkaline phosphatase (APase) which appears in murine lymphomas, two substrates were chosen. p-Nitrophenyl phosphatase (pNPP), the standard substrate, has an oxygen-phosphorus (O-P) bond. Cysteamine-S-phosphate (CASP) has a sulfur-phosphorus (S-P) bond. It has been reported that murine lymphoma APase does not cleave the S-P bond of CASP. These results were not confirmed. Under all conditions tested, the murine lymphoma APase showed consistent hydrolysis of CASP at approximately one-third the rate of hydrolysis of pNPP. Biochemical characteristics used to confirm this include pH optimum, heat inactivation, kinetics, magnesium activation, L-homoarginine inhibition, EDTA inhibition, and activity remaining during stages of partial purification. It is concluded that the murine lymphoma APase shows a preference of pNPP as a substrate but does hydrolyse CASP at a lower rate. The conflict between our laboratory and that of Neumann's may be one of interpretation rather than data. Neumann shows similar data to ours in terms of the 3-fold ratio, but considers all hydrolysis of CASP as representative of 'normal' APase. She uses a formula to calculate the amount of lymphoma APase based on the ratio 1.9 which is the ratio of hydrolysis of pNPP to that of CASP in kidney tissue. Our interpretation would be that the same isozyme is hydrolyzing the S-P bond at one-third the rate that it hydrolyzes the O-P bond.