COMPARISON OF THE PROPERTIES OF PHOSPHORIBOSYLPYROPHOSPHATE SYNTHETASE IN NORMAL AND LEUKEMIC HUMAN WHITE BLOOD-CELLS

Phosphoribosylpyrophosphate synthetase (PRPP synthetase) from normal human lymphocytes and granulocytes was compared with phytohemagglutinin (PHA)-stimulated lymphocytes and leukemic white blood cells of several types with respect to enzyme activity and kinetic constants. PRPP synthetase activity was determined by measuring the production of 14CO2 in a coupled reaction with |14C|orotic acid in the presence of orotidylate pyrophosphorylase and orotidylate decarboxylase. Enzyme activity was expressed both as nmoles/mg of protein/30 min and as nmoles/103 cells/30 min. In more than 50 per cent of the leukemic patients, activity was above normal values when compared on a per mg protein basis. However, when activity was compared on a per cell basis, only the acute myelocytic leukemic patients showed a change from the normal value, and in this case activity was inversely related to the percentage of blast cells in the peripheral circulation. Michaelis constants for ATP (KmATP) and ribose-5-phosphate (KmR5P) were found to be 17.6 ± 1.8 and 51.3 ± 2.4 μM, respectively, in normal lymphocytes, and 55.9 ± 8.0 and 82.5 ± 1.1 μM in normal granulocytes. The KmATP was found to decrease in all leukemic cell types, while the KmR5P showed deviation from normal values depending on the type of leukemia. The inhibition constants (KiATP and KiR5P) were also compared in all leukemic cell types and showed deviations from normal which were cell type dependent. These results suggest that sufficient alteration of the properties of PRPP synthetase from leukemic cells exists to warrant further investigation into the therapeutic implications of alterations of the properties of this enzyme. © 1979.