Alkaline phosphatase activities of 6-thiopurine-sensitive and -resistant sublines of Sarcoma 180.

Abstract

Abstract The alkaline phosphatase activities of 6-thiopurine-sensitive and -resistant sublines of Sarcoma 180 were examined in a further study of these enzymes, which have a role in the resistance of Sarcoma 180/TG to 6-thioguanine and 6-mercaptopurine. Both neoplasms possessed particulate-bound alkaline phosphatase enzymes labeled A and B on the basis of their ease of solubilization from the particulate fraction; more Enzyme A was present in both sublines, with the total activity of Enzyme Fractions A and B being about 100-fold greater in the resistant variant. A storage-labile, sulfhydryl reagent-sensitive factor, presumably a proteolytic enzyme, was demonstrated to be involved in the release of alkaline phosphatase A from the particulate fraction of Sarcoma 180/TG. p -Chloromercuribenzoate, iodoacetamide, phenol, and ethanol were all potent inhibitors of the solubilization of alkaline phosphatase A from the particulate fraction. Both alkaline phosphatases A and B of Sarcoma 180/TG were capable of degrading 6-thioinosine 5′-phosphate to its nucleoside form at optimum and physiological pH values; the rates of hydrolysis of the 6-thioinosine nucleotide were equivalent to those of inosine 5′-monophosphate. In contrast neither enzyme cleaved nucleoside triphosphates such as adenosine triphosphate under the conditions used. Significant differences exist in the properties of these enzymes. Thus, the optimum reaction pH of Enzymes A and B of Sarcoma 180 were considerably lower than those of the resistant variant. In addition, the thermostability and sensitivity of these enzymes to l-homoarginine inhibition differed. The findings, together with the immunological distinctions reported in the following paper (11), suggest that differences exist between the alkaline phosphatases of Sarcoma 180 and Sarcoma 180/TG.