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

The alkaline phosphatase activities of 6-thiopurine-sensitive and -resistant sublines of Sarcoma 180 were exam ined 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, pre sumably 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 phospha tase A from the particulate fraction. Both alkaline phos phatase s 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 equiva lent 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 inhi bition differed. The findings, together with the ¡mmunological distinctions reported in the following paper (11), sug gest that differences exist between the alkaline phosphatases of Sarcoma 180 and Sarcoma 180/TG.