RESISTANCE TO 6-MERCAPTOPURINE: I. BIOCHEMICAL DIFFERENCES BETWEEN THE EHRLICH ASCITES CARCINOMA AND A 6-MERCAPTOPURINE-RESISTANT SUBLINE

Abstract

A 6-mercaptopurine (6-MP)-resistant subline of the Ehrlich ascites carcinoma developed in this laboratory has been reported previously to lack the ability to synthesize thioinosinate, a characteristic believed to be related to mercaptopurine resistance. A comparison was made of various aspects of purine nucleotide metabolism in cells of the parent tumor line and the resistant subline in order to detect differences which, if they existed, might account for resistance to 6-MP.The synthesis of adenylate or of inosinate from adenine-8-C¹⁴was found to take place at similar rates in the two cell types under conditions in which adenylate renewal was independent of adenine-8-C¹⁴concentration. Both cell types synthesized adenylate and inosinate at comparable rates from hypoxanthine-8-C¹⁴and had equivalent adenylate deaminase activities. The size of the pool of inosinate in the tumor cells was measured by the isotope dilution method and that of the Ehrlich cells was found to be 1.6 times the pool size of the resistant cells. The conversion of guanine-8-C¹⁴to guanylate took place at similar rates in both tumors.In general, no differences in purine nucleotide metabolism were revealed which were obviously related to resistance or to the characteristic inability of the resistant subline to synthesize thioinosinate. This inability did not appear to be due to an inadequacy of the phosphoribosylpyrophosphate resources of the resistant cell, since the rates of purine nucleotide synthesis were similar in the two cell types. In Part II it is shown that extracts of the 6-MP-resistant cells catalyzed the synthesis of thioinosinate. It was concluded that the failure of intact resistant cells to synthesize thioinosinate was due to failure of 6-MP to enter the cell.