Asparagine synthetase in normal and malignant tissues; correlation with tumor sensitivity to asparaginase

Abstract

Asparagine synthetase activity has been measured in normal and malignant mouse and normal guinea pig tissues. The enzyme has a pH optimum of 8.0, requires ATP, uses either glutamine or NH 4Cl as a source of amide, and yields increased amounts of asparagine in the presence of puromycin. Significantly higher ASase activity has been observed in four asparaginase resistant mouse lymphomas than in two susceptible tumors or in normal mouse tissue. Liver is an exception, but exact values are rendered uncertain by the presence of asparaginase. When asparaginase is administered to tumor-bearing mice, resistant tumors respond with large increases (5 to 19-fold) in ASase activity over a 24-hr period while susceptible tumors respond only transiently, ASase levels reaching a maximum in 4 hr and then decreasing concomitantly with cell death. These observations confirm and extend the idea that ASase activity may provide a basis for tumor resistance or sensitivity to asparaginase. The moderate increases in ASase in normal tissue following asparaginase administration account for the low toxicity of the enzyme. The levels reached in normal mouse tissue are equivalent to those of the tissues of guinea pigs which have asparaginase in the circulation as a natural constituent. The increased ASase activity following asparaginase administration may be accounted for by contributions of both reduced product inhibition and derepression of ASase synthesis. Induction apparently plays no role in these tumors. ASase appears in the postmicrosomal supernates of tumor cells and to a lesser degree in microsomes. In liver essentially all of the detectable ASase is in the microsomes, and asparaginase is present in the supernate.