Abstract

Sarcoma 180 cells contain a highly specific guanylate kinase (ATP-GMP phosphotransferase, EC 2.7.4.8) that converts GMP to GDP. This tumor enzyme closely resembles in its kinetic parameters and molecular size the guanylate kinase of hog brain. With both enzymes the analogue nucleotide 6-thioguanosine 5'-phosphate behaves as a potent inhibitor, competitive with GMP, with an inhibition constant (K_i) of about 6 x 10^-5 M. When examined with large amounts of hog brain enzyme, 6-thioguanosine 5'-phosphate behaves as an alternative substrate with a very low maximum velocity that is less than 0.04% of the reaction rate attainable with GMP as the substrate. The possible role of these findings in the cytolytic action of 6-thioguanine is discussed.