INACTIVATION OF GUANOSINE 5'-PHOSPHATE REDUCTASE BY 6-CHLORO- 6-MERCAPTO- AND 2-AMINO-6-MERCAPTO-9-BETA-D-RIBOFURANOSYLPURINE 5'-PHOSPHATES

6-Chloro-9-β-D-ribofuranosylpurine 5′-phosphate (Cl-IMP) in high dilution rapidly inactivates guanosine 5′-phosphate (GMP) reductase purified from Aerobacter aerogenes; the process is slowed by GMP but not by reduced nicotinamide-adenine dinucleotide phosphate (NADPH) (the second substrate) unless GMP is present. Enzyme preparations which required glutathione (GSH) for activity were inactivated by Cl-IMP only when GSH was present. 6-Chloropurine nucleoside did not inactivate the enzyme; iodoacetamide inactivated reversibly with respect to GMP but was much less potent than Cl-IMP. When GSH is absent, the 6-mercapto analogs of IMP (inosine 5′-phosphate) and GMP strongly inactivate GMP reductase; 6-thioguanosine was ca. 20 times less effective. Inactivations by the 6-thio analogs were retarded by GMP and reversed by GSH. Inhibition by IMP was reversed by GMP but unaffected by GSH. The 6-chloro and 6-mercapto analogs appear to inactivate by forming thioether and disulfide bonds, respectively, with a sulfhydryl group at the GMP site. These inactivations of GMP reductase could play a part in inhibition of cell growth by the antineoplastic agents 6-chloropurine, 6- mercaptopurine, and 6-thioguanine which are anabolized to the nucleotide analogs of the present study. Inhibition of GMP reductase by adenosine 5′-triphosphate (ATP) is probably involved in cellular regulation of purine nucleotide interconversion; this inhibition appears to be exerted at a site other than the GMP and NADPH sites since NADPH did not reverse ATP inhibition and since strongly inhibitory levels of ATP did not retard inactivation of the enzyme by Cl-IMP. © 1968, American Chemical Society. All rights reserved.