title: Guanosine 5'-monophosphate reductase from Leishmania donovani. A possible chemotherapeutic target.
abstract: GMP reductase was highly purified from promastigotes of Leishmania donovani by chromatography on a single DEAE-cellulose column. Bimodal substrate saturation curves resulted in a 1/v versus 1/[GMP] plot that curved downward above 40 microM GMP. The kinetic constants were, therefore, obtained with GMP below this concentration. The K'm for GMP was 21 microM at pH 6.9. The enzyme was very sensitive to activation by GTP. At 20 microM GMP, a maximum of 600% activation occurred at 100 microM GTP. Half-maximal activation occurred at 8 microM GTP. GTP at 100 microM did not affect the K'm for GMP but did increase its V'max by 7-fold. Xanthosine monophosphate (XMP) and IMP analogs served equally well as competitive inhibitors versus GMP. The inhibition by the analogs and the activation by GTP were mutually antagonistic processes. The inhibition by the IMP analogs, allopurinol nucleotide and thiopurinol nucleotide is of chemotherapeutic interest because these compounds were shown previously to be produced in Leishmania from the anti-leishmanial agents allopurinol and thiopurinol. These nucleotides were 100- and 20-fold, respectively, more potent inhibitors of GMP reductase from L. donovani than of the corresponding enzyme from human erythrocytes.
mesh_terms: D000818:Animals; D004789:Enzyme Activation / Q000187:drug effects; D050899:GMP Reductase; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007700:Kinetics; D007891:Leishmania / Q000187:drug effects / Q000201:enzymology*; D009247:NADH, NADPH Oxidoreductases / Q000037:antagonists & inhibitors*; D009711:Nucleotides / Q000138:chemical synthesis / Q000627:therapeutic use*
chemical_list: D009711:Nucleotides; D006160:Guanosine Triphosphate; D009247:NADH, NADPH Oxidoreductases; D050899:GMP Reductase