Role of 5′-nucleotidase in thiopurine metabolism: Enzyme kinetic profile and association with thio-GMP levels in patients with acute lymphoblastic leukemia during 6-mercaptopurine treatment

Abstract

Thiopurines are used for treatment of several diseases. Cytotoxicity is caused by the derived compounds 6-thioguanine nucleotides (TGNs) and methyl-6-thioinosine monophosphate (methylthio-IMP). The 6-thiopurine mononucleotides 6-thio-IMP (thio-IMP), 6-thio-GMP (thio-GMP) and methylthio-IMP can be catabolized by purine 5′-nucleotidase. It has been shown that the various 5′-nucleotidases are key enzymes for (6-thio)-purine metabolism. We aimed to investigate whether the overall 5′-nucleotidase (5′NT) activity is correlated with the efficacy and toxicity of 6-thiopurine nucleotides. Substrate affinity of 5′NT for IMP, GMP, AMP, thio-IMP, thio-GMP and methylthio-IMP was studied in human lymphocytes. For each of the substrates, the pH for optimal overall enzyme activity has been determined at a pH range between 6 and 10. At the optimal pH, assays were performed to establish K m and V max values. Optimal pH values for the various substrates were between 7 and 8.5. K m values ranged from 33 to 109 μM, V max ranged from 3.99 to 19.5 nmol/10 6 peripheral mononuclear cells (pMNC) h, and V max/ K m ratios ranged from 105 to 250. The results did not show a distinct preference of 5′NT activity for any of the tested thiopurine nucleotides. The enzyme kinetic studies furthermore revealed substrate inhibition by thio-IMP and thio-GMP as a substrate. Inhibition by thio-GMP also seems to occur in patients treated with 6-mercaptopurine (6 MP); subsequently, this may lead to toxicity in these patients.