mercaptopurine and 3-4-dimethoxy-5-hydroxybenzoic-acid

Thiopurine drugs are used in the treatment of inflammatory bowel disease--as are sulphasalazine and its metabolite 5-aminosalicylic acid (ASA). S-Methylation catalyzed by thiopurine methyltransferase (TPMT) is a major pathway in the metabolism of thiopurines. The hypothesis was tested that TPMT might be inhibited by sulphasalazine or isomers of ASA. Sulphasalazine as well as 3-, 4- and 5-ASA inhibited recombinant human TPMT, with IC50 values of 78, 99, 2600 and 1240 microM, respectively. Kinetic studies demonstrated that the inhibition of TPMT by sulphasalazine and ASA isomers was non-competitive with regard to the thiopurine substrate, 6-MP, and was uncompetitive with regard to the methyl donor for the reaction, S-adenosyl-L-methionine. Our observations raise the possibility of a clinically significant drug-drug interaction in patients treated simultaneously with sulphasalazine and thiopurine drugs.

Topics: Aminosalicylic Acids; Anti-Inflammatory Agents, Non-Steroidal; Azathioprine; Binding, Competitive; Drug Interactions; Gallic Acid; Humans; Kinetics; Mercaptopurine; Mesalamine; Methylation; Methyltransferases; Pyridines; Recombinant Proteins; Substrate Specificity; Sulfapyridine; Sulfasalazine

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of heterocyclic and aromatic sulfhydryl compounds such as the thiopurine drug 6-mercaptopurine (6-MP). TPMT activity in human tissue is regulated by a common genetic polymorphism, and "pharmacogenetic" variation in TPMT activity is an important factor in individual differences in thiopurine drug metabolism, toxicity and therapeutic efficacy. Human renal tissue contains two isozymes of TPMT, Peak I and Peak II, that can be separated by ion exchange chromatography. Our experiments were performed to determine whether S-adenosyl-L-methionine (Ado-Met), the methyl donor for the TPMT reaction, could be used as a photoaffinity ligand for these isozymes as one step in the study of the molecular basis for the TPMT genetic polymorphism. When [3H-methyl]Ado-Met and partially purified preparations of either isozyme of human kidney TPMT were exposed to ultraviolet light at 254 nm, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a 35 kDa protein was the predominant species that was radioactively labeled. The same 35 kDa protein was photoaffinity labeled with [14C-carboxyl]Ado-Met, demonstrating that labeling involved covalent binding of Ado-Met rather than methylation of the protein. TPMT enzymatic activity co-eluted with the 35 kDa protein during sequential DEAE ion exchange, gel filtration and hydroxylapatite chromatography. Inhibitors of TPMT enzymatic activity including S-adenosyl-L-homocysteine, sinefungin, 6-methylmercaptopurine and 3,4-dimethoxy-5-hydroxybenzoic acid inhibited photoaffinity labeling of the 35 kDa protein in preparations of both TPMT Peak I and Peak II isozymes in a concentration-dependent fashion, as did 6-MP, the methyl acceptor substrate for the TPMT reaction. All of these results were compatible with the conclusion that the 35 kDa protein was TPMT. Photoaffinity labeling of TPMT with [3H]Ado-Met should make it possible to purify the enzyme to homogeneity and to study amino acid sequences at or near its active site.

Topics: Adenosine; Affinity Labels; Autoradiography; Chromatography, Ion Exchange; Gallic Acid; Humans; Isoenzymes; Kidney; Mercaptopurine; Methyltransferases; Photolysis; Polymorphism, Genetic; S-Adenosylhomocysteine; S-Adenosylmethionine