Thiopurine methyltransferase isozymes in human renal tissue

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine (6-MP). Levels of TPMT activity in human tissue are controlled by a common genetic polymorphism. On a genetic basis, 88.6% of humans have high, 11.1% have intermediate, and 0.3% have very low or undetectable levels of enzyme activity. Ion exchange chromatography of a pooled human kidney preparation yielded two peaks of TPMT activity. Approximately 83-88% of the enzyme activity eluted as a major peak (peak I), while 12-17% eluted in a distinct second peak (peak II). Each of these isozymes was then purified further by gel filtration chromatography. The two isozymes had similar pH optima, similar K(M) values for the reaction co-substrates, 6-MP and S-adenosyl-L-methionine, and similar K(i) and K(s) values for the TPMT inhibitor 3,4-dimethoxy-5-hydroxybenzoic acid. Electrophoretic mobilities of the two isozymes were identical, and both had molecular weights of approximately 30,000. To determine whether tissue from individual subjects with different presumed genotypes for the TPMT genetic polymorphism contained one, the other, or both isozymes, TPMT activities were measured in renal preparations from 48 patients. Of these patients, 40 (83%) had high and 8 (16%) had intermediate levels of activity. Ion exchange chromatography of renal preparations from three individuals with intermediate and three individuals with high TPMT activities showed that all six samples contained both isozymes in similar proportions. Our results demonstrate that two isozymes of TPMT are present in the human kidney. However, these isozymes do not appear to be the molecular basis for the genetic polymorphism which controls TPMT activity in human tissue, and this polymorphism is expressed in both of the two isozymes.