Human liver thiopurine methyltransferase pharmacogenetics: biochemical properties, liver-erythrocyte correlation and presence of isozymes.

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine (6-MP). TPMT activity in the human red blood cell (RBC) is controlled by a common genetic polymorphism. Gene frequencies for this polymorphism are such that approximately one in 300 subjects is homozygous for the allele for low activity and lacks RBC TPMT activity, 11% of subjects are heterozygous and have intermediate levels of enzyme activity and 89% are homozygous for the allele for high activity. Our experiments were performed to determine whether the properties of TPMT in an important human drug metabolizing organ, the liver, were similar to those of RBC TPMT and to test the hypothesis that the genetic polymorphism which controls TPMT activity in the human RBC might also regulate the level of this enzyme activity in hepatic tissue. Human liver TPMT is a cytoplasmic enzyme and the Km values for 6-MP and S-adenosyl-L-methionine, cosubstrates for the reaction, were 580 microM and 2.7 microM, respectively. These properties, as well as the sensitivity of human liver TPMT to a panel of methyltransferase inhibitors, were similar to those of RBC TPMT. The enzyme activity was then measured in 119 surgical biopsy samples of hepatic tissue. Average hepatic TPMT activity was 13.6% higher in samples from male than in those from female patients. Frequency distribution histograms demonstrated the presence of a subgroup with intermediate enzyme activity that included 8.4% of samples. In addition, when TPMT activity was measured in both RBCs and hepatic tissue for 35 patients, those with inherited intermediate levels of RBC TPMT activity also had intermediate hepatic enzyme activity. Finally, ion exchange chromatography demonstrated the presence of two isozymes of TPMT in human hepatic tissue, but the isozymes did not appear to explain the molecular mechanism responsible for the genetic polymorphism. These results were compatible with the conclusion that the genetic polymorphism which controls TPMT activity in the RBC also controls levels of this important enzyme activity in a major human drug metabolizing organ, the liver.