Human pharmacogenetics of methyl conjugation

Methyl conjugation is an important pathway in drug metabolism. Activities of three human drug-metabolizing methyltransferase enzymes, catechol-O-methyltransferase (COMT) (EC 2.1.1.6), thiopurine methyltransferase (TPMT) (EC 2.1.1.67), and thiolmethyltransferase (TMT) (EC 2.1.1.9), are controlled by inheritance. COMT activity in the red blood cell (RBC) is regulated by a single genetic locus with two alleles, COMT(L) for low activity and GOMT(H) for high activity. Gene frequencies of these two alleles were approximately equal in a white population sample of Northern European origin. The genetically controlled level of COMT activity in the RBC reflects the level of enzyme activity in other tissues and is significantly correlated with individual variations in the methyl conjugation of catechol drugs such as L-dopa and methyldopa. TPMT catalyzes the S-methylation of thiopurines and thiopyrimidines. RBC TPMT activity is also controlled by a single genetic locus with two alleles, TPMT(L) for low and TMPT(H) for high activity. The gene frequencies of these two alleles were 0.06 and 0.94, respectively, in a white population sample, RBC TPMT activity reflects the level of enzyme activity in other cells and tissues such as the lymphocyte and kidney TMT catalyzes the S-methylation of aliphatic sulfhydryl compounds such as the drugs captopril and D-penicillamine. The heritability of the level of RBC membrane TMT activity has been estimated on the basis of family studies to be approximately 0.98. Regulation of these three methyl-conjugating enzymes by inheritance raises the possibility that genetically determined methylator status may be one factor responsible for variations in drug metabolism in humans.