Methyl conjugation is an important pathway in the metabolism of many drugs, other xenobiotic compounds, neurotransmitters and hormones. During the past two decades, the applicant's laboratory has systematically explored the pharmacogenetics of enzymes that catalyze methylation in humans. As a result, we have discovered and characterized--both biochemically and at the molecular level--a series of functionally significant genetic polymorphisms for methyltransferase enzymes in humans. One of those, the genetic polymorphism for thiopurine methyltransferase (TPMT), represents a striking example of the potential contribution of pharmacogenetics to medicine. We have also discovered and characterized genetic polymorphisms for other enzymes that catalyze S- and N-methylation, including histamine N-methyltransferase (HNMT) and thiol methyltransferase (TMT). During the preceding funding cycle for this grant, we successfully cloned cDNAs and genes for both TPMT and HNMT in humans, discovered the molecular basis for functionally significant genetic polymorphisms within exons of those enzymes, and applied those observations to clinical medicine. We now propose to extend our studies of TPMT and HNMT molecular pharmacogenetics to include functionally significant genetic polymorphisms outside of the exons of the humans TPMT and HNMT genes. We also propose to expand our methylation pharmacogenetics research in humans to include studies of molecular mechanisms involved in the genetic polymorphism for TMT, a membrane-bound enzyme that catalyzes the S-methylation of important sulfhydryl drugs such as captopril and D-penicillamine. The results of these experiments will contribute to our understanding of molecular mechanisms responsible for the genetic regulation of enzymes that catalyze the methyl conjugation of drugs, other xenobiotic compounds and neurotransmitters in humans and may make it possible to predict individual variations in the methylation and, therefore, variation in the toxicity or therapeutic efficiency of substrates for these enzymes.