METTL7 Alkyl Thiol Methyltransferases: Implications in Drug Metabolism and Human Health

Abstract

Thiol metabolism in humans is catalyzed by two enzymes, thiopurine methyltransferase (TPMT) and thiol methyltransferase (TMT). TPMT selectively methylates thiopurine compound while TMT is selective for aliphatic thiol substrates. Both enzymes exhibit highly variable activity in vivo which can lead to toxicity for thiol-containing drugs that undergo methylation, such as azathioprine. While TPMT has been extensively characterized, TMT has never been purified despite prior attempts. As such, the gene and protein sequence remain unknown. We identified methyltransferase-like protein 7A and 7B, METTL7A and METTL7B respectively, as thiol methyltransferases specific for aliphatic thiol compounds. Modulation of their gene and protein expression in multiple human cell lines directly alters methylation of captopril, a TMT marker reaction. Purified recombinant METTL7B enzymatically catalyzes methylation of several known TMT substrates, including hydrogen sulfide, and exhibits classic Michaelis-Menten steady state kinetics. TMT activity is highly variable (~9-fold) between single donor human liver microsomes (HLMs) and is not explained by variation in the expression of METTL7A and METTL7B protein. METTL7A and METTL7B gene expression is not correlated in single donor human liver microsomes (HLMs), however relative protein expression is highly correlated, suggesting potential post-translational co-regulation. Both genes are downregulated in human ventricular cardiomyocytes upon exposure to doxorubicin-induced oxidative stress, possibly due to their role in hydrogen sulfide catabolism. Overall, TMT activity in humans is catalyzed, in part, by METTL7A and METTL7B. This is supported by modulating gene expression in human cell culture and using purified recombinant enzyme. Variation in TMT interindividual activity is not fully explained by protein expression of METTL7A and METTL7B and could be due to post-translational modification. Preliminary analysis suggests that both proteins are involved in oxidative stress response, however, future experiments should focus on their role in human health and disease.