Human METTL7B is an alkyl thiol methyltransferase that metabolizes hydrogen sulfide and captopril

Methylation of alkyl thiols is a biotransformation pathway designed to reduce thiol reactivity and potential toxicity, yet the gene and protein responsible for human alkyl thiol methyltransferase (TMT) activity remain unknown. Here we demonstrate with a range of experimental approaches using cell lines, in vitro systems, and recombinantly expressed enzyme, that human methyltransferase-like protein 7B (METTL7B) catalyzes the transfer of a methyl group from S-adenosyl-l-methionine (AdoMet) to hydrogen sulfide (H2S) and other exogenous thiol small molecules. METTL7B gene modulation experiments, including knockdown in HepG2 cells and overexpression in HeLa cells, directly alter the methylation of the drug captopril, a historic probe substrate for TMT activity. Furthermore, recombinantly expressed and purified wild-type METTL7B methylates several thiol compounds, including H2S, 7α-thiospironolactone, l-penicillamine, and captopril, in a time- and concentration-dependent manner. Typical for AdoMet-dependent small molecule methyltransferases, S-adenosyl-l-homocysteine (AdoHcy) inhibited METTL7B activity in a competitive fashion. Similarly, mutating a conserved aspartate residue, proposed to anchor AdoMet into the active site, to an alanine (D98A) abolished methylation activity. Endogenous thiols such as glutathione and cysteine, or classic substrates for other known small molecule S-, N-, and O-methyltransferases, were not substrates for METTL7B. Our results confirm, for the first time, that METTL7B, a gene implicated in multiple disease states including rheumatoid arthritis and breast cancer, encodes a protein that methylates small molecule alkyl thiols. Identifying the catalytic function of METTL7B will enable future pharmacological research in disease pathophysiology where altered METTL7B expression and, potentially H2S levels, can disrupt cell growth and redox state.

Human METTL7B Encodes an Alkyl Thiol Methyltransferase that Methylates Hydrogen Sulfide

Summary Paragraph/AbstractMethyltransferase-like protein 7B (METTL7B) is implicated in tumor growth and progression while gene expression is upregulated in several different disease states such as rheumatoid arthritis and breast cancer. Yet, the catalytic function of METTL7B has not been characterized. Here we demonstrate that METTL7B encodes a protein that catalyzes the transfer of a methyl group from S-adenosyl-L-methionine (SAM) to hydrogen sulfide (H2S) to form methanethiol (CH3SH). Several exogenous aliphatic thiols were also identified as substrates. Modulation of METTL7B gene expression in HepG2 and HeLa cell culture directly alters the methylation of captopril, a marker reaction of alkyl thiol methyltransferase (TMT) activity(1, 2). Furthermore, cloned and recombinantly expressed and purified METTL7B full length protein methylates several thiol compounds, including hydrogen sulfide, 7α-thiospironolactone, captopril, and L-penicillamine in a concentration dependent manner. Endogenous thiols such as glutathione and cysteine or classic probe substrates of other known small molecule S-, N-, and O- methyltransferases were not substrates for METTL7B. Our results unequivocally demonstrate, and for the first time, that METTL7B, a protein implicated in several disease states, is an alkyl thiol methyltransferase(3–5). Identifying the catalytic function of METTL7B will enable future pharmacological research in disease pathophysiology where METTL7B expression and H2S levels can potentially alter the redox state and growth cycle of cells.