Catalyzes the alpha,gamma-elimination of L-methionine to produce methanethiol, 2-oxobutanoate and ammonia; methanethiol (methyl mercaptan) is considered to be one of the main causes of the oral malodor in periodontal disease and may also play a role in the pathogenicity of P.gingivalis in that disease (PubMed:11083813). Is also able to catalyze the alpha,gamma-elimination of L-homocysteine (By similarity). H2O + L-methionine = 2-oxobutanoate + methanethiol + NH4(+) H2O + L-homocysteine = 2-oxobutanoate + H(+) + hydrogen sulfide + NH4(+) Homotetramer; dimer of active dimers. Cells lacking this gene show a marked decrease in the formation of methyl mercaptan from L-methionine and decreased virulence compared with the wild-type strain W83. The hydrogen sulfide content in the culture supernatants of mutant and wild-type strains is similar. Belongs to the trans-sulfuration enzymes family. L-methionine gamma-lyase subfamily. Truncated N-terminus.