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 associated with periodontitis and may also play a role in the pathogenicity of T.denticola. Also displays homocysteine desulfhydrase activity, degrading homocysteine to produce hydrogen sulfide, 2-oxobutanoate and ammonia. H2O + L-methionine = 2-oxobutanoate + methanethiol + NH4(+) H2O + L-homocysteine = 2-oxobutanoate + H(+) + hydrogen sulfide + NH4(+) Homotetramer; dimer of active dimers. Belongs to the trans-sulfuration enzymes family. L-methionine gamma-lyase subfamily.