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. Also displays homocysteine desulfhydrase activity, degrading homocysteine to produce hydrogen sulfide, 2-oxobutanoate and ammonia. L-cysteine and S-methyl-L-cysteine are poor substrates for the enzyme. Plays an important role in the resistance of F.nucleatum to the antibacterial agent 3-chloro-DL-alanine (3CA), thanks to its 3CA chloride-lyase (deaminating) activity. H2O + L-methionine = 2-oxobutanoate + methanethiol + NH4(+) H2O + L-homocysteine = 2-oxobutanoate + H(+) + hydrogen sulfide + NH4(+) Homotetramer. F.nucleatum strain ATCC 10953 is able to grow in medium containing 1 mM 3CA. Belongs to the trans-sulfuration enzymes family. L-methionine gamma-lyase subfamily.