Catalyzes the last step in the trans-sulfuration pathway from L-methionine to L-cysteine in a pyridoxal-5'-phosphate (PLP)-dependent manner, which consists on cleaving the L,L-cystathionine molecule into L-cysteine, ammonia and 2-oxobutanoate. Part of the L-cysteine derived from the trans-sulfuration pathway is utilized for biosynthesis of the ubiquitous antioxidant glutathione. Besides its role in the conversion of L-cystathionine into L-cysteine, it utilizes L-cysteine and L-homocysteine as substrates (at much lower rates than L,L-cystathionine) to produce the endogenous gaseous signaling molecule hydrogen sulfide (H2S). H2O + L,L-cystathionine = 2-oxobutanoate + L-cysteine + NH4(+) H2O + L-cysteine = H(+) + hydrogen sulfide + NH4(+) + pyruvate H2O + L-homocysteine = 2-oxobutanoate + H(+) + hydrogen sulfide + NH4(+) L-homoserine = 2-oxobutanoate + NH4(+) Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-homocysteine and L-serine: step 2/2. Homotetramer (By similarity). Interacts with CALM in a calcium-dependent manner (By similarity). Up-regulated in aspidocytes, a resistant cell type induced by a range of toxins including heavy metals and antibiotics. Belongs to the trans-sulfuration enzymes family.