Catalyzes the cleavage of cystathionine to homocysteine, pyruvate and ammonia during methionine biosynthesis. H2O + L,L-cystathionine = L-homocysteine + NH4(+) + pyruvate an S-substituted L-cysteine + H2O = a thiol + NH4(+) + pyruvate Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from L-cystathionine: step 1/1. Homotetramer. More than twofold more abundant in the small cell variant (SCV) stage than in the large cell variant (LCV) stage (at protein level). LCVs are more metabolically active than SCVs. Belongs to the trans-sulfuration enzymes family.