Catalyzes the cleavage of cystathionine to homocysteine, pyruvate and ammonia during methionine biosynthesis (By similarity). Also has cytotoxic activity toward osteogenic, osteosarcoma and tracheal cells, in vitro. The chemical basis for cell toxicity might be the formation and subsequent transfer of sulfane-sulfur to proteins, derived via beta-cystathionase cleavage of L-cystine (PubMed:8463265). 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. Homodimer. Belongs to the trans-sulfuration enzymes family.