(sis'tă-thī'ō-nēn gam'ă lī'ās) A liver enzyme that catalyzes the hydrolysis of l-cystathionine to l-cysteine and 2-ketobutyrate. A deficiency of this enzyme results in cystathioninuria. A step in methionine catabolism and in cysteine biosynthesis. Synonym(s): cystathionase.