A cystine-dependent inactivator of tyrosine aminotransferase co-purifies with gamma-cystathionase (cystine desulfurase).

: Tyrosine aminotransferase is stable in homogenates of rat liver, but not when L-cystine or L-cysteine is added, which causes the enzyme to be reversibly inactivated due to oxidation of thiol groups. By monitoring inactivation of the aminotransferase in the presence of L-cystine, a factor responsible for this loss of activity was purified from rat liver. The factor required vitamin B6 and co-purified with gamma-cystathionase during numerous steps. Highly purified inactivating factor contained a protein that was identical in size and isoelectric point to cystathionase but also contained a dissimilar peptide that appeared to be unrelated to cystathionase. Cystathionase and the cystine-dependent inactivator shared several catalytic activities, including the hydrolysis of cystathionine, desulfuration of cystine, and desulfhydration of cysteine. During incubation of L-cysteine with the purified factor, hydrogen sulfide was generated but no inactivation of the aminotransferase occurred, suggesting that cysteine-dependent inactivation requires additional mechanisms. An insoluble inactivator of tyrosine aminotransferase that is produced during the reaction may be elemental sulfur, since colloidal suspensions of sulfur also inhibited the enzyme. Another inhibitor fractionated with high molecular weight substances; this may be protein-bound sulfane.