Abstract
The activity of histidase (L-histidine ammonia-lyase EC 4. 3. 1. 3) of rat liver was inhibited by cystine competitively with the substrate histidine. Histidase was inactivated by preincubation with cystine, but the inactivated enzyme was reactivated by the addition of glutathione to the reaction mixture. Cysteine, at a lower level, partly reactivated the enzyme inactivated by cystine, but, at a higher level, did not reactivate it. Cysteine itself acted inhibitory on the enzyme. Glutathione did not reactivate histidase activity inhibited by cysteine. No other amino acid affected histidase activity at the level of 2mM. Apparent Km of rat liver histidase was l.8×lO-3M.
Neither inhibitory nor activating effect on histidase activity was observed by the addition of liver homogenate obtained from rats fed a histidine imbalanced diet to that from those fed a basal diet and vice versa.
