Adaptive Responses of Mammalian Histidine-degrading Enzymes

Effects of nutritional and hormonal treatments on the activities of rat liver histidine-catabolizing enzymes histidine pyruvate transaminase, histidase, and urocanase, have been examined and estimates have been made of the rates of induction and degradation of each enzyme. Histidase and urocanase activities were slightly elevated in animals fasted for 48 hours, but the transaminase was not affected. In animals fed for 14 days on diets containing various amounts of casein, the activity of histidase increased 20-fold, urocanase 6-fold, but the transaminase only 2-fold as the casein content of the diet was increased from 6 to 80%. When rats were fed a diet containing 4% of histidine for 14 days, histidase and urocanase activities per g of liver were only slightly above control values, but since liver weights increased greatly, the total activities of these two enzymes increased per unit of body weight. The transaminase, in contrast, was diluted as the liver increased in size. A high histidine intake did not stabilize the high histidase and urocanase activities induced by feeding rats an 80% casein diet when they were subsequently fed a low protein diet. Transaminase activity increased 6-fold in adrenalectomized rats fed an 18% casein diet and treated with daily injections of hydrocortisone acetate (2.5 mg per 150 g, body wt) for 14 days; histidase and urocanase activities responded much less (3-fold and 2-fold, respectively). Adrenalectomy depressed the activities of histidine-catabolizing enzymes, the greatest depression occurring in those rats fed a low protein diet. Although glucocorticoids were not necessary for protein-induced increases in transaminase and urocanase activities, hydrocortisone (5.0 mg per 150 g, body wt) and high protein together acted synergistically. In rats fed either a high protein diet or administered hydrocortisone, several days of treatment were required before substantial increases occurred in the activities of these enzymes. In rats fed a diet containing 80% casein for 14 days and then switched to a diet containing no casein, the transaminase, histidase, and urocanase decreased with apparent half-lives of 2.8, 2.5, and 3.5 days, respectively, indicating slow turnover rates in comparison with some other amino acid-degrading enzymes.