VITAMIN-B-6 DEFICIENCY AND LEVEL OF DIETARY-PROTEIN AFFECT HEPATIC TYROSINE AMINOTRANSFERASE ACTIVITY IN CATS

Total activity [pyridoxal 5'-phosphate (PLP) added in the assay] of he patic tyrosine aminotransferase (TAT) measured in cats at 0300, 0900, 1500 and 2100h was 10.3 +/- 1.1, 14.0 +/- 0.7, 9.8 +/- 1.3 and 11.0 +/ - 0.7 nkat/g liver, indicating little diurnal variation. Activity afte r 18 h of food deprivation was 10.0 +/- 0.3 nkat/g liver, also not dif ferent from cats that were eating ad libitum. These findings support t he idea that cats have only limited changes in the activity of hepatic TAT compared with rats. Total TAT activity was measured in cats fed h igh protein (550 g/kg) and low protein (180 g/kg) diets for 4 wk. Cats fed a high protein diet had activities significantly higher (about tw ice) than cats fed the low protein diet. Hepatic TAT activity of vitam in B-6-deficient cats (diet without pyridoxine for 9 wk) was compared with cats given the same diet with 8 mg pyridoxine/kg. Total hepatic T AT activity in deficient cats was significantly (P < 0.05) lower per g ram soluble or total protein (but not per gram liver) than control cat s; holoenzyme activity and percentage of active enzyme in deficient ca ts were also significantly lower by 75 and 64%, respectively. The appa rent K-m of TAT from cats for tyrosine (2.1 mmol/L) was similar to tha t for rats (1.9 mmol/L), but higher for PLP in cats (0.16 mu mol/L) th an rats (0.034 mu mol/L). Part of the reason for the higher plasma tyr osine in vitamin B-g-deficient cats than rats is the higher K-m of TAT for PLP in cats than rats.