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Vitamin B-6 Deficiency and Level of Dietary Protein Affect Hepatic Tyrosine Aminotransferase Activity in Cats

Manuscript received 15 December 1997. Initial reviews completed 22 January 1998. Revision accepted 16 June 1998.

Sungchul C. Bai, Quinton R. Rogers, Daniel L. Wong, David A. Sampson*, and James G. Morris

Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616 and * Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, CO 80523

Total activity [pyridoxal 5'-phosphate (PLP) added in the assay] of hepatic 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 after 18 h of food deprivation was 10.0 ± 0.3 nkat/g liver, also not different from cats that were eating ad libitum. These findings support the idea that cats have only limited changes in the activity of hepatic TAT compared with rats. Total TAT activity was measured in cats fed high 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 twice) than cats fed the low protein diet. Hepatic TAT activity of vitamin 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 TAT activity in deficient cats was significantly (P < 0.05) lower per gram soluble or total protein (but not per gram liver) than control cats; holoenzyme activity and percentage of active enzyme in deficient cats were also significantly lower by 75 and 64%, respectively. The apparent Km of TAT from cats for tyrosine (2.1 mmol/L) was similar to that for rats (1.9 mmol/L), but higher for PLP in cats (0.16 µmol/L) than rats (0.034 µmol/L). Part of the reason for the higher plasma tyrosine in vitamin B-6-deficient cats than rats is the higher Km of TAT for PLP in cats than rats.

Key words: pyridoxal phosphate, tyrosine aminotransferase, vitamin B-6 deficiency, feline, dietary protein.

The Journal of Nutrition Vol. 128 No. 11 November 1998, pp. 1995-2000
Copyright ©1998 by the American Society for Nutritional Sciences




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