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Loss of In Vitro Inactivation of Rat Liver Tyrosine Aminotransferase with Dietary Vitamin B₆ Restriction

Nutrition Institute, Science and Education Administration, U.S. Department of Agriculture, Agricultural Research Center, Beltsville, Maryland 20705

Tyrosine aminotransferase, in the presence of 8 mM L-cysteine, is inactivated in incubated liver homogenates prepared from normal rats, but not in those from rats deprived of vitamin B₆. In this study we fed rats a diet deficient in vitamin B₆ to determine the length of time required for in vitro inactivating activity to be lost from liver homogenates. After 2 weeks, the half-life of tyrosine aminotransferase in liver homogenates from vitamin B₆-deficient rats was 5.9 hours, and from control rats, 1.8 hours. After 3 weeks, tyrosine aminotransferase was no longer inactivated in homogenates prepared from livers of deficient rats. The pyridoxal phosphate (PLP) concentration of plasma from rats fed the vitamin B₆-deficient diet dropped from 89 ng/ml to 14 ng/ml after 1 week and to 7 ng/ml after 2 weeks. In 5 weeks the PLP concentration of liver from vitamin B₆-adequate rats increased from 2.9 µg/g to 6.6 µg/g while in deficient rats it dropped to 2 µg/g. The loss of tyrosine aminotransferase inactivating activity in the livers of vitamin B₆-deficient rats occurred at approximately the same time that the concentration of PLP in the livers of rats fed the two diets began to show marked differences.

KEY WORDS: • vitamin B₆ deficiency • pyridoxal phosphate • enzyme inactivation • tyrosine aminotransferase

Manuscript received 4 January 1978.