Loss of in vitro inactivation of rat liver tyrosine aminotransferase with dietary vitamin B6 restriction.

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 microgram/g to 6.6 microgram/g while in deficient rats it dropped to 2 microgram/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.