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Effects of Dietary Vitamin B₆ on the in Vitro Inactivation of Rat Tyrosine Aminotransferase in Host Liver and Morris Hepatomas¹

Vitamin and Mineral Nutrition Laboratory, Nutrition Institute, United States Department of Agriculture, Beltsville, Maryland 20705 [R. D. R.], Department of Biochemistry, Howard University, College of Medicine, Washington, D. C. 20059 [H. P. M.]

Control rats or rats bearing Morris hepatoma 5123C (intact), 5123C (adrenalectomized), 7794A, 7800, 8999, 9121, or 9618A were fed a purified diet either deficient or adequate for vitamin B₆. The concentration of pyridoxal phosphate in the plasma, host livers, and hepatomas was determined, as well as the in vitro rate of inactivation of induced tyrosine aminotransferase in homogenates of host livers and hepatomas. The results demonstrated the presence of a cysteine-independent inactivating system for tyrosine aminotransferase in hepatomas 5123C (adrenalectomized), 7800, 8999, and 9121. Only in hepatoma 9121 was there a dramatic influence of the dietary vitamin B₆ on the rate of cysteine-independent inactivation. A cysteine-dependent inactivating system for the enzyme was present in all host livers and hepatomas. The rate of this in vitro inactivation for both host livers and hepatomas apparently was a function of the concentration of pyridoxal phosphate, but inactivation of tyrosine aminotransferase occurred at a significantly lower concentration of pyridoxal phosphate in the hepatomas than in the host livers.

¹ Supported in part by USPHS Grant Ca 10729.

² To whom requests for reprints should be addressed, at Vitamin and Mineral Nutrition Laboratory, Nutrition Institute, Room 217, Building 307, Beltsville, Md. 20705.

Received 3/ 5/79. Accepted 4/25/79.