Studies on the Role of Vitamin B6 Derivatives in Regulating Tyrosine α-Ketoglutarate Transaminase Activity in Vitro and in Vivo

The role of pyridoxine and its coenzymically active derivatives in determining the stability of rat liver tyrosine α-ketoglutarate transaminase in vitro and in vivo was studied. Incubation of crude or purified holoenzyme preparations with cysteine and other amino acids causes removal of the coenzyme and subsequent inactivation of the inherently unstable apoenzyme; under these conditions the relatively stable glutamic-pyruvic transaminase retains its coenzyme and remains active. Inactivation of tyrosine transaminase is prevented by small amounts of 5'-phosphate derivatives of pyridoxine; the enzyme is also stabilized in solutions of high ionic strength. Evidence is presented that protection by coenzyme involves its reaction with enzymically active sites. The induction of tyrosine transaminase by pyridoxine administration does not reflect stabilization in vivo . Induction involves an increased rate of transaminase synthesis, determined immunochemically, and is inhibited by actinomycin D, and is thus similar to the inductions of this enzyme brought about by hydrocortisone and other hormones.