Liver cytosol tyrosine aminotransferase. Product inhibition and interaction with substrates.

The reaction catalyzed by purified rat liver cytosol tyrosine aminotransferase (L-tyrosine + 2 oxoglutarate = p-hydroxyphenylpyruvate + L-glutamate, EC 2.6.1.5) has been analyzed with regard to product inhibition and combination with each of its rate-limiting substrates by the Hill equation. Glutamate is a linear competitive inhibitor with respect to tyrosine, a linear noncompetitive inhibitor with respect to coenzyme or a-ketoglutarate. p-Hydroxyphenylpyruvate is a linear noncompetitive inhibitor with respect to tyrosine or pyridoxal-P but is a linear competitive inhibitor with respect to Lu-ketoglutarate. These data, considered with replots of slopes and intercepts, indicate a “ping-pang” mechanism random in the sense that either coenzyme or Schiff’s base of coenzyme and amino acid can add to the apoenzyme. A study of the half-reaction with apoenzyme, pyridoxamine-P, and cw-ketoglutarate showed that 90% or more of the complete chemical reaction rate was achieved when freshly isolated apoenzyme was used. The pH dependence of the half-reaction paralleled that of the complete chemical reaction and supports the conclusion that the same protein catalyzes the complete and half-reaction. Furthermore, the magnitude of the half-reaction compared to the complete transamination would dictate that the mechanism does not switch between ping-pong and ordered depending upon the level of substrates available. By use of the Hill equation, the reaction between enzyme and substrates is tirst order with respect to tyrosine, pyridoxal-P, and a-ketoglutarate. Reduction of the concentration of cu-ketoglutarate to a rate-limiting value does not change the order with respect to tyrosine or to pyridoxal-P. Reduction of the concentration of pyridoxal-P, or of tyrosine, or of both of these reactants to rate-limiting levels when the concentration of cY-ketoglutarate is varied does not a.lYect the order of the reaction with respect to cr-ketoglutarate. Thus, homotropic effects are not manifested by this system.