Studies on the induction and repression of enzymes in rat liver. Characterization and metabolic regulation of multiple forms of tyrosine aminotransferase.

Abstract

Tyrosine aminotransferase may be separated into four isolatable forms on hydroxyapatite columns. Form I is found in kidney, heart, brain and other tissues while forms II, III and IV appear to be unique to the liver in the rat. Forms II, III and IV were purified more than 1000‐fold from liver while form I was purified 30‐fold from kidney and heart. The several forms have been compared kinetically and physicochemically. The Km for tyrosine of form I is significantly higher than that of the other three forms. On the other hand, in the presence of glutamate, V for all substrates studied is significantly lower for form II than for the other three forms. In addition, Kis for 2‐oxoglutarate in the presence of glutamate is somewhat lower and in the presence of pyridoxal 5′‐phosphate is significantly higher than for forms III and IV. The moledular weights and absorption spectra of forms II, III and IV could not be distinguished one from another. In contrast to forms II, III and IV the activity of form I was destroyed by brief heating at 60°C in the presence or absence of cofactors. For I also had a higher pH optimum than the other three forms. In the fetus, glucagon, adenosine 3′:5′‐monophosphate and guanosine 3′:5′‐monophosphate induced predominantly form II. In the neonatal animal, hydrocortisone induced both forms II and III, while hydrocortisone in the presence of insulin induced form IV “prematurely”. This latter induction was insensitive to the action of actinomycin D. The rates in vivo of degradation of forms II, III and IV after hydrocortisone induction were indistinguishable. In the isolated perfused liver, hydrocortisone induced forms II, III and IV as had previously been shown in vivo, while adenosine 3′: 5′‐monophosphate induction was limited to forms II and III and insulin stimulated the level of form IV only. The significance of the differences in form I as compared to the hepatic forms, as well as the implications of selective metabolic regulation of the other three hepatic forms, are discussed.