Heterogeneity of Hepatic Tyrosine Aminotransferase. Separation of the Multiple Forms from Rat and Frog Liver by Isoelectric Focussing and Hydroxylapatite Column Chromatography and Their Partial Characterization.

Abstract

L-Tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5; TAT) and other enzymes that transaminate tyrosine in rat liver cytosol were separated into 4 fractions by isoelectric focussing. One of the forms is probably identical to mitochondrial L-aspartate:2-oxoglutarate aminotransferase (EC 2.6.1.1; mASAT). The other 3 forms have pI''s of 4.72, 4.98 and 5.30 and Km values of 1.3 and 0.3 mM for tyrosine and .alpha.-ketoglutarate. These heat stable forms have little or no ASAT activity. Rat liver TAT is also separated into 3 peaks by hydroxylapatite. Each fraction gives only 1 peak of activity when electrofocussed separately. In the frog 3 groups of peaks of TAT activity were separated by hydroxylapatite column chromatography. The 1st group is connected with ASAT activity. These peaks (pI''s 6.35, 6.50 and 6.90) are heat stable and have a Km value for tyrosine of 4 mM. These fractions probably represent cytoplasmic ASAT (sASAT). The 2nd group of peaks has at least 2 subforms (pI''s 9.0 and 9.4, Km for tyrosine 15 mM). These forms probably represent mASAT. The 3rd group consists of 3 forms that resemble the major forms of rat liver TAT. Heterogeneity is apparently common to many aminotransferases and independent of regulation by glucocorticoids.