Pseudoisozymes of hepatic tyrosine aminotransferase.

Dexamethasone, insulin, and serum induce an additive increase in the activity of tyrosine aminotransferase (L-tyrosine : Z-oxoglutarate aminotransferase, EC 2.6.1.5) in hepatoma cells in tissue culture. By the criteria of immunotitration, heat stability, and electrophoresis on polyacrylamide gels, we have shown previously that all three effecters increase the cellular concentration of the same protein (GELEHRTER, T. D., EMANUEL, J. R., SPENCER, C. J. (1972) J. Biol. Chem. 247, 6197-6203). During the course of these studies however, we observed an apparent isozyme of tyrosine aminotransferase whose activity is not enhanced by these three inducers. This minor, more cathodal form is also a heat-stable, cytoplasmic enzyme which differs from the major anodal form in net charge but not molecular weight. Unlike the anodal form, however, this enzyme can utilize oxalacetate in place of a-ketoglutarate in the transamination of tyrosine, and can transaminate aspartate as well as tyrosine. A similar, heatstable, cathodal form with these same biochemical properties has also been found in cytoplasmic extracts of adult and fetal rat liver. We have used immunologic criteria to characterize further the apparent heterogeneity of hepatic tyrosine aminotransferase. Incubation of extracts of hepatoma cells, and adult, and fetal rat liver with antiserum to purified rat liver soluble tyrosine aminotransferase inactivates only the anodal enzyme; whereas incubation with antiserum to purified pig heart soluble aspartate aminotransferase (L-aspartate:Z-oxoglutarate aminotransferase, EC 2.6.1.1) inactivates only the cathodal form. Thus, the latter activity presumably represents cytosol aspartate aminotransferase which, because of its broad substrate specificity for aromatic amino acids, was detected as a “pseudoisozyme” of tyrosine aminotransferase. These observations stress the need for caution in the interpretation of apparent multiple forms of enzymes.