STIMULATION OF TYROSINE AMINOTRANSFERASE SYNTHESIS BY DEXAMETHASONE PHOSPHATE IN CELL CULTURE

A specific antibody, prepared by injecting rabbits with purified rat liver tyrosine aminotransferase, has been used to study the glucocorticoid induction of this enzyme in HTC cells, a tissue culture system derived from a rat hepatoma. When the enzyme activity is induced, parallel immunotitrations show that there is a corresponding increase of the antigenic activity of tyrosine aminotransferase, indicating that the induction is due to an accumulation of enzyme protein. The rates of amino acid incorporation into tyrosine aminotransferase in induced and non-induced HTC cells were compared by using the double-label isotope technique and specific immunoprecipitation. When there was maximum induction (12-fold) of enzyme activity, there was about a 20-fold increase in the rate of enzyme synthesis. These studies show that the accumulation of aminotransferase on induction is due to an increased rate of its synthesis from free amino acids and not due to a decreased rate of degradation. Since there is amino acid incorporation into tyrosine aminotransferase under both basal and fully induced conditions (when the level of the enzyme is not changing), we conclude that in both conditions a steady state is maintained by a balance between enzyme degradation and synthesis. A simplified method for the preparation of the enzyme from rat liver, which gives 40 to 50% yields of highly purified material, is also presented. © 1968.