Abstract
Rat hepatoma cells (R-Y121B) which can grow under serum-free conditions were used to study the true effect of serum on the induction of the liver-specific enzymes, tyrosine aminotransferase and phenylalanine hydroxylase. R-Y121B was derived from Reuber hepatoma cells (H4-II-E).
Serum and dexamethasone similarly induced tyrosine aminotransferase and phenylalanine hydroxylase. Tyrosine aminotransferase activity increased gradually until the 8th hour when it reached a plateau. The increase in phenyl-alanine hydroxylase activity was much slower until the 12th hour, then a strik-ing increase in phenylalanine hydroxylase activity occurred up to the 24th hour. Insulin also increased tyrosine aminotransferase activity up to the 4th to 6th hour, then this activity gradually decreased.
Insulin did not, however, induce phenylalanine hydroxylase activity. The increase in both enzyme activities due to serum or dexamethasone after 24 h of incubation was partially abolished in the presence of insulin.
The degree of the induction of both enzymes by serum differed greatly for t he various preparations of sera, but no clear correlation between the prep-aration of sera and enzyme activities was found. Thus, we concluded that serum, which contains hormones, including insulin and glucocorticoids, as well as other unknown factors, plays an important role in the regulation of liver-specific enzyme activities.
