Kinetics of Steroid Induction and Deinduction of Tyrosine Aminotransferase Synthesis in Cultured Hepatoma Cells

The specific rate of synthesis of tyrosine aminotransferase (EC 2.6.1.5; L-tyrosine:2-oxoglutarate aminotransferase) is used as a measure of the level of functional, cytoplasmic, tyrosine aminotransferase-specific mRNA in cultured rat hepatoma cells. An analysis of the kinetics of change in this rate after the addition or withdrawal of glucocorticosteroids sets an upper limit on the half-life of the enzyme-specific mRNA of 1-1.5 hr, whether or not steroid is present. The inactivation rate of the enzyme mRNA is independent of the growth condition of the cells, occurring equally rapidly in the presence or absence of serum or insulin, both of which induce tyrosine aminotransferase in these cells. The implications of these results for the mechanism of steroid induction are discussed.