Transcortin from rats selectively binds cortisol but not the synthetic glucocorticoid dexamethasone. Both steroids are potent inducers of the enzyme tyrosine aminotransferase in hepatoma tissue culture cells in vitro under serum free conditions. Transcortin is shown to block selectively enzyme induction by cortisol but not dexamethasone. The inhibitory effect of added transcortin can be abolished by adding sufficient cortisol to overwhelm the transcortin binding capacity. It is also shown that transcortin can inhibit the entry of radiolabelled cortisol, but not dexamethasone, into hepatoma cells in vitro. This probably is the mechanism which inhibits enzyme induction by cortisol. In addition a rapid and reproducible technique for estimating transcortin binding capacity is described.
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