Mechanism of induction of &#916;<SUP>5</SUP>-3&#946;-hydroxysteroid dehydrogenase-isomerase activity in rat adrenocortical cells by corticotropin

The regulation of Δ<SUP>5</SUP>-3β-hydroxysteroid dehydrogenase-isomerase (3β-HSD) was studied in primary cultures of rat adrenocortical cells. In the absence of ACTH, this enzymic activity was found to decay with a half-life of 3.1 days, which was similar to the half-life of the enzyme activity induced by ACTH in vitro (3.5 days). The increase in 3β-HSD activity was highly specific for ACTH and dibutyryl cAMP; the activity was not increased by other hormones known to affect adrenocortical growth or function. The induction of 3β-HSD activity by ACTH or dibutyryl cAMP required a lag period of approximately 4 h and was dependent on RNA and protein syntheses. The increase in 3β-HSD activity observed after ACTH treatment was not a result of ACTH-induced inhibition of degradation of the enzyme, nor was it due to the synthesis of a soluble intermediate which could directly activate the enzyme. ACTH stimulated the incorporation of [<SUP>35</SUP>S]methionine into a protein associated with 3β-HSD activity detected on polyacrylamide gels after electrophoresis of Triton X-100 extracts of adrenocortical cells. The induction of this protein by ACTH was inhibited by actinomycin D. A protein band of a partially purified preparation of rat adrenal 3β-HSD was found to comigrate with the ACTH-induced protein on sodium dodecyl sulfate-polyacrylamide gel. These results suggest that ACTH caused the de novo synthesis of 3β-HSD by a mechanism dependent on RNA synthesis