The mechanism of corticotropin stimulation of the synthesis of a specific rat adrenal cytosolic protein was investigated. This protein (protein E) has a mol.wt. of approx. 30000. It is detected by polyacrylamide-gel electrophoresis of cytosol prepared from adrenal slices from rats treated with corticotropin in vivo and control rats, the slices being incubated with [3H]- and [14C]-leucine respectively. In rats 1–15 days after hypophysectomy, corticotropin, like dibutyryl cyclic AMP, induces an increase in protein E similar to that induced in control rats, even though both compounds no longer stimulate total protein synthesis. Corticotropin stimulation of protein E synthesis is mediated by cyclic AMP but not by corticosterone, since aminoglutethimide, a steroidogenic inhibitor, does not affect corticotropin stimulation, and dexamethasone alone has no effect. Actinomycin D, when injected in vivo 1h before or after corticotropin injection, prevents the effect of corticotropin on protein E synthesis, which is interpreted as evidence that mRNA synthesis is necessary for the stimulation of protein E synthesis. When injected more than 2h after corticotropin, actinomycin D does not prevent corticotropin stimulation of protein E synthesis, but completely blocks corticotropin stimulation of total protein synthesis. This is interpreted as meaning that, after stimulation of mRNA coding for protein E, corticotropin has no effect on the synthesis of protein E. On the other hand, corticotropin stimulation of protein E synthesis persists after hypophysectomy even though it no longer stimulates total protein synthesis. These data suggest that the factor(s) involved in the synthesis of protein E are more stable than those involved in total protein synthesis.

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