A method has been developed for investigation of the effect of adrenocorticotrophic hormone (ACTH) on the state of activation of a cyclic AMP-dependent protein kinase within cells of the adrenal cortex. Enzyme activity was measured in terms of the quantity of32P transferred from [γ-32P]ATP to histone under conditions in which bound cyclic AMP did not dissociate from the regulatory subunit of the protein kinase ACTH (1×10-2i.u./ml) caused a rapid and complete activation of the cyclic AMP-dependent protein kinase activity within 2min of hormone addition to the isolated cells. In response to a range of ACTH concentrations a sigmoid log dose–response curve for protein kinase activation was obtained, with half-maximal stimulation attained at about 1×10-3i.u./ml. However, some low doses of ACTH that elicited a marked (but submaximal) steroidogenic response failed to cause a clear stimulation of protein kinase activity in isolated adrenal cells. Theophylline (2mm) potentiated the effect of ACTH on protein kinase activity. The results implicate an important role for protein kinase in ACTH action on the adrenocortical cell.

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