1a) Lithium and tumor-promoting phorbol esters stimulate ACTH secretion by anterior pituitary tumor cells. Phorbal esters, which activate protein kinase C, also induce the translocation of this enzyme from the cytosol to the membranes. Membrane kinase activity then falls, corresponding to the desensitization of the cells to further stimulation. Lithium also desensitizes the cells, but neither causes translocation itself nor affects phorbol-ester induced translocation of protein kinase C. 1b) Phorbol esters also mimic the effects of certain neurotransmitters on brain. Treatment of hippocampal slices with phorbol ester caused translocation of protein kinase C activity from cytosol to membranes. However, experiments with carbachol, norepinephrine, glutamate, KC1, and lithium failed to demonstrate a similar translocation. 2. The acylation of rhodopsin by long chain fatty acids from acylcoenzyme A has been demonstrated in vivo and in vitro. Evidence has been obtained that the bond may be a thioester and that the transfer to mature rhodopsin, though physiologic, may not be enzymatic. Acylation of rhodopsin is prototypical of a new class of posttranslational modification of membrane receptors.
