Differential control of protein kinase activities of the retinal photoreceptor. Cation effects on phosphorylation by adenosine and guanosine 5'-triphosphates.

ATP-kinase activity in photoreceptor membranes is maximal at equimolar amounts of ATP and Mg2+. GTP-kinase activity is maximal with high concentrations of Mn2+. Under these conditions, calcium ion markedly inhibits phosphorylation with ATP but not with GTP. GTP-kinase activity is maximal at pH 6.5 and decreases at higher pH; little change is seen in ATP-kinase activity over the pH range of 6.0-7.5. GTP-kinase activity is inhibited by ATP and other adenine compounds; ATP-kinase activity is less sensitive, although I mM adenosine decreases activity by about threefold. No marked differences in the protein moieties phosphorylated by ATP or GTP were seen in bovine or frog outer segment membranes. Fluxes in cation and metabolite concentrations as well as availability of ATP and GTP could thus exert a major influence on protein kinase activity in the photoreceptor unit, affording the possibility of differential phosphorylation under various physiological conditions.