Some aspects of rat kidney plasma membrane cAMP-receptor and its connection with protein kinase activity

Rat kidney plasma membranes prepared by the method of FITZPATRICK et al. (J Biol. Chem. 244, 3561, 1969) show protein kinase activity as well as specific cAMP binding activity (diss. const. 1.3 x 10-9 M). However, no stimulation of kinase activity by cAMP is observed in presence of exogenous substrates (e.g. histone) and only poor stimulation with endogenous substrates in the membrane could be shown. At high ionic strength (1 M NaCl) cAMP independent protein kinase activity can be solubilized. Low ionic strength buffer (1 mM Tris-HCl pH 7.4 1 mM EDTA) and non-ionic detergents (Lubrol PX, Lubrol WX and Triton X 100) are able to solubilize both protein kinase activity and cAMP binding activity. Protein kinase activity seemed to be only loosely associated with the membrane, whereas cAMP binding protein appears to be more firmly fixed into the membrane structure. In addition we have found that membranes serve as a good substrate for cytosol protein kinase (s) and Ca-ion concentration influences the effect of cAMP on protein kinase activity. Dependent on the increase of Ca-ion concentration the effect of cAMP on protein kinase changes from activation to inhibition.