Interaction of EGTA with a hydrophobic region inhibits particulate adenylate cyclase from rat cerebral cortex: a study of an EGTA-inhibitable enzyme by using alamethicin.

Washed membranes isolated from rat cerebral cortex (gray matter) showed the presence of EGTA-inhibitable and EGTA-insensitive forms of adenylate cyclase activity. The former activity was stimulated by low concentrations (microM) of various divalent cations (Mn2+, Ca2+, Co2+ and Sr2+) assayed with MgATP2- and MgCl2. At higher concentrations (mM), only Mn2+ stimulated this enzyme whereas Ca2+, Co2+ and Sr2+ were inhibitory. Alamethicin markedly (up to 30-fold) increased the activity of EGTA-inhibitable form and only moderately of EGTA-insensitive form of the enzyme. The increased activity due to alamethicin does not result from solubilization of the enzyme from membranes. Our results suggest the presence of two distinct metal binding sites--one of high (Site I) and other of low (Site II) affinity. Divalent metals via interacting with these produce divergent effects on the enzyme. Site I appears to be located in the hydrophobic region of catalytic unit of the enzyme or of membrane-associated calmodulin. The likely significance of these results is briefly presented.