THE EFFECT OF GTP ON THE ALUMINUM FLUORIDE-ACTIVATED AND FORSKOLIN-ACTIVATED ADENYLYL-CYCLASE FROM HUMAN EMBRYONIC KIDNEY-293 CELLS

GTP has been shown to inhibit AlF4--stimulated, and to activate forsko lin-stimulated adenylyl cyclase activity in the presence of Mg2+ in ce ll membranes from human embryonic kidney 293 cells. The maximal inhibi tory response of ALF(4)(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the cas e of forskolin-activated activity at all. forskolin concentrations ass ayed. Mn2+ ions stimulated AlF4-- or forskolin-activated adenylyl cycl ase activity to a greater extent than Mg2+. The inhibition of ALF(4)(- )-stimulated cyclase by GTP was still observed with Mn2+, but the acti vation of forskolin-stimulated cyclase by GTP was not. When assayed to gether, Mn2+ and Mg2+ showed non-additive behaviours with respect to t he amount of cyclic AMP formed after ALF(4)(-)-stimulation of adenylyl cyclase, The temperature dependence of the activation of adenylyl cyc lase by forskolin, ALF(4)(-) or under basal conditions was observed to be somehow different in the presence of Mn2+ than in the presence of Mg2+ ions. Cholera toxin treatment produced a markedly increased cycla se activity, specially when assayed with ALF(4)(-). In the case of for skolin-activated adenylyl cyclase, UTP and CTP were unable to reproduc e the cyclase activation detected with GTP. However, in the case of AL F(4)(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibit ing cyclase activity, and CTP virtually eliminated the activation of t he cyclase with ALF(4)(-).