The effect of GTP on the aluminum fluoride- and forskolin-activated adenylyl cyclase from human embryonic kidney 293 cells.

GTP has been shown to inhibit AlF4(-)-stimulated, and to activate forskolin-stimulated adenylyl cyclase activity in the presence of Mg2+ in cell membranes from human embryonic kidney 293 cells. The maximal inhibitory response of AlF4(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the case of forskolin-activated activity at all forskolin concentrations assayed. Mn2+ ions stimulated AlF4(-)- or forskolin-activated adenylyl cyclase activity to a greater extent than Mg2+. The inhibition of AlF4(-)-stimulated cyclase by GTP was still observed with Mn2+, but the activation of forskolin-stimulated cyclase by GTP was not. When assayed together, Mn2+ and Mg2+ showed non-additive behaviours with respect to the amount of cyclic AMP formed after AlF4(-)-stimulation of adenylyl cyclase. The temperature dependence of the activation of adenylyl cyclase by forskolin, AlF4- 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 cyclase activity, specially when assayed with AlF4-. In the case of forskolin-activated adenylyl cyclase, UTP and CTP were unable to reproduce the cyclase activation detected with GTP. However, in the case of AlF4(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibiting cyclase activity, and CTP virtually eliminated the activation of the cyclase with AlF4-.