[Effects of GTP and NaF on rabbit heart adenylate cyclase activated by guanyl-5'-ilimidodiphosphate]

The GTP analog, guanyl-5'-ilimidodiphosphate (Gpp(NH)p) activates rabbit heart adenylate cyclase with the lag period eliminated by isoproterenol. The Gpp(NH)p-isoproterenol-induced state of adenylate cyclase is very stable and is retained after removal of the effector excess and a 40-min incubation at 37 degrees. GTP and NaF decrease the activity of Gpp (NH)p-stimulated adenylate cyclase. This effect of GTP appears with the lag period, which is not affected by isoproterenol. GTP and Gpp(NH)p compete for the same binding site; it is assumed that GTP can remove Gpp(NH)p from the regulatory center of heart adenylate cyclase. The inhibiting effect of NaF on Gpp(NH)p-stimulated enzyme is reversible, has no lag period and is not accompanied by the Gpp(NH)p removal from the regulatory center of the enzyme. The data obtained suggest that the hormones control the activity of adenylate cyclase during GTP substitution for GDP, but do not change the rate of guanosine triphosphate dissociation from the nucleotide binding site of the enzyme. NaF does not probably affect the binding of guanylic nucleotides, but changes the type of interaction of the catalytic subunit of adenylate cyclase with the regulatory protein binding guanylic nucleotides.