On the mechanism of activation of fat cell adenylate cyclase by guanine nucleotides. An explanation for the biphasic inhibitory and stimulatory effects of the nucleotides and the role of hormones.

Adenylate cyclase activity in purified plasma membranes from rat fat cells displays transient kinetic characteristics in the absence and presence of guanyl=5'=yl imidodiphosphate (Gpp(NH)p). Gpp(NH)p causes immediate inhibition of enzyme activity; the inhibitory phase is followed by a slow increase in activity which, depending on incubation temperature, exceeds activity stimulated in the presence of hormones (glucagon, secretin, epinephrine, or adrenocorticotropin). Basal activity displays an initial high rate of activity which decays to a low state of activity within 2 min of incubation. Hormones do not alter the initial rate but prevent the decay in enzyme activity. The inhibitory phase of Gpp(NH)p action and the previously reported (Harwood, J.P., Low, H., and Rodbell, M. (1973) J. Biol. Chem. 248, 6239-6245) inhibitory effects of GTP are abolished by increasing (Mg2+) and pH to 50 mM and 8.5, respectively. Under these conditions, Gpp(NH)p and GTP cause marked stimulation of activity, the stimulatory effect of Gpp(NH)p being greater than that of GTP both in the absence and presence of hormones...