Salts promote activation of fat cell adenylate cyclase by GTP: special role for sodium ion.

Abstract

The effects of GTP on adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] of human and rat fat cell membranes ("ghosts" and purified membranes) were examined in the absence and presence of added inorganic salts. With human ghosts GTP alone (0.1 mM) inhibited enzyme activity by 40% at 30°C and had no significant effect at 3rC. At both temperatures Na' salts of Cl1, N3-, and S042-stimulated activity (up to 4-fold basal activity for 200 mM NaN3), with maximal effects at salt concentrations of 100-200 mM. Over the same concentration range these salts also allowed temperature-dependent stimulation by GTP. GTP increased the maximal activity produced by salt alone by about 2-fold at 30°C and about 4-fold at 37C. Na+ (added as Cl-) was much more effective than other alkali metal cations in promoting activation by GTP. Na+ salts allowed activation of the human enzyme by the GTP analog 5'-guanylyl imidodiphosphate and also promoted stimulation of rat fat cell adenylate cyclase by both nucleotides. In time course studies of human and rat fat cell ghosts, GTP appeared to sustain an initial high rate of salt-stimulated activity, which in the absence of nucleotide subsequently fell to a lower rate, suggesting that salts might activate adenylate cyclase by promoting the stimulatory effect of endogenous membrane-bound GTP. However, with purified human fat cell membranes and a GTP-free system, salts were still stimulatory and promoted activation by added GTP. These results differ from those of previous reports in other systems in which Na+ has promoted only inhibitory effects in GTP regulation of adenylate cyclase.Guanine nucleotides regulate the activity of mammalian adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. Activation of the enzyme by a number of agents, notably hormones and cholera toxin, has been suggested to occur by one or more mechanisms that facilitate an activating effect of GTP at the nucleotide regulatory subunit of the enzyme (1-4). Fluoride anion, a powerful activator ofenzyme activity, also appears to affect adenylate cyclase at the regulatory subunit, but it is as yet uncertain whether activation by fluoride requires interaction ofguanine nucleotides with the regulatory subunit (5, 6). Recently inorganic salts ofnon-fluoride anions have been shown to stimulate adenylate cyclase activity in several tissues (7-13). Although studies of liver, kidney, and pancreas preparations (9-11) have shown that inorganic salts enhance activation of adenylate cyclase by the synthetic GTP analog 5'-guanylyl imidodiphosphate (GMP-P[NH]P), the mechanism of enzyme activation by salts, and specifically the interaction ofions with the naturally occurring stimulatory nucleotide GTP, remains speculative (14).In our own studies of human and rat fat cell membranes ("ghosts"), we have found that a number of non-fluoride inorganic salts stimulate fat cell adenylate cyclase activity. Moreover, fat cell ghosts have proven to be a convenient system in which to study salt interaction with GTP. G...

Adipocyte Hormone Responsiveness and Aging in the Rat: Problems in the Interpretation of Aging Research

“…Beta receptors are coupled by regulatory proteins (G or N proteins) to adenylate cyclase catalytic units, which convert adenosine triphosphate to cyclic AMP. Adenylate cyclase activity may be affected not only by beta‐receptor number and affinity but also by changes in membrane lipid composition, 28–30 adenosine, 3,31 guanine nucleotide, 32,33 sodium ion 34 and calcium ion concentrations, 35 the amount of G protein, the number of catalytic units, insulin 13,14,36 and free fatty acid levels, 3 and probably many other factors.…”

Design and properties of a fluorescent indicator of intracellular free Na+ concentration

“…Indirect evidence suggested that this could not be attributed to the block of the pHi response to the ionophore when external Na+ was removed, but might depend instead on a decrease in [Na+]i which we were unable to measure. More generally, indicators of [Na+]i would be useful, for example, in studies of Na+ exchange in heart [14] and smooth muscle [15], in stimulation-secretion coupling in exocytosis [16] and in the coupling of G protein(s) to adenylate cyclase [17].…”