Partial purification from rat and pig liver of cytosolic stimulators of hormone-sensitive adenylate cyclase: quantitation and resolution of two components

Apparent activation of rabbit lung membrane adenylate cyclase by cytosolic proteins possessing adenylate kinase activity

Lung cytosolic fraction (23500 x g supernatant) activates cAMP synthesis by lung membrane adenylate cyclase (AC). 23 kDa and 29 kDa proteins were isolated from rabbit lung cytosolic fraction in a homogeneous state, as 'activators' of lung membrane AC. Both of these proteins possess high adenylate kinase (AK) activity and are able to mimic the 'activating' effect of lung cytosol on the lung membrane AC in the standard incubation mixture devoid of adenylate kinase. The activating effect is abolished in the presence of adenylate kinase inhibitor DAPP and after heat- or trypsin-treatment of the cytosolic fraction. Commercial adenylate kinase or nonionic detergent Lubrol PX activate cAMP synthesis by lung membrane AC in a similar manner to that of cytosolic fraction. In the presence of commercial adenylate kinase or Lubrol PX no activating effect of the cytosolic fraction on lung membrane AC is revealed. The ability of cytosolic fraction, commercial adenylate kinase, Lubrol PX or purified 23 kDa and 29 kDa proteins to activate cAMP synthesis by lung membrane AC correlates with their ability to support the constant ATP (AC substrate) concentration in the AC assay mixture. Our data indicate that 'activation' of lung membrane AC in the presence of cytosolic fraction may be produced by cytosolic adenylate kinase activity which regenerates ATP from AMP in the presence of creatine kinase and creatine phosphate providing the substrate for cAMP synthesis by AC.

Enhancement of parathyroid hormone-responsive renal cortical adenylate cyclase activity by a cytosol protein activator from rat reticulocytes

The effects of the cytosol activator protein obtained from rat reticulocytes (RCAP) were investigated in a heterologous membrane system--partially purified cell membranes from dog renal cortex. RCAP enhanced the response of dog renal cortical adenylate cyclase to bovine parathyroid hormone (1-34) [bPTH (1-34)] from two- to three-fold. RCAP also enhanced the response to 5 microM arginine vasopressin, 10 microM glucagon, and 10 microM isoproterenol. Analysis of double-reciprocal plots of substrate concentration and enzyme activity indicated that bPTH (1-34) alone and together with RCAP increased the Vmax of the adenylate cyclase enzyme and did not alter the apparent Km of the enzyme for MgATP. Membranes from dog renal cortex contain 42K and 39K proteins that are ADP-ribosylated by cholera toxin and pertussis toxin, respectively, and appear to be the stimulatory (Ns) and inhibitory (Ni) guanine nucleotide binding proteins described in many other hormone-responsive membrane preparations. Similar to its effects in rat reticulocytes, RCAP inhibited ADP-ribosylation of Ns and enhanced ADP-ribosylation of Ni. The muscarinic agonist, carbachol, inhibited PTH-responsive adenylate cyclase activity in dog renal cortical membranes and this inhibition was reversed by RCAP. These results indicate that RCAP enhances stimulation of adenylate cyclase by a variety of hormones in a heterologous membrane preparation and supports the hypothesis that RCAP's site of action is common to all adenylate cyclase systems. RCAP may facilitate coupling between Ns and the catalytic unit of adenylate cyclase by a pertussis toxin-like effect to inactivate Ni. The dual effects of RCAP upon ADP-ribosylation of Ni and Ns alpha subunits suggest that a binding site for RCAP may exist at a site of homology between Ns alpha and Ni alpha.

Membrane association of soluble protein activators of rat liver adenylate cyclase. Evidence for distinctness from the guanine nucleotide-binding stimulating protein (Ns)

Sonication of a crude rat liver membrane preparation and centrifugation at 100,000 X g yielded a supernatant which activated basal and hormone-sensitive adenylate cyclases [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. The membrane origin of the stimulatory activity was confirmed by the use of lactate dehydrogenase as a marker for contamination by cytosol. The solubility of the activating factors was verified by their passage through 0.05 micron diameter pores of Millipore filters. The membrane-derived activators were nondialyzable and destroyed by heat and trypsin in the same manner as adenylate cyclase activators detectable in cytosol. Stimulation by factors from membranes and cytosol was not additive. The amount of the activators which could be freed from membranes by sonication was 12-15% of that contained in cytosol previously separated from the membranes. Soluble activators from the two sources had limited ability to restore adenylate cyclase activity to membranes from the cyclone of S49 mouse lymphoma cells which are deficient in the enzyme's guanine nucleotide-binding stimulatory protein, Ns. Cytosol did not contain a substrate for ADP-ribosylation by cholera toxin that corresponded electrophoretically to Ns. Furthermore, purified Ns did not affect adenylate cyclase activity in preparations stimulated by the soluble activators. These findings suggest that the activating factors found in cytosol may be released from membranes during tissue homogenization. Because these protein activators can be obtained from membranes without use of detergents and can neither substitute for nor be substituted for by Ns in functional assays, they are distinct from Ns.

Effects of aging on the lipid order and composition of rat adipocyte ghosts

An analysis of the cholesterol/phospholipid ratio of adipocyte ghosts from rat epididymal fat pads shows a significant increase with age (P less than 0.005). An attempt to correlate these changes with the order of the lipid matrix was made using the stearic acid spin label 2-(3-carboxypropyl)-4, 4-dimethyl-2-tridecyl-3-oxazolidinyloxyl [I(12,3)]. Although order was negatively correlated with temperature in preparations from both 6- and 24-month-old rats, no effect of age could be detected.