A regulatory component of adenylate cyclase is located on the inner surface of human erythrocyte membranes

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.

Identification and characterization of human erythrocyte muscarinic receptors

Cholinergic muscarinic but not nicotinic receptors could be detected on the outer surface of the human red blood cell membrane by direct labelled ligand binding. This 3H-QNB binding could be inhibited by atropine. The erythrocyte cholinergic receptor was similar to brain muscarinic receptor in regard to stimulation of cGMP production.

Cyclic GMP in the human erythrocyte. Intracellular levels and transport in normal subjects and chronic hemodialysis patients

We have studied the intracellular levels of cyclic GMP in the red blood cells of chronic hemodialysis patients before and after dialysis treatment and in normal subjects. Cyclic GMP is present at levels (mean +/- 1SD) of 19.6 +/- 4.7 and 18.4 +/- 5.8 nM/L before and after treatment, respectively. The levels in normal subjects are 3.6 +/- 0.9. The uptake of cyclic GMP was also studied and the data demonstrate that the red blood cell is slightly permeable to cyclic GMP, suggesting that cyclic GMP uptake could substitute for guanylate cyclase activity in the human erythrocyte.

Effects of trypsin-treatment on agonist binding affinity to the muscarinic acetylcholine receptor

Trypsin-treatment of the microsome fraction of the ileum and the synaptic membrane fraction of the cerebral cortex of guinea-pig caused selective reduction in the apparent affinity of an agonist (carbachol), but not an antagonist (atropine), to muscarinic acetylcholine receptors (mAChR), measured as inhibition of binding of 3H-quinuclidinyl benzilate (3H-QNB). This effect was similar to that of Gpp(NH)p. The effects of trypsin and Gpp(NH)p were not additive. On the other hand, treatment of these fractions with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) increased the apparent affinity of agonist, but not antagonist. The effect of DTNB predominated over those of trypsin and Gpp(NH)p, when the fractions were treated with two reagents simultaneously.

Role of membrane gangliosides in the binding and action of bacterial toxins

Gangliosides are complex glycosphingolipids that contain from one to several residues of sialic acid. They are present in the plasma membrane of vertebrate cells with their oligosaccharide chains exposed to the external environment. They have been implicated as cell surface receptors and several bacterial toxins have been shown to interact with them. Cholera toxin, which mediates its effects on cells by activating adenylate cyclase, bind with high affinity and specificity to ganglioside GM1. Toxin-resistant cells which lack GM1 can be sensitized to cholera toxin by treating them with GM1. Cholera toxin specifically protects GM1 from cell surface labeling procedures and only GM1 is recovered when toxin-receptor complexes are isolated by immunoadsorption. These results clearly demonstrate that GM1 is the specific and only receptor for cholera toxin. Although cholera toxin binds to GM1 on the external side of the plasma membrane, it activates adenylate cyclase on the cytoplasmic side of the membrane by ADP-ribosylation of the regulatory component of the cyclase. GM1 in addition to functioning as a binding site for the toxin appears to facilitate its transmembrane movement. The heat-labile enterotoxin of E. coli is very similar to cholera toxin in both form and function and can also use GM1 as a cell surface receptor. The potent neurotoxin, tetanus toxin, has a high affinity for gangliosides GD1b and GT1b and binds to neurons which contain these gangliosides. It is not yet clear whether these gangliosides are the physiological receptors for tetanus toxin. By applying the techniques that established GM1 as the receptor for cholera toxin, the role of gangliosides as receptors for tetanus toxin as well as physiological effectors may be elucidated.

Increased dose-response relationship of liver plasma membrane adenylate cyclase to glucagon stimulation in diabetic rats. A possible role of the guanyl nucleotide-binding regulatory protein

In view of controversial findings regarding the mechanism for the increased intracellular hepatic cyclic 3':5' adenosine monophosphate levels in diabetic rats, we studied the dose-response relationship of the adenylate cyclase to glucagon stimulation in severely diabetic and in diabetic, insulin-treated rats. An enhanced response to glucagon and an additional augmenting effect of guanosine triphosphate on hormonal stimulation of the adenylate cyclase activity were found in diabetes which were reversible with insulin treatment. The results suggest a role of the regulatory guanyl nucleotide-binding protein in diabetes leading to an increased dose response relationship of the hepatic adenylate cyclase system to glucagon.

Effect of proteolytic and lipolytic enzymes on the adenylate cyclase activity from brain membranes of Ceratitis capitata

1. The effect of various proteolytic enzymes was assayed on the adenylate cyclase activity in purified brain membrane preparations from the insect Ceratitis capitata. Trypsin, chymotrypsin, papain, thermolysin, elastase, subtilisin and prot. XIV were examined. 2. Trypsin treatment, at 37 degrees C, decreased the adenylate cyclase activity even in the presence of GppNHp that protects the activity from the thermal inactivation. 3. Residual basal, GppNHp- and F(-)-stimulated activities were similar when membrane preparations were preincubated either in the presence or in the absence of GppNHp and F-. 4. All proteolytic activities assayed on the brain membrane preparations, excepting papain, exerted an inhibition of adenylate cyclase in basal conditions. 5. The inhibition was stronger in the presence of F- than in the presence of other regulators. 6. Papain showed also a notable inhibition of adenylate cyclase in the presence of F-. 7. Phospholipase A2 treatment decreased both basal and stimulated activity; however, F(-)-sensitive activity was less affected than basal and GppNHp-sensitive activity. F(-)-stimulated activity was less affected by phospholipase A2 than either basal or GppNHp-stimulated activities. 8. Phospholipids are, then, essential for the highest basal activity, although the relationship between catalytic and nucleotide-regulatory components was unaffected by this treatment.

Molecular complexes involved in the regulation of adenylate cyclase

Selective extraction of the adenylate cyclase regulatory protein (N-protein) from pigeon erythrocyte plasma membranes provided evidence for its cytoskeletal association. Cholate, but not Triton X-100 or digitonin, was effective in solubilizing the ADP-ribosylated N-protein. The labeled protein complex or components thereof that were associated with the Triton-insoluble cytoskeleton (shells) could be partly released by 0.1 mM EDTA; 1 M KCl in the presence of Triton X-100 achieved complete solubilization. 5'-Guanylyl imidodiphosphate (p[NH]ppG) and NaF, activators of adenylate cyclase, promoted the release of the regulatory protein from the cytoskeleton but MnCl2, an "uncoupler" of the adenylate cyclase system, had the opposite effect. The solubilized, labeled N-protein was able to bind specifically to rat erythrocyte inside-out vesicles in the presence of divalent cations. A proteolytic product of inside-out vesicles inhibited the binding of the N-protein to fresh vesicles. Three molecular species which contained the Mr 45,000 polypeptide component of the N-protein were identified by gel permeation chromatography and by sucrose density gradient velocity sedimentation. p[NH]ppG appeared to convert the two larger molecular complexes to a smaller molecular entity. Such a molecular dissociation might be relevant to the effects of guanyl nucleotides on the activity of adenylate cyclase and on the affinity of hormone receptors.

Distinct effects of the C-terminal octapeptide of cholecystokinin and of a cholera toxin pretreatment of the kinetics of rat pancreatic adenylate cyclase activity

(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTP gamma S as nucleotide activator, cholecystokinin as peptide hormone, and GDP beta S and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (EA/ETOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant kappa+1), and a deactivation process (rate constant kappa off) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant kappa 2) and/or the dissociation of the intact nucleotide (rate constant kappa-1), so that EA/ETOT = kappa+1/(kappa+1 + kappa 2 + kappa-1). (2) The hormone CCK-8 increased the value of kappa+1 with GTP dose-dependently, from 0.2 to 10.9 min-1. The value of kappa-1 increased 0.01 to 0.3 min-1 but the value of kappa 2 was unaltered at 7 min-1, so that EA/ETOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 micrograms/ml allowed also a large increase in EA/ETOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the kappa off value of the GTP-activated enzyme (from 7 min-1 to 0.5 min-1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of kappa+1 (from 0.2 to 0.8 min-1) and the occurrence of a significant 0.3 min-1 value for kappa-1.

Defect of receptor-cyclase coupling protein in pseudohypoparathyroidism

Hormone-sensitive adenylate cyclase contains a recently discovered protein component that is required for stimulation of cyclic AMP synthesis by hormones and guanine nucleotides; the component presumably couples the membrane receptor to the cyclase. We studied this protein (termed "N") in erythrocyte membranes of patients with pseudohypoparathyroidism, using assays of the protein's biochemical activity and of its susceptibility to radiolabeling in the presence of [32P]NAD and cholera toxin. By both assays, the protein's activity was reduced by 40 to 50 per cent in erythrocytes of five of 10 patients with Type I pseudohypoparathyroidism as compared with those of normal and hypoparathyroid subjects and one patient with Type II pseudohypoparathyroidism. If activity of the N protein is reduced in other tissues, this deficiency could cause the resistance of target organs in pseudohypoparathyroidism to parathyroid hormone and other hormones that work via cyclic AMP. Erythrocytes of five patients with Type I pseudohypoparathyroidism, all in one family, showed no defect in activity of the N protein; the biochemical defect of this family remains undefined.