Exposure of latent prostaglandin binding sites in the rat epididymal adipocyte membrane and the effects of albumin, heating and alkylating agents

Brief incubation (1 min) of the adipocyte isolated membranes at 60 degrees C caused an increase in prostaglandin E2 binding, similar to that obtained with albumin. The increase in the membranal binding capacity after a short heating of the membranes was concomitant with a substantial decline in the ability of albumin to induce a further increase in the binding capacity of the treated membranes. Pretreatment of the isolated adipocyte membranes with 10 mM N-ethylmaleimide inhibited the enhancement of prostaglandin E2 binding in the presence of albumin, but did not affect the prostaglandin E2 binding in the absence of albumin. Identical treatment of the isolated membranes with glutathione-maleimide, an impermeable SH reagent with comparable alkylation reactivity, enhanced the binding of prostaglandin E2 in the absence of albumin and failed to inhibit the enhancement of prostaglandin E2 binding in its presence. In contrast to the effect of albumin on prostaglandin E2 binding to the isolated membranes, albumin failed to alter prostaglandin E2 specific binding to intact adipocytes.     

Characterization of high-density lipoprotein binding to rat adipocytes and adipocyte plasma membranes

The interaction of high-density lipoproteins (HDL) with adipocytes is important in the regulation of cellular cholesterol flux. To study the mechanisms of HDL binding and cellular processing, we incubated adipocytes isolated from epididymal and perirenal adipose tissue of male Wistar rats (300 g) with HDL1 (1.07-1.10 g/mL) and HDL2 (1.10-1.14 g/mL) fractions separated from rat plasma by gradient ultracentrifugation. Freshly isolated adipocytes were incubated with 125I-labeled HDL for 2 h at 37 degrees C to determine cell-associated uptake and degradation. Adipocytes from both fat regions showed significant cell-associated HDL1 and HDL2 uptake and very high medium degradation (2- to 6-fold higher than uptake). To assess 125I-labeled HDL binding independent of cellular metabolism, we purified adipocyte plasma membranes from isolated adipocytes and used them in binding assays. Binding of HDL1 and HDL2 in the membrane system was 85-95% specific, sensitive to high NaCl concentrations, and abolished by pronase treatment. In contrast to HDL2 binding, the maximum HDL1 binding to perirenal plasma membranes was significantly higher than its binding to epididymal membranes (7.2 +/- 1.3 vs. 4.4 +/- 0.2 micrograms/mg, n = 6, p less than 0.05). This increment in HDL1 binding to perirenal membranes represented an EDTA- sensitive, calcium-dependent component. These results indicate that HDL binding to adipocyte plasma membranes depends on both adipose tissue region and HDL subtype. The membrane binding characteristics, taken together with the cellular uptake results, suggest that adipocytes bind and metabolize HDL and that this interaction may involve a protein receptor.     

Comparison of bile acid binding to sinusoidal and bile canalicular membranes isolated from rat liver

Simon et al. (J. Clin. Invest., 70 (1982) 401) studied cholate binding to crude liver plasma membrane vesicles and suggested that the binding may represent mainly the binding to the receptor (carrier) on the canalicular membrane. This hypothesis was supported by finding a good correlation between the number of cholate binding sites on liver plasma membrane and the maximal rate of biliary secretion (Tm) for taurocholate. We studied bile acid binding to sinusoidal and canalicular membrane vesicles isolated from rat liver by a rapid filtration technique. Scatchard analysis of the saturation kinetics showed both [3H]cholate and [3H]chenodeoxycholate bind to two classes of binding site on each membrane. However, little difference was observed between the binding to sinusoidal and canalicular membrane vesicles for each bile acid (cholate, Kd1 = 10.4 and 19.8 microM, n1 = 31.0 23.6 pmol/mg protein, Kd2 = 1.32 and 1.73 mM, n2 = 13.1 and 23.4 nmol/mg protein; and chenodeoxycholate, Kd1 = 0.207 and 0.328 microM, n1 = 36.7 and 27.4 pmol/mg protein, Kd2 = 1.16 and 2.26 mM, and n2 = 20.6 and 24.2 nmol/mg protein; numbers show the mean values sinusoidal and canalicular membrane vesicles, respectively). Chenodeoxycholate binding to sinusoidal membrane vesicles was markedly inhibited by cholate but not by Rose bengal, an organic anion dye. These studies indicate that both membranes (sinusoidal and canalicular membrane vesicles) have two kinds of binding site for bile acids, although no clear difference in the binding properties was observed between the two membranes. Consequently, the cholate binding Simon detected may represent the binding not only to canalicular membrane vesicles but also to sinusoidal membrane vesicles.     

Characterization of low density lipoprotein binding to human adipocytes and adipocyte membranes

125I-labeled low density lipoprotein (LDL) binding to purified plasma membranes prepared from freshly isolated human adipocytes was saturable, specific, and displaceable by unlabeled ligand. The maximum specific binding capacity measured at saturating concentrations of 125I-LDL was 1.95 +/- 1.17 micrograms of LDL bound/mg of membrane protein (mean +/- S.D., n = 16). In contrast to cultured fibroblasts, specific binding of LDL to adipocyte membranes was calcium-independent, was not affected by EDTA or NaCl, and was not destroyed by pronase. Plasma membranes purified directly from homogenized adipose tissue also showed calcium-independent LDL specific binding (0.58 +/- 0.33 micrograms of LDL bound/mg of membrane protein, mean +/- S.D. n = 11). Specific binding, internalization, and degradation of 125I-methylated LDL was demonstrated in isolated adipocytes and competition experiments showed that native and methylated LDL interacted with adipocytes through some common recognition mechanism(s). Compared to native LDL, specific binding of methylated LDL to adipocyte membranes was significantly reduced (43%), indicating that interaction of LDL with adipocyte was dependent in part on the lysine residues of apolipoprotein B. LDL binding to adipocyte plasma membranes was also competitively inhibited by human high density lipoprotein subfractions HDL2 and HDL3. Thus, LDL metabolism in mature adipocytes appears to be regulated by mechanisms distinctly different from a variety of cultured mesenchymal cells. In addition, the ability of adipocytes to bind, internalize, and degrade significant amounts of methylated LDL supports the view that adipose tissue is involved in the metabolism of modified lipoproteins in vivo.     

Characterization of gastric mucosal prostaglandin E2 receptor

1. The binding characteristics of gastric mucosal prostaglandin (PG) E2 (PGE2) receptor were investigated using mucosal cell membranes from rat stomach. The binding was found to be dependent upon PGE2 and membrane protein concentration, the time of incubation and the pH of the mixture, being highest at pH 3.0. 2. Scatchard analysis of the binding data revealed a curvilinear plot with high affinity binding (Kd = 2 nM; Bmax = 0.106 pmol/mg protein) and low affinity binding (Kd = 319 nM; Bmax = 2.262 pmol/mg protein) sites. 3. Competitive displacement study indicated that the receptor was specific for PGs of the E series, as PGF2 alpha and 6-keto-PGF1 alpha failed to displace the PGE2. 4. The study is the first report to provide biochemical parameters of specific PGE receptors in rat gastric mucosa.     

Characterization of an 11,000-dalton beta-bungarotoxin: binding and enzyme activity on rat brain synaptosomal membranes

The binding and phospholipase A2 activity of an 11,000-dalton beta-bungarotoxin, isolated from Bungarus multicincutus venom, have been characterized using rat brain subcellular fractions as substrates. 125I-labeled beta-bungarotoxin binds rapidly (k = 0.14 min-1 and 0.11 min-1), saturably (Vmax = 130.1 +/- 5.0 fmoles/mg and 128.2 +/- 7.1) fmoles/mg), and with high affinity (apparent Kd = 0.8 +/- 0.1 nM and 0.7 +/- 0.1 nM) to rat brain mitochondria and synaptosomal membranes, respectively, but not to myelin. The binding to synaptosomal membranes is inhibited by divalent cations and by pretreatment with trypsin. The binding results suggest that the toxin binds to specific protein receptor sites on presynpatic membranes. The 11,000-dalton toxin rapidly hydrolyzes synaptosomal membrane phospholipids to lysophosphatides and manifests relative substrate specificity in the order phosphatidyl ethanolamine greater than phosphatidyl choline greater than phosphatidyl serine. These results indicate that the 11,000-dalton beta-bungarotoxin is a phospholipase A2 and can use presynaptic membrane phospholipids as substrates. The binding, phospholipase activity and other biological properties of the 11,000-dalton toxin are contrasted with those of the beta-bungarotoxin found in highest concentration in the venom (the 22,000-dalton beta-bungarotoxin), and the two toxins are shown to have qualitatively similar properties. Finally the results are shown to support the hypothesis that beta-bungarotoxins act in a two-step fashion to inhibit transmitter release: first, by binding to a protein receptor site on the presynatic membrane associated with Ca2+ entry, and second, by perturbing through enzymatic hydrolyses the phospholipid matrix of the membrane and thereby causing an increase in passive Ca2+ permeability.     

Stimulation of prostaglandin E1 binding to human blood platelet membrane by insulin and the activation of adenylate cyclase

Incubation of human platelet-rich plasma with physiological amounts of insulin resulted in the increase of the binding of prostaglandin E1 by more than 2-fold when compared to the control platelets. Scatchard analysis of the binding of the prostaglandin to the hormone-treated platelets showed that the increased binding was due to the increase of receptor numbers rather than the increase of affinity of the binding sites. The membranes prepared from the insulin-treated platelets also showed similar enhanced binding of the prostaglandin. However, addition of insulin directly to the membranes isolated from the platelets which had not been previously incubated with the hormone failed to show any effect. This increased binding of the prostanoid to the membranes prepared from the insulin-treated platelets resulted in the stimulation of adenylate cyclase by more than 2-fold when compared with the control of membrane preparation by the prostaglandin alone. In contrast, treatment of platelets with the hormone which increased the prostanoid binding to these cells did not influence the cyclic AMP phosphodiesterase activity of either the membrane or cytosolic fraction. The increase in the cellular level of cyclic AMP by prostaglandin E1 was 2-fold greater in the hormone-treated cells than in the case of untreated platelets stimulated by the agonist alone. The incubation of platelet-rich plasma with insulin, as expected, decreased the amount of prostaglandin E1 needed to inhibit platelet aggregation by more than 50% when compared to the control platelets.     

High-affinity prostaglandin E receptors attenuate adenylyl cyclase activity in isolated bovine myometrial membrane

Purified bovine myometrial plasma membranes were used to characterize prostaglandin (PG) E2 binding. Two binding sites were found: a high-affinity site with a dissociation constant (KD) of 0.27 +/- 0.08 nM and maximum binding (Bmax) of 102.46 +/- 8.6 fmol/mg membrane protein, and a lower affinity site with a KD = 6.13 +/- 0.50 nM and Bmax = 467.93 +/- 51.63 fmol/mg membrane protein. Membrane characterization demonstrated that [3H]PGE2 binding was localized in the plasma membrane. In binding competition experiments, unlabelled PGE1 displaced [3H]PGE2 from its receptor at the same concentrations as did PGE2. Neither PGF2 alpha nor PGD2 effectively competed for [3H]PGE2 binding. Adenylyl cyclase activity was inhibited at concentrations of PGE2 that occupy the high-affinity receptor. These data demonstrate that two receptor sites, or states of binding within a single receptor, are present for PGE2 in purified myometrial membranes. PGE2 inhibition of adenylyl cyclase activity support the view that cAMP has a physiological role in the regulation of myometrial contractility by PGE2.     

The adipocyte fatty acid-binding protein binds to membranes by electrostatic interactions

The adipocyte fatty acid-binding protein (AFABP) is believed to transfer unesterified fatty acids (FA) to phospholipid membranes via a collisional mechanism that involves ionic interactions between lysine residues on the protein surface and phospholipid headgroups. This hypothesis is derived largely from kinetic analysis of FA transfer from AFABP to membranes. In this study, we examined directly the binding of AFABP to large unilamellar vesicles (LUV) of differing phospholipid compositions. AFABP bound LUV containing either cardiolipin or phosphatidic acid, and the amount of protein bound depended upon the mol % anionic phospholipid. The K(a) for CL or PA in LUV containing 25 mol % of these anionic phospholipids was approximately 2 x 10(3) M(-1). No detectable binding occurred when AFABP was mixed with zwitterionic membranes, nor when acetylated AFABP in which surface lysines had been chemically neutralized was mixed with anionic membranes. The binding of AFABP to acidic membranes depended upon the ionic strength of the incubation buffer: >/=200 mM NaCl reduced protein-lipid complex formation in parallel with a decrease in the rate of FA transfer from AFABP to negatively charged membranes. It was further found that AFABP, but not acetylated AFABP, prevented cytochrome c, a well characterized peripheral membrane protein, from binding to membranes. These results directly demonstrate that AFABP binds to anionic phospholipid membranes and suggest that, although generally described as a cytosolic protein, AFABP may behave as a peripheral membrane protein to help target fatty acids to and/or from intracellular sites of utilization.     

Gonadotropin and prostaglandins binding sites in nuclei of bovine corpora lutea.

Highly purified nuclei isolated from bovine corpora lutea showed marked enrichment of NAD pyrophosphorylase, a marker for this organelle. Rough endoplasmic reticulum and lysosomal markers were undetectable, whereas plasma membrane and Golgi markers were detectable but not enriched in nuclei. These highly puridied nuclei exhibited specific binding with 125I-labeled human choriogonadotropin, [3H]prostaglandin E1 and [3H]prostaglandin F2 alpha. However, these bindings were only 15.4% (human choriogonadotropin), 7.9% (prostaglandin E1) and 8.9% (prostaglandin F2 alpha) of the plasma membrane binding observed under the same conditions. Washing of nuclei and plasma membranes twice with buffer containing 0.1% Triton X-100 resulted in gonadotropin and prostaglandin F2 alpha binding site and 5'-nucleotidase (EC 3.1.3.5) losses from nuclei that were different from those observed for plasma membranes. More importantly, the washed nuclei exhibited 44% (human choriogonadotropin), 21--26% (prostaglandins) of original specific binding despite virtual disappearance of 5'-nucleotidase activity. The nuclear membranes isolated from nuclei, specifically bound 125I-labeled human choriogonadotropin and [3H]prostaglandin F2 alpha to the same extent or significantly more ([3H]prostaglandin E1, P less than 0.05) than nuclei themselves, despite the marked losses of chromatin. In summary, our data suggest that gonadotropin and prostaglandins bind to nuclei and that this binding was intrinsic and was primarily associated with the nuclear membrane.     

Specific binding of the thromboxane A2 antagonist 13-azaprostanoic acid to human platelet membranes

In the present study we characterized the interaction between the thromboxane A2/prostaglandin H2 antagonist, trans-13-azaprostanoic acid (13-APA), and isolated human platelet membranes. In these studies, we developed a binding assay using trans [3H] 13-APA as the ligand. It was found that trans [3H] 13-APA specific binding was rapid, reversible, saturable and temperature dependent. Scatchard analysis of the binding data yielded a curvilinear plot which indicated the existence of two classes of binding sites: a high-affinity binding site with an estimated dissociation constant (Kd) of 100 nM; and a low-affinity binding site with an estimated Kd of 3.5 microM. At saturation, approximately 1 pmol/mg protein of [3H] 13-APA was bound to the high affinity site. In order to further characterize the nature of the [3H] 13-APA binding site, we evaluated competitive binding by cis 13-APA, cis 15-APA, prostaglandin F2 alpha, U46619, 6-ketoprostaglandin F1 alpha and thromboxane B2. It was found that the [3H] 13-APA binding site was stereospecific and structurally specific. Thus, the cis isomer of 13-APA exhibited substantially reduced affinity for binding. Furthermore, the prostaglandin derivatives, thromboxane B2 and 6-ketoprostaglandin F1 alpha, which do not possess biological activity, also did not compete for [3H] 13-APA binding. On the other hand, U46619 which acts as a thromboxane A2/prostaglandin H2 mimetic, and prostaglandin F2 alpha which acts as a thromboxane A2/prostaglandin H2 antagonist, both effectively competed for [3H] 13-APA binding. These findings indicate that trans 13-APA binds to a specific site on the platelet membrane which presumably represents the thromboxane A2/prostaglandin H2 receptor.     

Solubilization and purification of the prostaglandin E2 receptor from cardiac sarcolemma.

A prostaglandin E2 (PGE2) receptor was solubilized and isolated from cardiac sarcolemma membranes. Its binding characteristics are almost identical to those of the membrane bound receptor. [3H]PGE2 binding to solubilized and membrane bound receptor was sensitive to elevated temperature and no binding was observed in the absence of NaCl. No significant effects of DTT, ATP, Mg2+, Ca2+ or of changes in buffer pH were observed on [3H]PGE2 binding to either solubilized or membrane-bound receptor. Unlabelled PGE1 displaced over 90% of [3H]PGE2 from the CHAPS-solubilized receptor. PGD2, PGI2, PGF2 alpha and 6-keto-PGF1 alpha were not effective in displacing [3H]PGE2 from the receptor. Scatchard analysis of [3H]PGE2 binding to CHAPS-solubilized receptor revealed the presence of two types of PGE2 binding sites with Kd of 0.33 +/- 0.05 nM and 3.00 +/- 0.27 nM and Bmax of 0.5 +/- 0.04 and 2.0 +/- 0.1 pmol/mg of protein. The functional PGE2 receptor was isolated from CHAPS-solubilized SL membrane using two independent methods: first by a WGA-Sepharose chromatography and second by sucrose gradient density centrifugation. Receptor isolated by these two methods bound [3H]PGE2. Unlabelled PGE1 and PGE2 displaced [3H]PGE2 from the purified receptor. Scatchard analysis of [3H]PGE2 binding to purified receptor revealed the presence of the two binding sites as observed for the membrane bound and CHAPS-solubilized receptor. SDS-polyacrylamide gel electrophoresis of the purified receptor fractions revealed the presence of a protein band of M(r) of approx. 100,000. This 100-kDa was photolabelled with [3H]azido-PGE2, a photoactive derivative of PGE2. We propose that this 100-kDa protein is a cardiac PGE2 receptor.     

The binding of d-glucosyl-neoglycoproteins to the hepatic asialoglycoprotein receptor

The binding of D-glucosyl-neoglycoproteins and D-galactose-terminated glycoproteins to the hepatic asialoglycoprotein receptor of rabbit liver membranes were characterized and compared. The binding of both types of glycoproteins showed the same dependence on calcium concentration, sensitivity to neuraminidase, and degree of inhibition by various carbohydrate derivatives. These results, along with the observation that the rabbit liver membranes bound both the D-glucosyl- and D-galactosyl-terminated glycoproteins to the same extent, indicated that both types of glycoproteins bound to the same receptor. To confirm this hypothesis, receptors were isolated from rabbit livers by affinity chromatography using D-galactosyl-bovine serum albumin or D-glucosyl-bovine serum albumin immobilized on Sepharose. These receptors were shown to be identical by several chemical and immunological criteria as well as in their ability to bind equal amounts of D-galactosyl- and D-glucosyl-terminated glycoproteins. The conclusion is that the rabbit hepatic asialoglycoprotein receptor cannot discriminate between D-galactosyl and D-glucosyl-terminated glycoproteins and binds both.     

Effect of membrane fluidity upon binding of Electrophorus acetylcholinesterase to lipid vesicles

A previous report (Watkins, M.S., Hitt, A.S. and Bulger, J.E. (1977) Biochem. Biophys. Res. Commun. 79, 640-647) has indicated that the asymmetric forms of Electrophorus acetylcholinesterase bind exclusively to sphingomyelin vesicles through interaction with the collagen-like 'tail' portion of the enzyme. We report here that acetylcholinesterase also binds to phosphatidylcholine vesicles containing saturated fatty acyl chains and to egg phosphatidylcholine vesicles containing cholesterol. This suggests preferential binding of acetylcholinesterase to membranes of lower fluidity. Surface charge of vesicles and density of zwitterionic lipid headgroups do not significantly affect binding of native acetylcholinesterase. The presence of chondroitin sulfate or hyaluronic acid slightly increases the binding of native acetylcholinesterase to sphingomyelin vesicles, while the presence of 1 M NaCl, bovine serum albumin, or tissue fractions enriched in basement membrane diminish binding. The dissociation constant for native acetylcholinesterase and sphingomyelin vesicles is (1.0-1.5) X 10(-7) M, as measured by a flotation binding assay. The globular, 11S form of acetylcholinesterase also binds to lipid vesicles, although not to the same degree as native acetylcholinesterase. This suggests that the collagen tail of the enzyme enhances binding, but is not essential for binding to occur. These results are consistent with the location of acetylcholinesterase on the surface of the postsynaptic plasma membrane in vivo.     

Prostaglandin E1 binds to Z protein of rat liver

Z protein or fatty-acid-binding protein is abundant in the cytosol of many cell types including liver cells. It is considered to play an important role in intracellular transport and metabolism of long-chain fatty acids and other organic anions. We studied the role of Z protein in the metabolism of prostaglandin E1 (PGE1). Binding of tritiated prostaglandin E1 to this fatty-acid-binding protein (Z protein) purified from rat liver was determined. The binding of [3H]prostaglandin E1 to Z protein is rapid, saturable and reversible. Scatchard analysis of [3H]PGE1 binding to Z protein showed a single class of binding sites with a dissociation constant (Kd) of 37 nM. The binding capacity is 110 nmol/mg Z protein. Optimal [3H]PGE1 binding occurred at pH 7.4. The presence of 3 mM MgCl2 stimulated the prostaglandin E1 binding to Z protein. Competition experiments show that the binding of this autacoid to Z protein is highly specific. It could not be displaced by other prostaglandins (PGA1, PGA2, PGE2, PGB1, PGI2, PGD2, PGF2 alpha, and 6-keto PGF1 alpha). Z protein might be involved in the metabolism of prostaglandins in the cytosol.     

3H]GDP release from rat and hamster adipocyte membranes independently linked to receptors involved in activation or inhibition of adenylate cyclase. Differential susceptibility to two bacterial toxins

When rat adipocyte membranes had been labeled with [3H]GTP in the presence of a beta-adrenergic agonist, the subsequent [3H]GDP release was stimulated by beta-agonists or agonists (e.g. glucagon and secretin) of other "activatory" receptors involved in activation of adenylate cyclase, but was not stimulated by agonists (e.g. prostaglandin E1 and adenosine) of "inhibitory" receptors involved in cyclase inhibition. On the contrary, agonists of inhibitory receptors were effective in stimulating GDP release from hamster adipocyte membranes that had been labeled via inhibitory alpha 2-adrenergic receptors, but an activatory receptor agonist such as isoproterenol was not. Thus, the guanine nucleotide regulatory protein (Ni) involved in adenylate cyclase inhibition is an entity distinct from the regulatory protein (Ns) involved in cyclase activation, and multiple activatory or inhibitory receptors are coupled to a respective common pool of Ns or Ni. Preactivated cholera toxin added together with NAD enhanced GDP release from rat adipocyte membranes prelabeled with isoproterenol but was without effect on the release from hamster adipocyte membranes that had been labeled with an alpha-agonist. In sharp contrast, the active subunit of islet-activating protein, pertussis toxin, failed to alter GDP release from the former membrane but completely abolished inhibitory agonist-induced stimulation of GDP release from the latter membrane preparation in the presence of NAD. Thus, the site of action of cholera toxin is Ns, while that of islet-activating protein is Ni. The function of Ni to communicate between inhibitory receptors and adenylate cyclase was lost when it was ADP-ribosylated by islet-activating protein.     

Enhanced binding of low and high density lipoproteins to human adipocyte plasma membranes: effects of temperature and proteases

The specific binding of 125I-labelled low density lipoprotein ([125I]LDL to human adipocyte plasma membranes was higher at 37 than at 0 degree C. Prior treatment of membranes with pronase had no effect on LDL binding measured at 0 degree C but consistently stimulated binding at 37 degrees C. Plasmin was similar to pronase in enhancing LDL-specific binding, but thrombin was not as effective. 125I-labelled high density lipoprotein ([125I]HDL2) specific binding to human adipocyte plasma membranes was similarly sensitive to temperature and pronase treatment. Addition of the protease inhibitor aprotinin in the adipocyte membrane binding assay significantly reduced [125I]LDL binding at 37 degrees C (p less than 0.05), suggesting the involvement of a protease activity intrinsic to the lipoproteins and (or) membranes. These data demonstrate that both LDL and HDL binding in human adipocyte plasma membranes can be "up-regulated" by specific proteolytic perturbations in a temperature-dependent manner.     

Effect of prostaglandin E2, cAMP and histamine on glycoprotein synthesis in isolated cells of the rat gastric mucosa

It was discovered that prostaglandin E2 (PGE2), but not histamine, increased the incorporation of 3H-N-acetyl-D-glucosamine and 14C-amino acids into the acid-insoluble protein fraction of isolated, mainly mucoid cells of rat gastric mucosa. The cAMP at the dose of 1 mM enhanced, like the PGE2, the synthesis of gastric mucoids. Cycloheximide inhibited the basal incorporation of labelled N-acetyl-D-glucosamine and the amino acid mixture by 28 and 72%, respectively, and blocked completely the PGE2 effect on glycoproteins formation. It is suggested that the PGE2, unlike histamine, enhances the biosynthesis of glycoproteins in the mucoid cells of rat gastric mucosa. The cAMP is believed to be a messenger of the PGE2 effect.     

Characterization of lipophorin binding to the fat body of Rhodnius prolixus

In insects, lipids are transported by a hemolymphatic lipoprotein, lipophorin. The binding of lipophorin to the fat body of the hematophagous insect Rhodnius prolixus was characterized in a fat body membrane preparation, obtained from adult females. For the binding assay, purified lipophorin was radiolabelled in the protein moiety (125I-HDLp), and it was shown that iodination did not affect the affinity of the membrane preparation for lipophorin. Under incubation conditions used, lipophorin binding to membranes achieved equilibrium after 40-60 min, but this time was longer when a low concentration of lipophorin was present in the medium. The capacity of the fat body membrane preparation to bind lipophorin was abolished when membranes were pre-treated with trypsin, and it was also affected by heat. When 125I-HDLp was incubated with increasing concentrations of membrane protein, corresponding increases in binding were observed. Lipophorin binding was sensitive to pH, and it was maximal between pH 6.0 and 7.0. The specific binding of lipophorin to the fat body membrane preparation was a saturable process, with a Kd of 2.1 +/- 0.4 x 10(-7)M and a maximal binding capacity of 289 +/- 88 ng lipophorin/microgram of membrane protein. Binding to the fat body membranes did not depend on calcium, but it was affected by ionic strength, being totally inhibited at high salt concentrations. Suramin also interfered with lipophorin binding and it was abolished in the presence of 2 mM suramin, but at concentrations of 0.05 and 0.1 mM it seemed to increase binding activity slightly. Fat body membrane preparation from Rhodnius prolixus was able to bind lipophorin from Manduca sexta larvae.     

Prostaglandin E2 can bimodally inhibit and stimulate the epididymal adipocyte adenylyl cyclase activity.

Measurements of prostaglandin E2 (PGE2)-induced adenylyl cyclase activity in membranes isolated from epididymal rat adipocytes revealed inhibition of cAMP production at low concentrations of PGE2 (less than 10 mM) and stimulation at higher concentrations. This biphasic effect of PGE2 was obtained when adenylyl cyclase was stimulated with GTP or NaF. In the presence of forskolin only the inhibitory phase by PGE2 was observed. Sulprostone, a PGE2 analogue, did not affect cAMP synthesis in the presence of either GTP or NaF; however, in the presence of forskolin, it inhibited cAMP production similarly to PGE2. Treatment of the membranes with cholera or pertussis toxin did not alter the biphasic effect of PGE2 on cAMP production. These findings raise the possibility that PGE2 acts through several receptor subtypes which are coupled to GTP binding proteins different from the classical Gi or Gs proteins.