Hydrodynamic characterization of vasoactive intestinal peptide receptors extracted from rat lung membranes in Triton X-100 and n-octyl-beta-D-glucopyranoside

Rat lung membrane vasoactive intestinal peptide (VIP) receptors were covalently labeled with 125I-VIP, extracted in Triton X-100 and n-octyl-beta-D-glucopyranoside, and analyzed by gel filtration and sucrose density gradient sedimentation. The fractions were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, and the identity of the 125I-VIP.receptor complex was demonstrated by its co-migration with the covalently labeled 55-kDa receptor unit identified previously. Furthermore, the radioactivity in the peak corresponding to the 125I-VIP.receptor complex was displaced in the presence of unlabeled VIP in a dose-dependent manner. The following hydrodynamic properties were determined for VIP receptors in each detergent solution: in Triton X-100, Stokes radius of 6.1 +/- 0.4 nm, sedimentation coefficient (S20,w) of 7.35 +/- 0.45 S, and partial specific volume (v) of 0.809 +/- 0.015 ml/g; in n-octyl-beta-D-glucopyranoside, Stokes radius of 5.6 +/- 0.00 nm, S20,w of 10.87 +/- 0.22 S, and partial specific volume of 0.783 +/- 0.020 ml/g. The apparent molecular weight of the 125I-VIP.receptor.detergent complex was calculated as 270,000 +/- 36,000 in Triton X-100 and 320,000 +/- 32,000 in n-octyl-beta-D-glucopyranoside. The amount of detergent bound to the receptor was estimated by using the two sets of hydrodynamic data and the significantly different partial specific volumes of the two detergents. Thus, the molecular weight of the receptor alone was calculated as 54,600 daltons, indicating that approximately 3.9 g of Triton X-100 and 4.9 g of n-octyl-beta-D-glucopyranoside were bound per g of receptor. This species contained the 55-kDa binding unit and appeared to be glycosylated as evidenced by its specific binding to wheat germ agglutinin-Sepharose. These results indicate that the rat lung VIP receptor is a glycoprotein with a single polypeptide chain of 55 kDa. The large amount of detergent bound suggests that the receptor is extensively embedded in the membrane.     

Nonionic detergents increase the stoichiometry of ligand binding to the rat hepatic galactosyl receptor

When digitonin is used to expose intracellular galactosyl (Gal) receptors in isolated rat hepatocytes, only about half of the binding activity for 125I-asialoorosomucoid (ASOR) is found as compared to cells solubilized with Triton X-100. The increased ligand binding in the presence of detergent is not due to a decrease in Kd but could be due either to an increase in the number of ASORs bound per receptor or to exposure of additional receptors. Several experiments support the former explanation. No additional activity is exposed even when 80% of the total cell protein is solubilized with 0.4% digitonin. It is, therefore, unlikely that receptors are in intracellular compartments not permeabilized by digitonin and inaccessible to 125I-ASOR. Digitonin-treated cells are not solubilized by Triton X-100 if they are first treated with glutaraldehyde under conditions that retain specific binding activity. 125I-ASOR binding to these permeabilized/fixed cells increases about 2-fold in the presence of Triton X-100 and a variety of other detergents (e.g., Triton X-114, Nonidet P-40, Brij-58, and octyl glucoside) but not with the Tween series, saponin, or other detergents. When these fixed cells are washed to remove detergent, 125I-ASOR binding decreases almost to the initial level. Affinity-purified Gal receptor linked to Sepharose 4B binds approximately twice as much 125I-ASOR in the presence of Triton X-100 as in its absence. The results suggest that the increase in Gal receptor activity in the presence of nonionic detergents is due to an increase in the valency of the receptor rather than to exposure of additional receptors.     

VIP receptors from porcine liver: high yield solubilization in a GTP-insensitive form.

Vasoactive intestinal peptide (VIP) receptors were solubilized from porcine liver membranes using CHAPS. The binding of 125I-VIP to solubilized receptors was reversible, saturable and specific. Scatchard analysis indicated the presence of one binding site with a Kd of 6.5 +/- 0.3 nM and a Bmax of 1.20 +/- 0.15 pmol/mg protein. Solubilized and membrane-bound receptors displayed the same pharmacological profile since VIP and VIP-related peptides inhibited 125I-VIP binding to both receptor preparations with the same rank order of potency e.g. VIP greater than helodermin greater than rat GRF greater than rat PHI greater than secretin greater than human GRF. GTP inhibited 125I-VIP binding to membrane-bound receptors but not to solubilized receptors supporting functional uncoupling of VIP receptor and G protein during solubilization. Affinity labeling of solubilized and membrane-bound VIP receptors with 125I-VIP revealed the presence of a single molecular component with Mr 55,000 in both cases. It is concluded that VIP receptors from porcine liver can be solubilized with a good yield, in a GTP-insentive, G protein-free form. This represents a major advance towards the purification of VIP receptors.     

Solubilization of rat lung vasoactive intestinal peptide receptors in the active state. Characterization of the binding properties and comparison with membrane-bound receptors

We demonstrate here that rat lung membrane vasoactive intestinal peptide (VIP) receptors can be extracted in the active state using digitonin. Sepharose 4B gel filtration chromatography was utilized to demonstrate the formation of specific binding complexes between 125I-VIP and solubilized receptors. A rapid soluble receptor assay was established to separate 125I-VIP-receptor complexes from free 125I-VIP, which entailed differential precipitation of the 125I-VIP-receptor complex with polyethylene glycol and bovine gamma-globulin. Using this assay, several detergents were tested for their suitability to extract active VIP receptors, and most favorable results were obtained with digitonin, as judged by specific binding of 125I-VIP to the solubilized receptors. Time course studies indicated that the binding of 125I-VIP to digitonin extract was more rapid than to rat lung membranes. Scatchard analyses of competitive binding data indicated the presence of two classes of binding sites in the digitonin extract, as in the membrane. The values for the dissociation constants (Kd) were 200 pM for Class I and 8 nM for Class II receptors while the values for binding capacity (Bmax) were 200 and 2300 fmol/mg for Class I and II sites, respectively. Although the binding parameters of the two classes were similar to those in the membrane, the pharmacological properties were different, as evidenced by the inability of rat growth hormone releasing factor, a potent VIP agonist in the membrane, to displace specifically bound 125I-VIP from solubilized receptors. The ability to solubilize active VIP receptors represents an important step toward purification of the functional protein.     

Solubilization of the liver vasoactive intestinal peptide receptor. Hydrodynamic characterization and evidence for an association with a functional GTP regulatory protein

Vasoactive intestinal peptide (VIP) receptors were solubilized using the nondenaturing detergent Triton X-100 after occupancy of rat liver membrane-bound receptors with 125I-VIP. Gel filtration and ultracentrifugation on sucrose density gradients revealed the existence in the soluble macromolecular fraction of two labeled components: a major (80%) heavy component and a minor (20%) light one. The two components exhibit the following hydrodynamic parameters: Stokes radius, 5.8 nm: s20,w, 5.98 s; Mr, 150,000; frictional ratio, 1.52 for the major; and Stokes radius, 3.0 nm: s20,w, 3.98 s; Mr = 52,000; frictional ratio, 1.12 for the minor component. The labeling of these components was specific in that it dramatically decreased when unlabeled VIP was added together with 125I-VIP. The pharmacological specificity was also assessed by using 10 nM histidylisoleucineamide (a VIP agonist). Many lines of evidence indicate that the light component (Mr = 52,000) is the VIP-receptor complex while the heavy component (Mr = 150,000) is a ternary complex consisting of VIP, the receptor, and a guanine nucleotide regulatory protein, probably Ns. GTP is required to dissociate 125I-VIP from the heavy component whereas it is ineffective on the light component. This effect is nucleotide specific. After cholera toxin-induced [32P]ADP ribosylation of liver membranes, a high peak of 32P radioactivity containing the alpha subunit (Mr = 42,000) of the Ns protein is coeluted with the heavy component on Sephacryl S-300. By mild urea (2 M) treatment, the heavy component is converted into the light without significant dissociation of 125I-VIP. When a Triton extract of membranes prelabeled with 125I-VIP is treated with dithiobis(succinimidyl propionate) subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis reveals a major band corresponding to Mr = 150,000. Alternatively, when prelabeled membranes are directly treated with the cross-linker, a major complex of Mr = 51,000 is observed. This may be related to different accessibility of the cross-linker to the site at which the receptor and the Ns protein interact in the two conditions. In conclusion, these data represent initial reports on the successful solubilization of functional VIP-receptor complexes and provide evidence for an interaction between liver VIP-receptor complexes and a GTP-binding protein.     

Solubilization of active and stable receptors for vasoactive intestinal peptide from rat liver

Vasoactive intestinal peptide (VIP) receptors were solubilized from rat liver using the zwitterionic detergent CHAPS. Optimal conditions of solubilization were obtained with 5 mM CHAPS and 2.5 mg protein/ml. The binding of 125I-VIP to CHAPS extracts was time- and pH-dependent, saturable and reversible. The following order of potency of unlabeled VIP-related peptides for inhibiting 125I-VIP binding was observed: VIP greater than helodermin greater than peptide histidine isoleucine amide (PHI) greater than rat growth hormone releasing factor (rGRF) greater than secretin. This peptide specificity is identical to that of rat liver membrane-bound receptors. VIP binding activity in the CHAPS extract was destroyed by trypsin or dithiothreitol in accordance with the known sensitivity of membrane-bound receptors to these agents. VIP receptors in CHAPS extracts were stable for at least 5 days at 4 degrees C. Scatchard analysis of equilibrium binding data indicated the presence in CHAPS extracts of high (H) and low (L) affinity binding sites with the following characteristics: KdH = 0.27 nM and BmH = 34 fmol/mg protein; KdL = 51 nM and BmL = 1078 fmol/mg protein. The guanine nucleotide GTP inhibited 125I-VIP binding to soluble receptors and enhanced the dissociation of soluble VIP-receptor complexes, suggesting that GTP-binding proteins were functionally associated with VIP receptors in solution. Gel filtration of solubilized VIP receptors on Sephacryl S-300 revealed a single binding component with a Stokes radius of 6.1 nm. It is concluded that active VIP receptors can be extracted from liver membranes by CHAPS. The availability of this CHAPS-soluble, stable and functional receptor from a tissue which can be obtained in large amounts represents a major step toward the purification of VIP receptors.     

A new competition assay for the solubilized hepatic galactosyl receptor

The present method of quantitating soluble asialoglycoprotein (galactosyl) receptor activity relies on the selective precipitation of receptor-ligand complexes to allow separation from free ligand. To provide an alternative to selective precipitation procedures, a simple and rapid method to assay for detergent-solubilized galactosyl receptor activity has been developed which uses permeabilized, fixed cells as a source of immobilized solid-phase receptors. Isolated rat hepatocytes were treated with digitonin to make available the internal as well as the external receptors. The permeable cells were also treated with glutaraldehyde to prevent further protein loss during subsequent exposure to detergents such as Triton X-100. The permeable/fixed cells, which retained about 70% of their total 125I-asialo-orosomucoid (125I-ASOR)-binding activity, with 89% specific binding, were insoluble even in 0.5% Triton X-100 and were easily pelleted. The permeable/fixed cells can be prepared in advance and stored frozen for months. A detergent extract of receptor is mixed with a constant amount of both 125I-ASOR and permeable/fixed cells. Soluble active receptors compete with immobilized receptors on the treated cell for binding of the 125I-ASOR. The assay is reproducible, linear over a broad range of soluble receptor concentration, and can quantitate receptor activity from as few as 10(5) hepatocytes. A modified purification procedure for the rat hepatic galactosyl receptor using this competition assay is also described.     

An investigation of sites that bind human somatotropin (growth hormone) in the liver of the pregnant rabbit.

The binding of 125I-labelled human somatotropin (growth hormone) to a crude membrane preparation from the liver of pregnant rabbit, and to receptors solubilized from this fraction by Triton X-100, was dependent on time, temperature and receptor concentration. At 4 degrees C a steady state was reached after 20 h, and maximum specific binding (as a percentage of total tracer added) was approx. 50% for both membrane-bound and solubilized receptors. Solubilization did not significantly affect the binding properties of the receptor at low concentrations of Triton X-100 (less than 0.05%, v/v, in the assay tube). However, at higher concentrations (approx. 0.1%, v/v), the detergent lowered the ability of some hormones, for example ovine prolactin, to displace 125I-labelled human somatotropin, but did not affect other hormones such as bovine somatotropin. Some somatogenic hormones, such as bovine somatotropin, and some lactogenic hormones, such as ovine prolactin, displaced 125I-labelled human somatotropin from membrane-bound and solubilized receptor preparations. Furthermore, 85% of 125I-labelled bovine somatotropin was displaced from membrane-bound receptors by ovine prolactin, and 125I-labelled ovine prolactin was almost completely displaced by bovine somatotropin. Scatchard analysis of the binding data for human somatotropin suggested a single class of binding sites in the membrane-bound receptor preparation, with an affinity (Ka) of 1.9 X 10(9) M-1 and a capacity of 1726 fmol/mg of protein; these values were slightly increased by solubilization (Ka = 3.2 X 10(9) M-1, capacity = 2103 fmol/mg of protein). Scatchard analysis of binding to membrane-bound receptors also indicated a single class of high-affinity binding sites for bovine somatotropin (Ka = 4.8 X 10(9) M-1, capacity = 769 fmol/mg) and for ovine prolactin (Ka = 6.1 X 10(9) M-1, capacity = 187 fmol/mg).     

Amphiphilic Detergent-Soluble Acetylcholinesterase from Torpedo marmorata: Characterization and Conversion by Proteolysis to a Hydrophilic Form

The membrane-bound acetylcholinesterase (AChE) from the electric organ of Torpedo marmorata was solubilized by Triton X-100 or by treatment with proteinase K and purified to apparent homogeneity by affinity chromatography. Although the two forms differed only slightly in their subunit molecular weight (66,000 and 65,000 daltons, respectively), considerable differences existed between native and digested detergent-soluble AChE. The native enzyme sedimented at 6.5 S in the presence of Triton X-100 and formed aggregates in the absence of detergent. The digested enzyme sedimented at 7.5 S in the absence and in the presence of detergent. In contrast to the detergent-solubilized AChE, the proteolytically derived form neither bound detergent nor required amphiphilic molecules for the expression of catalytic activity. This led to the conclusion that limited digestion of detergent-soluble AChE results in the removal of a small hydrophobic peptide which in vivo is responsible for anchoring the protein to the lipid bilayer.     

Solubilization of the active vasoactive intestinal peptide receptor from human colonic adenocarcinoma cells

The non-ionic detergent n-octyl-beta-D-glucopyranoside was used to solubilize the VIP (vasoactive intestinal peptide) receptor from human colonic adenocarcinoma cell line HT29-D4. The binding of monoiodinated 125I-VIP to the solubilized receptor was specific, time-dependent, and reversible. Scatchard analysis of data obtained from competitive displacement of monoiodinated 125I-VIP by native VIP suggested the presence of two classes of VIP binding sites with Kd values of 0.32 and 46.7 nM. The binding capacities of these two classes were 1.7 x 10(10) and 30.2 x 10(10) sites/mg of proteins, respectively. The solubilized receptor retained the specificity of the human VIP receptor towards the peptides of the VIP/secretin/glucagon family. The order of potency in inhibiting monoiodinated 125I-VIP binding was VIP (IC50 = 1.0 x 10(-9) M) much greater than peptide histidine methionine amide (IC50 = 10(-7) M) greater than growth hormone-releasing factor (IC50 = 3 x 10(-7) M) greater than secretin (IC50 greater than 10(-6) M); glucagon had no effect on VIP binding. The reducing agent dithiothreitol inhibited in a dose-dependent manner the binding of 125I-VIP. Covalent cross-linking experiments between the solubilized receptor and 125I-VIP showed that after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography two major and one minor polypeptides of Mr 67,000, 72,000, and 83,000 were specifically labeled. When analyzed by gel filtration, the n-octyl-beta-D-glucopyranoside-solubilized 125I-VIP-receptor complex was resolved into two major peaks with molecular mass in the range of 60-70 and 270-300 kDa. Thus, the soluble form of the VIP receptor was probably a multimeric complex in which disulfide bonds may play an important role to hold the receptor in an active configuration.     

Solubilization of pituitary receptors for thyrotropin-releasing hormone

Receptors for thyrotropin-releasing hormone were solubilized by Triton X-100. Membrane fractions from GH₃ pituitary tumor cells were incubated with thyrotropin-releasing hormone in order to saturate specific receptor sites before the addition of detergent. The amount of protein-bound hormone solubilized by Triton X-100 was proportional to the fractional saturation of specific membrane receptors. Increasing detergent: protein ratios from 0.5 to 20 led to a progressive loss of hormone · receptor complex from membrane fractions with a concomitant increase in soluble protein-bound hormone. The soluble hormone · receptor complex was not retained by 0.22 μm filters and remained soluble after ultracentrifugation. Following incubation with high (2.5–10%) concentration of Triton X-100 and other non-ionic detergents, or following repeated detergent extraction, at least 18% of specifically bound thyrotropin-releasing hormone remained associated with particulate material. Unlike the hormone receptor complex, the free hormone receptor was inactivated by Triton X-100. A 50% loss of binding activity was obtained with 0.01% Triton X-100, corresponding to a detergent: protein ratio of 0.033.The hormone · receptor complex was included in Sepharose 6B and exhibited an apparent Stokes radius of 46 Å in buffers containing Triton X-100. The complex aggregated in detergent-free buffers. Soluble hormone receptors were separated from excess detergent and thyrotropin-releasing hormone by chromatography on DEAE-cellulose. Thyrotropin-releasing hormone dissociated from soluble receptors with a half-time of 120 min at 0°c, while the membrane hormone · receptor complex was stable for up to 5 h at 0°C.     

Solubilization of pituitary receptors for thyrotropin-releasing hormone.

Receptors for thyrotropin-releasing hormone were solubilized by Triton X-100. Membrane fractions from GH3 pituitary tumor cells were incubated with thyrotropin-releasing hormone in order to saturate specific receptor sites before the addition of detergent. The amount of protein-bound hormone solubilized by Triton X-100 was proportional to the fractional saturation of specific membrane receptors. Increasing detergent:protein ratios from 0.5 to 20 led to a progressive loss of hormone . receptor complex from membrane fractions with a concomitant increase in soluble protein-bound hormone. The soluble hormone . receptor complex was not retained by 0.22 micron filters and remained soluble after ultracentrifugation. Following incubation with high (2.5--10%) concentrations of Triton X-100 and other non-ionic detergents, or following repeated detergent extraction, at least 18% of specifically bound thyrotropin-releasing hormone remained associated with particulate material. Unlike the hormone receptor complex, the free hormone receptor was inactivated by Triton X-100. A 50% loss of binding activity was obtained with 0.01% Triton X-100, corresponding to a detergent:protein ratio of 0.033. The hormone . receptor complex was included in Sepharose 6B and exhibited an apparent Stoke radius of 46 A in buffers containing Triton X-100. The complex aggregated in detergent-free buffers. Soluble hormone receptors were separated from excess detergent and thyrotropin-releasing hormone by chromatography on DEAE-cellulose. Thyrotropin-releasing hormone dissociated from soluble receptors with a half-time of 120 min at 0 degrees C, while the membrane hormone . receptor complex was stable for up to 5 at 0 degrees C.     

An improved procedure for the purification of ethanolaminephosphotransferase. Reconstitution of the purified enzyme with lipids

Ethanolaminephosphotransferase (CDPethanolamine:1,2-diacylglycerol ethanolaminephosphotransferase, EC 2.7.8.1) has been purified in active form from rat brain microsomes by a two-step chromatographic procedure. Enzyme preparations characterized by high specific activity and stability were obtained supplementing the solubilization and elution buffers, containing 1% Triton X-100, with 0.01% 2,6-di-tert-butyl-4-methylphenol. The specific activity of the purified enzyme was about 1200-times higher than that of the crude solubilized enzyme. The lipid dependence of ethanolaminephosphotransferase was studied both in the presence of Triton X-100 and in detergent-free enzyme preparations. The activity of the detergent-solubilized ethanolaminephosphotransferase was strongly modified by phospholipids. The kinetic behaviour of the enzyme was also dependent on the lipids contained in the aggregates obtained by removal of the detergent from detergent/lipid/protein suspensions. A regulatory role of phospholipids on the activity of the membrane-bound ethanolaminephosphotransferase is discussed.     

Purification of vasoactive intestinal peptide receptor from porcine liver by a newly designed one-step affinity chromatography

Vasoactive intestinal peptide (VIP) receptors were solubilized from porcine liver membrane using the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. The solubilized VIP receptor has been purified approximately 50,000-fold to apparent homogeneity by a one-step affinity chromatography using a newly designed VIP-polyacrylamide resin. The purified receptor bound 125I-VIP with a Kd of 22.3 +/- 0.7 nM and retained its peptide specificity toward VIP-related peptides. The specific activity of the purified receptor (16,400 pmol/mg of protein) was very close to the theoretical value (18,900 pmol/mg of protein) calculated assuming one binding site/protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of purified receptor revealed a single band with an Mr of 53,000 after either silver staining or radioiodination. Affinity labeling of the purified receptor with 125I-VIP using dithiobis(succinimidyl propionate) gave a single radioactive band, the labeling of which was completely inhibited by an excess of unlabeled VIP. In conclusion, an Mr 53,000 protein containing the VIP-binding site was purified to homogeneity by a one-step affinity chromatography using immobilized VIP.     

Characterization of a lipid-associated gonadotropin receptor from the luteinized rat ovary

Gonadotropin receptors which bind luteinizing hormone (lutropin) and human chorionic gonadotropin (hCG) in the ovaries of immature female rats showed a 30-fold increase after treatment of animals with pregnant mare serum gonadotropin (PMSG) and hCG. This marked induction of lutrophin/hCG receptors in the rat ovary was not accompanied by a change in binding affinity for labeled hCG. Such luteinized ovaries have been found consistently to contain a small proportion of soluble receptor sites, which comprised about 5% of the total receptor population. The soluble receptor sites were present in the floating lipid fraction of the 360 000 × g supernatant of homogenate prepared from luteinized ovaries, and could not be detected in similar fractions prepared from interstitial cells or homogenates of the normal rat testis.The physico-chemical properties of the spontaneously soluble ovarian receptors were similar to those derived for detergent-solubilized receptors prepared by extraction of particulate ovarian binding fractions with Triton X-100. The affinity constant to the soluble ovarian receptor sites for [¹²⁵I]hCG was 0.70 · 10¹⁰ M^?1, and that of the receptors solubilized by Triton X-100 was 0.72 · 10¹⁰ M^?1. The sedimentation pattern of the soluble receptors during sucrose density gradient centrifugation showed extensive aggregation into rapidly sedimenting forms. However, centrifugation of the cytosol receptor in the presence of Triton X-100 gave a single 6.5 S component, corresponding to the solubilized receptors previously characterized in detergent extracts of the rat ovary and testis.The pesence of a spontaneously soluble lutropin/hCG receptor in ovarian cytosol fractions suggests that rapid synthesis and assembly of receptors in ovaries of PMSG-hCG-treated rats is accompanied by increased production of cytoplasmic receptor precursors; alternatively, this receptor population may represent a fraction that has been internalized or processed as during receptor turnover in the cell membrane.     

Covalent affinity labeling, detergent solubilization, and fluid-phase characterization of the rabbit neutrophil formyl peptide chemotaxis receptor

The formyl peptide chemotaxis receptor of rabbit neutrophils and purified rabbit neutrophil plasma membranes has been identified by several affinity labeling techniques: covalent affinity cross-linking of N-formyl-Nle-Leu-Phe-Nle-125I-Tyr-Lys (125I-hexapeptide) to the membrane-bound receptor with either dimethyl suberimidate or ethylene glycol bis(succinimidyl succinate) and photoactivation of N-formyl-Nle-Leu-Phe-Nle-125I-Tyr-N epsilon-[6-[(4-azido-2-nitrophenyl)amino]hexanoyl]Lys(125I-PAL). These techniques specifically identify the receptor as a polypeptide that migrates as a broad band on sodium dodecyl sulfate-polyacrylamide electrophoresis, with Mr 50 000-65 000. The receptor has been solubilized in active form from rabbit neutrophil membranes with the detergents 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and digitonin and from whole cells with CHAPS. Chemotaxis receptor activity was measured by the ability of the solubilized membrane material to bind 125I-hexapeptide or fMet-Leu-[3H]Phe with gel filtration or rapid filtration through poly(ethylenimine)- (PEI) treated filters as assay systems. 125I-PAL was specifically cross-linked to the same molecular weight material in the CHAPS and digitonin solubilized extract, but no specific labeling of the receptor was seen when membranes were extracted with Nonidet P-40 and Triton X-100. Therefore, although a large number of detergents are able to solubilize the receptor, it appears that some release the receptor in an inactive form. The ligand binding characteristics of fMet-Leu-[3H]Phe to the CHAPS-solubilized receptor shared properties with the membrane-bound formyl peptide receptor, both of which showed curvilinear, concave-upward Scatchard plots. Computer curve fitting with NONLIN and statistical analyses of the binding data indicated that for both the membrane-bound and solubilized receptors a two saturable sites model fitted the data significantly better (p less than 0.01) than did a one saturable site model. The characteristics of the two saturable sites model for the soluble receptor were a high-affinity site with a KD value of 1.25 +/- 0.45 nM and a low-affinity site with a KD value of 19.77 +/- 3.28 nM. A total of 35% of the two sites detected was of the higher affinity. In addition, a Hill coefficient of 0.61 +/- 0.12 was observed.     

Protection of soluble benzodiazepine receptors from heat inactivation by GABAergic ligands

Benzodiazepine receptors were solubilized from calf brain cortex by the ionic detergent deoxycholate and by the nonionic detergent Triton X-100. Approximately 90% of the soluble benzodiazepine receptors of both preparations were heat inactivated within 30 min at 55 degrees C. 100 microM of gamma-aminobutyric acid (GABA) protected 80% of Triton X-100 solubilized benzodiazepine receptors and 56% of the deoxycholate soluble benzodiazepine receptors from heat inactivation. Time course of heat inactivation showed that the deoxycholate soluble receptors are more sensitive to heat than the Triton X-100 soluble receptors.     

Crossed immunoelectrophoretic studies of the solubility immunogenicity of herpes simplex virus antigens.

The nonionic detergent Triton X-100 was capable of solubilizing 90% of the protein content in herpes simplex virus (HSV)-infected rabbit cornea cells. The solubilized HSV antigens formed well-characterized precipitates by crossed immunoelectrophoresis in Triton X-100-containing agarose gel, allowing both identification and relative quantitation. Water-soluble and detergent-requiring HSV antigens were identified by different solubilization procedures in buffer with and without detergent. Five glycoprotein antigens were solubilized only in the presence of detergent, indicating their membrane-bound state. One non-glycosylated antigen was present in both a water-soluble and a membrane-bound form. Based upon the crossed immunoelectrophoretic precipitating patterns of Triton X-100-solubilized HSV antigens, it has been estimated that infected cells yield an amount of virus-specific protein equivalent to 2,000 enveloped virions per cell. Rabbits inoculated intracutaneously with Triton X-100-solubilized HSV antigens developed neutralizing antibodies against HSV almost as effectively as rabbits with an active HSV infection. Precipitins against individual HSV antigens in sera from rabbits infected with HSV and immunized with the Triton X-100-solubilized HSV antigens were assayed by the crossed immunoelectrophoretic technique. Sera from infected rabbits reacted more strongly and with a higher number of HSV antigens than sera from immunized rabbits.     

Detergent solubilization of mammalian cardiac and hepatic beta-adrenergic receptors.

The solubilization of canine cardiac and hepatic β-adrenergic receptors was characterized with 19 different ionic and nonionic surfactants and surfactant combinations. The effects of alterations in the hydrophobic and hydrophilic moieties of the nonpolar detergents were examined in relation to their efficacy in solubilizing these receptor molecules. Within this group of surfactants the more effective agents contained an average of 9–10 polyoxyethylene units per molecule. The best degree of β-receptor solubilization for both heart and liver was obtained with 1% Brij 96 or a combination of 1% digitonin with 0.25% Triton X-100. Hepatic but not cardiac β-receptors were solubilized by polyoxyethylene ether W-1 or by Triton X-100. Solubilization time courses indicated that the maximum degree of receptor solubilization occurred within 5 min at 0–4 °C. Solubilization temperatures were varied from 0 to 37 °C. Temperatures up to 30 °C increased numbers of cardiac receptors solubilized by 30% over those obtained at 0 °C. The same temperature changes had no significant effects on liver β-receptor solubilization. Increasing the solubilization temperature to 37 °C decreased the detectable number of β-receptors by 91 (liver) and 72% (heart). β-Receptors solubilized in the absence of receptor ligand were extremely labile with a half-life on the order of 90 min at 4 °C for both heart and liver. Occupation of the receptors by [¹²⁵I]-iodohydroxybenzylpindolol prior to solubilization conferred a certain degree of stability on the receptors.     

Solubilization and hydrodynamic characterization of guanine nucleotide sensitive vasoactive intestinal peptide-receptor complexes from rat intestine

The purpose of this work was to solubilize vasoactive intestinal peptide (VIP) receptors from rat small intestinal plasma membranes and to analyze the nature and function of its molecular form(s) in a nondenaturing environment. Membranes were incubated with 3 nM 125I-VIP, washed, and treated with 1% Triton X-100. Chromatography on Sephadex G-50 showed that 60% of the extractable radioactivity was eluted with macromolecular components in the void volume. This radioactive material was dramatically reduced when 1 microM unlabeled VIP was present in the incubation medium or when membranes were pretreated with trypsin or dithiothreitol. Macromolecular components that had bound 125I-VIP were further chromatographed on Sephacryl S-300. Two peaks were observed: a major one (80%) and a minor one (20%) with Stokes radii of 5.2 and 3.1 nm, respectively. The labeling of both components was inhibited by unlabeled VIP or peptide with NH2-terminal histidine and COOH-terminal isoleucine amide (a VIP agonist). The presence of GTP (0.1 mM) in the incubation medium of membranes completely abolished the labeling of the 5.2-nm component but did not affect that of the 3.1-nm one. Moreover, GTP induced dissociation of 125I-VIP from the 5.2-nm component isolated by Sephacryl S-300 chromatography. This effect was time dependent and nucleotide specific. In contrast, GTP did not affect the stability of the 3.1-nm component. After cholera toxin catalyzed [32P]ADP-ribosylation of membranes, chromatography of solubilized material on Sephacryl S-300 showed that a peak of 32P radioactivity was coeluted with the 5.2-nm component.(ABSTRACT TRUNCATED AT 250 WORDS)