Stereoselective photoaffinity labelling of the purified 1,4-dihydropyridine receptor of the voltage-dependent calcium channel

The voltage-dependent calcium channel from guinea-pig skeletal muscle T-tubules has been isolated with a rapid, two-step purification procedure. Reversible postlabelling of the channel-linked 1,4-dihydropyridine receptor and stereoselective photolabelling as a novel approach were employed to assess purity. A 135-fold purification to a specific activity of 1311 +/- 194 pmol/mg protein (determined by reversible equilibrium binding with (+)-[3H]PN200-110) was achieved. Three polypeptides of 155 kDa, 65 kDa and 32 kDa were identified in the purified preparation. The 155-kDa band is a glycoprotein. The arylazide photoaffinity probe (-)-[3H]azidopine bound with high affinity to solubilized membranes (Kd = 0.7 +/- 0.2 nM) and highly purified fractions (Kd = 3.1 +/- 2 nM), whereas the optical antipode (+)-azidopine was of much lower affinity. Irradiation of (-)-[3H]azidopine and (+)-[3H]azidopine receptor complexes with ultraviolet light led to preferential incorporation of the (-) enantiomer into the 155-kDa polypeptide in crude solubilized and purified preparations. The pharmacological profile of irreversible labelling of the 155-kDa glycoprotein by (-)-[3H]azidopine is identical to that found in reversible binding experiments. Specific photolabelling of the 155-kDa band by (-)-[3H]azidopine per milligram of protein increases 150-fold upon purification, whereas incorporation into non-specific bands in the crude solubilized material is identical for both, (-) and (+)-[3H]azidopine.     

Specific binding of the calcium antagonist [3H]verapamil to membrane fractions from plants

Specific binding of the Ca2+ channel blocker [3H] verapamil to a membrane fraction from plants has been characterized. Binding to zucchini membranes was saturable and reversible. The apparent equilibrium dissociation constant is KD = 102 nM and the maximum number of binding sites is Bmax = 60 pmol/mg of protein. The KD determined from the association and dissociation rate constants is 130 nM. [3H]Verapamil binding to zucchini membranes could not be inhibited by the Ca2+ antagonists nifedipine and diltiazem. However, [3H]verapamil could be displaced by diltiazem but not by nifedipine from corn membranes. Sucrose density fractionation of zucchini membrane preparations revealed that [3H]verapamil binding sites are located primarily at the plasma membrane.     

Characterization of [3H]Vesamicol binding in rat brain preparations

The binding of (1)-[³H]vesamicol was characterized in several subcellular fractions and brain regions of the rat. Binding to a lysed P₂ fraction from the rat cerebral cortex reached equilibrium within 4 min at 37°C and was reversible (dissociation half-time 4.9 min). At least two binding affinities were found in P₂ fractions from the cerebral cortex (K_d:21 nM and 980 nM), striatum (K_d:28 nM and 690 nM), and cerebellum (K_d:22 nM and 833 nM). High affinity B_max values were highest in striatum (1.17 pmol/mg protein), followed by cerebellum (0.67 pmol/mg protein), and cerebral cortex (0.38 pmol/mg protein). Low affinity B_max values were highest in cerebellum (5.2 pmol/mg protein), with similar values for cerebral cortex (3.7 pmol/mg protein) and striatum (3.8 pmol/mg protein). High affinity but not low affinity binding in each brain region was stereospecific. Another inhibitor of vesicular ACh-transport also displaced 1-vesamicol binding potently (IC₅₀:17 nM) and efficaciously (over 90%). Both high affinity and low affinity B_max values for [³H]vesamicol-binding were highest in a partially purified synaptic vesicle fraction, followed by puriffied synaptosomes, crude membranes and P₂ fractions. Specific binding was not observed in a mitochondria-enriched fraction. Crude membrane preparations of primary, neuron-enriched whole brain cultures also exhibited high (64 nM) and low affinity (1062 nM) [³H]vesamicol binding. Isoosmotic replaement of 0.18 M KCl in the binding-buffer with NaCl had no effect on binding. These results suggest that at least some high affinity [³H]vesamicol binding in rat brain preparations may be associated with synaptic vesicles, some of which may not be cholinergic in origin.     

Increase in number of myocardial [3H]BAY K 8644 binding sites during adult maturation of rat.

In the present study we investigated the binding properties of [3H]BAY K 8644 to the purified sarcolemmal membrane, isolated from 2- and 12-month old Sprague-Dawley rats. Specific binding of [3H]BAY K 8644 was saturable and the Scatchard plot analysis revealed a single class of binding sites in purified sarcolemmal membrane. The estimated maximum number of binding sites in the membrane of 12-month-old rat was 2.4 +/- 0.1 pmol/mg protein, which was significantly greater than the maximum number of binding sites in 2-month-old rats (1.7 +/- 0.2 pmol/mg protein). The affinity to bind [3H]BAY K 8644 was, however, reduced in older rats (KD, 14.5 +/- 0.8 vs. 4.8 +/- 0.3 nM). Measurement of activities of sarcolemmal and subcellular marker enzymes showed that the purification of membrane was virtually identical in two age groups. This would suggest that membrane purity was not a contributing factor to the observed increase in [3H]BAY K 8644 receptor density. Since dihydropyridine receptor sites are very likely to represent voltage-gated calcium channels of sarcolemma, it is concluded that the density of myocardial voltage-gated calcium channels increases during adult maturation.     

Direct binding of verapamil to the ryanodine receptor channel of sarcoplasmic reticulum.

Radioligand binding experiments and single channel recordings demonstrate that verapamil interacts with the ryanodine receptor Ca2+ release channel of the sarcoplasmic reticulum of rabbit skeletal muscle. In isolated triads, verapamil decreased binding of [3H]Ryanodine with an IC50 of approximately 8 microM at an optimal pH 8.5 and pCa 4.3. Nitrendipine and d-cis-diltiazem did not interfere with binding of [3H]Ryanodine to triads, suggesting that the action of verapamil does not involve the dihydropyridine receptor. Single channel recordings showed that verapamil blocked Ca2+ release channels by decreasing open probability, duration of open events, and number of events per unit time. A direct interaction of verapamil with the ryanodine receptor peptide was demonstrated after purification of the approximately 400 kDa receptor protein from Chaps-solubilized triads. The purified receptor displayed high affinity for [3H]Ryanodine with a Kd of approximately 5 nM and a Bmax of approximately 400 pmol/mg. Verapamil and D600 decreased [3H]Ryanodine binding noncompetitively by reducing the Bmax. Thus the presence of binding sites for phenylalkylamines in the Ca2+ release channel was confirmed. Verapamil blockade of Ca2+ release channels may explain some of the paralyzing effects of phenylalkylamines observed during excitation-contraction coupling of skeletal muscle.     

Subcellular distribution of leukotriene C4 binding units in cultured bovine aortic endothelial cells

The subcellular distribution of specific binding sites for [3H]leukotriene C4 ([3H]LTC4) was analyzed after sedimentation of organelles from disrupted bovine aortic endothelial cells on sucrose density gradients and was shown to be in membrane fractions I (20% sucrose) and IV (35% sucrose). Saturation binding studies of [3H]LTC4 on endothelial cell monolayers at 4 degrees C demonstrated high-affinity binding sites with a dissociation constant (Kd) of 6.8 +/- 2.2 nM (mean +/- SD) and a density of 0.12 +/- 0.02 pmol/10(6) cells. At 4 degrees C, the specific binding of [3H]LTC4 by each of the subcellular fractions reached equilibrium at 30 min and remained stable for an additional 60 min. After 30 min of incubation with [3H]LTC4, the addition of excess unlabeled LTC4 to each subcellular fraction reversed more than 70% of [3H]LTC4 binding in 10 min. The [3H]LTC4 binding activities of subcellular fractions were enhanced approximately twofold to fourfold in the presence of Ca2+, Mg2+, and Mn2+, whereas Na+, K+, and Li+ were without effect. As measured by saturation experiments, the Kd and density of LTC4 binding sites in fraction I were 4.8 +/- 1.6 nM and 16.5 +/- 1.9 pmol/mg of protein, respectively, and in fraction IV were 4.7 +/- 1.5 nM and 81.4 +/- 19 pmol/mg of protein, respectively. Inhibition of [3H]LTC4 binding in membrane-enriched subcellular fractions I and IV by LTC4 occurred with molar inhibition constant (Ki) values of 4.5 +/- 0.1 nM and 4.7 +/- 1.2 nM, respectively, whereas Ki values for LTD4 were 570 +/- 330 nM and 62.5 +/- 32.8 nM, respectively, and for LTE4 were greater than 1000 nM for each fraction; LTB4 and reduced glutathione were even less active. FPL55712, a putative antagonist of the sulfidopeptide LT components of slow reacting substance of anaphylaxis, had Ki values of 1520 +/- 800 nM and 1180 +/- 720 nM for [3H]LTC4 binding sites on membrane-enriched subcellular fractions I and IV, respectively. Thus as defined by Kd, Ki, and specificity, the LTC4 binding units that are distributed to the plasma membrane and the binding units in the subcellular fraction of greater density were similar to each other. Pretreatment of the isolated subcellular membrane fractions with trypsin abolished [3H]LTC4 binding by fraction I, enriched for the plasma membrane marker 5' nucleotidase, and that by fraction IV, enriched for the mitochondrial membrane marker succinate-cytochrome C reductase.(ABSTRACT TRUNCATED AT 400 WORDS)     

Adenosine transporters in chromaffin cells: subcellular distribution and characterization

Adenosine transporters in freshly isolated and cultured chromaffin cells were quantified by the [3H]dipyridamole binding technique, showing a maximal bound capacity of 0.4 +/- 0.05 pmol/10(6) cells (240,000 +/- 20,000 transporters by cell). Scatchard analysis showed a similar affinity for [3H]dipyridamole in isolated cells and subcellular fractions (Kd = 5 +/- 0.6 nM). For enriched plasma membrane preparations and chromaffin granule membranes, the maximal binding capacities were also very similar, 2.3 +/- 0.3 and 1.8 +/- 0.4 pmol/mg protein, respectively. When [3H]nitrobenzylthioinosine was employed as a radioligand, the maximal bound capacity in cultured chromaffin cells was 0.053 +/- 0.004 pmol/10(6) cells (32,000 +/- 3000 transporters per cell) with a high affinity constant (Kd = 0.25 +/- 0.03 nM); similar values were obtained in all subcellular fractions (Kd = 0.1 +/- 0.01). Also, plasma and chromaffin granule membranes showed similar maximal binding values (0.4 +/- 0.06 pmol/mg protein). Photoincorporation studies with [3H]nitrobenzylthioinosine into plasma membrane polypeptides showed the presence of three molecular species of 115 +/- 10; 58 +/- 6 and 42 +/- 5 kDa. Chromaffin granule membranes showed only the 105 +/- 9 and 51 +/- 4 molecular species.     

The lobster nerve sodium channel: solubilization and purification of the tetrodotoxin receptor protein

Solubilization and purification of the tetrodotoxin (TTX) binding protein of the lobster walking-leg nerve Na+ channel were carried out utilizing [3H]tetrodotoxin [( 3H]tetrodotoxin) as a marker. The nerve membrane was solubilized with Lubrol-PX and the Na+ channel protein was purified with diethylaminoethyl Bio-Gel A, Bio-Gel hydroxylapatite powder and two Sepharose 6B columns. Care was taken to keep the temperature of the Na+ channel preparation as close to 1 degrees C as possible and to use solutions (pH 7.5) that contain Na channel protectors, i.e., egg phosphatidylcholine/Lubrol-PX mixture, TTX, EDTA, EGTA, phenylmethylsulfonyl fluoride, pepstatin A, iodoacetamide, antipain, phosphoramidon, soybean trypsin inhibitor, leupeptin and bacitracin. From an initial specific binding of 20.1 pmol of [3H]TTX/mg protein for the solubilized membrane, the binding increased to 1241 pmol/mg protein for the most active fraction of the last Sepharose 6B column. The [3H]TTX specific binding of the Sepharose 6B fractions correlated with a large peptide of Mr 260,000 (240-280K), although other peptides were also present in lesser amounts.     

[3H] verapamil binding sites in skeletal muscle tranverse tubule membranes

[³H]verapamil binding to muscle tubule membrane has the following properties. K_D = 27 ± 5 nM and maximum binding capacity B_max = 50 ± 5 pmol/mg of protein. A 1 = 1 stoichiometry of binding was found for the ratio of [³H]verapamil versus [³H] nitrendipine binding sites. The dissociation constant found at equilibrium is near that determined from the ratio of the rate constants for association (k₁) and dissociation (k_?1). Antiarrhythmic drugs like D600, diltiazem and bepridil are competitive inhibitors of [³H]verapamil binding with K_D values between 40 and 200 nM. Dihydropyridine analogs are apparent non competitive inhibitors of [³H]verapamil binding with half-maximum inhibition values (K_0.5) between 1 and 5 nM.     

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.     

Binding of l-[3H]glutamate to repeatedly frozen-thawed rat brain membranes

l-[³H]Glutamate binding to synaptic plasma membranes from rat cerebral cortices was carried out at 2–4°C in 50 mM Tris-acetate buffer (pH 7.4) using a microfuge centrifugation method. Binding was increased by repeated freezing-thawing and washing in either crude or partially purified synaptic membranes. Scatchard analysis showed a single binding site (dissociation constant, K_D = 697 nM; maximal binding capacity, B_max = 7.5 pmol/mg protein) in four times distilled water washed crude synaptic membrane. After six times freezing-thawing and washing, a new high affinity site (K_D1 = 26 nM, B_max1 = 1.8 pmol/mg protein) appeared and the number of low affinity site was increased with no apparent change in affinity (K_D2 = 662 nM, B_max2 = 10.5 pmol/mg protein). l-[³H]Glutamate binding was inhibited by acidic amino acid analogues that interact with N-methyl-d-aspartate- and quisqualate-sensitive sites of glutamate receptors. Binding was marginally inhibited by kainate and l-2-amino-4-phosphonobutyrate. These results indicate that repeatedly frozen-thawed and washed synaptic plasma membrane is suitable for studying the subtypes and regulation of glutamate receptors.     

Ion pathways in transverse tubules. Quantification of receptors in membranes isolated from frog and rabbit skeletal muscle

The presence of four cation pathways in membrane vesicles isolated from transverse tubules of frog and rabbit skeletal muscle was studied by measuring binding of specific blockers. Transverse tubules purified from frog muscle have a maximal binding capacity for [3H]nitrendipine (a marker for voltage-dependent calcium channels) of 130 pmol/mg of protein; this binding is strongly dependent on temperature and, at 37 degrees C, on the presence of diltiazem. Receptors for [3H]ethylenediamine tetrodotoxin (a marker for voltage-dependent sodium channels) and for 125I-labeled alpha-bungarotoxin (a marker for acetylcholine-mediated channels) showed maximal binding values of about 5 pmol/mg. The number of sodium-pumping sites in the isolated tubule vesicles, inferred from [3H]ouabain binding, was 215 pmol/mg. The high purity of this preparation makes feasible the use of these values as a criterion to judge the degree of purity of isolated preparations, and it allows investigation of transverse tubule contamination in other muscle membrane fractions.     

Oxytocin binding sites in lactating rabbit mammary gland.

Material which specifically binds oxytocin was prepared from a crude preparation of lactating rabbit mammary gland by purification on a sucrose density gradient. On examination of activities of enzyme markers and the molar ratio of cholesterol to phospholipid, this material was considered to be a highly purified plasma membrane fraction. For the determination of specificity and time course of oxytocin binding, a Scatchard plot analysis was carried out for the crude and purified fractions. Dissociation constant (Kd) and binding capacity values were found to be as follows: crude, Kd equals 1.83 X 10(-9) M, capacity equals 670 fmol/mg protein; purified, Kd equals 2.8 X 10(-9) M, capacity equals 1700 fmol/mg protein. Treatment of the purified material with different detergents resulted in loss of all [3H]oxytocin binding capacity. However, preincubation of this material with [3H]oxytocin prior to detergent treatment resulted in solubilization of a receptor-hormone complex. This complex remained in the supernatant even after centrifugation at 210 000 X g for 30 min. Using oxytocin analogs, we have shown this solubilized complex to be oxytocin specific.     

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.     

(+)-PN200-l 10 and Ouabain Binding Sites in Purified Bovine Adrenomedullary Plasma Membranes and Chromaffin Cells

Bovine adrenal medulla plasma membranes were purified by a differential centrifugation procedure using sucrose and Urografin discontinuous density gradients; the membranes were enriched 10-12-fold in acetylcholinesterase activity and [3H]ouabain binding sites. Specific (+)-[3H]PN200-110 binding to these membranes amounted to 90% of total binding and was saturable and of high affinity (KD = 41 pM; Bmax = 119 fmol/mg of protein) with a Hill coefficient close to 1, a result suggesting the presence of a single, homogeneous population of dihydropyridine receptors. The association and dissociation rate constants were, respectively, 7.5 X 108 M-1 min-1 and 0.023 min-1. Unlabeled (+)-PN200-110 displaced (+)-[3H]PN200-110 binding with a potency 100-fold higher than (-)-PN200-110 (IC50,0.5 and 45nM, respectively). Although the two enantiomers of BAY K 8644 completely displaced (+)-[3H]PN200-110 binding, they exhibited no stereoselectivity (IC50, 69 and 83 nM,respectively). Whereas ( +/- )-nitrendipine very potently displaced (+)-[3H]PN200-110 binding (IC50 = 1.3 nM) verapamil and cinnarizine displaced the binding by only 30 and 40% at 1 microM, and diltiazem increased it by 20% at 10 microM. [3H]Ouabain bound to plasma membranes with a KD of 34 nM and a Bmax of 9.75 pmol/mg of protein, a figure 80-fold higher than the Bmax for (+)-PN200-110. [3H]Ouabain also bound to intact chromaffin cells with a Bmax of 244 fmol/10(6) cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of platelet-activating factor binding sites on uterine membranes from pregnant rabbits

One of the earliest signs of endometrial preparation for blastocyst implantation is a localized increase in capillary permeability, an event that is essentially inflammatory in character and thought to be a prerequisite for subsequent decidual tissue formation. Platelet-activating factor (PAF), chemically identified as 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine, is a very potent vasoactive compound that recently has been implicated in the implantation process. In the present study, PAF binding sites are characterized in the rabbit uterus. A specific, reversible, saturable, and thermally labile binding of [3H]PAF to uterine membranes has been demonstrated, exhibiting multiple binding sites. The equilibrium dissociation constant (Kd) of the higher affinity binding site (type 1) was 3.6 +/- 0.4 nM (mean +/- SD) with a binding capacity (Bmax) of 3.4 +/- 1.6 pmol/mg protein. The second (lower affinity) binding site (type 2) had an apparent Kd of 114.6 +/- 13.5 nM and a Bmax of 164.3 +/- 17.6 pmol/mg membrane protein, under the conditions of maximal [3H]PAF binding, 25 degrees C, 150 min. Incubations at 4 degrees C for up to 3 h yielded only 30% of the Bmax observed at 25 degrees C. In crude and purified endometrial membrane preparations in which the PAF binding was predominantly located, the affinity of the binding for PAF was significantly higher than for the whole uterus, giving Kds of 1.5 +/- 0.8 and 0.8 +/- 0.5 nM; these latter values were not significantly different. However, the Bmax values of 3.9 +/- 0.9 pmol/mg protein and 376.8 +/- 163.3 fmol/mg protein for the two endometrial preparations, respectively, did differ significantly. Kinetic analysis at 25 degrees C resulted in a calculated Kd of 3.28 +/- 1.14 nM, which did not differ from the value for for the whole uterus at the same temperature, but was greater than for the endometrial preparations. Using 4 nM [3H]PAF to selectively label only the type 1 binding sites, the relative potencies of PAF and its antagonists in displacing [3H]PAF were lyso-PAF greater than CV3988 greater than PAF greater than U66985 greater than A02405 greater than BN52021 greater than U66982. The antagonists SRI 63,441 and L652,731 were ineffective in displacing [3H]PAF at up to 5000-fold molar excess of [3H]PAF. [3H]Lyso-PAF binding at 4 nM was displaceable by PAF. All cations tested, i.e. Ca2+, Mg2+, K+, Na+, and Li+, inhibited [3H]PAF binding. Serine hydrolase inhibitors, diisopropylfluorophosphate (DFP) and phenylmethylsulfonyl fluoride (PMSF), inhibited binding, but bacitracin, leupeptin, and antipain stabilized it.(ABSTRACT TRUNCATED AT 400 WORDS)     

Localisation of the [32P]IP3 binding site on human platelet intracellular membranes isolated by high-voltage free-flow electrophoresis.

This study reports the localisation of the [32P]IP3 binding site on highly purified membrane fractions prepared using high-voltage free-flow electrophoresis. Binding studies on mixed membranes, carried out at 4 degrees C, revealed a binding site with a Kd = 86 nM and beta max = 5.3 pmol/mg protein. The binding was potently inhibited by heparin. High-voltage free-flow electrophoresis was used to further purify surface and intracellular membranes. The intracellular membranes showed a 5-fold enrichment of binding sites with respect to the parent mixed membranes with the same Kd (80 nM), but the surface membranes showed an absence of binding activity. The results indicate the localisation of the IP3 receptor on highly purified intracellular membranes.     

Characterization of leukotriene C4 binding sites in the brain of the American bullfrog, Rana catesbeiana.

In this study, specific binding sites for [3H]-LTC4 on membrane preparations from American bullfrog (Rana catesbeiana) brain were characterized. Binding assays were done in the presence of serine (5mM) borate (10 mM) for 30 min at 23 degrees C. Under these conditions, no metabolism of LTC4 to LTD4 occurred. Specific binding of [3H]-LTC4 reached steady state within 10 min, remained constant for 60 min, and was reversible with the addition of 1,000-fold excess unlabelled LTC4. Scatchard analysis of the binding data indicated a single class of binding sites with an estimated Kd of 89.83 nM and Bmax of 43.79 pmol/mg protein. Competition binding studies demonstrated that LTD4 and LTE4 were ineffective in displacing [3H]-LTC4 from its binding site. The Ki for LTC4 was 51 nM. S-decylglutathione, glutathione and hematin had Ki values of 44, 312,602, and 25,576 nM, respectively. The mammalian cysteinyl leukotriene antagonist L-660,711 inhibited specific binding of [3H]-LTC4, with a Ki of 87,149 nM. Guanosine-5'-0-3-thiotriphosphate (GTP gamma S) did not affect specific binding of [3H]-LTC4 indicating that, like mammalian LTC4 receptors, a Gi protein is not involved in the transduction mechanism. The LTC4 binding site in bullfrog brain demonstrates both similarities and differences from its mammalian counterpart.     

Purification and characterization of the rat pancreatic cholecystokinin receptor

Cholecystokinin (CCK) is a peptide hormone that has a variety of physiologically important functions in the gastrointestinal tract, in which distinct high affinity receptors have been identified. We describe here the purification of the digitonin-solubilized rat pancreatic receptor as an initial step in the determination of its primary structure. Solubilization of total pancreatic membranes using 1% digitonin resulted in a single class of binding sites with a specific content of 4 pmol/mg as measured in a soluble binding assay using the nonpeptidyl CCK antagonist [3H]3S[-]-N-[2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4- benzodiazepine-3-yl]-1H-indole-2-carboxamide [( 3H]364,718). The solubilized receptor was purified using the following chromatographic steps: 1) cation exchange; 2) Ulex europaeus agglutinin-I-agarose; and 3) Sephacryl S-300. The final preparation of the purified receptor had a specific content of 8,055 pmol/mg, which represented a 9,051-fold purification from intact membranes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the purified receptor preparation under reducing conditions resulted in a predominant polypeptide with an Mr = 85,000-95,000 and minor polypeptides of Mr = 57,000 and 26,000 as determined by radiolabeling and silver staining. Solubilized pancreatic membranes were affinity labeled with the peptidyl CCK agonist 125I-D-Tyr-Gly-[(Nle28,31,6-NO2-Phe33)CCK-26-33] and chromatographed under conditions similar to those described for untreated membranes. Elution of radioactive peaks from each chromatographic column was coincident with [3H]364,718 binding activity and resulted in a labeled polypeptide having the same electrophoretic mobility as receptor derived from freshly labeled membranes and purified from untreated membranes. High performance liquid-gel exclusion chromatography of the crude digitonin-solubilized membrane preparation revealed an estimated molecular size for the [3H]364,718-binding activity of 370,000, which was consistent with the size determined by nondenaturing gel electrophoresis of the purified receptor complexed with the labeled nonpeptidyl antagonist. Binding of [3H]364,718 to the purified receptor preparation was comparable to that observed with the crude solubilized pancreatic membrane preparation; and both the homologous ligand 364,718 (Ki = 0.5 nm) and CCK-8 (Ki = 1.4 microM) competed for binding to both preparations in a similar manner.     

[3H]ethylpropylamiloride, a radio-labelled diuretic for the analysis of the Na+/H+ exchange system. Its use with kidney cell membranes.

The interaction of amiloride and several amiloride derivatives with the Na+/H+ exchange system in Madin-Darby canine kidney cells and in rabbit renal microvillus membrane vesicles was studied from 22Na+ uptake experiments. On both types of preparation, the order of potency of the different molecules tested is: ethylisopropylamiloride greater than ethylpropylamiloride (EPA) greater than amiloride greater than benzamil. 3H-labelled EPA was prepared and used to titrate amiloride binding sites in solubilized microvillus membranes. Kinetics experiments, equilibrium binding studies and competition experiments between [3H]EPA and unlabelled EPA indicate that EPA recognizes a single family of binding sites with a Kd value of 45 nM and a maximum binding capacity of 2 pmol/mg of protein. The order of potency of different amiloride analogs tested in [3H]EPA competition experiments is identical to that found for the inhibition of 22Na+ uptake by the Na+/H+ exchange system, suggesting that [3H]EPA binding sites are associated with the Na+/H+ exchange system. [3H]EPA binding sites are pharmacologically distinct from those of [3H]benzamil and [3H]bumetanide in kidney membranes.