Kinetic properties of rat hepatic prolactin receptors

Porcine endometrial prolactin receptors detected by homologous radioreceptor assay

Heterologous radioreceptor assays, commonly using ovine prolactin, may generate inconsistent results since prolactin (PRL) from one species may be recognized as growth hormone in another. Homologous radioreceptor assays (RRA) are most similar to the in vivo hormone-tissue receptor environment; however, lactogenic homologous RRAs have been reported only for mouse hepatic membranes. In this study, an assay system was developed to investigate homologous binding for porcine PRL in porcine uterine tissue. The pig does not produce a decidual PRL or a placental lactogen; yet, PRL affects uterine physiology during reproductively important events. Optimal binding conditions established for porcine PRL homologous RRA include 150 micrograms membrane, 45,000 cpm labeled porcine PRL and 500 microliters sodium phosphate buffer pH 7.6, incubated at 25 degrees C for 24 h. Binding of porcine PRL tracer is very low (less than 3%); however, when tissue is treated with the chaotropic agent, MgCl2 (4 M), binding increases from 3 to 28%. Dissociation kinetics show a rate of 3.79 X 10(-6)/s initially, and then 1.63 X 10(-6)/s. Competition for labeled PRL on binding sites with unlabeled porcine PRL results in 80% displacement with unlabeled porcine prolactin (NSB) of 7% at 1000 ng. Affinity constant generated from homologous inhibition assays is 0.326 X 10(8) M-1. Porcine growth hormone (GH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) do not displace porcine PRL tracer. These data describe a lactogenic homologous RRA for porcine endometrial membranes similar to that previously reported for murine hepatic tissue. Homologous RRAs may allow elucidation of PRL receptor characteristics with more similarity to the in vivo hormone-receptor milieu.

Lactogenic and somatogenic binding sites in intact and detergent-solubilized membrane preparations of female rat liver

The presence of lactogenic and somatogenic binding sites in intact microsomal membranes and in detergent-solubilized microsomal membrane preparations of female rat liver has been studied by affinity cross-linking-SDS/polyacrylamide-gel electrophoresis. In microsomal membrane preparations an Mr 40,000 lactogenic binder is present which is not disulphide-linked to another protein. Triton X-100 solubilization of membranes results in the appearance of three lactogenic 125I-human growth hormone (125I-hGH) binders with Mr values of 87,000, 40,000 and 35,000, and one somatogenic 125I-hGH binder with Mr 32,000. Treatment of rats with oestrogen increased the amount of lactogenic and somatogenic binding species in liver. The lactogenic binding sites are present as one entity in Triton X-100-solubilized preparations, clearly separated from the somatogenic binder as analysed by gel chromatography. Furthermore, 125I-hGH interacts with an Mr 95,000 somatogenic binder in membrane preparations to which the hormone can be cross-linked only following Triton X-100 solubilization.

Immunological relationship and binding capacity of prolactin receptors in cytosolic and membrane fractions of rabbit mammary gland

We have recently identified and partially characterized a specific lactogen binding protein in rabbit mammary gland cytosol. In this report, studies using pregnant or lactating rabbits are described which further characterize the cytosolic lactogen binding protein in relation to the membrane-bound lactogen receptor. The data show that in pregnant or lactating rabbits the binding capacity (fmol/mg protein) of membranes is at least double that of the cytosol preparation although when expressed on a tissue content basis (fmol/g tissue) there was no membrane-cytosol difference in receptor number. Treatment of lactating rabbits with CB-154, however, caused a marked increase (100-150%) in the binding capacity of membrane-bound receptors with comparatively little effect (+20%) on the cytosolic lactogram binding protein. There was also a marked difference in the association constants for 125I-hGH, with the cytosolic lactogen binding protein exhibiting a 6-fold higher affinity than the membrane-bound receptor. Three anti-prolactin receptor monoclonal antibodies (M110, A82 (antagonists) and A917 (agonist) have also been used to assess the relative immunological characteristics of the cytosolic lactogen binding protein and the membrane lactogen receptor. Each monoclonal antibody was able to inhibit the specific binding of 125I-hGH to both membranes and cytosol in a dose-dependent manner. However, the order of potency was not identical being M110 greater than A917 greater than A82 in membranes and M110 greater than A82 greater than A917 in cytosol. A917 was at least 10 times more active in membranes than cytosol whereas A82 was at least 10 times more active in cytosol.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding and structural characteristics of a soluble lactogen-binding protein from rabbit mammary-gland cytosol

Specific receptors for prolactin (PRL) are known to be present on plasma membranes and intracellular membranes of mammary gland. We now report, however, the detection and characterization of a soluble lactogen-specific binding protein in high-speed (200,000 g) cytosolic preparations from pregnant- and non-pregnant-rabbit mammary gland. The binding protein was not detectable by poly(ethylene glycol) precipitation; instead, bound and free 125I-labelled human growth hormone (hGH; a potent lactogen) was separated using a mini-gel filtration technique. Specific binding of 125I-hGH reached an apparent equilibrium between 10 and 14 h at 21-23 degrees C. It was dependent on mammary-gland protein concentration and, partially, on Ca2+ or Mg2+ concentrations. Scatchard analysis revealed steep curvilinear plots, the high-affinity component having a KA of approximately 3 X 10(10) M-1. Gel filtration on calibrated Ultrogel AcA34 columns of 125I-hGH-cytosol complexes or of cytosol alone, followed by measurement of 125I-hGH binding in each eluted fraction, indicated that the binding protein had an Mr of 33,000-43,000. A specific binding protein of the same size was observed when 125I-ovine or -human PRL, but not 125I-bovine GH, was used as ligand. The apparent lactogenic specificity was confirmed by a lack of cross-reactivity of the binding protein with an anti-[GH receptor (rabbit liver)] monoclonal antibody. Polyacrylamide-gel electrophoresis of 125I-hGH covalently cross-linked to cytosol with disuccinimidyl suberate revealed binding proteins of Mr 35,000 (non-reduced) and 37,000 (reduced), results comparable with those obtained by gel filtration and indicating an absence of inter-subunit disulphide bonds. These studies have shown the presence of an apparently naturally soluble lactogen-binding protein in the cytosolic fraction of rabbit mammary gland. The relationship between this binding protein and the membrane PRL receptor is not yet known.

Thermodynamics and kinetics of mouse prolactin-hepatic receptor interaction

Kinetic and thermodynamic parameters associated with the binding of secreted mouse prolactin (smPRL) to mouse hepatic receptors were investigated. When the reaction temperature was increased from 8 degrees C to 37 degrees C, the association rate constant k+1, increased approximately 5-fold, from 2.3 X 10(4) M-1 . S-1 to 12.6 X 10(4) M-1 . S-1. An Arrhenius plot indicated that there was a linear relationship between ln (k+1) and 1/T. When the reaction temperature was increased from 8 degrees C to 37 degrees C, the equilibrium binding constant, Ka, decreased approximately 1.5-fold, from 2.8 X 10(8) M-1 to 1.9 X 10(8) M-1. When the pH of the binding reaction was lowered from 9.0 to 6.2, Ka decreased approximately 3-fold, from 2.6 X 10(8) M-1 to 0.9 X 10(8) M-1. Transition state thermodynamic parameters for the formation of the smPRL-receptor complex, represented by delta G+', delta H+' and delta S+', were +45.7 kJ/mol, +41.2 kJ/mol and -15.1 J/(mol . K), respectively. Parameters for the equilibrium reaction, described by delta G0', delta H0' and delta S0', were -47.6 kJ/mol, -10.6 kJ/mol and +124 J/(mol . K), respectively. Over the temperature range studied, a Van't Hoff plot of the binding constants demonstrated a linear relationship between ln (ka) and 1/T, indicating that changes in enthalpy for the binding reaction were temperature independent. The binding reaction was largely entropically driven (delta S0' greater than 0), suggesting that hydrophobic interactions are involved in forming the smPRL-receptor complex.

Properties of hepatic binding sites for prolactin in ring doves (Streptopelia risoria)

Specific binding of 125I-labeled ovine prolactin (125I-oPRL) was detected in crude membrane fractions prepared from ring dove (Streptopelia risoria) liver homogenates. In characterization studies, specific binding was found to depend upon pH, incubation time, incubation temperature, and membrane protein concentration. Competitive inhibition of specifically bound 125I-oPRL was observed with human growth hormone, human and rat prolactin, and dove pituitary extract but not with turkey prolactin, human placental lactogen, and several nonlactogenic hormone preparations. Dove liver membranes showed high affinity (Kd = 3 X 10(-10) M) for binding to oPRL but had relatively low binding capacity (Bmax less than 20 fmol/mg protein). PRL binding activity in pooled liver fractions from breeding doves during early stages of incubation prior to crop sac growth did not differ markedly from that observed in doves sampled at the end of incubation when crop sac weight and serum PRL were elevated. However, binding activity was higher in pooled male liver fractions than in pooled female liver fractions at both reproductive stages. A two- to threefold increase in binding capacity was observed in pooled liver fractions from late-incubating doves following MgCl2-induced binding site desaturation. The MgCl2 treatment did not eliminate the differences in specific binding observed between male and female liver fraction pools, thus suggesting the possibility of sex-specific mechanisms of hepatic PRL binding site regulation in this species. It is concluded that the dove liver possesses specific binding sites for PRL with properties similar, but not identical, to those found in other vertebrate target tissues.

Uptake and processing of prolactin by alternative pathways in rat liver

The uptake and subcellular processing of radiolabelled prolactin has been studied in male and female rats. Analytical subcellular fractionation of liver homogenates from rats injected with 125I-prolactin showed that in female rats the prolactin was primarily internalised to low density (1.12 g X cm-3) membranes. Approx. 10-15% of the total label was found in high density membranes, similar in distribution to lysosomal marker enzymes. In the normal male rat, prolactin was internalised solely to lysosomal type membranes. However, in male rats treated with estrogen, the distribution of prolactin was very similar to that seem in the female, indicating that internalisation to low density membrane is dependent on the presence of prolactin receptors. Gel exclusion chromatography showed that the prolactin internalised to the lysosomal membranes was extensively degraded whereas that associated with the low density membrane remained intact. Use of digitonin, to establish the identity of the low density membrane gave inconclusive results, but suggested that the prolactin was associated with membrane bearing NADH pyrophosphatase rather than the classical Golgi marker, galactosyltransferase.

Hydrodynamic properties of rat hepatic prolactin receptors

The hydrodynamic properties of rat hepatic prolactin receptors have been determined by a combination of gel chromatography and ultracentrifugation. Prolactin receptors were detergent extracted from partially purified plasma membranes prepared from female rat livers. Fifteen different nonionic detergents were tested for solubilizing prolactin receptors, including Triton X-100, Polyoxyethylene W-1, Lubrol WX, detergents of the Tween and Brij series, and digitonin. When the receptors were detergent solubilized after ligand was bound to the receptor, 1% Triton X-100 had the highest efficacy of solubilization. However, if the receptors were solubilized prior to exposure to ligand, maximum binding was to receptors solubilized with 0.25% Triton X-100. The Kd of 43.2-74.5 pM for binding to the soluble receptor was three- to fivefold lower than the Kd for the membrane receptor. Gel chromatography (Bio-Gel A-1.5m, 2.5 x 50 cm) of the soluble receptor indicated a Stokes radius (Rs) of 5.0 nm for the hormone-receptor-detergent complex. The hydrodynamic properties of the receptor-detergent-ligand complex were determined by centrifugation in 5-20% sucrose gradients in H2O and in D2O. They are v = 0.7; S20,w = 4.7; f/f0 = 1.49; Mr = 118,000 for the complex, 73,000 for the receptor alone. Approximately 0.22 mg of Triton X-100 is estimated bound per milligram of protein. This represents about 25 mol detergent/mol receptor.