Extracellular Binding Protein Research Articles

Corticosteroid binding globulin, testosterone-estradiol binding globulin, and androgen binding protein belong to protein families distinct from steroid receptors

The cDNA nucleotide sequences and the deduced amino acid sequences of human corticosteroid binding globulin (hCBG), human testosterone-estradiol binding globulin (hTeBG), and rat androgen binding protein (rABP) were determined. Studies of the steroid binding sites suggest they are toward the carboxy-terminus in hTeBG and rABP and more central in hCBG. hCBG has remarkable sequence homology with members of a superfamily whose functions have diverged; these include thyroxine-binding protein, serine protease inhibitors, egg white proteins, and angiotensinogen. hTeBG and rABP have a 68% amino acid sequence identity. Hybridization studies suggest that hTeBG is probably even more closely related, if not identical, to hABP. The carboxy-terminal sequences of hTeBG and rABP are also similar to that of protein S, a vitamin-K-dependent clotting factor. There were no nucleotide or amino acid sequence homologies between hCBG, hTeBG, or rABP and other steroid binding proteins such as steroid receptors, albumin, α-fetoprotein, and vitamin D binding protein. We conclude that the “extracellular steroid binding proteins” and steroid receptors do not appear to have descended from a common ancestor.

Influence of glycolipids on immune reactions of phospholipid antigens in liposomes

Complement-dependent immune damage to liposomes mediated by a murine monoclonal antibody to the liposomal bilayer was completely inhibited by ceramide tetrasaccharide (globoside) at an 8% concentration in the liposomes. Lower concentrations of globoside, or higher concentrations of ceramide tri-, di-, or monohexoside, were not inhibitory. At a globoside concentration of 5.8%, inhibition of the monoclonal antibody activity was reduced and inhibition was related to antibody concentration; but at 2% globoside, suppression of antibody activity was not observed at all. Analysis of space-filling models revealed that at 8% globoside the distance between adjacent tetrasaccharides of globoside approached the dimensions of the antigen-binding end of a 7S immunoglobulin molecule. We therefore propose that globoside, and perhaps other glycolipids, can exert steric hindrance to the binding of extracellular proteins to nonglycolipid constituents of the lipid bilayer. We conclude that microheterogeneity among polar groups of glycolipids may be a novel mechanism for allowing selective access of proteins to phospholipids on the lipid bilayer.

Pharmacokinetics of the hepatic transport of organic anions: influence of extra- and intracellular binding on hepatic storage of dibromosulfophthalein and interactions with indocyanine green.

The influence of intracellular and extracellular protein binding on the hepatic storage and biliary elimination of dibromosulfophthalein (DBSP) was studied in isolated perfused rat liver. Under first order kinetic conditions the amount of DBSP in the liver at a given plasma concentration (hepatic storage) was determined by extracellular binding to albumin and intracellular binding to the cytosolic Y and Z proteins as well as concentrative membrane transport from plasma into the liver. At higher doses, extensive binding of DBSP to intracellular organelles also occurred while liver cytosol/plasma concentration gradients of unbound DBSP were much lower. Hepatic storage increased with decreasing albumin concentration in the perfusate of isolated perfused rat livers. However, it was shown that this parameter is dose-dependent, and errors can be introduced in its calculation if nonlinearity of sinusoidal and canalicular transport processes as well as nonlinear protein binding are not taken into account. The influence of another organic anion, indocyanine green (ICG) on the hepatic storage, subcellular distribution, and elimination of DBSP was subsequently studied. At equimolar amounts the presence of ICG resulted in a 50% decrease in hepatic clearance and hepatic distribution volume of DBSP. It was inferred that these changes are due to an inhibition of carrier-mediated transport across the sinusoidal and canalicular membrane and preferential displacement from intracellular binding sites. In contrast DBSP in equimolar amount enhanced the initial disappearance rate and biliary excretion of ICG, probably due to increasing its free fraction in plasma. It is concluded that the level and mechanism of interaction of two drugs within the eliminating organ can be characterized by combining clearance studies with data on subcellular and extracellular binding.

Hepatic drug transport in the rat: A comparison between isolated hepatocytes, the isolated perfused liver and the liver in vivo

The hepatic transport of three different drugs, the organic anion dibromosulphophthalein, the organic cation d-tubocurarine and the uncharged compound ouabain was studied in vivo in the isolated perfused rat liver and isolated hepatocytes. The respective clearances by uptake were determined for the various substrates and corrected for differences in hepatic blood flow and extracellular protein binding in the three liver preparations. The corrected uptake values in the intact organ, in vivo and in the isolated perfused liver were highly comparable; for dibromosulphophthalein a clearance of 2.1ml/minper 10 6 hepatocytes was found in vivo, whereas in perfusion a value of 2.4 ml/min per 10 6 cells was calculated. For d-tubocurarine, the values were 34 × 10 −4 and 55 × 10 −4ml/min per 10 6 cells obtained in vivo and in the isolated perfused organ, respectively. With ouabain as the substrate, the in vivo clearance amounted to 5.1 × 10 −2, whereas in the isolated perfused liver a value of 4.8 × 10 −2ml/min per 10 6 cells was calculated. The clearance by uptake obtained for dibromosulphophthalein and ouabain in the isolated hepatocytes appeared to be a factor of 2–3 lower than in the intact organ. In the case of d-tubocurarine however the clearance was identical to that in vivo and the isolated perfused liver. The rate of secretion from isolated hepatocytes was, for dibromosulphophthalein identical to, and for d-tubocurarine and ouabain lower than that in the intact organ, especially as compared with the in vivo preparation. It is concluded that transport function is well preserved in the isolated perfused liver and isolated hepatocytes. For certain substrates freshly isolated hepatocytes may exhibit a somewhat lower uptake and/or secretion rate, in spite of a good cell quality as judged by generally accepted criteria for cell viability. Whether this is due to changes in membrane composition (not detected by our viability tests) or a selection of a subpopulation of hepatocytes, is discussed.

Combination chemotherapy of the epipodophyllotoxin derivatives, teniposide and etoposide. A pharmacodynamic rationale?

Previous studies in vitro on the influence of extracellular protein binding of Teniposide (VM26) and Etoposide (VP16-213) on subsequent cellular uptake by experimental murine tumor cells [Cancer Res 38:2549 (1978); Drug Metab Rev 8:119 (1978)] suggested that a timed-sequential combination of VM26 and VP16-213 may increase the bioavailability of VP16-213. This was studied clinically in six cancer patients with ascites (five ovarian, one rectal) whereby VM26 (20 mg/m2) was given i.p. 2 h prior to VP16-213 (100 mg/m2; i.p.) In some patients, this regimen was administered i.v. The i.v. regimen was found to be more toxic (myelosuppression, nausea, vomiting) than i.p. regimen at same doses of drugs. Several patients remained stable to disease during 1-2 courses of therapy (3 weeks per course), one patient had partial remission, and has been stable in her disease for more than 4 months. In two patients, plasma and ascites fluid was analyzed for VP16-213 and VM26 by a new reverse-phase high performance liquid chromatography method. Both VM26 and VP16-213 could be eluted isocratically (28% v/v acetonitrile in water) from a c18 column with retention times of 6.6 and 13.3 min, respectively. Subsequent pharmacokinetic analysis of one patient suggests that protein binding displacement of VP16-213 in plasma and perhaps ascites fluid increased the pharmacokinetic volume of distribution (28 l) and reduced the elimination half-life (12 h). The data suggests that VP16-213 is distributed more widely in the body and is represented by a single compartment pharmacokinetic model. Analysis of VM26 in ascites and plasma suggests that the so-called "deep pharmacokinetic compartment" represents ascites equivalent space and that the plasma concentration represents VM26 as free and protein-bound drug in kinetic distinguishable compartments. Determinants of drug action are potentially composed of a multiplicity of physiological, biochemical, and other factors. The potential for manipulating the pharmacodynamic properties of drugs to achieve greater therapeutic potential needs further study.

The polynucleotide binding sites of estradiol receptor complexes.

As a model for interaction of steroid receptors with DNA, the binding of estradiol receptor complexes (E2R) to oligodeoxynucleotides, covalently linked to cellulose, was studied in detail. Binding was optimal at concentrations of monovalent cationic salts at, or near, isotonic levels and was selective for intracellular receptors in contrast to extracellular steroid binding proteins. Among the oligomers, the order of affinity was oligo dG greater than oligo dT greater than oligo dC greater than oligo dA greater than oligo dI. The binding to oligo dG was stable to 37 degrees C exposure and the processes of adsorption and desorption, while reactivity with oligo dT, oligo dC and oligo dA was labile. The decrease in binding following purification was restored by histone 2B. Oligo dG binding was the most resistant to inhibition by cibacron blue F3GA (CB) and pyridoxal-5-phosphate. On the basis of these data, a hypothesis is proposed for the interaction of mouse uterine cytosol E2R with prevalent nonspecific and putative specific sequences of DNA.

Extracellular androgen binding proteins.

Macrophage polarization refers to how macrophages have been activated at a given point in space and time. Polarization is not fixed, as macrophages are sufficiently plastic to integrate multiple signals, such as those from microbes, damaged tissues, and ...Read More

Extracellular and Intracellular Steroid Binding Proteins. Properties, Discrimination, Assay and Clinical Application

Extracellular and Intracellular Steroid Binding Proteins. Properties, Discrimination, Assay and Clinical Application

Physiological role for androgen binding protein-steroid complex in testis?

The concentration of unbound androgens in the lumen of rat testis seminiferous tubules is not dependent on the presence of the extra-cellular androgen binding protein (ABP); other parameters such as permeability properties, fluid dynamics and metabolic activities in different testicular compartments appear to have a much greater influence. It has been shown that in other target tissues the metabolic effects of steroids on cells depend on the concentration of the steroids which are not bound to extra-cellular proteins. It is therefore unlikely that the physiological function of ABP is related to the accumulation of androgens around germinal cells.

La fixation des hormones thyroidiennes par Escherichia coli et son mécanisme

Resting cells of Escherichia coli rapidly bind 3,5,3′-triiodo- l-thyronine (T 3) and l-thyroxine (T 4) to a considerable degree. Neither iodotyrosines nor iodides are concentrated by the bacterial cells under the same conditions. Concentration of thyroid hormones by the bacteria is due to the interaction of iodothyronines with endocellular proteins (bTBP, or bacterial thyroxine-binding proteins). Fixation of T 3 is greater, and proceeds at a higher speed, than does fixation of T 4; bTBP shows a greater affinity for T 3 than for T 4. Electrophoretic analysis on a cellulose column of the acellular extracts obtained by ultrasonic breaking of bacteria and incubated with T 3 or T 4, shows that each of these hormones is bound to a particular protein system. The binding of T 3 or T 4 to E. coli is inhibited by the addition of human serum or of bacterial extract. A reversible equilibrium is established between the hormone and the intracellular and extracellular binding proteins. Compared to human serum thyroxine-binding globulin, the binding capacity of bTBP for T 3 or T 4 is high and its affinity is relatively low. The E. coli system could serve as a model for the partition of thyroid hormones between extracellular and intracellular binding proteins.