Multiple binding of bilirubin to human serum albumin and cobinding with laurate

Numerical analysis of multiple binding of two ligands to one carrier has been accomplished, using the principle of several sets of acceptable binding constants, with bilirubin-laurate-albumin as an example. Binding of bilirubin to defatted human serum albumin was investigated by a spectroscopic method, based upon a difference of light absorption spectrum for free and bound bilirubin. The observations were supplemented with previous data from an independent technique, measurement of oxidation rates of free bilirubin with hydrogen peroxide and peroxidase. A continuous isotherm was obtained, showing binding of at least 4 mol bilirubin per mole albumin with the following stoichiometric binding constants, 1.11 X 10(8), 1.7 X 10(7), 8 X 10(5), and 4 X 10(4) M-1 at pH 8.2, ionic strength 0.15 M, 25 degrees C. The binding is anticooperative at all steps. A saturation level was not reached. Cobinding of bilirubin and laurate was studied, with up to 2 mol of each ligand per mole albumin, using the peroxidase method for determination of free equilibrium concentrations of bilirubin, and a dialysis rate technique for free laurate. The findings could be described in terms of a stoichiometric model. Heterotropic cooperativity was present among the first bilirubin and the first and second laurate molecules. More than two molecules of either ligand can be bound at the same time.     

Spectroscopic properties of bilirubin-human serum albumin complexes: a stoichiometric analysis

Light absorption spectra, fluorescence of bound bilirubin, fluorescence of albumin as quenched by bilirubin, and circular dichroism spectra have been studied in mixtures of bilirubin and defatted human serum albumin in variable proportions at 25 degrees C and at pH 7.4, 8.2, and 9.0. Corresponding spectral data have been calculated for the stoichiometric bilirubin-albumin complexes, 1:1, 2:1, and 3:1. Light absorption spectra as well as the bound bilirubin fluorescence indicate that all three bound bilirubin dianions are internalized. These data were obtained by curve fitting to least sum of squared deviations. In addition to the best fit we obtained 30 acceptable curves, located within an F contour, thus producing a rough estimate of the variation of the resulting spectral data.     

Cobinding of bilirubin and sulfonamide and of two bilirubin molecules to human serum albumin: a site model

Differential light absorption spectra of the bilirubin-albumin 1:1 complex, obtained on addition of 20 different sulfonamides, differ with respect to shape and amplitude. This finding seems to indicate that the sulfonamide molecule is bound in direct touch with the bilirubin. The light absorption spectrum of bilirubin-albumin 1:1 undergoes changes on cobinding of a fatty acid anion, laurate, and on variation of pH, previously explained by a change of dihedral angle between the two chromophores of the bilirubin molecule. In bilirubin-albumin 2:1, binding of laurate and variation of pH cause little change of the spectrum. This is best explained by binding of the two bilirubin molecules in close proximity, preventing conformational changes in the complex. From measurements of fluorescence of the lone tryptophan group in albumin and quenching on binding of bilirubin, we calculated the distance of 22 A from tryptophan to the first bound bilirubin molecule, and of 18 A to the second. Mutual quenching of the bilirubin fluorescence from two bound bilirubin molecules seemed to indicate that the two are bound closely together. A model of bilirubin-albumin with a binding site capable of accommodating one bilirubin and one sulfonamide molecule, or two molecules of bilirubin, is compatible with our findings.     

Bilirubin and haeme binding to human serum albumin studied by spectroscopy methods

Cobinding of bilirubin and of haeme to human serum albumin was investigated by means of difference absorption spectroscopy and fluorescence spectroscopy. Two specific sites for bilirubin and two for haeme binding occur on the albumin molecule. The primary binding site for bilirubin (Ka = 2.5 microM-1) is different from the primary heame binding site (Ka = 50 microM-1; Beaven et al., Eur J. Biochem. 41, 539-546, 1974), the former, however, might be identical with the secondary center for haeme binding. Similarly, the primary haeme binding center might be identical with the secondary bilirubin binding site.     

Kinetics of bilirubin oxidase and modeling of an immobilized bilirubin oxidase reactor for bilirubin detoxification

The unbound bilirubin concentration and the enzymatic rate of bilirubin degradation by bilirubin oxidase in bilirubin-serum albumin solutions have been investigated experimentally and theoretically. A stoichiometric bilirubin-serum albumin binding analysis shows that the unbound bilirubin concentration depends only on the molar ratio of the total bilirubin concentration to the total serum albumin concentration. From the theoretical analysis and the measured unbound bilirubin concentrations, serum albumin may be modelled as a molecule having two binding sites, primary and secondary, with stoichiometric equilibrium constants of K(1) = 6 x 10(7)M(-1) and K(2) = 4.5 x 10(6)M(-1), respectively. The rate of total bilirubin degradation in bilirubin-serum albumin mixtures is zero order. An immobilized bilirubin oxidase reactor model, which shows good agreement with experimental bilirubin conversions, is presented. At a flow rate of 1 mL/min with a 8-mL reactor volume, a 50% bilirubin conversion per pass was observed with an inlet bilirubin concentration of 350muM and a serum albumin concentration of 500muM.     

Role of high-density lipoprotein in transport of circulating bilirubin in rats

The analbuminemic rat strain established by Nagase et al. (Nagase, S., Shimamune, K., and Shumiya, S. (1979) Science 205, 590-591) exhibits hereditary deficiency in albumin biosynthesis. Serum bilirubin concentration is rather lower in homozygous (aa) rats (0.009 +/- 0.002 mg/dl) as compared with heterozygous (Aa) rats (0.047 +/- 0.009 mg/dl) or wild-type Sprague-Dawley (AA) rats (0.034 +/- 0.006 mg/dl) as evidenced by high pressure liquid chromatography analysis of bilirubin. After intravenous administration of various amounts of [heme-3H]hemoglobin in rats, [3H]bilirubin derived from [3H]heme of hemoglobin in vivo is more efficiently excreted into bile in aa rats than in Aa or AA rats. [3H]Bilirubin is exclusively bound with high-density lipoprotein (HDL) in aa rats, and a significant amount of [3H]bilirubin is shown to bind with HDL in Aa or AA rats in vivo. Scatchard plots revealed that [3H]bilirubin is bound with HDL in three binding modes depending on the molar ratio of [3H]bilirubin to HDL: Kd = 0.8 X 10(-7) M (molar ratio, 0.02-0.06), Kd = 1.6 X 10(-6) M (molar ratio, 0.06-0.41), and Kd = 1.2 X 10(-4) M (molar ratio, 0.79-9.02). Even under extreme conditions of excess hemoglobin administration, the molar ratio remains under 0.041; and thus, expected the Kd value would remain around 0.8 X 10(-7) M. Binding of [3H]bilirubin to rat serum albumin revealed two distinct binding modes depending on the molar ratio of [3H]bilirubin to rat serum albumin: Kd = 3.6 X 10(-7) M (molar ratio, 0.03-0.21), and Kd = 5.0 X 10(-6) M (molar ratio, 0.21-2.46). Under physiological conditions in Aa or AA rats, the former mode would be more reliable than the latter. Thus, HDL could bind with approximately 4.5 times higher affinity than rat serum albumin in Aa or AA rats under physiological conditions in vivo.     

Physical and binding properties of large fragments of human serum albumin.

Three large fragments of human serum albumin were produced by peptic digestion of the native protein [Geisow & Beaven (1977) Biochem. J. 161, 619-625]. Fragment P44 represents residues 1-386 and fragments P29 and P31 represent residues 49-307 and residues 308-584 respectively of the albumin molecule. The large N-terminal fragment P44 has a similar percentage of alpha-helix to stored defatted albumin, although the alpha-helix content of all the fragments is significantly less than that of freshly prepared albumin. The fragment P44 appears to account for all the binding of the hydrophobic probe 8-anilinonaphthalene-1-sulphonate to albumin. N-Acetyl-L-tryptophan binds to this fragment and displaces one of the bound molecules of 8-anilinonaphthalene-1-sulphonate. Bilirubin binds to fragments P44 and P29, and the complexes show similar circular-dichroism spectra to that of the complex between bilirubin and whole albumin. These results are in agreement with affinity-labeling work on albumin with reactive ligands where substitution occurs in the N-terminal region of the molecule. The sharp conformational transitional transition in albumin which is observed between pH4 and 3.5 was absent from the fragments. This isomerization, usually called the N-F transition, probably occurs in intact albumin as a result of the unfolding or separation of the C-terminal third of the protein from the remainder of the molecule.     

Circular dichroism studies of cobalt substituted lentil lectin

A recent method has been developed to effect metal ion substitution at the Mn2+ site in the lentil lectin (Bhattacharyya et al. (1984) Biochem. Biophys. Res. Commun. 124, 857-862). We report here the preparation of cobalt substituted lentil lectin, containing Co2+ at the S1 site and Ca2+ at the S2 site. The cobalt derivative possesses full saccharide binding activity and can be used for spectroscopic studies. The near UV and visible CD spectra of the derivative are shown, and its spectral properties are compared with various cobalt complexes of concanavalin A.     

Stopped-flow studies of spectral changes in bilirubin-human serum albumin following an alkaline pH jump and following binding of bilirubin

A stopped-flow technique was used to study the spectral changes occurring in bilirubin-albumin following a pH jump as well as following binding of bilirubin at 25 degrees C. The changes were studied in two wavelength ranges, 280-310 nm (tyrosine residues) and 400-510 nm (bound bilirubin). The changes were analyzed according to a scheme of consecutive unimolecular reactions. Spectral monitoring of a pH jump from 11.3 to 11.8 reveals that the bilirubin-albumin complex changes its structure in several steps. The UV absorption spectra show that 3.8 tyrosine residues ionize in the first step, 2.5 in the second, none in the third, and 0.8 in the fourth and following steps. The visible absorption spectrum of bound bilirubin changes in the second, third, and fourth steps. The bilirubin spectra of the different bilirubin-albumin complexes occurring in the transition show a common isosbestic point at 445 nm, indicating a change of the dihedral angle between the two bilirubin chromophores in a three-step reaction. It is suggested that 1 tyrosine residue is located close to the bilirubin site and is externalized in the second step. Bilirubin binding to albumin was monitored at two pH values, 11.3 and 11.8. At pH 11.3 the complex changes its structure in a three-step relaxation sequence. A change of the dihedral angle between the bilirubin chromophores can explain the spectral changes observed in the second and third relaxations. Protonation of 0.7 tyrosine residues occurs in the third relaxation, suggesting internalization of a tyrosine residue as a late consequence of bilirubin binding. At pH 11.8 a two-step relaxation sequence follows bilirubin binding. No tyrosine protonation occurs. Bilirubin is probably bound more superficially at pH 11.8 than at pH 11.3.     

Mechanism of hepatocellular uptake of albumin-bound bilirubin

We previously demonstrated that unconjugated bilirubin spontaneously diffuses through phospholipid bilayers at a rate which exceeds albumin dissociation, suggesting that solvation from albumin represents the rate-limiting step in hepatic bilirubin clearance. To further examine this hypothesis, we studied the uptake of bovine serum albumin (BSA)-bound bilirubin by cultured hepatoblastoma (HepG2) cells. Uptake of bilirubin was saturable, with a K(m) and V(max) of 4.2+/-0.5 microM (+/-S.E.M.) and 469+/-41 pmol min(-1) mg(-1) at 25 degrees C. Substantial bilirubin uptake also was observed at 4 degrees C (K(m)=7.0+/-0.8 microM, V(max)=282+/-26 pmol min(-1) mg(-1)), supporting a diffusional transport mechanism. Consistent with reported solvation rates, the cellular uptake of bilirubin bound to human serum albumin was more rapid than for BSA-bound bilirubin, indicative of dissociation-limited uptake. Counterintuitively, an inverse correlation between pH and the rate of bilirubin flip-flop was observed, due to pH effects on the rate of dissociation of bilirubin from albumin and from the membrane bilayer. The identification of an inflection point at pH 8.1 is indicative of a pK(a) value for bilirubin in this range. Taken together, our data suggest that hepatocellular uptake of bilirubin is dissociation-limited and occurs principally by a mechanism involving spontaneous transmembrane diffusion.     

Determination of protein secondary structure from circular dichroism spectra. III. Protein-derived base spectra of circular dichroism for antiparallel and parallel beta-structures

Protein-derived basic CD spectra for alpha-helix, antiparallel and parallel beta-structures, beta-bends and irregular form of proteins have been determined from the experimental CD spectra of six (myoglobin, lysozyme, ribonuclease A, papain, lactate dehydrogenase, subtilisin BPN') or seven (glyceraldehyde-3-phosphate dehydrogenase added) reference proteins and the analysis of the X-ray data. The secondary structures of thirteen proteins (seven reference and six additional ones) have been analysed using the basic CD spectra thus obtained. The data obtained have been compared with the results of the X-ray data analysis. It is shown that the accuracy of determination of the beta-structure and beta-bends contents using our basic CD spectra is about 2-3 times better than using the basic spectra reported by Chang et al. (Analyt. Biochem. 91, 13-31, 1978).     

A dynamic model for bilirubin binding to human serum albumin

Site-directed mutagenesis of human serum albumin was used to study the role of various amino acid residues in bilirubin binding. A comparison of thermodynamic, proteolytic, and x-ray crystallographic data from previous studies allowed a small number of amino acid residues in subdomain 2A to be selected as targets for substitution. The following recombinant human serum albumin species were synthesized in the yeast species Pichia pastoris: K195M, K199M, F211V, W214L, R218M, R222M, H242V, R257M, and wild type human serum albumin. The affinity of bilirubin was measured by two independent methods and found to be similar for all human serum albumin species. Examination of the absorption and circular dichroism spectra of bilirubin bound to its high affinity site revealed dramatic differences between the conformations of bilirubin bound to the above human serum albumin species. The absorption and circular dichroism spectra of bilirubin bound to the above human serum albumin species in aqueous solutions saturated with chloroform were also examined. The effect of certain amino acid substitutions on the conformation of bound bilirubin was altered by the addition of chloroform. In total, the present study suggests a dynamic, unusually flexible high affinity binding site for bilirubin on human serum albumin.     

Adenosine deaminase-complexing protein from bovine kidney. Isolation of two distinct subunits

We have isolated a complex of two proteins from bovine kidney that bind to adenosine deaminase immobilized on Sepharose 4B. One protein, with Mr = 110,000, comigrates on both PAGE and SDS-PAGE gels with complexing protein isolated from rabbit kidney by the method of Schrader et al. (Schrader, W.P., Harder, C. M., and Schrader, D. K. (1983) Comp. Biochem. Physiol. B Comp. Biochem. 75, 119-126). The second protein has a Mr = 70,000. Both proteins bind to the adenosine deaminase-Sepharose but not to a control resin of bovine serum albumin bound to Sepharose. Based on a comparison of partial and complete denaturation on SDS-PAGE the two proteins appear to be bound to each other. At adenosine concentrations of 0.5-1 mM the isolated complexing protein increases small subunit adenosine deaminase catalytic activity by 20-30%. There may be some inhibition of catalytic activity at low adenosine concentrations. We have designated the 110,000 Mr protein CP-I, the 70,000 Mr protein CP-II and the complex of these two CP.     

Evidence for the extent of insertion of the active site loop of intact alpha 1 proteinase inhibitor in beta-sheet A.

The extent of insertion of beta-strand s4A into sheet A in intact serpin alpha 1-proteinase inhibitor (alpha 1PI has been probed by peptide annealing experiments [Schulze et al. (1990) Eur. J. Biochem. 194, 51-56]. Twelve synthetic peptides of systematically varied length corresponding in sequence to the unprimed (N-terminal) side of the active site loop were complexed with alpha 1PI. The complexes were then characterized by circular dichroism spectroscopy and tested for inhibitory activity. Four peptides formed complexes which retained inhibitory activity, one of which was nearly as effective as the native protein. Comparison with the three dimensional structures of cleaved alpha 1PI [L?bermann et al. (1984) J. Mol. Biol. 177, 531-556] and plakalbumin [Wright et al. (1990) J. Mol. Biol. 213, 513-528] supports a model in which alpha 1PI requires the insertion of a single residue, Thr345, into sheet A for activity.     

Regulated nuclear polyadenylation of Xenopus albumin pre-mRNA.

Cytoplasmic regulation of the length of poly(A) on mRNA is a well-characterized process involved in translational control during development. In contrast, there is no direct in vivo evidence for regulation of the length of poly(A) added during nuclear pre-mRNA processing in somatic cells. We previously reported that Xenopus serum albumin [Schoenberg et al. (1989) Mol. Endocrinol. 3, 805-815] and transferrin [Pastori et al. (1992) J. Steroid Biochem. Mol. Biol. 42, 649-657], mRNA have exceptionally short poly(A) tails ranging from 12 to 17 residues, whereas vitellogenin mRNA has long poly(A). An RT-PCR protocol was adapted to determine the length of poly(A) added onto pre-mRNA, defined here as that species bearing the terminal intron. Using this assay we show that vitellogenin pre-mRNA has the same long poly(A) tail as mature vitellogenin mRNA. In contrast, albumin pre-mRNA has the same short poly(A) as found on fully-processed albumin mRNA. These results indicate that the short poly(A) tail on albumin mRNA results from regulation of poly(A) addition during nuclear 3' processing.     

Orientation of pigments in the thylakoid membrane and in the isolated chlorophyll-protein complexes of higher plants. I. Determination of optimal conditions for linear dichroism measurement

The nature and possible causes of polarized light-scattering artefacts in linear dichroism measurements are investigated. Using criteria described in this article, the available orientation techniques have been critically assessed in order to obtain the linear dichroism spectra of thylakoids and of pigment-protein complexes isolated from pea. It is demonstrated here that the polyacrylamide gel squeezing technique of Abdourakhmanov et al. (Abdourakhmanov, I.A., Ganago, A.O., Erokhim, Yu.E., Solov'ev, A.A. and Chugunov, V.A. (1979) Biochim. Biophys. Acta 546, 183–186) does not lead to pigment degradation and that the linear dichroism spectra obtained in these conditions are essentially free of scattering artefacts. The linear dichroism spectra of light-harvesting complex isolated in different states of aggregation or incorporated into phospholipid vesicles are compared to the spectra of thylakoids. This comparison indicates: (1) that the isolation procedure of Burke et al. (Burke, J.J., Ditto, C.L. and Arntzen, C.J. (1978) Arch. Biochem. Biophys. 187, 252–263) leads to light-harvesting complex in which the in vivo orientation of pigments is preserved; (2) that the antenna chlorophyll a molecules of this complex have a significant degree of orientation with respect to the plane of the thylakoid.     

Comparison of kinetic study of the photochemical changes of (ZZ)-bilirubin IX alpha bound to human serum albumin with that bound to rat serum albumin.

It has been stated by McDonagh, Palma & Lightner [(1982) J. Am. Chem. Soc. 104, 6867-6871] that complexing of bilirubin with serum albumin has a marked species-dependent influence on bilirubin photoisomerization in vitro and in vivo. Therefore the kinetics for the quantitatively important reaction: (Formula: see text) of the photochemical interconversion between bilirubin and its photoisomers bound to human or rat serum albumin in aqueous solution, assayed by h.p.l.c., was used to elucidate the observed species-dependent difference. The relative rate constants for bilirubin bound to human serum albumin, except for k4, the rate of interconversion from (ZZ)-bilirubin into (EZ)-bilirubin, proved to be considerably larger than those for bilirubin bound to rat serum albumin. In accordance with these rate constants, the formation of photoisomers of bilirubin bound to human serum albumin, except for (EZ)-bilirubin, is very rapid and much greater than that for bilirubin bound to rat serum albumin.     

M?ssbauer studies of solid thionin-oxidized MoFe protein of nitrogenase

Recently Hagen et al. (Hagen, W. R., Wassink, H., Eady, R. R., Smith, B. E., and Haaker, H. (1987) Eur. J. Biochem. 169, 457-465) reported the observation of S = 7/2 EPR signals for thionin-oxidized nitrogenase MoFe protein. Here we have studied the protein from Azotobacter vinelandii and Klebsiella pneumoniae with M?ssbauer and EPR spectroscopies, with the following results: when the MoFe protein is oxidized by addition of stoichiometric amounts (6-8 equivalents) of dissolved thionin, the well characterized P-cluster state Pox results. Pox has an as yet undetermined, but half-integer electronic spin; however, the state is EPR-silent. In contrast, oxidation by addition of a large excess of solid thionin powder, the method used by Hagen et al., yields mixtures with variable proportions of two oxidized P-cluster forms, namely the familiar Pox and the new state Pox(S = 7/2) observed by Hagen et al. The M?ssbauer data suggest that Pox and Pox(S = 7/2) are isoelectronic. The two states, however, have distinct electronic structures; the M?ssbauer spectra of Pox exhibit the characteristic trapped-valence Fe2+ site, whereas the spectra of Pox(S = 7/2) lack this feature. Hagen et al. have proposed two new P-cluster models. We conclude that one of the models is incompatible with the M?ssbauer data and that the basic assumptions of the other model are not supported by the available data. Finally, the M?ssbauer data show that either oxidation method puts the cofactor centers into the diamagnetic state Mox.     

Porphyromonas gingivalis fimbriae binds to neoglycoproteins: evidence for a lectin-like interaction

Porphyromonas gingivalis is a likely major pathogen in adult periodontitis. Fimbriae in particular have been suggested as playing an important role in facilitating the initial interaction between the bacteria and the host and triggers host responses. Murakami et al. [Biochem. Biophys. Res. Commun. 192 (1993) 826] have shown that fimbriae of P. gingivalis strongly induced TNF-alpha gene expression in macrophages and expression of TNF-alpha was inhibited by N-acetyl-D-galactosamine, but not inhibited by other sugars. Studies by Sojar et al. [FEBS Lett. 422 (1998) 205] suggested that the oligosaccharide moiety of lactoferrin is involved in the interaction of P. gingivalis fimbriae and human lactoferrin. In the present study, purified fimbriae from P. gingivalis and neoglycoproteins were used to assess lectin-like interaction of fimbriae. In dot blot and overlay assays, iodinated purified P. gingivalis fimbriae as well as biotinylated purified P. gingivalis fimbriae bound strongly to albumin-fucosylamide (albumin-1-amido-1-deoxy-L-fucose) and by lesser extent to albumin-N-acetyl-D-galactosamine (albumin-p-aminophenyl-N-acetyl-beta-D-galactosaminide). However, fimbriae failed to bind carbohydrate free bovine serum albumin, which was used in preparation of the neoglycoproteins. These results suggests that P. gingivalis fimbriae bind to glycoconjugates through lectin-like interaction with carbohydrate. This protein-carbohydrate interactions may be important for triggering events in these cells, which mediate the host response of this pathogen.     

Phosvitin isolation from fish eggs: methodological improvements including 'specific' phosvitin precipitation with ferric ion

Modifications of the method of Wallace et al. [Can. J. Biochem. 44, 1647-1655 (1966)] for phosvitin isolation from vertebrate eggs were devised to enhance the method's general effectiveness. Phosvitins which do not precipitate on dilution from solutions of their lipovitellin complexes may be selectively adsorbed onto, and desorbed from, DEAE-cellulose. Phosvitins which are too small for dialysis or ultrafiltration may be concentrated or desalted by precipitation with stoichiometric amounts of ferric ions, followed by iron removal with EDTA on gel filtration. Since phosvitin distribution among ammonium sulfate fractions depends on initial protein and salt concentrations in a species-specific manner, pilot experiments are needed to establish conditions for optimal fractionation.