Binding and circular dichoism data on bilirubin-albumin in the presence of oleate and salicylate

Effects of Heme Site (FA1) Ligands Bilirubin, Biliverdin, Hemin, and Methyl Orange on the Albumin Binding of Site I Marker Warfarin: Complex Allosteric Interactions

Human serum albumin (HSA) is the most abundant plasma protein in circulation. The three most important drug-binding sites on HSA are Sudlow's Site I (subdomain IIA), Sudlow's Site II (subdomain IIIA), and Heme site (subdomain IB). Heme site and Site I are allosterically coupled; therefore, their ligands may be able to allosterically modulate the binding affinity of each other. In this study, the effects of four Heme site ligands (bilirubin, biliverdin, hemin, and methyl orange) on the interaction of the Site I ligand warfarin with HSA were tested, employing fluorescence spectroscopic, ultrafiltration, and ultracentrifugation studies. Our major results/conclusions are the following. (1) Quenching studies indicated no relevant interaction, while the other fluorescent model used suggested that each Heme site ligand strongly decreases the albumin binding of warfarin. (2) Ultrafiltration and ultracentrifugation studies demonstrated the complex modulation of warfarin-HSA interaction by the different Heme site markers; for example, bilirubin strongly decreased while methyl orange considerably increased the bound fraction of warfarin. (3) Fluorescence spectroscopic studies showed misleading results in these diligand-albumin interactions. (4) Different Heme site ligands can increase or decrease the albumin binding of warfarin and the outcome can even be concentration dependent (e.g., biliverdin and hemin).

The relationship of oleic acid/albumin molar ratio and clinical outcomes in leptospirosis

Human leptospirosis is an acute infectious zoonosis presenting specific lipid disorders. Previous in vitro studies showed both leptospira glycolipoprotein endotoxin, and high oleic acid levels were associated with Na/K-ATPase inhibition that is amplified by the reduction of circulating albumin levels. In this study, we aimed to investigate the relationship of oleic acid/albumin (OA/A) molar ratio and clinical outcomes in Leptospirosis. Through a prospective observational cohort study employing high-performance liquid chromatography (HPLC) we sequentially determined serum concentrations of nonesterified fatty acids (NEFA) and albumin in twenty-eight patients with severe leptospirosis since their hospital admission. Twenty patients recovered, and eight died. Data was distributed in two groups according to clinical outcomes. Oleic acid/albumin molar ratios (OA/A), initial samples, were higher than those in healthy donors. The ratio OA/A, however, persisted high in dying patients, whereas patients who survived had a reduction matching to healthy donors. Biochemical alterations suggest that cure is correlated to the reestablishment of the OA/A molar ratio, while fatal outcomes related to persisting OA/A imbalances. Analysis by receiver operating characteristic (ROC) showed the area under the curve of 0.864 and the cutoff value of 0.715 being associated with a high odds ratio. Lipid analysis from patients with leptospirosis had an acute high serum OA/A molar ratio, and sustained imbalance has a high odds ratio and strong correlation with mortality.

Serum protein-based nanoparticles for cancer diagnosis and treatment

Serum protein as naturally essential biomacromolecules has recently emerged as a versatile carrier for diagnostic and therapeutic drug delivery for cancer nanomedicine with superior biocompatibility, improved pharmacokinetics and enhanced targeting capacity. A variety of serum proteins have been utilized for drug delivery, mainly including albumin, ferritin/apoferritin, transferrin, low-density lipoprotein, high-density lipoprotein and hemoglobin. As evidenced by the success of paclitaxel-bound albumin nanoparticles (Abraxane^TM), serum protein-based nanoparticles have gained attractive attentions for precise biological design and potential clinical application. In this review, we summarize the general design strategies, targeting mechanisms and recent development of serum protein-based nanoparticles in the field of cancer nanomedicine. Moreover, we also concisely specify the current challenges to be addressed for a bright future of serum protein-based nanomedicines.

Serum albumin saturation test based on non-esterified fatty acids imbalance for clinical employment

Fatty acids are fundamental as energy and structural source to the human cells. They are not usually found free in human circulation. Alteration in fatty acids metabolism is linked to diseases such as diabetes, preeclampsia, heart disease, and some infectious diseases. Increased levels of non-esterified fatty acids (NEFA) may cause cell dysfunction and lipotoxicity. Since physiologically fatty acids are transported bound to albumin, we propose here a simple and cheap test that consists of albumin isoelectric focusing determination to measure the potential systemic NEFA cytotoxicity. For validation of this method, albumin isoelectric focusing in 51 serum samples from 40 critically ill patients and 11 controls was compared with NEFA/albumin ratios measured by HPLC. We called this approach an albumin saturation test. This test may indicate to physicians the potential NEFA lipotoxicity guiding them throughout better patient management. The albumin saturation test can point out serum albumin-NEFA saturation through a cheap assay that could be performed by any care facility.

Albumin Carriers for Cancer Theranostics: A Conventional Platform with New Promise

Theranostic nanoplatforms with integrated diagnostic and therapeutic functions, aiming at imaging-guided therapy to improve treatment planning, as well as combination therapy to enhance treatment efficacy, have received tremendous attention in recent years. Among numerous types of functional nanomaterials explored in this field, protein-based nanocarriers with inherent biocompatibility have also been selected as building blocks to construct multifunctional theranostic platforms. In particular, albumin, which has been extensively used as drug-delivery carriers for decades, has shown great new promise in the construction of novel imaging and therapeutic nanoagents, as demonstrated by a number of recent studies. IHere, the motivations of using albumins to build up nanoscale theranostics are discussed, and the latest progress/future perspectives in this direction are summarized.

Studies of the affinity of human serum albumin for binding of bilirubin at different temperatures and ionic strength

The association constants for the binding of bilirubin to human serum albumin (HSA) have been determined at four different temperatures by measurements of the rate of the peroxidase catalyzed oxidation of unbound bilirubin. The change of enthalpy is determined from a van't Hoff plot (ln Kass versus 1/T) to about -13.5 kcal/mol. deltaG degrees is calculated from the binding constants, and deltaS degrees is obtained from: deltaG degrees = deltaH degrees--TdeltaS degrees. The results show that the large negative deltaG degrees (--11 kcal/mol) for binding of bilirubin to HSA is a consequence of the negative deltaH degrees. The entropy was found to be about--8.5 cal/mol/degree and tends to diminish the numerical value of deltaG degrees. The binding constant has also been determined at varying ionic strength. The results show a decrease in binding for increasing salt concentration. The data from the two sets of experiments suggest that hydrogen bonds and salt linkages rather than hydrophobic interactions are the main factor in the binding of bilirubin to its primary site on HSA.

Interaction of bilirubin with sealed and human serum albumin-entrapped sealed membranes

In order to study the mechanism of entry and localization of bilirubin (BR) into cell membrane, binding of BR to sealed and human serum albumin (HSA)-entrapped sealed membranes was studied by CD spectroscopy. An induced bisignate CD cotton effects (CDCEs) of BR-bound sealed membranes were observed with maxima at 515 nm and minima at 470 nm with a shoulder at 430 nm. BR-bound HSA-entrapped sealed membranes produced CD spectra with additional positive peaks at 450 and 475 nm and negative troughs at 390 and 415 nm. The induced CDCEs of BR-bound sealed membranes and BR-bound HSA-entrapped sealed membranes were perturbed by the addition of drugs (ceftriaxone and sodium salicylate) with the effect of ceftriaxone being more pronounced. Drugs' being the displacer of BR from albumin, their incorporation in the incubation mixture was paralleled by reduction in CDCEs. Taken together, these results suggest that BR can traverse the membrane bilayer towards the inner surface instead of remaining intercalated in the exterior half of the bilayer.

Nonregenerative stimulation of hepatocyte proliferation in the rat: variable effects in relation to spontaneous liver growth; a possible link with metabolic induction

Three procedures were used to stimulate hepatocyte proliferation in the rat without reducing liver mass, resulting in a supplementary growth which differs from the regenerative growth observed after loss of liver mass by hepatectomy or toxic necrosis. They were: (a) the ingestion of cyproterone, a cytochrome P450 inducing drug (b) the injection of an irritant which provokes glycogenesis and synthesis of acute-phase proteins (c) the injection of albumin-bound bilirubin leading to elimination of glucuronated bilirubin in bile. This ensuing supplementary growth was studied in the rat under several conditions of hepatic proliferation: 1. In normal adult rats, in which hepatocyte proliferation is very low, the effect on proliferation was either weak or undetectable. 2. In suckling rats, with a rapid body and liver growth, all the stimulants provoked a synchronized wave of proliferation with a steep increase of the percentage of S-phase hepatocytes from 4.5% in controls to 15-30% in treated rats. This increase was followed by a compensatory period of low proliferation during which a treatment with a second stimulant was much less effective. 3. In 2/3 hepatectomized adult rats, the proliferation induced by cyproterone was higher than the spontaneous regenerative proliferation alone and additional to it during all of the regenerative process. The proliferation induced by acute inflammation was competitive with the synchronous spontaneous proliferation during the early period of synchronized proliferation following surgery, suggesting that both are similar acute responses. Differently, during the late period of lower and unsynchronized regenerative proliferation, the proliferation provoked by acute inflammation was additional to the spontaneous one. A stimulation of proliferation by injection of the albumin-bilirubin complex was observed during the late period after 2/3 hepatectomy. The highest level of stimulation occurred when the liver growth and the hepatocyte proliferation were already high. This suggests that these stimulants are not complete mitogenic stimuli and need cofactors which are present during the spontaneous growth or, alternatively, that the effect of stimulants is opposed by an inhibitory mechanism present in the adult rat.

Spectroscopic probes of the individual and combined effects of Triton X-100 and chloroform on serum albumins and serum-albumin.bilirubin complexes

The effects of the non-ionic surfactant Triton X-100 on the biphasic induced CD spectra of bilirubin complexes of human and bovine serum albumins (HSA and BSA) are divergent. While Triton X-100 inverts the induced CD spectrum of HSA.bilirubin, this surfactant enhances the ellipticity values of induced CD of BSA.bilirubin without inversion. The effect of Triton X-100 on the characteristic ultraviolet-CD spectra of the albumins are similar; both the albumins are denatured from their native globular structures. The anionic surfactant SDS, unlike non-ionic Triton X-100, dislodges the ligand from its protein complexes, indicating that both electrostatic and hydrophobic forces are involved in binding of bilirubin to the albumins. The aprotic solvent chloroform inverts the biphasic induced CD spectra of HSA.bilirubin and BSA.bilirubin, whereas CHCl3 has relatively little effect on the ultraviolet CD spectra of the albumins. The combined effect of Triton X-100 and CHCl3 shows that the effect of CHCl3 predominates over that of Triton X-100. The perturbing effects of Triton X-100 and CHCl3 on the CD or induced CD spectra of the proteins or their bilirubin complexes are reversible, and independent of the order in which components were added. The observations suggest that the denaturation of the albumins by Triton X-100 or solvation of CHCl3 within albumins markedly alter the internal topography or dynamics of the receptor sites, triggering alterations of the chirality of the bound pigment in sign and/or magnitude.

Effects of albumin-bilirubin complexes with syngeneic or allogeneic albumin on DNA and protein synthesis in liver and spleen of partially hepatectomized rats

The effects of non-covalently bound complexes of allogeneic or syngeneic albumin with bilirubin and of albumin alone on DNA and protein synthesis in rat liver and spleen cells after partial hepatectomy were studied. The assay procedure was based on different intravenous doses of these compounds in rats after partial hepatectomy. The allogeneic albumin-bilirubin complex (at protein doses of 0.9 and 90 micrograms/100 g body weight) stimulated DNA and protein synthesis in liver cells irrespective of the dose. At a dose of 0.9 micrograms the syngeneic albumin-bilirubin complex enhanced DNA synthesis insignificantly and produced no effect on protein synthesis, while at a dose of 90 micrograms, both DNA and protein synthesis were considerably increased. Allogeneic or syngeneic albumin at the above doses stimulated only protein, not DNA, synthesis in the liver, while the highest stimulation was at 90 micrograms allogeneic albumin. It was found also that partial hepatectomy decreased DNA and protein synthesis in spleen cells. Albumin-bilirubin complex with allogeneic or syngeneic albumin and albumin alone either significantly enhanced DNA and protein synthesis in the spleen, compared to controls, or only restored synthesis to control levels. Thus DNA and protein synthesis in the regenerating liver and spleen was significantly enhanced after the injection of small doses of the albumin-bilirubin complex, indicating the existence of small amounts of a similar endogenous complex in the blood stream.

Bilirubin helps to overcome the two-cell block in mouse oocyte cultures

PURPOSE

In vitro fertilization and culture of mouse oocytes, under normal atmospheric oxygen tension, subjects them to severe oxidative stress. Oocytes from some strains of mice lack the natural protective mechanism that guards them against this oxidative stress and fail to develop beyond the two-cell stage.

METHODS

We could overcome the toxic effects of oxygen metabolites by adding 0.2-0.4 mg/dl bilirubin in a lactate-pyruvate culture medium defined by Whitten (1971). Six- to 8-week-old ICR (Institute of Cancer Research) female mice were super ovulated by intra peritoneal injection of 5 IU PMSG (pregnant mare serum gonadotropin) followed by 10 IU hCG 48 h later. The oocytes were collected from the distended fallopian tubes and inseminated with 1-2 million sperm from 3-4-month-old ICR male mice. The eggs were scored at 24, 48, and 72 h after the hCG injection.

CONCLUSIONS

With 0.4 mg/dl bilirubin supplement, by the end of 72 h, 82% of the eggs progressed from the two-cell stage to the four-cell stage. Routine inclusion of bilirubin can improve embryo development in vitro.

On the interaction of polypeptides with bile salts or bilirubin-IX alpha

Aqueous solutions formed by polypeptides, simple models of proteins, and bile salts (sodium cholate and deoxycholate, NaC and NaDC, respectively) or bilirubin-IX alpha (BR) have been studied by CD measurements. They could mimic more complicated biliary systems, thus supplying a possible interpretation of the behavior of some amino acid residues in the biliary proteins. The aggregation of NaDC and NaC in water can be monitored by CD measurements. Bile salts, in submicellar and micellar form, stabilize poly(L-Lys) (PLL) in alpha-helical conformation. The alpha-helix content increases with increasing bile salt concentration and ionic strength. NaDC seems to be a slightly better stabilizing agent of the alpha-helix conformation than NaC. Models characterized by hydrogen bonds between bile salts and PLL are proposed, also resorting to previous data available on the systems formed by NaDC and poly(L-Leu-L-Leu-L-Lys) (PLLL) or poly(L-Leu-L-Leu-L-Asp) (PLLA). Binding of BR to PLL, poly(D-Lys), poly(L-Glu), PLLL, and PLLA in water has been investigated by CD spectra in order to clarify the nature of the association complexes and the mechanism of the BR enantioselective complexation. Potential energy calculations provide binding models capable of explaining the enantioselective ability of the PLL and PLLL alpha-helices toward the left- and right-handed enantiomer of BR, respectively. BR is bound to -NH2 groups of PLL and PLLL lying on a right- and left-handed spiral, respectively. These results, together with those formerly obtained for some bile salts-BR systems, indicate that the selectivity originates from a binding that involves large regions of the BR molecule and gives rise, very probably, to moderate conformational changes from the "ridge tile" structure observed in the crystals. In some cases van der Waals forces can play a crucial role in the chiral recognition of bilirubin. Moreover, possible interaction models of BR with human serum albumin are proposed on the basis of a recent x-ray crystal structure of the protein.

The effects of hypoxanthine, xanthine oxidase and hyperoxia on the accumulation of bilirubin and albumin in young rat brain

Hyperoxia has been suggested as a risk factor for kernicterus. The toxicity of hyperoxia may be mediated by free radicals. We investigated the effects of free radicals, formed by the hypoxanthine/xanthine oxidase system, with and without additional hyperoxia, on the accumulation of bilirubin and albumin in rat brain. Hypoxanthine was infused for 60 min into retrograde carotid catheters in awake, young, male SPRD rats. After 30 min the infusion was briefly interrupted to inject xanthine oxidase 1 U/kg through the same catheter. Group I (controls) received 0.9% NaCl in lieu of hypoxanthine/xanthine oxidase. Groups I and II breathed room air at all times, while group III breathed 90% O2. After 60 min all groups received a bolus dose of 125I-albumin through a peripheral venous catheter, followed by bilirubin 25 mg/kg for 5 min, then bilirubin 35 mg/kg for 55 min. There were no significant differences between the groups as regards serum bilirubin, serum albumin, brain bilirubin, or brain albumin. Neither during normoxic nor hyperoxic conditions did the hypoxanthine/xanthine oxidase system increase the accumulation of bilirubin or albumin in rat brain.

Effect of volatile anesthetics on the circular dichroism of bilirubin bound to human serum albumin

The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding.

The fatty acid analogue 11-(dansylamino)undecanoic acid is a fluorescent probe for the bilirubin-binding sites of albumin and not for the high-affinity fatty acid-binding sites

1. The fluorescent fatty acid probe 11-(dansylamino)undecanoic acid (DAUDA) binds with high affinity to bovine and human serum albumin (BSA and HSA) at three sites. 2. The Kd of the primary binding site could not be determined; however, the two secondary sites appeared to be equivalent, with an apparent Kd of 8 x 10(-7) M for both BSA and HSA. 3. The spectral characteristics of DAUDA when bound to the primary site of the two albumins were different, with HSA producing a greater fluorescence enhancement and emission maximum at a shorter wavelength (480 nm) than for BSA (495 nm). 4. Displacement studies indicated that the DAUDA-binding sites were not equivalent to the primary long-chain fatty acid-binding sites on albumin, but corresponded to the bilirubin sites. Fatty acyl-CoAs also bind to the bilirubin sites, as do medium-chain fatty acids. 5. The solubility, stability and spectral properties of DAUDA make it an excellent probe for investigating the bilirubin-binding sites of albumin, particularly HSA.

Influence of aspirin and iron(III) tetrasulfonated phthalocyanine on bilirubin binding by human serum albumin

The interaction of bilirubin with aspirin-modified human serum albumin (HSA) and the influence of iron tetrasulfonated phthalocyanine on bilirubin binding by the native protein has been studied by difference spectroscopy and circular dichroism measurements. Spectroscopic studies of the systems containing bilirubin and aspirin-modified HSA compared to the analogous systems with the native protein have shown that selective acetylation of albumin at lysine 199 inhibits bilirubin binding by this protein. In both cases, interaction between bilirubin and albumin leads to complex formation at a molar ratio of ligand to protein of 2:1. The studies of the reaction of bilirubin with fragments of albumin produced by reaction with CNBr have demonstrated that one of the strong bilirubin binding sites is located in the M fragment and is close to the high-affinity binding site of aspirin. The other one was found in fragment C. Acetylation of albumin brings about marked conformational change in the protein, which probably accounts for the decrease in its ability to react with anti-HSA antibody. Bilirubin does not change the secondary structure of albumin but, like aspirin, lowers its antigenicity. It has been suggested that the decrease in antigenic properties in this case results from cooperation of the closely neighboring antigenic and bilirubin-binding sites. The studies of the influence of iron(III) tetrasulfonated phthalocyanine on bilirubin binding by HSA suggest that there is no competition between strong sites for iron(III) tetrasulfonated phthalocyanine and bilirubin, but these compounds compete for some of the weaker sites.

The induced circular dichroism of bilirubin complexed with the alpha-helix form of poly(L-lysine)

Binding of bilirubin by the alpha-helix conformation of poly(L-lysine) in water induces optical activity. The bisignate circular dichroism spectrum exhibits exciton bands centred at 444 nm, negative, and at 525 nm, positive. The magnitude of the induced circular dichroism depends on the concentration of total bilirubin and total lysine residues, the molar ratio of total lysine residues-to-total bilirubin molecules, the pH and the degree of polymerization of poly(L-lysine). Although bilirubin binds to the random coil conformation of poly(L-lysine), as evidence by the absorption spectrum, the complex is optically inactive. The results suggest that bilirubin binds to the poly(L-lysine) in the form of dimers and oligomers.

Ligand-binding properties of albumin Parklands: Asp365----His

An albumin variant, isolated from the plasma of a patient with bisalbuminemia, was compared with albumin A from the same patient for binding of long-chain fatty acids and bilirubin. No differences in binding of [14C]palmitate, cis-parinaric acid or bilirubin could be detected for the variant form. These results suggest that the region adjacent to residue 365 is unlikely to be part of a major binding site for any of these ligands.

Identification of biliprotein as a liver growth factor

We determined the concentration of biliprotein in plasma of rats at different times after partial hepatectomy. From the same plasma samples, we purified a liver growth factor previously characterized by our group. When we plotted the 14 points studied, a linear relationship was obtained (r = 0.999; p less than 0.001). This result, in addition to our group's recent identification of this liver growth factor as an albumin-bilirubin complex, strongly suggests that biliprotein is a liver growth factor.

Influence of the N-B transition of human serum albumin on the structure of the warfarin-binding site

The fluorescence quantum yield of warfarin increased with the viscosity of the medium and showed good correlation with it. The internal rotation of the acetonylbenzyl group of a warfarin molecule may thus possibly decrease in a viscous medium. the fluorescence quantum yield of warfarin bound to human serum albumin increased with the pH of the medium in the pH range of 6.2-9.0. Fluorescence-emission maximum wavelengths of warfarin bound to human serum albumin indicated a small blue-shift with the pH of the medium and that of free warfarin in the absence of albumin also shifted slightly to a shorter wavelength with the viscosity of the medium. Warfarin is bound more strongly to human serum albumin at basic pH than at neutral pH, and the increase in the bound fraction of warfarin correlated well with the increase in the fluorescence quantum yield of bound warfarin in the same pH range. Thus, the structure of the warfarin-binding site in the B (base) form appears more spatially confined than that in the N (neutral) form. The motion of the warfarin molecule bound to its binding site on human serum albumin in the N-B transition may thus be more restricted at basic than at neutral pH, and this may possibly be the reason for the stronger binding of warfarin to human serum albumin in the B form.

Fluorescence energy transfer study of the relationship between the lone tryptophan residue and drug binding sites in human serum albumin

The relationship between the lone tryptophan residue at position 214 and drug binding sites (Sites I and II) in human serum albumin (HSA) was studied by fluorescence energy transfer. The distance between the lone tryptophan residue and ligands bound to HSA was estimated by Förster's equation, taking into consideration the degree of ligand binding at these sites, as determined from binding parameters (binding constant, k, and the number of binding sites, n). For all ligands investigated, the distance in each case appeared to asymptotically decrease when the occupation ratio of the binding sites increased with ligand concentration. When the primary binding site of each ligand in HSA was almost saturated, the distance attained a constant value, making possible a somewhat more exact determination of the distance. The distance ranged from approximately 22 to 23 A for ligands typical of Site I (warfarin, dansylamide, dansylglutamine), and approximately 16.1 to 17.5 A for ligands typical of Site II (dansylsarcosine, dansylproline, dansylglycine, diazepam, flufenamic acid).

Identification of a liver growth factor as an albumin-bilirubin complex

We have reported the purification and characterization of a protein that behaves as a liver growth factor, showing activity either in vivo or in vitro [Díaz-Gil et al. (1986) Biochem. J. 235, 49-55]. In the present paper, we identify this liver growth factor (LGF) as an albumin-bilirubin complex. This conclusion is supported by the results of chemical and spectroscopic characterization of this protein as well as by experiments in vivo. Incubation of albumin isolated from normal rats with bilirubin/albumin molar ratios (r) resulted (when r = 1 or 2) in a complex with liver DNA synthesis promoter activity identical with that of LGF. The exact amount of bilirubin bound to albumin was assessed by fluorescence and c.d. spectra. This albumin-bilirubin complex showed the same dose-dependence profile as LGF either at low or high dose of protein injected per mouse. Both LGF and albumin-bilirubin complex produced similar increases in the mitotic index of mouse hepatocytes in vivo. A new mechanism for the onset of the hepatic regenerative process is proposed.

Evidence for the fatty acid-induced heterogeneity of the N and B conformations of human serum albumin

The influence of oleic acid on the interaction between albumin and warfarin, oxyphenbutazone or diazepam has been studied by circular dichroism and equilibrium dialysis. The pH dependences of the molar ellipticity of the drug-albumin complexes and of the free fraction of drug are completely changed by the presence of oleic acid. This phenomenon is attributed to an oleic acid-induced conformational change in both the neutral (N) and the basic (B) conformation of albumin, a change to which the warfarin-oxyphenbutazone binding area and the diazepam binding site is sensitive. The oleic acid-induced conformational states of albumin, the so-called N* and B* conformations, show binding properties that are different from the binding properties of the N and B conformations.

Brain uptake of a food dye, erythrosin B, prevented by plasma protein binding

Although food colors have been held responsible for several behavioral disorders and do affect neuronal function when directly applied, there is no information on whether significant quantities of the dyes appear in the brain after consumption or parenteral administration. [14C]erythrosin B was administered directly into the circulation of mature rats and radioactivity was measured thereafter in brain regions at several times. Although insignificant parenchymal radioactivity was detected in brains perfused with dye in whole blood, significant concentrations of [14C]erythrosin B were detected in all brain regions when perfused with protein-free Ringers, as predicted from the octanol-water partition coefficient of the dye. Thus, significant brain uptake of intravascular dye is normally prevented by its binding to plasma protein (greater than 99% bound) and by the blood-brain barrier impermeability to the dye-protein complex. Sensitivity to food dyes such as erythrosin B in some individuals may reflect altered plasma protein binding capacity, which can vary with age and disease.

Influence of free fatty acids and glucose infusion on serum bilirubin and bilirubin binding to albumin: clinical implications

We studied the risk of a large group of jaundiced neonates for bilirubin encephalopathy by serial assessment of their reserve serum albumin binding capacity as measured by the saturation index test. In 1271 infants with serum bilirubin concentration greater than 10 mg/dl, 12% had a saturation index (SI) of 7% or greater and therefore were clinically at or near risk for bilirubin encephalopathy. Treatment with glucose infusion (1 gm/kg over one hour) was highly effective in lowering the SI (delta = -3.7%. P less than 0.001). In none of the infants did SI rebound to 7% or greater within 24 hours after the infusion. In a detailed study of 19 infants who received glucose, the highly significant (P less than 0.001) fall in SI (delta = -3.7%) was accompanied by an equally significant rise in serum values for insulin (delta = +21.6 mcu/ml) and fall in serum free fatty acids (delta = -0.51 mEq/L). Many factors in the study, such as prematurity, hemolysis, acidosis, and hypoxemia, could have predisposed the infants to the risk of bilirubin encephalopathy. However, the facility by which most (93%) of the infants with high SI, including those who were premature or had evidence of hemolysis or respiratory insufficiency, responded to infusion of glucose indicates that serum free fatty acids may be the principal factor contributing to the high saturation index and therefore an underestimated factor in bilirubin binding to albumin.

The effects of N-B transition of human serum albumin on the specific drug-binding sites

In the pH range 6-9, human serum albumin undergoes a conformational change termed the neutral-base (N-B) transition. Recently, it has been shown that the N-B transition causes enhanced binding at the warfarin-binding site (site I). The present study used fluorescence and equilibrium dialysis to investigate the effects of the N-B transition, chloride, calcium and fatty acids on the specific binding sites I and II on human serum albumin. The effect of the N-B transition of human serum albumin provides a further distinction between site I and II binding characteristics. The N-B transition of albumin caused a change in conformation at site I which resulted in increased binding of drugs and fluorescent probes at this site, whereas there was no effect on acidic drug binding at site II. These effects on site I and II are qualitatively similar to those induced by fatty acids (increased drug binding at site I and no change at site II). However, the effects of increasing pH and fatty acids were additive, showing that they were caused by two different conformational changes. The effect of Cl- on site I binding was pH-dependent and was abolished by the presence of fatty acid. Ca2+ reduced the fluorescence of site I probes but had no effect on a site II fluorescent probe. Effects of pH were also investigated with drugs not binding to site I or II. Increasing pH caused a decrease in binding to indomethacin, increases in binding of L-tryptophan, tolmetin and quinidine and no change in the binding of salicylic acid, diflunisal and phenytoin.

The effect of ligand presaturation on the interaction of serum albumins with an immobilized Cibacron Blue 3G-A studied by affinity gel electrophoresis

The interaction of the immobilized triazine dye Cibacron Blue 3G-A with rat, rabbit, sheep, goat, bovine and human serum albumins was studied by affinity gel electrophoresis. Dissociation constants were estimated in each instance and showed human serum albumin to have a significantly higher affinity for the dye than did albumin from any other species. Pretreatment of the defatted proteins with bilirubin (3 mol of bilirubin/mol of protein) did not increase the dissociation constants of the serum albumins, whereas pretreatment with palmitate (7 mol of palmitate/mol of protein) increased the dissociation constant in all cases: 3-fold for human serum albumin, 15-fold for other serum albumins. Increasing the bilirubin/albumin ratio (to 7:1) did not affect the dissociation constant of the albumins studied. Decreasing the palmitate/albumin ratio decreased the dissociation constant for human serum albumin, but did not affect those of bovine and rat albumins. Altering the chain length of the presaturating fatty acid dramatically changed the dissociation constant of both human and bovine serum albumins. Butyrate, hexanoate, octanoate and decanoate did not significantly influence the dissociation constants of bovine and human serum albumins for Cibacron Blue, whereas laurate, myristate and palmitate greatly increased the dissociation constant. These data are discussed in relationship to the behaviour of albumins during dye--agarose column chromatography. In Addendum the effect of nucleotide presaturation on the interaction between Bacillus stearothermophilus 6-phosphogluconate dehydrogenase and the immobilized triazine dyes Cibacron Blue 3G-A and Procion Red HE-3B was examined, and the implications for dye--ligand chromatography are discussed.

Kinetics of neutral transition of human serum albumin

The fast step in the conformational change of human serum albumin from the alkaline to the neutral form of the albumin-bilirubin complex is studied by various pH jump experiments in a stopped-flow apparatus. The results indicate that the first step is caused by electrostatic attraction between a carboxylate group of bilirubin and a histidine residue of albumin.

Studies on the mechanism of binding of serum albumins to immobilized cibacron blue F3G A

The interaction of Cibacron Blue F3G A-Sepharose 4B with several serum albumins was studied. Although all albumins used were fond to bind to this adsorbent, human serum albumin was bound to a far greater extent than were the others. From the results of competition experiments and n.m.r. studies of Cibacron Blue and/or bilirubin binding to human serum albumin it is proposed that the mechanism of the interaction between human serum albumin and cibacron Blue is consistent wit Cibacron Blue binding to bilirubin-binding sites. In contrast with these findings with human serum albumin, there is little or no interaction of Cibacron Blue and the bilirubin-binding sites of albumins from rabbit, horse, bovine or sheep sera, although some interaction occurs between Cibacron Blue and the fatty acid-binding sites of these proteins. Structural analogues of Cibacron Blue have been used to investigate the binding of albumins to these ligands.