The kinetics of interactions of bilirubin with lipid bilayers and with serum albumin

Unconjugated bilirubin modulates neuronal signaling only in wild-type mice, but not after ablation of the R-type/Cav 2.3 voltage-gated calcium channel

INTRODUCTION

The relationship between blood metabolites and hemoglobin degradation products (BMHDPs) formed in the cerebrospinal fluid and the development of vasospasm and delayed cerebral ischemia (DCI) after aneurysmal subarachnoid hemorrhage (aSAH) has been the focus of several previous studies, but their molecular and cellular targets remain to be elucidated.

METHODS

Because BMHDP-induced changes in Ca_v 2.3 channel function are thought to contribute to DCI after aSAH, we studied their modulation by unconjugated bilirubin (UCB) in an organotypical neuronal network from wild-type (WT) and Ca_v 2.3-deficient animals (KO). Murine retinae were isolated from WT and KO and superfused with nutrient solution. Electroretinograms were recorded before, during, and after superfusion with UCB. Transretinal signaling was analyzed as b-wave, implicit time, and area under the curve (AUC).

RESULTS

Superfusion of UCB significantly attenuated the b-wave amplitude in the isolated retina from wild-type mice by 14.9% (P < 0.05), followed by gradual partial recovery (P = 0.09). Correspondingly, AUC decreased significantly with superfusion of UCB (P < 0.05). During washout, the b-wave amplitude returned to baseline (P = 0.2839). The effects of UCB were absent in Ca_v 2.3-deficient mice, lacking the expression of Ca_v 2.3 as proofed on the biochemical level.

CONCLUSIONS

Ex vivo neuronal recording in the murine retina is able to detect transient impairment of transretinal signaling by UCB in WT, but not in KO. This new model may be useful to further clarify the role of calcium channels in neuronal signal alteration in the presence of BHMDPs.

Protein-bound molecules: a large family with a bad character

Many small solutes excreted by the kidney are bound to plasma proteins, chiefly albumin, in the circulation. The combination of protein binding and tubular secretion allows the kidney to reduce the free, unbound concentrations of such solutes to lower levels than could be obtained by tubular secretion alone. Protein-bound solutes accumulate in the plasma when the kidneys fail, and the free, unbound levels of these solutes increase more than their total plasma levels owing to competition for binding sites on plasma proteins. Given the efficiency by which the kidney can clear protein-bound solutes, it is tempting to speculate that some compounds in this class are important uremic toxins. Studies to date have focused largely on two specific protein-bound solutes: indoxyl sulfate and p-cresyl sulfate. The largest body of evidence suggests that both of these compounds contribute to cardiovascular disease, and that indoxyl sulfate contributes to the progression of chronic kidney disease. Other protein-bound solutes have been investigated to a much lesser extent, and could in the future prove to be even more important uremic toxins.

Adsorption of amphiphilic hyperbranched polyglycerol derivatives onto human red blood cells

Hydrophobically derivatized hyperbranched polyglycerol (HPG)-polyethylene glycol (PEG) polymers bearing stearoyl chains (HPG-C18-PEG) were originally developed as human serum albumin substitutes and further as a unimolecular drug delivery system. In view of these in vivo applications and the potential for membrane interaction by these materials due to their amphiphilic structure, determining the adsorption of the polymers to human red blood cells (RBCs) is an important issue. This paper reports on the in vitro adsorption to RBCs of tritium-radiolabeled HPG-C18-PEG polymers. The morphological changes of RBCs associated with the adsorption were also examined by light and scanning electron microscopy (SEM). Laser scanning confocal microscopy (LSCM) suggests that the binding site of the polymers on RBCs is the cell membrane. Adsorption experiments show that, in the medium of either saline or plasma, the binding amount of the polymers to RBCs increases with increased polymer concentration in a manner which implies simple Langmurian behavior. The binding amount in saline is of the order of 10(5) molecules/cell at an equilibrium concentration of 1 mg/mL of HPG-C18-PEG polymer. The RBC morphology depends on the adsorbed amount; the cells become crenated in high concentrations (5 and 10 mg/mL) of the polymer solutions in the absence of plasma proteins. Interestingly, a large amount of polymers remain bound to RBCs even after washes with plasma (of the order of 10(4) molecules/cell). Thus, the bound polymers might have an extended circulating time by "hitchhiking" on RBCs in the bloodstream. These results provide significant information and insight for related studies of the interaction of amphiphilic molecules with cell membranes and for in vivo applications of biopolymers as drug delivery systems.

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.

Effects of bilayer surface charge density on molecular adsorption and transport across liposome bilayers

Second harmonic generation (SHG) was used to study both the adsorption of malachite green (MG), a positively charged organic dye, onto liposomes of different lipid compositions, and the transport kinetics of MG across the liposome bilayer in real time. We found that the dye adsorption increased linearly with the fraction of negatively charged lipids in the bilayer. Similarly, the transport rate constant for crossing the bilayer increased linearly with the fraction of charged lipid in the bilayer.

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.

Inhibition of glutamate uptake by unconjugated bilirubin in cultured cortical rat astrocytes: role of concentration and pH

The molecular basis of bilirubin toxicity to nerve cell function is still unclear. Since astrocytes are the main transporters of synaptically released glutamate and impaired glutamate uptake results in neuronal death, we investigated the effect of unconjugated bilirubin (UCB) on [(3)H]glutamate uptake in cultured rat astrocytes and the role of bilirubin ionization on toxicity. Astrocytes were incubated for 5-15 min, with UCB concentrations from 17 to 342 microM and UCB/albumin molar ratios of 0.2-3.0, at pH 7.0, 7.4, and 8.0. Exposure of astrocytes for 15 min to 85.5 microM UCB and 28.5 microM albumin resulted in a 63.1% decrease of glutamate uptake (p < 0.01). Interestingly, the effect demonstrated to be correlated with the UCB/albumin molar ratio (r = -0.986, p < 0.01) and a significant decrease was observed for a UCB/albumin molar ratio as low as 0.8. Inhibition of glutamate transport was also pH-dependent as it occurred at 7.4 (p < 0.05) and 8.0 (p < 0.01), but not at 7.0, suggesting that the monoanionic species of UCB accounted for the inhibition. These findings indicate that UCB, and more precisely the monoanionic species, impairs a crucial function of astrocytes such as glutamate transport and support a potential role of astrocyte function in the pathogenesis of UCB-related brain damage (kernicterus).

Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

The liver is responsible for the clearance and metabolism of unconjugated bilirubin, the hydrophobic end-product of heme catabolism. Although several putative bilirubin transporters have been described, it has been alternatively proposed that bilirubin enters the hepatocyte by passive diffusion through the plasma membrane. In order to elucidate the mechanism of bilirubin uptake, we measured the rate of bilirubin transmembrane diffusion (flip-flop) using stopped-flow fluorescence techniques. Unconjugated bilirubin rapidly diffuses through model phosphatidylcholine vesicles, with a first-order rate constant of 5.3 s-1 (t(1)/(2) = 130 ms). The flip-flop rate is independent of membrane cholesterol content, phospholipid acyl saturation, and lipid packing, consistent with thermodynamic analyses demonstrating minimal steric constraint to bilirubin transmembrane diffusion. The coincident decrease in pH of the entrapped vesicle volume supports a mechanism whereby the bilirubin molecule crosses the lipid bilayer as the uncharged diacid. Transport of bilirubin by native rat hepatocyte membranes exhibits kinetics comparable with that in model vesicles, suggesting that unconjugated bilirubin crosses cellular membranes by passive diffusion through the hydrophobic lipid core. In contrast, there is no demonstrable flip-flop of bilirubin diglucuronide or bilirubin ditaurate in phospholipid vesicles, yet these compounds rapidly traverse isolated rat hepatocyte membranes, confirming the presence of a facilitated uptake system(s) for hydrophilic bilirubin conjugates.

Effect of bilirubin on erythrocyte shape and haemolysis, under hypotonic, aggregating or non-aggregating conditions, and correlation with cell age

The effect of unconjugated bilirubin on the morphology and haemolysis of human erythrocytes was accomplished under distinct incubation conditions: (i) hypotonic medium, with bilirubin concentrations ranging from 1 x 10(-9) to 1 x 10(-4) mol l-1; (ii) isotonic medium, with 171 mumol l-1 bilirubin, in the absence of albumin (aggregating conditions), using non-separated and age-fractionated erythrocytes; (iii) isotonic medium, with 171 mumol l-1 bilirubin, in the presence of a surplus of human serum albumin (non-aggregating conditions), and using sulfisoxazole as a bilirubin displacer (bilirubin/albumin and sulfisoxazole/ albumin molar ratios of 0.5 and 4.0, respectively). Our data showed that low concentrations of bilirubin (1 x 10(-7) to 1 x 10(-5) mol l-1) protect against hypotonic haemolysis and induce crenation, while higher bilirubin concentrations induce haemolysis and lead to membrane disruption. When aggregating conditions were used, these phenomena were reproduced, the younger cells being significantly more susceptible to crenation while the older erythrocytes showed increased susceptibility to haemolysis. In non-aggregating conditions, haemolysis was virtually absent, though crenation was evident. Based on the above observations we conclude that the first step of erythrocyte bilirubin toxicity is crenation due to an expansion of the outer membrane leaflet by bilirubin mono-anion location. This effect is more evident in younger cells and explains the protection against the hypotonic haemolysis. Insertion of bilirubin deeper into the bilayer, facilitated by higher concentrations (> or = 1 x 10(-4) mol l-1) and cell age, produces an unstable situation, where bilirubin acid aggregation is apparently the main cause for haemolysis and cell destruction.

Cytotoxicity of bilirubin for human fibroblasts and rat astrocytes in culture. Effect of the ratio of bilirubin to serum albumin

The sensitivity of rat brain astrocytes and human fibroblasts in culture to unconjugated bilirubin was investigated. Medium containing 6 mumol/1 bilirubin and increasing concentrations of human serum albumin giving ratios of 0.5-1.5 that resulted in an increase of the free bilirubin concentrations. The LDH activity in the culture medium was an index of cytolysis and the MTT assay was used as an index of mitochondrial impairment. The ratios producing half-maximum cell lysis after 24, 48, and 72 h, were 1.1, 0.9 and 0.85, for astrocytes, and 1.2, 0.75 and 0.75, for fibroblasts. Mitochondrial activity decreased after 24 h for ratio = 0.7 and partly recovered at 48 h. Mitochondrial activity was more impaired in fibroblasts than in astrocytes above ratio = 0.7. The cytotoxic effects were linked to the free bilirubin concentration. We conclude that astrocytes are less sensitive to bilirubin cytotoxic effects than are fibroblasts.

Alterations of erythrocyte morphology and lipid composition by hyperbilirubinemia

Morphology and membrane lipid composition of erythrocytes from neonates (jaundiced and healthy) and adults (before and after incubation with bilirubin) were studied. The morphological index, expressing the relative proportions of the different stages of cell distortion, and the membrane cholesterol, phospholipids and cholesterol/phospholipids molar ratio, were determined. In jaundiced neonates a significant increase in the morphological index (P < 0.01) was found. After incubation with bilirubin, adult erythrocytes also showed an increase in the morphological index (P < 0.01). Hemolysis occurred under these conditions, and the red cell ghosts obtained (vesicles) showed a rounded morphology. Higher cholesterol/phospholipid ratio and lower phospholipid content were found in jaundiced neonates compared with healthy babies (P < 0.05) and adults (P < 0.01), as well as in the cells (P < 0.05) and vesicles (P < 0.01) obtained after bilirubin incubation. Bilirubin cytotoxicity may occur in a stepwise manner: deposition of bilirubin in membrane produces echinocytosis, which is followed by disintegration of the lipid bilayer with loss of phospholipids from the membrane.

Bilirubin glucuronidation by intact Gunn rat fibroblasts expressing bilirubin UDP-glucuronosyltransferase

Crigler-Najjar (CN) disease is an inherited disorder of bilirubin metabolism. The disease is caused by a deficiency of the hepatic enzyme bilirubin UDP-glucuronosyltransferase (B-UGT). Patients with CN disease have high serum levels of the toxic compound, unconjugated bilirubin. The only defect in bilirubin metabolism of CN patients is the absence of B-UGT activity. The transplantation of cells able to glucuronidate bilirubin should therefore lower serum bilirubin levels. The Gunn rat is the animal model of CN disease. Primary Gunn rat fibroblasts (GURF) were transduced with a recombinant retrovirus, capable of transferring B-UGT cDNA. A cell line was obtained expressing B-UGT at a level comparable to hepatocytes. Bilirubin added to the culture medium of these cells was glucuronidated and excreted. The B-UGT activities of transduced GURF and freshly isolated Wistar hepatocytes were compared at different bilirubin concentrations. The specific B-UGT activities of these two cell types were comparable when physiological bilirubin concentrations (5-10 microM) were present in the culture media. At higher bilirubin concentrations (20-80 microM) the hepatocytes were more active than the transduced GURF. We conclude that with the addition of only one enzyme (B-UGT) fibroblasts can perform the complete set of reactions necessary for bilirubin glucuronidation. The difference in B-UGT activity between transduced GURF and hepatocytes at 20-80 microM bilirubin can be explained by lower UDP-glucuronic acid and glutathione S-transferase levels in GURF. Our findings also indicate that these cells could be used to develop extrahepatic gene therapy for CN disease.

Stable inducible expression of a functional rat liver organic anion transport protein in HeLa cells

Recently we expression cloned a rat liver organic anion transport protein in Xenopus laevis oocytes (Jacquemin, E. Hagenbuch, B, Stieger, B., Wolkoff, A.W., and Meier, P.J.,(1994) Proc. Natl. Acad. Sci. U.S.A. 91, 133-137). In the present study, we have stably transfected the cDNA encoding this protein into HeLa cells by using a vector containing a zinc-inducible promoter. The parent cells have virtually no baseline transport of [35S]sulfobromophthalein, whereas the induced transfected cells express a novel 74-kDa protein and avidly transport this ligand. Transport by these cells is saturable (Km = 3.3 microM, Vmax = 257 pmol/min/mg protein), bidirectional, and highly temperature-dependent. In the presence of albumin, uptake of [35S]sulfobromophthalein requires the presence of extracellular Cl, whereas in the absence of albumin, this C1- dependence is not seen. These studies indicate that cellular uptake of sulfobromophthalein does not result from direct interaction with the plasma membrane lipid bilayer but rather requires the presence of a specific plasma membrane transporter.

Kinetics of bilirubin transfer between serum albumin and membrane vesicles. Insight into the mechanism of organic anion delivery to the hepatocyte plasma membrane

Unconjugated bilirubin is transported in the plasma bound primarily to serum albumin, from which it is taken up and metabolized by the liver. To better characterize the mechanism of bilirubin delivery to the hepatocyte, stopped-flow techniques were utilized to study the kinetics of bilirubin transfer between serum albumin and both model phospholipid and native hepatocyte plasma membrane vesicles. The transfer process was best described by a single exponential function, with rate constants of 0.93 +/- 0.04, 0.61 +/- 0.03, and 0.10 +/- 0.01 s-1 (+/- S.D.) at 25 degrees C for human, rat, and bovine serum albumins, respectively. The observed variations in rate with respect to donor and acceptor concentrations provide strong evidence for the diffusional transfer of free bilirubin. Thermodynamic analysis suggests that the binding site on bovine serum albumin demonstrates higher specificity for the bilirubin molecule than that on human or rat serum albumin, which exhibit similar binding characteristics. Kinetic analysis of bilirubin transfer from rat serum albumin to isolated rat basolateral liver plasma membranes indicates that the delivery of albumin-bound bilirubin to the hepatocyte surface occurs via aqueous diffusion, rather than a collisional process, thereby mitigating against the presence of an "albumin receptor."

Discrimination between Crigler-Najjar type I and II by expression of mutant bilirubin uridine diphosphate-glucuronosyltransferase

Crigler-Najjar (CN) disease is classified into two subtypes, type I and II. The molecular basis for the difference between these types is not well understood. Several mutations in the bilirubin UDP-glucuronosyl-transferase (B-UGT) gene of six CN type I and two CN type II patients were identified. Recombinant cDNAs containing these mutations were expressed in COS cells. B-UGT activity was measured using HPLC and the amount of expressed protein was quantitated using a sandwich ELISA. This enabled us to determine the specific activities of the expressed enzymes. All type I patients examined had mutations in the B-UGT1 gene that lead to completely inactive enzymes. The mutations in the B-UGT1 gene of patients with CN type II only partially inactivated the enzyme. At saturating concentrations of bilirubin (75 microM) CN type II patient A had 4.4 +/- 2% residual activity and CN type II patient B had 38 +/- 2% residual activity. Kinetic constants for the glucuronidation of bilirubin were determined. The affinities for bilirubin of B-UGT1 expressed in COS cells and B-UGT from human liver microsomes were similar with Km of 5.1 +/- 0.9 microM and 7.9 +/- 5.3 microM, respectively. B-UGT1 from patient B had a tenfold decreased affinity for bilirubin, Km = 56 +/- 23 microM. At physiological concentrations of bilirubin both type II patients will have a strongly reduced conjugation capacity, whereas type I patients have no B-UGT activity. We conclude that CN type I is caused by a complete absence of functional B-UGT and that in CN type II B-UGT activity is reduced.

Studies on the efflux of heme from biological membranes

It is unknown how heme is distributed intracellularly from its site of synthesis in the mitochondria to other organelles. In previous work (Biochemistry 23, 3715, 1984) the transfer of heme from lipid bilayers to soluble proteins had been found to be independent of the recipient proteins' affinity for heme. Here, we investigated whether proteins are involved in the transfer of heme from biological membranes into aqueous media. We followed the release of 14C-labeled heme, from mitochondria preloaded with the heme, to BSA and found that only about 28%, of the heme was extracted on the first wash. After the third wash 35-50% of the heme that had been partitioned into the membranes was extracted. Fourth and fifth washes with BSA or a cytosolic heme-binding protein (HBP, also known as liver fatty acid binding protein) removed only insignificant amounts of 14C-labeled heme. Similarly, a large portion of the preloaded 14C-labeled heme could not be extracted from a variety of isolated membranes (inner and outer mitochondrial membranes, plasma membranes of liver cells, kidney cortex cells and erythrocyte membranes). By contrast, essentially all [14C]palmitate preloaded in biological membranes and all 14C-labeled heme preloaded in synthetic membranes was released to albumin (Biochemistry 23, 3715, 1984). These observations suggest that, in general, heme associates with membrane components which can be distinguished into two compartments. One compartment releases its heme spontaneously, while another compartment binds heme so tightly that a specific process has to be evoked for its release.

Greater selective uptake by Hep G2 cells of high-density lipoprotein cholesteryl ester hydroperoxides than of unoxidized cholesteryl esters

We have observed recently that high-density lipoproteins (HDL) are the predominant carriers of cholesteryl ester hydroperoxides (CEOOH), the major class of lipid hydroperoxides detectable at nanomolar concentrations in the plasma of healthy fasting humans. The present study investigates the effect of such very low levels of CEOOH in apolipoprotein E-free HDL3 on lipoprotein particle metabolism and 'selective uptake' of its CE by human Hep G2 cells. Minimal oxidation with aqueous peroxyl radicals had a negligible effect on the binding, internalization and degradation of 125I-labelled HDL3. In contrast, with an increasing degree of radical-mediated oxidation of labelled HDL3, [3H]cholesteryl linoleate ([3H]Ch18:2) was taken up at an increasingly greater rate than were 125I-apoproteins. When [3H]cholesteryl linoleate hydroperoxide ([3H]Ch18:2-OOH was incorporated into unoxidized HDL3 by exchange from donor liposomes, it was taken up at a more than 8-fold higher rate than was incorporated [3H]Ch18:2. The same degree of preferential uptake of oxidized CE was observed when HDL3 was used that was doubly labelled with [3H]Ch18:2-OOH and cholesteryl [14C]oleate ([14C]Ch18:1). In both situations, uptake of [3H]Ch18:2-OOH exceeded that of 125I-apolipoprotein A-I some 40-fold. This increased selective uptake of [3H]Ch18:2-OOH from very mildly oxidized HDL3 was accompanied by a parallel increase in the intracellular levels of labelled free cholesterol. In contrast, lipid hydroperoxides were not detectable within Hep G2 cells, suggesting efficient detoxification of CEOOH by these cells. Neither the increased selective uptake of Ch18:2-OOHs nor the levels of intracellular free cholesterol were influenced by the presence of 50 microM chloroquine, suggesting extralysosomal hydrolysis of oxidized CEs. These results show that the selective uptake of HDL CEOOH by Hep G2 cells is more efficient than that of unoxidized CE, and support a protective role for rapid selective uptake in the removal of circulating HDL CEOOH.

The effects of bilirubin on the thermal properties of phosphatidylcholine bilayers

The thermotropic properties of multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC), as a function of the concentration of bilirubin in the range of 0.1 to 1 mol%, were measured. The exact effects of bilirubin depended on the chain length of the polymethylene chains. But the general effects of bilirubin were the same in all systems. At the lowest concentrations tested (0.1 mol bilirubin/100 mol phospholipid (0.1 mol%)), bilirubin broadened and shifted to higher temperatures the main phase transitions of all bilayers. For DPPC and DSPC, but not DMPC, this concentration of bilirubin was associated with a new transition at 25 degrees C (DPPC) or 34 degrees C (DSPC). Bilirubin at 0.2 mol% was required for the detection of a similar transition (at 13.7 degrees C) in DMPC. Higher concentrations of bilirubin (> 0.2 mol%) suppressed completely the main phase transitions in all bilayers but increased the enthalpy of the new transition. Maximal values of delta H for these transitions were reached at 0.5, 0.25, and 0.2 mol% bilirubin in DMPC, DPPC, and DSPC, respectively. Values of delta H and delta S for these transitions were far larger than for the corresponding gel-to-liquid crystal transitions in pure lipid bilayers but were equal to those expected for a transition between crystalline and liquid crystalline phases.