Bilirubin transport in the newborn infant, reviewed with relation to kernicterus

Circular dichroism study of the interaction between mutagens and bilirubin bound to different binding sites of serum albumins

Although recent investigations have shown that bilirubin not only has a negative role in the organism but also exhibits significant antimutagenic properties, the mechanisms of interactions between bilirubin and mutagens are not clear. In this study, interaction between bilirubin bound to different binding sites of mammalian serum albumins with structural analogues of the mutagens 2-aminofluorene, 2,7-diaminofluorene and mutagen 2,4,7-trinitrofluorenone were investigated by circular dichroism and absorption spectroscopy. Homological human and bovine serum albumins were used as chiral matrices, which preferentially bind different conformers of bilirubin in the primary binding sites and make it observable by circular dichroism. These molecular systems approximated a real system for the study of mutagens in blood serum. Differences between the interaction of bilirubin bound to primary and to secondary binding sites of serum albumins with mutagens were shown. For bilirubin bound to secondary binding sites with low affinity, partial displacement and the formation of self-associates were observed in all studied mutagens. The associates of bilirubin bound to primary binding sites of serum albumins are formed with 2-aminofluorene and 2,4,7-trinitrofluorenone. It was proposed that 2,7-diaminofluorene does not interact with bilirubin bound to primary sites of human and bovine serum albumins due to the spatial hindrance of the albumins binding domains. The spatial arrangement of the bilirubin bound to serum albumin along with the studied mutagens was modelled using ligand docking, which revealed a possibility of an arrangement of the both bilirubin and 2-aminofluorene and 2,4,7-trinitrofluorenone in the primary binding site of human serum albumin.

The location of the high- and low-affinity bilirubin-binding sites on serum albumin: ligand-competition analysis investigated by circular dichroism

The locations of three bilirubin (BR)-binding sites with different affinities were identified as subdomains IB, IIA and IIIA for five mammalian serum albumins (SAs): human (HSA), bovine (BSA), rat, (RSA), rabbit (RbSA) and sheep (SSA). The stereoselectivity of a high-affinity BR-binding site was identified in the BR/SA=1/1 system by circular dichroism (CD) spectroscopy, the sites with low affinity to BR were analyzed using difference CD. Site-specific ligand-competition experiments with ibuprofen (marker for subdomain IIIA) and hemin (marker for subdomain IB) did not reveal any changes for the BR/SA=1/1 system and showed a decrease of the bound BR at BR/SA=3/1. Both sites were identified as sites with low affinity to BR. The correlation between stereoselectivity and the arrangement of Arg-Lys residues indicated similarity between the BR-binding sites in subdomain IIIA for all of the SAs studied. Subdomain IB in HSA, BSA, SSA and RbSA has P-stereoselectivity while in RSA it has M-selectivity toward BR. A ligand-competition experiment with gossypol shows a decrease of the CD signal of bound BR for the BR/SA=1/1 system as well as for BR/SA=3/1. Subdomain IIA was assigned as a high-affinity BR-binding site. The P-stereoselectivity of this site in HSA (and RSA, RbSA) was caused by the right-hand localization of charged residues R257/R218-R222, whereas the left-hand orientation of R257/R218-R199 led to the M-stereoselectivity of the primary binding site in BSA (and SSA).

Chiroptical properties of bilirubin-serum albumin binding sites

Although the interactions between bilirubin and serum albumin are among the most studied serum albumin-ligand interactions, the binding-site location and the participation of bilirubin-serum albumin complexes in pathological and physiological processes are under debate. In this article, we have benefited from the chiral structure of bilirubin and used CD spectroscopy to characterize the structure of bilirubin bound to human and bovine serum albumins. We determined that in a phosphate buffer at pH 7.8 there are three binding sites in both human and bovine serum albumins. While the primary binding sites in human and bovine serum albumins bind bilirubin with P- and M-helical conformations, respectively, the secondary binding sites in both albumins bind bilirubin in the P-helical conformation. We have shown that the bonding of bilirubin to the serum albumin matrix is a more favorable process than the self-association of bilirubin under the studied conditions, with a maximum of three bound bilirubins per serum albumin molecule. Although bilirubin bound to the primary binding site has attracted the most attention, the presented results have documented the impact of the secondary binding sites which are relevant in the displacement reactions between BR and drugs and in the phenomena where bilirubin plays antioxidant, antimutagenic, and anti-inflammatory roles.

Role of biokinetics in risk assessment of drugs and chemicals in children

Whether children incur different risks from xenobiotics than adults will depend on the exposure, biokinetics, and dynamics of compound. In this paper, current knowledge on developmental physiology and possible effects on biokinetics are evaluated and the role of biokinetics in risk assessment both for drugs and chemicals is discussed. It is concluded that most dramatic age-related physiological changes that may affect biokinetics occur in the first 6-12 months of age. The difference in internal exposure between children and adults can generally be predicted from already known developmental physiological differences. However, for risk assessment it will also be necessary to determine whether internal exposure is within the drug's therapeutic window or if it will exceed the NOAEL of a chemical. Furthermore, the effects of internal exposure of potentially harmful compounds on developing organ systems is of utmost importance. However, knowledge on this aspect is very limited. Risk assessment in children could be improved by: (1) application of pediatric PBPK-models in order to gain insight into internal exposure in children, (2) studies in juvenile animals for studying effects on developing systems, and (3) extrapolation of knowledge on the relationship between internal exposure and dynamics for drugs to other chemicals.

Changes in globus pallidus with (pre)term kernicterus

OBJECTIVE

We report serial magnetic resonance (MR) and sonographic behavior of globus pallidus in 5 preterm and 3 term infants with kernicterus and describe the clinical context in very low birth weight preterm infants. On the basis of this information, we suggest means of diagnosis and prevention.

METHODS

Charts and MR and ultrasound images of 5 preterm infants and 3 term infants with suspected bilirubin-associated brain damage were reviewed. Included were preterm infants with severe hearing loss, quadriplegic hypertonia, and abnormal hypersignal of globus pallidus on T2-weighted MR imaging (MRI). In 1 infant who died on day 150, the diagnosis was confirmed during the neonatal period. The others were picked up as outpatients and scanned at 12 or 22 months' corrected age. Three instances of term kernicterus were included for comparison of serial MRI in the neonatal period and early infancy: they were caused by glucose-6-phosphate dehydrogenase deficiency, urosepsis, and dehydration plus fructose 1-6 biphosphatase deficiency.

RESULTS

Five preterm infants of 25 to 29 weeks' gestational age presented with total serum bilirubin (TSB) levels below exchange transfusion thresholds commonly advised. Mixed acidosis was present in 3 infants around the TSB peak. The bilirubin/albumin molar ratio was >0.5 in all, in the absence of displacing drugs. All failed to pass bedside hearing screen tests and had severe hearing loss on auditory brain response testing. Symmetrical homogeneous hyperechogenicity of globus pallidus was the alerting feature in 1 infant. Globus pallidus was hyperintense on T1-weighted MR images in this child. The other infants presented with severe developmental delay as a result of dyskinetic quadriplegia and hearing loss. Globus pallidus was normal on T1- but hyperintense on T2-weighted MR images at 12 or 22 months' corrected age. Subthalamic involvement was documented in coronal fluid attenuated inversion recovery MRI in 2 infants. The term infants with classical clinical presentation in the neonatal period had MR behavior similar to the preterms, but pallidal injury was not recognized with targeted sonographic examination. Their neonatal MR images demonstrated pallidal T1 hyperintensity and mild T2 hyperintensity.

CONCLUSION

Acidotic very low birth weight preterm infants with low serum albumin levels develop MR-confirmed pallidal injury and hearing loss facing "accepted" TSB levels. Serial MRI documents a shift from acute mainly T1 hypersignal to permanent T2 hypersignal in globus pallidus within the late neonatal period. Subthalamic and not thalamic involvement helps to differentiate from ischemic or metabolic disorder. As newborns, these infants are rigid and have severe apnea, before developing hypertonic quadriplegia in infancy.

Mechanisms of bilirubin toxicity: clinical implications

The basic mechanism of kernicterus and bilirubin encephalopathy has not been unequivocally determined. Much knowledge has been gained about phenomena that contribute to bilirubin neurotoxicity, and this knowledge has implications for clinical practice. Conditions that impact on blood-brain barrier function, increase brain blood flow, or impact on bilirubin metabolism, including its transport in serum, should be avoided, if possible. Such conditions include drugs and drug stabilizers that compete with bilirubin binding to albumin, or that inhibit P-glycoprotein in the blood-brain barrier, prematurity/immaturity, and clinically significant illness in the infant that involves hemolysis, respiratory and metabolic acidosis, infection, asphyxia, hypoxia and (perhaps) hyperoxia, and hyperosmolality. If these conditions are not avoidable then there should be a more aggressive approach to the treatment of hyperbilirubinemia. The limits of tolerance for hyperbilirubinemia varies among neonates and there are no tools to determine with certainty when a particular infant is approaching the danger zone. Neurological symptoms in a jaundiced infant require extreme vigilance, and, in most cases, immediate intervention.

Magnetic resonance imaging findings in patients with severe neonatal indirect hyperbilirubinemia

The aim of this study was to document the magnetic resonance imaging (MRI) findings of cases with a history of severe neonatal indirect hyperbilirubinemia. Ten cases (eight cases with neurologic findings, two normal cases) with a history of severe neonatal indirect hyperbilirubinemia were studied. Neurologic findings and MRI results were described and correlated. Seven of eight cases with neurologic findings demonstrated symmetric and uniform increased T2 signal changes limited to globus pallidi. MRI scans of two cases without neurologic findings showed no abnormality. Severe neonatal indirect hyperbilirubinemia should be considered in the differential diagnosis of bilateral symmetric hyperintense signal changes in the globus pallidus on MRI. However, high levels of unconjugated bilirubin concentrations in the neonatal period may not always cause such lesions of globus pallidus on MRI despite the presence of neurologic findings.

Role of apolipoprotein D in the transport of bilirubin in plasma

Apolipoprotein D (apo D) is a 30-kDa glycoprotein of unknown function that is associated with high-density lipoproteins (HDL). Because unconjugated bilirubin has been shown to bind apo D with a 0. 8:1 stoichiometry, we examined the contribution of this protein to transport of bilirubin in human plasma. Density gradient centrifugation analysis using physiological concentrations of [(14)C]bilirubin reveals that 9% of unconjugated bilirubin is associated with HDL, with the remaining pigment bound primarily to serum proteins (i.e., albumin). The percentage of total plasma bilirubin bound to HDL was found to increase proportionally with bilirubin concentration. Affinity of human apo D for bilirubin was determined by steady-state fluorescence quenching, with Scatchard analysis demonstrating a single binding site for unconjugated bilirubin with an affinity constant (K(a)) of approximately 3 x 10(7) M(-1). Incorporation of apo D into phosphatidylcholine vesicles had no effect on K(a), suggesting that a lipid environment does not alter the affinity of the protein for bilirubin. Using stopped-flow techniques, the first-order rate constant for bilirubin dissociation from apo D was measured at 5.4 s(-1) (half-time = 129 ms). Our findings indicate that HDL is the principal nonalbumin carrier of bilirubin in human plasma and further support the proposition that the affinity of HDL for bilirubin is primarily the result of binding to apo D.

Significance of serum delta-bilirubin in patients with obstructive jaundice

BACKGROUND

Delta-bilirubin is a bilirubin covalently bound with albumin, which is nontoxic and excreted neither in urine nor in bile. We previously reported that the percentage of delta-bilirubin increased after biliary drainage and that the rapidly excretable bilirubin fraction (total minus delta-bilirubin) was a better parameter to predict the effectiveness of biliary decompression in the dog model. The aim of the present study was to elucidate whether it is applicable to humans.

MATERIALS AND METHODS

The serum bilirubin concentration was measured and its fractions were analyzed by high-performance liquid chromatography in 22 patients with obstructive jaundice before and after biliary drainage. In addition, the patients were subgrouped into good and poor drainage groups according to the decline index of serum bilirubin to examine the significance of delta-bilirubin.

RESULTS

The concentration of total bilirubin decreased from 14.1 mg/dl before biliary drainage to 5.4 mg/dl 28 days after drainage. During this period, the percentage of conjugated bilirubin steeply declined from 47.1 to 8.8% and that of excretable bilirubin from 63.4 to 28.6%. In contrast, the proportion of serum delta-bilirubin increased from 36.6 to 71.4%. There was an inverse correlation between percentage of delta-bilirubin and total bilirubin concentration (r = -0.69, P < 0.01). In the good drainage group, the percentage of delta-bilirubin increased above 60% within 7 days after biliary drainage, but it did not reach 60% by 28 days in the poor drainage group. A decreasing rate of total bilirubin minus delta-bilirubin, the excretable bilirubin fraction, was a better index than that of total bilirubin to assess the efficacy of biliary drainage (P< 0.01).

CONCLUSIONS

The increase in the percentage of serum delta-bilirubin indicates an effectiveness of biliary drainage in man. An analysis of serum delta-bilirubin for 7 days can distinguish the good drainage patients from the poor drainage patients.

Modification of the MTT method for the study of bilirubin cytotoxicity

BACKGROUND

We propose a modification of the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] method to study the cytotoxicity of bilirubin. The original method involves reading the intensity of a purplish blue color resulting from the conversion of MTT to formazan crystals by the mitochondria of viable cells. We have found that when the method is applied to study the effect of bilirubin on growing cells, precipitation of the yellow bilirubin pigment interferes with the colorimetric reading.

METHODS

A human liver cell line was used. The interference of bilirubin deposition on the MTT assay was investigated by comparing the value of optical density of the MTT solution in the presence and absence of bilirubin. The effect of 0.04 mol/L HCL-isopropanol on the bilirubin precipitate was tested by recovering the amount of bilirubin from the wells after the isopropanol treatment.

RESULTS

Bilirubin deposition increases MTT reading by 10-24%. Hydrochloride-isopropanol (0.04 mol/L) dissolves MTT formazan only without disturbing the bilirubin precipitates. The bilirubin extracted into the supernatant was less than 5% of the total bilirubin deposited.

DISCUSSION

This indirect MTT assay, as developed in this study, could eliminate the interference of bilirubin deposits and serve as a good method for the study of bilirubin cytotoxicity.

[Metabolism of albumin]

Albumin is quantitatively the most important plasma protein. Albumin is a major factor for the maintenance of intravascular oncotic pressure; it also plays a significant role in the transport of exogenous or endogenous substances. Serum albumin is synthesized at a high rate in the liver. This synthetic process is modified by many interrelated factors. Oncotic pressure, malnutrition, toxins and trauma may reduce albumin production or secretion. Hormones regulate synthesis and degradation. Albumin is widely distributed, however the concentration in the extravascular space is low, compared to that in the plasma or lymph. While considerable data are available concerning albumin synthesis, little is known about degradation and further studies are required.

[Role of substitution albumin therapy in drugs, hormones, electrolytes and miscellanous substances transport]

Human serum albumin (HSA) is quantitatively the most important non specific transport protein. HSA binds a wide variety of both endogenous and exogenous ligands. Hypoalbuminaemia may lead to a decreased plasma binding capability of some compounds. Biological/pharmacological consequences depend on the ligand and the target tissue. Many experimental studies suggest that hypoalbuminaemia may influence the metabolism and toxicity of endogenous ligands (bilirubin, metallic ions, oxygen radicals) and the pharmacological effect of some drugs (among others: furosemide, phenytoin, warfarin). The relevance of such information for human surgical situations remains unclear. Clinical studies are scarce and inconclusive. There is a lack of pertinent data supporting the necessity of HSA infusions in order to maintain a minimal plasma concentration and a convenient plasma transport. However, experimental data indicate that major hypoalbuminaemia should be considered with caution.

Postnatal maturation of the blood-brain barrier for unbound bilirubin in newborn piglets

The postnatal maturation of the blood-brain barrier (BBB) for unbound bilirubin was studied in 2-day- and 2-week-old piglets. Hyperbilirubinemia was induced by bolus infusion of bilirubin at 25 mg/kg followed by continuous infusion of 20 mg/kg/h for 3 h. During the study period, arterial blood pH and blood gas tensions, serum osmolarity, and mean arterial blood pressures were within the physiologic range. Brain bilirubin content and the brain/blood distribution ratio for bilirubin were higher in the 2-day-old than in the 2-week-old piglets. In both age groups, regional brain bilirubin concentration and brain/blood ratios were higher in subcortical regions (cerebellum and brainstem) than in the cerebral cortex. We conclude that in newborn piglets the blood-brain barrier for unbound bilirubin matures with increasing postnatal age and that irrespective of maturity the relative permeability of the BBB for bilirubin appears higher in subcortical than in cortical regions.

Effects of bilirubin on visual evoked potentials in term infants

To determine bilirubin-induced neurotoxicity, serial visual evoked potentials (VEPs) of 72 infants with neonatal hyperbilirubinaemia and 22 controls were evaluated and compared in four sessions for 8 weeks after birth. The levels of maximal serum bilirubin were found positively related to the wave latencies of first VEP. Within 8 weeks after birth, the wave latencies were significantly prolonged in infants in the severe and moderate groups than in the controls. The amplitudes of VEPs were apparently lower in severe and moderate groups than in the control group only in the 1st week after birth. At 1 year, 4 of the 18 infants in the severe group had poor motor skills and one had general hypotonia.

CONCLUSION

These results suggest that bilirubin may affect the visual pathways, and that VEP is a useful adjunct to the neurological assessment of neonatal hyperbilirubinaemia.

Dynamic aspects of glutathione metabolism in obstructive jaundice

The dynamic aspects of glutathione metabolism during obstructive jaundice were analyzed in rats. Plasma bilirubin levels increased after ligation of the bile duct, with a concomitant increase in hepatorenal glutathione levels. When the bile duct was recanalized, plasma bilirubin levels rapidly decreased, with a concomitant decrease in hepatorenal glutathione levels. The half-life of hepatic glutathione turnover increased markedly after bile duct obstruction, returning to normal after recanalization of the bile duct. Intravenous administration of a loading dose of bilirubin inhibited the biliary secretion of glutathione in a dose-dependent manner. On the other hand, renal glutathione efflux increased markedly after bile duct obstruction. These observations suggest that glutathione status is significantly affected in obstructive jaundice, predominantly due to the inhibition of hepatic secretion by increased bilirubin.

Peritoneal dialysis for the removal of protein-bound markers of hepatic insufficiency

The addition of human serum albumin (HSA) to peritoneal dialysate increases the clearance of bilirubin in rats suffering from obstructive jaundice. The acceptor properties of the fluid can be enhanced by using HSA that does not contain standard stabilizing additives and has been purified by further adsorption on activated carbon. Bilirubin-containing dialysate fluid, as well as the ascitic fluid of cirrhotic patients, can be regenerated by a combination of membrane ultrafiltration and carbon adsorption. These observations suggest a potentially useful scheme for continuous, regenerative peritoneal dialysis in the treatment of hepatic insufficiency.

Na+,K(+)-ATPase and acetylcholinesterase activities: changes in postnatally developing rat brain induced by bilirubin

Na+,K(+)-ATPase, Mg(++)-ATPase, and acetylcholinesterase activities were determined in brain homogenates of rats in different ages, which were decapitated 30 min after administration of various bilirubin doses. Bilirubin serum and brain tissue levels should be dependent upon the dose administered. At these concentrations, a progressive enzyme inactivation was observed, which reached 25-30% for acetylcholinesterase and 70-80% for Na+,K(+)-ATPase in neonate rats and 15-20% for acetylcholinesterase and only 30-40% for Na+,K(+)-ATPase in the brain of aged rats (20 mo). However, Mg(++)-ATPase activity was not affected by bilirubin deposition in the developing brain. Moreover, brain albumin content increased 53% in suckling, 40% in adult, and 33% in aged rats at high drug administration. These results may indicate an opening of the blood-brain barrier and a bilirubin entry into the rat brain. The bilirubin immediate toxic effects on brain acetylcholinesterase and Na+,K(+)-ATPase, and probably on brain electrical activity, may be modulated by the developmental state of membrane-bound enzymes.

Assessment of neonatal jaundice in low birth weight infants comparing transcutaneous, capillary and arterial bilirubin levels

Total serum bilirubin level was assessed in a group of jaundiced low birth weight infants using three different methods. Transcutaneous bilirubinometry was compared with conventional capillary and arterial methods to investigate the over-or underestimation of neonatal jaundice. Sampling site did not influence bilirubin levels. Capillary and arterial results showed a linear correlation (r = 0.9) suggesting no influence of environmental light on peripheral bilirubin isomerization. Similar results were obtained comparing both serum levels with transcutaneous values (r = 0.7). We conclude that treatment decisions may be made on the basis of one of the three mentioned methods in healthy low birth weight infants with neonatal jaundice.

Antimicrobial therapy for pediatric patients

New antimicrobial agents are being introduced for clinical use at an increasingly rapid rate. This has required physicians continually to review relevant data and determine unique properties that might guide selection among any new antibiotics as well as older ones. Efficacy, potential toxicity, and comparative cost (in that order) generally guide selection. The present comprehensive review examines currently available antibiotics along with some under investigation, emphasizing these three basic areas of consideration.

Prediction of the development of neonatal jaundice by increased umbilical cord blood bilirubin

Umbilical cord serum bilirubin concentration as a predictor of subsequent jaundice was studied in 291 newborns. It was possible to define subgroups of infants with significantly higher or lower risks of developing jaundice. If cord bilirubin was below 20 mumol/l, 2.9% became jaundiced as opposed to 85% if cord bilirubin was above 40 mumol/l. Furthermore, 57% of jaundiced infants with cord bilirubin above 40 mumol/l required phototherapy, but only 9% if cord bilirubin was 40 mumol/l or lower (p less than 0.003). Knowledge of infants at risk of developing jaundice allows simple bilirubin reducing methods to be implemented before jaundice is present and could influence a decision regarding early discharge from hospital. Since the ability of plasma to bind bilirubin in cord blood from jaundiced and non-jaundiced infants showed no significant differences, the increased cord bilirubin among infants who later became jaundiced is presumably caused by increased fetal bilirubin production or decreased removal of bilirubin from the fetal circulation.

Serum albumin

The liver manufactures albumin at a massive rate and decreases production in times of environmental, nutritional, toxic and trauma stress. Osmotic pressure is a basic evolutionary regulatory factor, and hormonal control over albumin production has been demonstrated. Where and why new or old albumin is degraded are questions which have not been clarified, although the vascular endothelium may well be the degradative site. Albumin is important as a transport protein, as a measure of evolution and as a model to study secretion following synthesis without the intervening steps of glycosylation. Investigations as to how this protein enters the endoplasmic membrane may well answer some of the questions concerning signal peptide insertion (288). The role of the urea cycle intermediate ornithine and its participation in polyamine synthesis, which has a positive effect on albumin synthesis, is under study. Likewise, the inverse relation between acute-phase protein synthesis and albumin synthesis regulated by interleukin 1 and other cytokines will merit further study. These are a few of the concepts which will be tested in the future.

Relationship of serum bilirubin levels and hearing impairment in newborn infants

A comparison has been made between 39 infants with a birthweight of 1500 g or less and a bilirubinlevel of 240 mumol/l or above, born between January 1980 and December 1983 and 19 infants with the same criteria, born between January 1984 and December 1985. Eight of the 22 high risk and two of the 17 low risk infants were diagnosed to have sensorineural deafness (SND) during the first period and this was strongly associated with the duration of the hyperbilirubinaemia. During the second period, more active intervention for hyperbilirubinaemia led to an increased number of exchange transfusions and a marked drop in the mean duration of hyperbilirubinaemia (less than 240 mumol/l). None of the very low birth weight (VLBW) infants born in the second period have developed SND. To investigate the independent effect of hyperbilirubinaemia on hearing, six low risk infants with bilirubin levels less than 320 mumol/l were studied by serial auditory brainstem responses (ABR). Impairment of the ABRs was found in four infants, with further deterioration with the persistence of high bilirubin levels in two. Although recovery of hearing thresholds was noted in all infants with impaired ABRs, an absence of wave I was noted in three infants at 6 months of age, which could indicate damage to the auditory nerve-cochlear complex. These findings suggest that hyperbilirubinaemia in itself can have an adverse effect on hearing and that careful management of hyperbilirubinaemia may reduce the incidence of sensorineural deafness.

Effect of bilirubin on the membrane potential of rat brain synaptosomes

The effect of the neurotoxic pigment bilirubin on the membrane potential of rat brain synaptosomes was studied by using the tetraphenylphosphonium ion (TTP+) technique. Bilirubin induces a rapid depolarization of synaptosomes, as reflected by an efflux of previously accumulated [3H]TTP+. This phenomenon persisted when the membrane potential across either the plasma membrane of the synaptosome or the inner membrane of the entrapped mitochondria was selectively depressed, thus indicating that both components of the synaptosomal membrane potential were affected by bilirubin. Bovine serum albumin, used at a albumin/bilirubin molar ratio of 1:1, had the capacity to completely prevent and reverse the effect of bilirubin. This fact demonstrates that the bilirubin-induced TPP+ release from synaptosomes is a reversible process that requires the presence of bilirubin interacting with the synaptosomal membranes. These results, together with the inhibition by bilirubin of [3H]TPP+ and [2-14C]acetate uptake by synaptosomal plasma membrane vesicles isolated from rat brain, suggest that bilirubin depresses the membrane potential across the synaptosomal plasma membrane by a mechanism involving alterations in ion permeability. This effect could be of relevance in the pathogenesis of bilirubin encephalopathy.

An experimental model of postnatal jaundice in the suckling rat. Suppression of induced hyperbilirubinemia by Sn-protoporphyrin

A model of experimental postnatal hyperbilirubinemia in the rat has been developed utilizing the heme precursor delta-aminolevulinic acid (ALA) to produce jaundice during a selective time period after birth. This time period is defined as that between 7 d postnatally, when the initial postpartum alterations of serum bilirubin and heme metabolism in the neonate have subsided, and 21 d, when the hepatic conjugation mechanism for the bile pigment appears fully developed. Administration of ALA in this time period led to a rapid, consistent, and significant dose-dependent increase in serum bilirubin levels in the newborn animals. Heme administration produced a qualitatively similar but enhanced effect. Both compounds, in addition, induced a dose-dependent increase in hepatic heme oxygenase activity concomitant with the increase in serum bilirubin levels. Neither compound increased serum bilirubin levels significantly when administered at or after 21 d postnatally. Administration of the synthetic metalloporphyrin, Sn-protoporphyrin, to ALA-treated neonates resulted in a dose-dependent decrease in serum bilirubin levels and hepatic heme oxygenase activity. Mn- and Zn-protoporphyrin in comparable doses did not significantly inhibit ALA-induced hyperbilirubinemia. Sn-protoporphyrin also inhibited the hyperbilirubinemia produced by heme in the suckling animals. ALA administration to newborn rats during the specific postnatal period described provides a simple and convenient model of experimental jaundice in the developing neonate which permits an examination of the potential ability of synthetic metalloporphyrins or other compounds to suppress induced hyperbilirubinemia in the newborn animal. The ability to induce a consistent and significant degree of jaundice in the postnatal rat by the method described may also be useful for other types of studies concerned with the biological disposition and effects of endogenously formed bilirubin in the neonate. The results of this study confirm in another model system the potent ability of Sn-protoporphyrin to suppress jaundice in the neonate, and suggest that suppression of heme oxidation by synthetic heme analogues may represent a useful therapeutic approach to the problem of severe hyperbilirubinemia in human premature newborn.

Reserve albumin and bilirubin toxicity index in infant serum

Reserve albumin concentration (the concentration of albumin available for binding of unconjugated bilirubin) was determined in 95 sera from 76 subjects by dialysis with 14C-monoacetyl diamino diphenyl sulfone (MADDS). An index, I of bilirubin toxicity in the plasma was calculated for each subject, based on the bilirubin and reserve albumin concentrations, the affinity of bilirubin for serum albumin, and the pH-dependent solubility of bilirubin in the plasma. The values of reserve albumin and of I varied significantly with gestational age, clinical condition (whether sick or well), and serum bilirubin level. The value of reserve albumin was decreased and I was increased in association with clinical factors (e.g., hyperbilirubinemia, hypoxia, acidosis, or sepsis) recognized as increasing the risk for bilirubin encephalopathy. The lowest values of reserve albumin and the highest values of I were found in the least mature and sickest infants.

Interaction of bilirubin with human erythrocyte membranes

The kinetics of [3H]bilirubin binding to human erythrocyte ghost membranes was investigated. The binding occurred rapidly and was saturable with respect to [3H]bilirubin and membrane concentration. The apparent dissociation constant (Kd) and maximum binding (Bmax.) for bilirubin of the membranes were 2.3 microM and 0.93 nmol/mg of protein respectively. Low-affinity binding, non-saturable at 400 microM, was observed. Thermal dependency of the saturable binding showed a U-shaped curve with the lowest value around 37 degrees C. Affinity labelling of the membrane proteins using [3H]bilirubin-Woodward's reagent K complex did not define individual proteins. The Kd (12 microM) and Bmax. (4.4 nmol/mg of protein) for bilirubin of the tryptic membranes increased 5.0 and 5.2 times the respective control values (2.4 microM and 0.85 nmol/mg of protein). Heat-treatment of the membranes for 3 min at 100 degrees C increased the saturable binding as much as by 222%. These results indicate that there exist saturable bilirubin-binding sites on the erythrocyte membranes and also suggest that they are not composed of proteins.

The genesis and ultrastructural appearance of spontaneous renal lesions in the homozygous Gunn rat

Deposition of bilirubin at the tip of the renal papilla has been studied in the infant homozygous Gunn rat. The deposits form during the 4th wk of life in ground substance along basement membranes of capillaries and tubules and along the borders of interstitial cells. Deposits also form in capillary lumens. The early ultrastructural appearances indicate that unconjugated bilirubin is not directly cytotoxic. This conclusion has important implications for the pathogenesis of enhanced nephrotoxicity in the homozygous Gunn rat and ultimately for the pathogenesis of analgesic nephropathy.

Risk of bilirubin acid precipitation in preterm infants with respiratory distress syndrome: considerations of blood/brain bilirubin transfer equilibrium

Twenty-six preterm infants with respiratory distress syndrome (RDS), were examined daily during the first 6 days of life. Twenty-six equally preterm but clinically well infants served as controls. In the RDS infants, plasma albumin concentration was low, hyperbilirubinemia was prolonged, plasma pH was decreased during the first two days, and the concentration of reserve albumin for binding of monoacetyldiaminodiphenylsulfone (MADDS), a deputy ligand for bilirubin, was decreased on the second throughout the sixth day, when compared with the controls. These factors concur in increasing the likelihood of bilirubin acid precipitation in RDS above the increased risk present in preterm infants. The plasma of the preterm controls was supersaturated with respect to crystalline bilirubin acid by an average factor 5 (index of plasma bilirubin toxicity = 0.7) on the first day of life, peaking at a factor 10 (index 1.0) on the third and fourth days while these factors were 10 and 20 (index 1.0 and 1.3), respectively, in the RDS infants. Two of the latter surpassed a level of 60 times supersaturation (index 1.8) where acute precipitation of amorphous bilirubin acid becomes possible.

The effect of two intravenous fat emulsions and their components on bilirubin binding to albumin

The effect of two intravenous lipid emulsions on the binding of bilirubin to albumin was investigated in vitro. Various concentrations of a soybean (Intralipid) or a safflower (Liposyn) oil emulsion in 0.48 mM albumin were assayed for changed in bilirubin displacement using Sephadex G-25 gel filtration and for alternations of albumin reserve binding capacity by a novel difference spectroscopy technique. Two major components of the emulsions, glycerol and egg phosphatides, were also assayed by difference spectroscopy. A significant enhancement of the total reserve bilirubin binding capacity of albumin was noted with concentrations of 200 to 500 mg/100 ml lipid emulsion using difference spectroscopy. Using salicylate to block secondary albumin binding sites for bilirubin, reserve bilirubin binding capacity at nonsalicylate competitive sites was shown to have increased (maximum 61%) over the range of 50 to 1000 mg/100 ml lipid emulsion. Only changes in nonsalicylate competitive binding sites could be detected at concentrations normally achieved in vivo. Egg phosphatides had little effect and glycerol decreased reserve bilirubin binding capacity. Sephadex studies were unable to detect any significant change in bilirubin retained on the column with 50 or 500 mg/100 ml lipid emulsion at any bilirubin/albumin ratio. The enhancement of reserve bilirubin binding capacity caused by lipid emulsions is mediated through changes in nonsalicylate competitive albumin binding sites. Product differences demonstrated in vitro point out the complexity of their effects on bilirubin-albumin binding and the necessity for in vivo investigations with structurally different nutritional products designed for use in newborns.

A modified kit method and a miniphotometer used for the rapid determination of total bilirubin in neonatal sera

The results of the determination of total bilirubin in neonatal sera using the Compur kit and miniphotometer are reported. The kit method was modified somewhat by halving the volume of serum. This smaller volume (10 microliters) minimizes the interference of haemoglobin and turbidity. In a study of 68 neonatal sera, the method correlated excellently with the assay according to Hertz et al. ((1974) Scand. J. Clin. Lab. Invest. 33,215-230) and precision was better than 3.5%. The method described is rapid and reliable and ideally-suited for emergency requests for monitoring neonatal hyperbilirubinaemia.

Studies of the conformation of bilirubin and its dimethyl ester in dimethyl sulphoxide solutions by nuclear magnetic resonance

The conformation of bilirubin and its dimethyl ester in dimethyl sulphoxide (DMSO) was investigated by n.m.r. spectroscopy. The chemical shifts of the pyrrole NH and Lactam protons of bilirubin and its dimethyl ester in DMSO indicate a strong interaction with the solvent. Inter-proton distances were calculated from nuclear Overhauser effects (NOE), selective and non-selective relaxation times (T1) and rotational correlation times taken from 13C relaxation times. The interproton distances indicate that the conformation of the skeleton of bilirubin and its dimethyl ester in DMSO is similar to that of bilirubin and mesobilirubin in the crystalline state and in chloroform solutions, except for a possible slight twist of the pyrrolenone rings about the methine bonds, which may be a consequence of solvation of the NH groups by DMSO. Unlike in chloroform solutions, no direct hydrogen-bonding occurs between the carboxylic acid and the lactam groups of bilirubin in DMSO, as shown by the absence of an NOE between these groups. The fast exchange of the pyrrole NH protons with 2H shows that no hydrogen-bonding occurs between these protons and the propionic residues, in line with their solvation by DMSO. From the above results, and from the slowness of the internal motion of the propionic residues of bilirubin and its dimethyl ester, it is concluded that these residues are tied to the skeleton via bound solvent molecules.

Effect of exchange transfusion on serum reserve albumin for binding of bilirubin and index of serum bilirubin toxicity

Seventeen newborn infants, who received their first exchange transfusion due to hyperbilirubinaemia and/or rhesus haemolytic disease, were studied. The exchange transfusions were performed with fresh, citrated blood. During the exchange transfusion a marked increase in the serum reserve albumin concentration for binding of bilirubin measured by the [14C]-MADDS method was observed, followed by a smaller decrease after the transfusion. Plasma pH increased both during and after the exchange transfusion. During the exchange transfusion a drastic fall in index of serum bilirubin toxicity was observed, followed by a smaller increase after the transfusion. Citrate was not found to interfere in the binding of bilirubin to albumin. The results are in agreement with the clinical finding that an exchange transfusion performed with fresh, citrated blood effectively reduced the risk of bilirubin encephalopathy. The ratio in serum of binding albumin, i.e. bilirubin plus reserve albumin, to total albumin failed to be increased by the exchange transfusion, and a decrease occurred after the transfusion. These findings indicate the presence in infant serum of non-binding albumin. Donor albumin with intact binding potential is partly transformed into the non-binding variety in the course of one hour after the transfusion. In the most severely rhesus sensitized infant a drastic decline of the serum albumin binding capacity was seen during the first day of life.