The effect of bilirubin photoisomers on unbound-bilirubin concentrations estimated by the peroxidase method

Fetal and neonatal bilirubin metabolism

Human fetal and neonatal bilirubin metabolism is centered on 4Z,15Z-bilirubin IXα (BR) due to the extremely low BR conjugating capacity of the liver. BR is a unique, highly lipophilic substance with physiological and toxic effects in the cell membranes of organs and body tissues. The fetus excretes BR through the placenta to the maternal circulation. After birth, BR is thought to act as an antioxidant against the increase in reactive oxygen species caused by the rapid increase in oxygen concentration during the adaptation process from in amniotic fluid to in air. However, bilirubin encephalopathy is a toxic effect of bilirubin. Due to the lipophilic nature of BR, it must be bound to a carrier to be distributed to various parts of the body by hydrophilic blood. This carrier of BR is human serum albumin (HSA). In humans, BR can be excreted efficiently after undergoing photochemical reactions upon high affinity binding to HSA. HSA also plays an important role in the prevention of bilirubin encephalopathy. This review focuses on the developmental and physiological role of bilirubin metabolism during the fetal and neonatal periods.

Bilirubin Binding Capacity in the Preterm Neonate

Total serum/plasma bilirubin (TB), the biochemical measure currently used to evaluate and manage hyperbilirubinemia, is not a useful predictor of bilirubin-induced neurotoxicity in premature infants. Altered bilirubin-albumin binding in premature infants limits the usefulness of TB in premature infants. In this article, bilirubin-albumin binding, a modifying factor for bilirubin-induced neurotoxicity, in premature infants is reviewed.

Biology of Bilirubin Photoisomers

Phototherapy is the main treatment for neonatal hyperbilirubinemia. In acute treatment of extreme hyperbilirubinemia, intensive phototherapy may have a role in 'detoxifying' the bilirubin molecule to more polar photoisomers, which should be less prone to crossing the blood-brain barrier, providing a 'brain-sparing' effect. This article reviews the biology of bilirubin isomers. Although there is evidence supporting the lower toxicity of bilirubin photoisomers, there are studies showing the opposite. There are methodologic weaknesses in most studies and better-designed experiments are needed. In an infant acutely threatened by bilirubin-induced brain damage, intensified phototherapy should be used expediently and aggressively.

Disorders of bilirubin binding to albumin and bilirubin-induced neurologic dysfunction

Bilirubin-induced neurologic dysfunction (BIND) is a syndrome of subtle bilirubin neurotoxic disorders. The risk for developing BIND in newborns usually increases with elevated serum/plasma concentrations of unconjugated bilirubin. This risk is further increased by disorders of bilirubin binding to albumin, which includes a reduction in serum albumin concentrations or in the bilirubin-binding capacity and affinity of albumin, and the presence of displacing substances or infection. Serum unbound bilirubin (UB) concentration may be an ideal marker that reflects changes in bilirubin binding to albumin. Kernicterus, the chronic and with the most severe manifestations beyond BIND, is diagnosed by the presence of motor impairments with athetosis, abnormal magnetic resonance imaging, and/or brainstem auditory-evoked potential findings during infancy and childhood. Preterm infants sometimes have acute bilirubin encephalopathy without marked hyperbilirubinemia, such that bilirubin neurotoxicity occurs at bilirubin thresholds lower than usually associated with kernicterus. Disorders of bilirubin binding to albumin may be associated with the clinical signs of neurological injury associated with the lower bilirubin levels observed in preterm infants.

Influence of bilirubin photoisomers on unbound bilirubin measurement in clinical settings

Background

Measured unbound bilirubin concentration is influenced by bilirubin photoisomers. Bilirubin photoisomers are produced even with only a slight light exposure, and clinical samples are inevitably exposed to light. The objective of the study was to evaluate the influence of bilirubin photoisomers on the measurement of unbound bilirubin using serum of jaundiced neonates during blue light phototherapy.

Methods

Five neonates treated with phototherapy for hyperbilirubinaemia were enrolled. The samples were taken 12 h after initiation of phototherapy. Samples were processed by irradiation with blue light, by indoor ceiling light, by both blue light and indoor ceiling light or shaded. Bilirubin subfractions, total bilirubin and unbound bilirubin were measured.

Results

Compared with the non-irradiated samples, the (EZ)-cyclobilirubin concentration and (ZE)-bilirubin/(ZZ)-bilirubin ratio significantly increased in the blue light-irradiated samples, the (ZE)-bilirubin/(ZZ)-bilirubin ratio significantly increased in the indoor ceiling light-irradiated samples, and the (EZ)-cyclobilirubin, (EZ)-bilirubin and (ZE)-bilirubin/(ZZ)-bilirubin ratio significantly increased in the samples irradiated with both lights. No change was noted in unbound bilirubin in any group.

Conclusions

We consider that changes in bilirubin photoisomers induced by light exposure during clinical practice do not influence the measured unbound bilirubin concentration.

Unbound (free) bilirubin: improving the paradigm for evaluating neonatal jaundice

BACKGROUND

The serum or plasma total bilirubin concentration (B(T)) has long been the standard clinical laboratory test for evaluating neonatal jaundice, despite studies showing that B(T) correlates poorly with acute bilirubin encephalopathy (ABE) and its sequelae including death, classical kernicterus, or bilirubin-induced neurological dysfunction (BIND). The poor correlation between B(T) and ABE is commonly attributed to the confounding effects of comorbidities such as hemolytic diseases, prematurity, asphyxia, or infection. Mounting evidence suggests, however, that B(T) inherently performs poorly because it is the plasma non-protein-bound (unbound or free) bilirubin concentration (B(f)), rather than B(T), that is more closely associated with central nervous system bilirubin concentrations and therefore ABE and its sequelae.

CONTENT

This article reviews (a) the complex relationship between serum or plasma bilirubin measurements and ABE, (b) the history underlying the limited use of B(f) in the clinical setting, (c) the peroxidase method for measuring B(f) and technical and other issues involved in adapting the measurement to routine clinical use, (d) clinical experience using B(f) in the management of newborn jaundice, and (e) the value of B(f) measurements in research investigating bilirubin pathochemistry.

SUMMARY

Increasing evidence from clinical studies, clinical experience, and basic research investigating bilirubin neurotoxicity supports efforts to incorporate B(f) expeditiously into the routine evaluation of newborn jaundice.

Photoisomers: obfuscating factors in clinical peroxidase measurements of unbound bilirubin?

OBJECTIVES

The objectives of the study were to measure the effect of 4Z,15E-bilirubin on peroxidase free bilirubin measurements and to review the literature on this topic.

METHODS

4Z,15E-Bilirubin was generated in situ in serum or serum albumin solution through controlled irradiation of isomerically pure 4Z,15Z-bilirubin IXalpha, under conditions in which the total amount of bilirubin remained constant. Reactions were monitored by difference spectroscopy, to ensure that solutions were not irradiated beyond the initial photostationary state and that concentrations of other isomers were kept to a minimum. Prepared in this way, 10% to 25% of the total bilirubin in the final solutions was in the form of the 4Z,15E-isomer. Free bilirubin in the solutions was measured with a peroxidase method, before and after irradiation. The use of bovine serum albumin as a surrogate for human albumin in in vitro studies also was investigated.

RESULTS

The findings of previous studies are not altogether consistent, with a common flaw in several being the failure to measure photoisomer concentrations. For bilirubin in serum albumin solution, conversion of approximately 25% of the 4Z,15Z-isomer to 4Z,15E-bilirubin led to a much smaller decrease (<20%) in the apparent free bilirubin concentration; for bilirubin in serum, conversion of approximately 15% of the 4Z,15Z-isomer to photoisomers resulted in a much larger increase ( approximately 40%). Irradiation of bilirubin in bovine serum albumin solution generated a very different array of photoisomers than that observed in human albumin solutions.

CONCLUSIONS

The effect of photoisomers on the accuracy and specificity of free 4Z,15Z-bilirubin measurements remains uncertain. In a clinical setting, free bilirubin measurements need to be interpreted with caution when samples contain photoisomers. Irradiated bovine albumin solutions of isomerically impure bilirubin used in previous studies are poor models for investigating the effects of phototherapy in humans and the albumin binding of photoisomers.

Ontogeny of bilirubin-binding capacity and the effect of clinical status in premature infants born at less than 1300 grams

BACKGROUND

Bilirubin is toxic to the brain and enters the brain in unbound form. Serum unconjugated, unbound bilirubin may be a good predictor of bilirubin encephalopathy. Unbound bilirubin levels may depend on the bilirubin-binding capacity of albumin, which has not been described for neonates of <28 weeks' gestation.

OBJECTIVE

The purpose of this work was to determine the ontogeny of bilirubin-binding capacity and the effect of clinical status in very preterm neonates.

METHODS

A total of 152 neonates (23-31 weeks' gestational age; 440-1300 g) were enrolled prospectively. At 5 days of age, total serum bilirubin and unbound bilirubin were measured with the unbound bilirubin-A1 analyzer (Arrows Co, Osaka, Japan) and albumin with the Bromocresol-purple method. Scatchard plots were used to estimate bilirubin-binding affinity and capacity. Clinical status for each infant was rated as high, moderate, or low risk by using a modified Score for Neonatal Acute Physiology model. Low risk was considered clinically stable.

RESULTS

Unbound bilirubin has a significant, direct correlation to total bilirubin and is greater in unstable than in stable neonates. For the entire cohort, bilirubin-binding capacity had a direct relationship to gestational age. The bilirubin-binding capacities of infants in the low- and high-risk groups also had a direct relationship to gestational age. Bilirubin-binding capacity was greater in the low-risk group (20.8 +/- 4.6 mg/dL; 356 +/- 79 micromol/L) than in the moderate- (17.8 +/- 3.5 mg/dL; 304 +/- 60 micromol/L) or high- (17.3 +/- 3.4 mg/dL; 296 +/- 58 micromol/L) risk groups. Bilirubin-binding affinity did not differ by clinical risk status or gestational age.

CONCLUSIONS

In very preterm, very low birth weight infants, bilirubin-binding capacity is directly proportional to gestational age. Bilirubin-binding capacity is lower and unbound bilirubin higher in unstable than in stable neonates. These data may be useful in guiding the management of hyperbilirubinemia in very low birth weight infants.

Cytotoxicity is predicted by unbound and not total bilirubin concentration

Although it has been suggested that the unbound, free, (B(f)) rather than total (B(T)) bilirubin level correlates with cell toxicity, direct experimental evidence supporting this conclusion is limited. In addition, previous studies never included a direct measurement of B(f), using newer, accurate methods. To test "the free bilirubin hypothesis", in vitro cytotoxicity was assessed in four cell lines exposed to different B(f) concentrations obtained by varying B(T)/Albumin ratio, using serum albumins with different binding affinities, and/or displacing unconjugated bilirubin (UCB) from albumin with a sulphonamide. B(f) was assessed by the modified, minimally diluted peroxidase method. Cytotoxicity varied among cell lines but was invariably related to B(f) and not B(T). Light exposure decreased toxicity parallel to a decrease in B(f). In the absence of albumin, no cytotoxicity was found at a B(f) of 150 nM whereas in the presence of albumin a similar B(f) resulted in a 40% reduction of viability indicating the importance of total cellular uptake of UCB in eliciting toxic effect. In the presence of albumin-bound UCB, bilirubin-induced cytotoxicity in a given cell line is accurately predicted by B(f) irrespective of the source and concentration of albumin, or total bilirubin level.

Factors affecting the binding of bilirubin to serum albumins: validation and application of the peroxidase method

The unbound "free" bilirubin concentration (Bf), not the total bilirubin concentration, is the critical determinant of cellular uptake and toxicity of bilirubin. We compared Bf measured by a modified peroxidase method with published data obtained with ultrafiltration and examined conditions that affect the affinity (KF) of human (HSA) and bovine (BSA) serum albumin for bilirubin. The peroxidase and ultrafiltration methods yielded similar KF values that decreased with increasing HSA concentration and the presence of 50 mM chloride. When related to ionic strength, inhibition of BSA-bilirubin binding by chloride, bromide, and sulfate were similar, whereas phosphate buffer had a smaller effect. KF was lower at 37 degrees C than at 25 degrees C for HSA but not for BSA. KF for BSA was similar at pH 7.4 and 8.0. BSA and FCS had similar binding properties. The close agreement of Bf and KF values determined by the peroxidase method with published results obtained by ultrafiltration validates both methods and supports the use of the peroxidase method as a practical technique for measuring Bf under steady state conditions in minimally diluted serum or culture medium.

Measurement of unbound bilirubin by the peroxidase test using Zone Fluidics

BACKGROUND

Measuring plasma unbound bilirubin concentration by the peroxidase test is useful in the management of jaundiced newborns. However, the commercially available peroxidase technology is manual, and the unbound bilirubin may be seriously underestimated at the 42-fold sample dilution and single peroxidase concentration used. We investigated improving the test by adapting it to Zone Fluidics, which is a system for automating reactant handling that requires small sample volumes and dilution.

METHODS

A computer-directed Zone Fluidics system was constructed using small diameter tubing to connect in series a water-surfactant reservoir, a bi-directional pump, a multiport selection valve to which peroxidase test reactants (45 mul of sample) are attached with one port open to air, and a spectrophotometer flow cell. Test reactants and air are sequentially aspirated through the valve into the tubing connecting the pump and valve to form a reactant "zone" surrounded by air. The zone is advanced to the spectrophotometer flow cell where total and unbound bilirubin are determined (37 degrees C) from the absorbance at 460 nm at a 2-fold sample dilution and 4 peroxidase concentrations. Imprecision was assessed in artificial controls and newborn plasma. Plasma results were compared with those obtained using the commercial method.

RESULTS

The CV for unbound bilirubin in the various controls ranged from 11% to 38% (within day) and 12% to 27% (between days). Triplicate CV measurements for newborn plasma measurements ranged from 0.6% to 31% (mean 11%, n=47). Mean unbound bilirubin by Zone Fluidics was 5-fold higher than that by the commercial method.

CONCLUSION

Zone Fluidics can be used to automate the peroxidase test and overcome many of the limitations of the commercially available peroxidase technology.

Influence of glucuronosyl bilirubin and (EZ)-cyclobilirubin on determination of serum unbound bilirubin by UB-analyser

BACKGROUND

In the enzyme reaction for the determination of the unbound (free) bilirubin concentration by glucose oxidase and peroxidase, materials with low affinity for serum protein are reactive. The influence of these materials on the determination of serum unbound bilirubin was investigated. METHODS Serum samples from patients with neonatal hyperbilirubinaemia were analysed by high-performance liquid chromatography for total glucuronosyl bilirubin concentration (TGC) and (EZ)-cyclobilirubin concentration [(EZ)-C]. Based on these measurements, the samples were classified into three groups: group I [13 samples, TGC <2 micromol/L and (EZ)-C < 2.5 micromol/L]; group II [four samples, TGC < 2 micromol/L and (EZ)-C > or = 2.5 micromol/L]; and group III (five samples, TGC > or = 2 micromol/L). The concentrations of total bilirubin and unbound bilirubin were measured in these same samples with a UB-analyser. When the absorbance at 460nm was monitored, the decrease in absorbance was non-linear (concave curve). The degree of concavity was estimated (D15 value) as the deviation from linearity at 15s.

RESULTS

The D15 value was significantly higher in groups II and III than in group I. D15 value correlated significantly with TGC, (EZ)-C and unbound bilirubin concentration, and the unbound bilirubin concentration correlated significantly with TGC and (EZ)-C.

CONCLUSION

These results indicated that determination of serum unbound bilirubin concentration using the UB-analyser could be positively skewed by high concentrations of TGC and (EZ)-C.