Bilirubin oxidation in brain

Models of bilirubin neurological damage: lessons learned and new challenges

OBJECTIVE

Jaundice (icterus) is the visible manifestation of the accumulation of bilirubin in the tissue and is indicative of potential toxicity to the brain. Since its very first description more than 2000 years ago, many efforts have been undertaken to understand the molecular determinants of bilirubin toxicity to neuronal cells to reduce the risk of neurological sequelae through the use of available chemicals and in vitro, ex vivo, in vivo, and clinical models. Although several studies have been performed, important questions remain unanswered, such as the reasons for regional sensitivity and the interplay with brain development. The number of new molecular effects identified has increased further, which has added even more complexity to the understanding of the condition. As new research challenges emerged, so does the need to establish solid models of prematurity.

METHODS

This review critically summarizes the key mechanisms of severe neonatal hyperbilirubinemia and the use of the available models and technologies for translational research.

IMPACT

We critically review the conceptual dogmas and models used for studying bilirubin-induced neurotoxicity. We point out the pitfalls and translational gaps, and suggest new clinical research challenges. We hope to inform researchers on the pro and cons of the models used, and to help direct their experimental focus in a most translational research.

The Direct Spectrophotometric Method Is Reliable for Initial Assessment of Total Bilirubin in Neonatal Venous Plasma

OBJECTIVE

To determine the consistency of bilirubin values between the direct spectrophotometric (DS) and colorimetric diazo (diazo) methods in neonatal venous plasma specimens.

METHODS

We measured the total bilirubin via the DS and diazo methods in 255 neonatal venous plasma specimens and compared the overall and subgroup results.

RESULTS

Slight underestimation of total bilirubin values in most specimens using the DS method was observed, with higher mean biases found in higher concentrations. Significantly high positive correlations were found in all groups in which most of the different values were within the limits of agreement. DS cutoff of > 12 mg/dL showed 100% for all predictive indices in comparison with the diazo cutoff > 15 mg/dL.

CONCLUSIONS

Measurement of total bilirubin in neonatal venous plasma using the DS method had favorable agreement and high correlation with the diazo method. Therefore, the direct spectrophotometric method can be used as a reliable screening method.

Alterations of Cytochrome P450s and UDP-Glucuronosyltransferases in Brain Under Diseases and Their Clinical Significances

Cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs) are both greatly important metabolic enzymes in various tissues, including brain. Although expressions of brain CYPs and UGTs and their contributions to drug disposition are much less than liver, both CYPs and UGTs also mediate metabolism of endogenous substances including dopamine and serotonin as well as some drugs such as morphine in brain, demonstrating their important roles in maintenance of brain homeostasis or pharmacological activity of drugs. Some diseases such as epilepsy, Parkinson's disease and Alzheimer's disease are often associated with the alterations of CYPs and UGTs in brain, which may be involved in processes of these diseases via disturbing metabolism of endogenous substances or resisting drugs. This article reviewed the alterations of CYPs and UGTs in brain, the effects on endogenous substances and drugs and their clinical significances. Understanding the roles of CYPs and UGTs in brain provides some new strategies for the treatment of central nervous system diseases.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Kernicterus in a Neonatal Foal

A 5-day-old Thoroughbred foal was submitted to the necropsy service at the University of Kentucky Livestock Disease Diagnostic Center. The foal had a clinical history of seizure activity and severe icterus. A complete blood count and serum chemistry analysis indicated that the foal was anemic (hematocrit, 16%), hyperbilirubinemic (45 mg/dl), and hypoglycemic. At necropsy, all tissues were discolored various shades of yellow. Microscopically, there was degeneration and necrosis of cerebral neurons and cerebellar Purkinje cells; severe hepatocellular degeneration and necrosis; and deposition of amorphous golden-yellow material in the cerebellar granular cell layer, pulmonary alveoli, renal tubular epithelium, splenic trabecula, and the lamina propria of the small and large intestine. The golden-yellow material in the brain, lung, spleen, and small intestine was identified as bilirubin by histochemistry. Based on the macroscopic and microscopic findings, a diagnosis of kernicterus (bilirubin encephalopathy) was made. This report describes a rare case of equine neonatal kernicterus.

Difference in cerebral blood flow velocity in neonates with and without hyperbilirubinemia

PURPOSE

To evaluate the difference in cerebral blood flow velocity (CBFV) in neonates with and without hyperbilirubinemia.

METHODS

CBFV of 70 healthy late-preterm and term newborns with unconjugated hyperbilirubinemia (UCH) reaching the threshold of phototherapy requirement was compared with 70 gestational- and postnatal age-matched controls without hyperbilirubinemia. Resistance index (RI), pulsatility index (PI), peak systolic velocity (PSV) and vascular diameter were measured in internal carotid, vertebral and middle cerebral arteries by transcranial color Doppler ultrasound at the beginning of phototherapy, after 48-72h of starting phototherapy and at 5-7days after its stoppage. In controls CBFV was assessed once at inclusion.

RESULTS

Both the groups were comparable. An increase in CBFV (decreased RI and PI, increased PSV and vasodilation) was observed in the UCH group. A further increase in CBFV was noticed after 48h of phototherapy. After 5-7days of stoppage of phototherapy, though there was a significant reduction in CBFV in mild-to-moderate UCH (serum bilirubin ⩽25mg/dL), in severe UCH (serum bilirubin >25mg/dL), CBFV remained increased. Four neonates developed features of acute bilirubin encephalopathy and had significantly higher CBFV compared to those with normal outcome.

CONCLUSIONS

An increase in CBFV was observed in neonates with UCH compared to those without hyperbilirubinemia.

Metyrapone, an inhibitor of cytochrome oxidases, does not affect viability in a neuroblastoma cell model of bilirubin toxicity

Background

Unconjugated hyperbilirubinemia may cause brain damage in infants, and globally remains a source of neonatal morbidity and mortality. A significant inter-individual variability in vulnerability to bilirubin toxicity remains largely unexplained. An enzyme located in mitochondria oxidizes bilirubin. We hypothesized that inhibiting bilirubin oxidation in human neuronal cell cultures exposed to bilirubin would increase cell death.

Methods

The ability of mitochondrial membranes from CHP-212 human neuroblastoma cells to oxidize bilirubin was verified by spectrophotometry. Intact cells in culture were exposed to bilirubin (75 μM) with or without metyrapone (250 μM) for 24 h, stained with Annexin-V and Propidium iodide and analyzed for apoptosis and necrosis by flow cytometry.

Results

Bilirubin caused a significant reduction of viability, from 84 ± 2.0% (mean ± SEM) vs 67 ± 2.7% (p < 0.05), but adding metyrapone to the bilirubin-exposed cells did not further impact cell viability. Metyrapone alone did not influence cell viability.

Conclusion

Herein we have shown that metyrapone does not increase cell death in neuroblastoma cells in culture exposed to bilirubin. Our results question the relationship between the oxidative mechanism evaluated by spectrophotometry and cell viability. Our findings add to the discussion on whether bilirubin oxidation represents a potentially important protective mechanism in neurons challenged by hyperbilirubinemia.

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Neonatal jaundice may cause brain damage in newborn infants.

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Oxidation of bilirubin in the brain has been proposed as a protective mechanism.

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In a cell model, blocking the enzyme responsible for bilirubin oxidation did not increase toxicity.

The evolving landscape of neurotoxicity by unconjugated bilirubin: role of glial cells and inflammation

Unconjugated hyperbilirubinemia is a common condition in the first week of postnatal life. Although generally harmless, some neonates may develop very high levels of unconjugated bilirubin (UCB), which may surpass the protective mechanisms of the brain in preventing UCB accumulation. In this case, both short-term and long-term neurodevelopmental disabilities, such as acute and chronic UCB encephalopathy, known as kernicterus, or more subtle alterations defined as bilirubin-induced neurological dysfunction (BIND) may be produced. There is a tremendous variability in babies' vulnerability toward UCB for reasons not yet explained, but preterm birth, sepsis, hypoxia, and hemolytic disease are comprised as risk factors. Therefore, UCB levels and neurological abnormalities are not strictly correlated. Even nowadays, the mechanisms of UCB neurotoxicity are still unclear, as are specific biomarkers, and little is known about lasting sequelae attributable to hyperbilirubinemia. On autopsy, UCB was shown to be within neurons, neuronal processes, and microglia, and to produce loss of neurons, demyelination, and gliosis. In isolated cell cultures, UCB was shown to impair neuronal arborization and to induce the release of pro-inflammatory cytokines from microglia and astrocytes. However, cell dependent sensitivity to UCB toxicity and the role of each nerve cell type remains not fully understood. This review provides a comprehensive insight into cell susceptibilities and molecular targets of UCB in neurons, astrocytes, and oligodendrocytes, and on phenotypic and functional responses of microglia to UCB. Interplay among glia elements and cross-talk with neurons, with a special emphasis in the UCB-induced immunostimulation, and the role of sepsis in BIND pathogenesis are highlighted. New and interesting data on the anti-inflammatory and antioxidant activities of different pharmacological agents are also presented, as novel and promising additional therapeutic approaches to BIND.

Evidence for maternal-fetal genotype incompatibility as a risk factor for schizophrenia

Prenatal/obstetric complications are implicated in schizophrenia susceptibility. Some complications may arise from maternal-fetal genotype incompatibility, a term used to describe maternal-fetal genotype combinations that produce an adverse prenatal environment. A review of maternal-fetal genotype incompatibility studies suggests that schizophrenia susceptibility is increased by maternal-fetal genotype combinations at the RHD and HLA-B loci. Maternal-fetal genotype combinations at these loci are hypothesized to have an effect on the maternal immune system during pregnancy which can affect fetal neurodevelopment and increase schizophrenia susceptibility. This article reviews maternal-fetal genotype incompatibility studies and schizophrenia and discusses the hypothesized biological role of these ‘‘incompatibility genes”. It concludes that research is needed to further elucidate the role of RHD and HLA-B maternal-fetal genotype incompatibility in schizophrenia and to identify other genes that produce an adverse prenatal environment through a maternal-fetal genotype incompatibility mechanism. Efforts to develop more sophisticated study designs and data analysis techniques for modeling maternal-fetal genotype incompatibility effects are warranted.

Two different tests for total bilirubin in neonates and infants

BACKGROUND

The most readily available methods for testing serum total bilirubin in neonates are the capillary tube-directed optics color method and serum biochemistry. Because inconsistency between these two methods may cause confusion in clinical practice, this study was designed to quantify their differences.

METHODS

In 46 neonates with clinical jaundice, total bilirubin was measured by two different methods, using a nonchemical photometric device and a laboratory analyzer.

RESULTS

Differences in results between these two methods were statistically significant, especially when total bilirubin level exceeded 15 mg/dL.

CONCLUSION

Clinicians should be aware of the differences between the two methods when making decisions in patient care.

Effect of Rhesus D incompatibility on schizophrenia depends on offspring sex

Rhesus D incompatibility increases risk for schizophrenia, with some evidence that risk is limited to male offspring. The purpose of this study is to determine whether risk for schizophrenia due to Rhesus D incompatibility differs by offspring sex using a nuclear family-based candidate gene approach and a meta-analysis approach. The genetic study is based on a sample of 277 nuclear families with RHD genotype data on at least one parent and at least one child diagnosed with schizophrenia or related disorder. Meta-analysis inclusion criteria were (1) well-defined sample of schizophrenia patients with majority born before 1970, (2) Rhesus D incompatibility phenotype or genotype data available on mother and offspring, and by offspring sex. Two of ten studies, plus the current genetic study sample, fulfilled these criteria, for a total of 358 affected males and 226 affected females. The genetic study found that schizophrenia risk for incompatible males was significantly greater than for compatible offspring (p=0.03), while risk for incompatible and compatible females was not significantly different (p=.32). Relative risks for incompatible males and females were not significantly different from each other. Meta-analysis using a larger number of affected males and females supports their difference. Taken together, these results provide further support that risk of schizophrenia due to Rhesus D incompatibility is limited to incompatible males, although a weak female incompatibility effect cannot be excluded. Sex differences during fetal neurodevelopment should be investigated to fully elucidate the etiology of schizophrenia.

Rh and ABO maternal-fetal incompatibility and risk of autism

We hypothesized that maternal-fetal incompatibility at the Rh or ABO loci may contribute to the risk of autism. There are biologically plausible reasons to believe such effects may play a role, and two previous epidemiologic studies have provided suggestive evidence. To further test this hypothesis, we genotyped the Rh and ABO loci in a sample of 389 independent case-parent trios from the AGRE repository and analyzed the data using a modification of the log-linear model for case-parent trios in which the effects of maternal-fetal genotype incompatibility are modeled jointly with the effects of the affected child's or mother's genotypes. We did not find any evidence that incompatibility at the Rh or ABO loci increases the risk of autism. Furthermore, we did not find any evidence for the presence of a high-risk susceptibility allele at or near these two loci operating either through the mother or child.

Bilirubin measurement for neonates: comparison of 9 frequently used methods

OBJECTIVE

High blood concentrations of bilirubin are toxic to the brain and may cause kernicterus. Therefore, determination of bilirubin levels is performed for many newborns, and several different methods are available. We compared 9 frequently used methods for bilirubin determination among newborns under routine conditions, to define their sequence of use.

METHODS

In a prospective study, bilirubin concentrations were determined with 9 different methods, ie, 3 skin test devices, 3 nonchemical photometric devices (including 2 blood gas analyzers), and 3 laboratory analyzers.

RESULTS

A total of 124 samples were obtained. All 3 laboratory methods showed very strong correlations with each other, and their means were used as comparison values. To these comparison values, the skin test devices had correlation coefficients between 0.961 and 0.966, and the nonchemical photometric devices between 0.980 and 0.994. Bland-Altman plots demonstrated good agreement with the comparison values for all nonchemical photometric devices. All skin test devices and 1 nonchemical photometric device underestimated bilirubin levels, particularly at high concentrations.

CONCLUSIONS

In the routine care of newborns, the first method for bilirubin testing should be a skin test. If the skin test result exceeds 200 micromol/L and other analytes are to be determined with a nonchemical photometric device, then bilirubin can be included in this analysis and the result trusted up to 250 micromol/L. If the skin test result exceeds 200 micromol/L and only bilirubin concentrations are needed, then a standard laboratory method is the first choice, to avoid repeated blood sampling. Bilirubin concentrations from nonchemical photometric devices that exceed 250 micromol/L should be confirmed with standard laboratory methods.

Heme oxygenase-1 activity after excitotoxic injury: immunohistochemical localization of bilirubin in neurons and astrocytes and deleterious effects of heme oxygenase inhibition on neuronal survival after kainate treatment

An increased expression of the inducible form of heme oxygenase (HO), HO-1, is found in the hippocampus after kainate injection, but thus far it is unclear whether the HO-1 is enzymatically active. The present study was carried out, using monoclonal antibodies to bilirubin and HO-1 and histochemical staining for iron, to compare the products of HO enzymatic activity, bilirubin and iron, with HO-1 expression in the kainate-lesioned hippocampus. There was a close correlation between bilirubin and HO-1 expression, and both bilirubin and HO-1 were observed in damaged neurons at early times, and astrocytes at later times (weeks), after kainate injection. These results indicate that the increased HO-1 in the hippocampus is enzymatically active. Too determine whether HO-1 activity after kainate could have a protective or, perhaps, destructive effect, kainate-injected rats were injected intraperitoneally with a blood-brain barrier-permeable inhibitor of HO, tin protoporphyrin (SnPP), and the effects of such treatment were compared with effects in rats that received kainate and saline injection. It was found that SnPP treatment did not improve neuronal survival. Instead, increased mortality was observed in rats treated with SnPP. Four SnPP-injected rats vs. one saline-injected rats died after kainate treatment. The surviving SnPP-treated rats showed significantly less hippocampal field that containing Nissl or MAP2 staining (an indicator of surviving neurons) compared with the saline-injected rats. These results indicate that HO-1 induction had a net protective effect on neurons in the kainate model of excitotoxic injury.

Recent advances in the pharmacotherapy for hyperbilirubinaemia in the neonate

Jaundice is a common cause for diagnostic works-up and therapeutic intervention in neonates. This is motivated by the risk for severe neurological sequelae (kernicterus). The mainstays of treatment for the past decades have been exchange transfusion and phototherapy. Exchange transfusion is now becoming rare due to immune prophylaxis in Rhesus-negative women, and treatment of sensitised infants with intravenous immunoglobulin. Several different pharmacological approaches have been studied as far as the treatment of neonatal jaundice. Of these, the focus of attention in recent years has been on the haem oxygenase inhibitors (metal meso- and protoporphyrins). These are effective inhibitors of bilirubin production and have been shown to significantly reduce peak serum bilirubin levels in several clinical trials, both when used prophylactically and therapeutically. However, questions remain regarding long-term safety, as well as the advisability of whole-scale inhibition of bilirubin production. Nevertheless, in selected infants with a high risk of severe jaundice, the use of haem oxygenase inhibitors may be acceptable. Pharmacotherapy in jaundiced infants is fraught with risks, as many drugs may increase the entry of bilirubin into the brain and presumably, the risk for neurotoxicity. Both the displacement of bilirubin from its albumin binding and interference with the function of phosphoglycoprotein in the blood-brain barrier are documented mechanisms in this respect.

Kernicterus: still a concern

The presence of yellow staining and damage to the brain caused by unconjugated bilirubin was first described by Hervieux in 1847. Kernicterus, the technical term used to describe the intense yellow staining in the basal ganglia of the brain, was first used by Schmorl in 1903. Perhaps as many as 60 percent of all babies born each year in the U.S. are diagnosed with clinical jaundice. Kernicterus is a preventable brain injury caused by severe jaundice, yet it remains a threat today. Because of this, anyone caring for newborns must be aware of the risks and treatment for hyperbilirubinemia and the sequelae of this seemingly benign entity.

Bilirubin protects astrocytes from its own toxicity by inducing up-regulation and translocation of multidrug resistance-associated protein 1 (Mrp1)

Unconjugated bilirubin (UCB) causes encephalopathy in severely jaundiced neonates by damaging astrocytes and neurons. Astrocytes, which help defend the brain against cytotoxic insults, express the ATP-dependent transporter, multidrug resistance-associated protein 1 (Mrp1), which mediates export of organic anions, probably including UCB. We therefore studied whether exposure to UCB affects the expression and intracellular localization of Mrp1 in cultured mouse astroglial cells (>95% astrocytes). Mrp1 was localized and quantitated by confocal laser scanning microscopy and double immunofluorescence labeling by using specific antibodies against Mrp1 and the astrocyte marker glial fibrillary acidic protein, plus the Golgi marker wheat germ agglutinin (WGA). In unexposed astrocytes, Mrp1 colocalized with WGA in the Golgi apparatus. Exposure to UCB at a low unbound concentration (Bf) of 40 nM caused rapid redistribution of Mrp1 from the Golgi throughout the cytoplasm to the plasma membrane, with a peak 5-fold increase in Mrp1 immunofluorescence intensity from 30 to 120 min. Bf above aqueous saturation produced a similar but aborted response. Exposure to this higher Bf for 16 h markedly decreased Trypan blue exclusion and methylthiazoletetrazoilum activity and increased apoptosis 5-fold by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay. These toxic effects were modestly increased by inhibition of Mrp1 activity with 3-([3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl-(3-dimethylamino-3-oxopropyl)-thio-methyl]thio)propanoic acid (MK571). By contrast, Bf=40 nM caused injury only if Mrp1 activity was inhibited by MK571, which also blocked translocation of Mrp1. Our conclusion is that in astrocytes, UCB up-regulates expression of Mrp1 and promotes its trafficking from the Golgi to the plasma membrane, thus moderating cytotoxicity from UCB, presumably by limiting its intracellular accumulation.

RHD maternal–fetal genotype incompatibility and schizophrenia: extending the MFG test to include multiple siblings and birth order

Rh incompatibility disease (ie Rh hemolytic disease of the fetus and newborn) has been implicated as a risk factor for schizophrenia. Here, we extend the maternal-fetal genotype incompatibility (MFG) test used in an earlier case-parent trio study that found significant evidence for an increased risk of schizophrenia in RHD MFG-incompatible children. We modify the MFG test for case-parent trios to include any number of siblings. This modified test enables us to use more of the available data from the earlier study. The increased sample size not only gives us greater power to test for MFG incompatibility but it also enables us to model the impact of previous RHD MFG-incompatible pregnancies on the relative risk of RHD MFG incompatibility in later-born siblings. This modeling is important, because RHD MFG incompatibility is a proxy for Rh incompatibility disease, and the risk of Rh incompatibility disease increases with the number of previous RHD MFG-incompatible pregnancies. The best-fitting models are consistent with the hypothesized effect that previous incompatible pregnancies increase the risk of schizophrenia due to RHD MFG incompatibility. There was significant evidence that the relative risk of schizophrenia in the second- and later-born RHD MFG-incompatible children is 1.7, consistent with earlier estimates. Our extension of the MFG test has general application to family-based studies of maternal-genotype and MFG interaction effects.

RHD maternal-fetal genotype incompatibility increases schizophrenia susceptibility

Fetal events and obstetric complications are associated with schizophrenia. Here we report the results of a family-based candidate-gene study that assesses the role of maternal-fetal genotype incompatibility at the RHD locus in schizophrenia. We adapted the case-parent-trio log-linear modeling approach to test for RHD maternal-fetal genotype incompatibility and to distinguish this effect from a high-risk allele at or near the RHD locus and from a direct maternal effect alone. Eighty-eight patient-parent trios, 72 patient-mother pairs, and 21 patient-father pairs were genotyped at the RHD locus. Of the 181 patients, 62% were male and 81% were second born or later. Only three patients were born after prophylaxis against maternal isoimmunization had become common practice. There was significant evidence for an RHD maternal-fetal genotype incompatibility, and the incompatibility parameter was estimated at 2.6. There was no evidence to support linkage/association with schizophrenia at or near the RHD locus nor any evidence to support the role of maternal genotype effect alone. Our results replicate previous findings that implicate the RHD locus in schizophrenia, and the candidate-gene design of this study allows the elimination of alternative explanations for the role of this locus in disease. Thus, the present study provides increasing evidence that the RHD locus increases schizophrenia risk through a maternal-fetal genotype incompatibility mechanism that increases risk of an adverse prenatal environment (e.g., Rh incompatibility) rather than through linkage/association with the disorder, linkage disequilibrium with an unknown nearby susceptibility locus, or a direct maternal effect alone. This is the first candidate-gene study to explicitly test for and provide evidence of a maternal-fetal genotype incompatibility mechanism in schizophrenia.

Bilirubin brain toxicity

Bilirubin is toxic in most biological systems tested. Several mechanisms have been suggested for this toxic effect, including inhibition of enzyme systems and inhibition of cell regulatory reactions (protein/peptide phosphorylation). The identity of the basic mechanism(s) has not been conclusively proven, but inhibition of peptide phosphorylation, perhaps mediated or modulated by lysine at the active site(s), appears to be compatible with many of the observations currently found in the literature. Bilirubin entry into brain is facilitated by drug displacement of bilirubin from its albumin binding, reduced albumin binding capacity, increased brain bloodflow, increased permeability of the blood-brain barrier, and other factors. The rate of bilirubin entry into brain, as well as the degree of retention and rate of clearance from brain, depends on which of these circumstances are operative. It is as yet unclear whether the mechanism responsible for increased brain bilirubin is important for toxicity. The mechanism for preferential localization of bilirubin to the basal ganglia in kernicterus is also not known. Bilirubin appears to distribute differentially to brain subcellular compartments and is oxidized in brain by an enzyme localized on the inner mitochondrial membrane. This enzyme is found both in neurons and in glia, but appears to be more active in the latter. The activity increases with postnatal age, and is subject to genetic variability in animals. The enzyme is cytochrome c-dependent. It is as yet not clear whether the activity of this enzyme serves a brain-protective effect in severe hyperbilirubinemia.