Biliverdin-IX alpha reductase and biliverdin-IX beta reductase from human liver. Purification and characterization

Update on heme biosynthesis, tissue-specific regulation, heme transport, relation to iron metabolism and cellular energy

Heme is a primordial macrocycle upon which most aerobic life on Earth depends. It is essential to the survival and health of nearly all cells, functioning as a prosthetic group for oxygen-carrying proteins and enzymes involved in oxidation/reduction and electron transport reactions. Heme is essential for the function of numerous hemoproteins and has numerous other roles in the biochemistry of life. In mammals, heme is synthesised from glycine, succinyl-CoA, and ferrous iron in a series of eight steps. The first and normally rate-controlling step is catalysed by 5-aminolevulinate synthase (ALAS), which has two forms: ALAS1 is the housekeeping form with highly variable expression, depending upon the supply of the end-product heme, which acts to repress its activity; ALAS2 is the erythroid form, which is regulated chiefly by the adequacy of iron for erythroid haemoglobin synthesis. Abnormalities in the several enzymes of the heme synthetic pathway, most of which are inherited partial enzyme deficiencies, give rise to rare diseases called porphyrias. The existence and role of heme importers and exporters in mammals have been debated. Recent evidence established the presence of heme transporters. Such transporters are important for the transfer of heme from mitochondria, where the penultimate and ultimate steps of heme synthesis occur, and for the transfer of heme from cytoplasm to other cellular organelles. Several chaperones of heme and iron are known and important for cell health. Heme and iron, although promoters of oxidative stress and potentially toxic, are essential cofactors for cellular energy production and oxygenation.

An enzyme that selectively S-nitrosylates proteins to regulate insulin signaling

Acyl-coenzyme A (acyl-CoA) species are cofactors for numerous enzymes that acylate thousands of proteins. Here, we describe an enzyme that uses S-nitroso-CoA (SNO-CoA) as its cofactor to S-nitrosylate multiple proteins (SNO-CoA-assisted nitrosylase, SCAN). Separate domains in SCAN mediate SNO-CoA and substrate binding, allowing SCAN to selectively catalyze SNO transfer from SNO-CoA to SCAN to multiple protein targets, including the insulin receptor (INSR) and insulin receptor substrate 1 (IRS1). Insulin-stimulated S-nitrosylation of INSR/IRS1 by SCAN reduces insulin signaling physiologically, whereas increased SCAN activity in obesity causes INSR/IRS1 hypernitrosylation and insulin resistance. SCAN-deficient mice are thus protected from diabetes. In human skeletal muscle and adipose tissue, SCAN expression increases with body mass index and correlates with INSR S-nitrosylation. S-nitrosylation by SCAN/SNO-CoA thus defines a new enzyme class, a unique mode of receptor tyrosine kinase regulation, and a revised paradigm for NO function in physiology and disease.

Dysregulated anti-oxidant signalling and compromised mitochondrial integrity negatively influence regulatory T cell function and viability in liver disease

BACKGROUND

Dysregulated inflammatory responses and oxidative stress are key pathogenic drivers of chronic inflammatory diseases such as liver cirrhosis (LC). Regulatory T cells (Tregs) are essential to prevent excessive immune activation and maintain tissue homeostasis. While inflammatory cues are well known to modulate the function and stability of Tregs, the extent to which Tregs are influenced by oxidative stress has not been fully explored.

METHODS

The phenotypic and functional properties of CD4+CD25+CD127lo/- Tregs isolated from patients with LC were compared to healthy controls (HC). Treg redox state was investigated by characterizing intracellular reactive oxygen species (ROS), NADPH oxidase-2 (Nox2) activity, mitochondrial function, morphology, and nuclear factor-erythroid 2-related factor (Nrf2) antioxidant signalling. The relevance of Nrf2 and its downstream target, Heme-oxygenase-1 (HO-1), in Treg function, stability, and survival, was further assessed using mouse models and CRISPR/Cas9-mediated HO-1 knock-out.

FINDINGS

Circulating Tregs from LC patients displayed a reduced suppressive function, correlating with liver disease severity, associated with phenotypic abnormalities and increased apoptosis. Mechanistically, this was linked to a dysregulated Nrf2 signalling with resultant lower levels of HO-1, enhanced Nox2 activation, and impaired mitochondrial respiration and integrity. The functional deficit in LC Tregs could be partially recapitulated by culturing control Tregs in patient sera.

INTERPRETATION

Our findings reveal that Tregs rely on functional redox homeostasis for their function, stability, and survival. Targeting Treg specific anti-oxidant pathways may have therapeutic potential to reverse the Treg impairment in conditions of oxidative damage such as advanced liver disease.

FUNDING

This study was funded by the Wellcome Trust (211113/A/18/Z).

Metabolic Functions of Biliverdin IXβ Reductase in Redox-Regulated Hematopoietic Cell Fate

Cytoprotective heme oxygenases derivatize heme to generate carbon monoxide, ferrous iron, and isomeric biliverdins, followed by rapid NAD(P)H-dependent biliverdin reduction to the antioxidant bilirubin. Recent studies have implicated biliverdin IXβ reductase (BLVRB) in a redox-regulated mechanism of hematopoietic lineage fate restricted to megakaryocyte and erythroid development, a function distinct and non-overlapping from the BLVRA (biliverdin IXα reductase) homologue. In this review, we focus on recent progress in BLVRB biochemistry and genetics, highlighting human, murine, and cell-based studies that position BLVRB-regulated redox function (or ROS accumulation) as a developmentally tuned trigger that governs megakaryocyte/erythroid lineage fate arising from hematopoietic stem cells. BLVRB crystallographic and thermodynamic studies have elucidated critical determinants of substrate utilization, redox coupling and cytoprotection, and have established that inhibitors and substrates bind within the single-Rossmann fold. These advances provide unique opportunities for the development of BLVRB-selective redox inhibitors as novel cellular targets that retain potential for therapeutic applicability in hematopoietic (and other) disorders.

Molecular Mechanisms behind Safranal's Toxicity to HepG2 Cells from Dual Omics

The spice saffron (Crocus sativus) has anticancer activity in several human tissues, but the molecular mechanisms underlying its potential therapeutic effects are poorly understood. We investigated the impact of safranal, a small molecule secondary metabolite from saffron, on the HCC cell line HepG2 using untargeted metabolomics (HPLC–MS) and transcriptomics (RNAseq). Increases in glutathione disulfide and other biomarkers for oxidative damage contrasted with lower levels of the antioxidants biliverdin IX (139-fold decrease, p = 5.3 × 10⁵), the ubiquinol precursor 3-4-dihydroxy-5-all-trans-decaprenylbenzoate (3-fold decrease, p = 1.9 × 10⁻⁵), and resolvin E1 (−3282-fold decrease, p = 4⁵), which indicates sensitization to reactive oxygen species. We observed a significant increase in intracellular hypoxanthine (538-fold increase, p = 7.7 × 10⁻⁶) that may be primarily responsible for oxidative damage in HCC after safranal treatment. The accumulation of free fatty acids and other biomarkers, such as S-methyl-5′-thioadenosine, are consistent with safranal-induced mitochondrial de-uncoupling and explains the sharp increase in hypoxanthine we observed. Overall, the dual omics datasets describe routes to widespread protein destabilization and DNA damage from safranal-induced oxidative stress in HCC cells.

The Trypanosoma brucei-Derived Ketoacids, Indole Pyruvate and Hydroxyphenylpyruvate, Induce HO-1 Expression and Suppress Inflammatory Responses in Human Dendritic Cells

The extracellular parasite and causative agent of African sleeping sickness Trypanosoma brucei (T. brucei) has evolved a number of strategies to avoid immune detection in the host. One recently described mechanism involves the conversion of host-derived amino acids to aromatic ketoacids, which are detected at relatively high concentrations in the bloodstream of infected individuals. These ketoacids have been shown to directly suppress inflammatory responses in murine immune cells, as well as acting as potent inducers of the stress response enzyme, heme oxygenase 1 (HO-1), which has proven anti-inflammatory properties. The aim of this study was to investigate the immunomodulatory properties of the T. brucei-derived ketoacids in primary human immune cells and further examine their potential as a therapy for inflammatory diseases. We report that the T. brucei-derived ketoacids, indole pyruvate (IP) and hydroxyphenylpyruvate (HPP), induce HO-1 expression through Nrf2 activation in human dendritic cells (DC). They also limit DC maturation and suppress the production of pro-inflammatory cytokines, which, in turn, leads to a reduced capacity to differentiate adaptive CD4⁺ T cells. Furthermore, the ketoacids are capable of modulating DC cellular metabolism and suppressing the inflammatory profile of cells isolated from patients with inflammatory bowel disease. This study therefore not only provides further evidence of the immune-evasion mechanisms employed by T. brucei, but also supports further exploration of this new class of HO-1 inducers as potential therapeutics for the treatment of inflammatory conditions.

Recombinant Production of Biliverdin IXβ and δ Isomers in the T7 Promoter Compatible Escherichia coli Nissle

The ability to obtain purified biliverdin IX (BVIX) isomers other than the commercially available BVIXα is limited due to the low yields obtained by the chemical coupled oxidation of heme. Chemical oxidation requires toxic chemicals, has very poor BVIX yields (<0.05%), and is not conducive to scalable production. Alternative approaches utilizing recombinant E. coli BL21 expressing a cyanobacterial heme oxygenase have been employed for the production BVIXα, but yields are limited by the rate of endogenous heme biosynthesis. Furthermore, the emerging roles of BVIXβ and BVIXδ in biology and their lack of commercial availability has led to a need for an efficient and scalable method with the flexibility to produce all three physiologically relevant BVIX isomers. Herein, we have taken advantage of an optimized non-pathogenic E. coli Nissle (EcN(T7)) strain that encodes an endogenous heme transporter and an integrated T7 polymerase gene. Protein production of the Pseudomonas aeruginosa BVIXβ and BVIXδ selective heme oxygenase (HemO) or its BVIXα producing mutant (HemOα) in the EcN(T7) strain provides a scalable method to obtain all three isomers, that is not limited by the rate of endogenous heme biosynthesis, due to the natural ability of EcN(T7) to transport extracellular heme. Additionally, we have optimized our previous LC-MS/MS protocol for semi-preparative separation and validation of the BVIX isomers. Utilizing this new methodology for scalable production and separation we have increased the yields of the BVIXβ and -δ isomers >300-fold when compared to the chemical oxidation of heme.

Physics of biomolecular recognition and conformational dynamics

Biomolecular recognition usually leads to the formation of binding complexes, often accompanied by large-scale conformational changes. This process is fundamental to biological functions at the molecular and cellular levels. Uncovering the physical mechanisms of biomolecular recognition and quantifying the key biomolecular interactions are vital to understand these functions. The recently developed energy landscape theory has been successful in quantifying recognition processes and revealing the underlying mechanisms. Recent studies have shown that in addition to affinity, specificity is also crucial for biomolecular recognition. The proposed physical concept of intrinsic specificity based on the underlying energy landscape theory provides a practical way to quantify the specificity. Optimization of affinity and specificity can be adopted as a principle to guide the evolution and design of molecular recognition. This approach can also be used in practice for drug discovery using multidimensional screening to identify lead compounds. The energy landscape topography of molecular recognition is important for revealing the underlying flexible binding or binding-folding mechanisms. In this review, we first introduce the energy landscape theory for molecular recognition and then address four critical issues related to biomolecular recognition and conformational dynamics: (1) specificity quantification of molecular recognition; (2) evolution and design in molecular recognition; (3) flexible molecular recognition; (4) chromosome structural dynamics. The results described here and the discussions of the insights gained from the energy landscape topography can provide valuable guidance for further computational and experimental investigations of biomolecular recognition and conformational dynamics.

Regulation of inflammation by the antioxidant haem oxygenase 1

Haem oxygenase 1 (HO-1), an inducible enzyme responsible for the breakdown of haem, is primarily considered an antioxidant, and has long been overlooked by immunologists. However, research over the past two decades in particular has demonstrated that HO-1 also exhibits numerous anti-inflammatory properties. These emerging immunomodulatory functions have made HO-1 an appealing target for treatment of diseases characterized by high levels of chronic inflammation. In this Review, we present an introduction to HO-1 for immunologists, including an overview of its roles in iron metabolism and antioxidant defence, and the factors which regulate its expression. We discuss the impact of HO-1 induction in specific immune cell populations and provide new insights into the immunomodulation that accompanies haem catabolism, including its relationship to immunometabolism. Furthermore, we highlight the therapeutic potential of HO-1 induction to treat chronic inflammatory and autoimmune diseases, and the issues faced when trying to translate such therapies to the clinic. Finally, we examine a number of alternative, safer strategies that are under investigation to harness the therapeutic potential of HO-1, including the use of phytochemicals, novel HO-1 inducers and carbon monoxide-based therapies.

Divergent erythroid megakaryocyte fates in Blvrb-deficient mice establish non-overlapping cytoprotective functions during stress hematopoiesis

Cytoprotective mechanisms of heme oxygenases function by derivatizing heme to generate carbon monoxide, ferrous iron, and isomeric biliverdins, followed by rapid NAD(P)H-dependent biliverdin reduction to the antioxidant bilirubin using two non-overlapping biliverdin reductases that display biliverdin isomer-restricted redox activity. Although cytoprotective functions of heme oxygenases are widely recognized, concomitant effects of downstream biliverdin reductases remain incomplete. A computational model predicated on murine hematopoietic single-cell transcriptomic data identified Blvrb as a biological driver linked to the tumor necrosis factor stress pathway as a predominant source of variation defining hematopoietic cell heterogeneity. In vivo studies using Blvrb-deficient mice established the dispensable role of Blvrb in steady-state hematopoiesis, although model validation using aged Blvrb-deficient mice established an important cytoprotective function in stress hematopoiesis with dichotomous megakaryocyte-biased hematopoietic recovery. Defective stress erythropoiesis was evident in Blvrb^-/- spleens and in bone marrow erythroid development, occurring in conjunction with defective lipid peroxidation as a marker of oxidant mishandling. Cell autonomous effects on megakaryocyte lineage bias were documented using multipotential progenitor assays. These data provide the first physiological function of murine Blvrb in a non-redundant pathway of stress cytoprotection. Divergent effects on erythroid/megakaryocyte lineage speciation impute a novel redox-regulated mechanism for lineage partitioning.

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.

Biochemical characterization of biliverdins IXβ/δ generated by a selective heme oxygenase

The pro-oxidant effect of free heme (Fe2+-protoporphyrin IX) is neutralized by phylogenetically-conserved heme oxygenases (HMOX) that generate carbon monoxide, free ferrous iron, and biliverdin (BV) tetrapyrrole(s), with downstream BV reduction by non-redundant NADPH-dependent BV reductases (BLVRA and BLVRB) that retain isomer-restricted functional activity for bilirubin (BR) generation. Regioselectivity for the heme α-meso carbon resulting in predominant BV IXα generation is a defining characteristic of canonical HMOXs, thereby limiting generation and availability of BVs IXβ, IXδ, and IXγ as BLVRB substrates. We have now exploited the unique capacity of the Pseudomonas aeruginosa (P. aeruginosa) hemO/pigA gene for focused generation of isomeric BVs (IXβ and IXδ). A scalable system followed by isomeric separation yielded highly pure samples with predicted hydrogen-bonded structure(s) as documented by 1H NMR spectroscopy. Detailed kinetic studies established near-identical activity of BV IXβ and BV IXδ as BLVRB-selective substrates, with confirmation of an ordered sequential mechanism of BR/NADP+ dissociation. Halogenated xanthene-based compounds previously identified as BLVRB-targeted flavin reductase inhibitors displayed comparable inhibition parameters using BV IXβ as substrate, documenting common structural features of the cofactor/substrate-binding pocket. These data provide further insights into structure/activity mechanisms of isomeric BVs as BLVRB substrates, with potential applicability to further dissect redox-regulated functions in cytoprotection and hematopoiesis.

Bilin-metabolizing enzymes: site-specific reductions catalyzed by two different type of enzymes

In mammals, the green heme metabolite biliverdin is converted to a yellow anti-oxidant by NAD(P)H-dependent biliverdin reductase (BVR), whereas in O₂-dependent photosynthetic organisms it is converted to photosynthetic or light-sensing pigments by ferredoxin-dependent bilin reductases (FDBRs). In NADP⁺-bound and biliverdin-bound BVR-A, two biliverdins are stacked at the binding cleft; one is positioned to accept hydride from NADPH, and the other appears to donate a proton to the first biliverdin through a neighboring arginine residue. During the FDBR-catalyzed reaction, electrons and protons are supplied to bilins from ferredoxin and from FDBRs and waters bound within FDBRs, respectively. Thus, the protonation sites of bilin and catalytic residues are important for the analysis of site-specific reduction. The neutron structure of FDBR sheds light on this issue.

Association between decreased serum TBIL concentration and immediate memory impairment in schizophrenia patients

Cognitive impairment is a core feature of schizophrenia (SCH). In addition to the toxic effect of Bilirubin (BIL), it has antioxidant properties that were associated with the psychopathology and cognitive impairment of psychiatric disorders. The aim of this study was to examine the correlation of serum total BIL (TBIL) concentration with cognitive impairment in SCH patients. We recruited 34 SCH patients and 119 healthy controls (HCs) in this case-control design. Cognition was assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Serum TBIL concentration was measured using the immunoturbidimetric method. Serum TBIL concentration was significantly decreased in SCH patients compared to HCs after adjusting for age, gender, and education. Serum TBIL concentration in SCH patients was also positively correlated with the RBANS immediate memory score. Further stepwise multiple regression analysis confirmed the positive association between serum TBIL concentration and immediate memory score in SCH patients. Our findings supported that the decline in serum TBIL concentration was associated with the immediate memory impairment and psychopathology of SCH.

Biliverdin Reductase B Dynamics Are Coupled to Coenzyme Binding

Biliverdin reductase B (BLVRB) is a newly identified cellular redox regulator that catalyzes the NADPH-dependent reduction of multiple substrates. Through mass spectrometry analysis, we identified high levels of BLVRB in mature red blood cells, highlighting the importance of BLVRB in redox regulation. The BLVRB conformational changes that occur during conezyme/substrate binding and the role of dynamics in BLVRB function, however, remain unknown. Through a combination of NMR, kinetics, and isothermal titration calorimetry studies, we determined that BLVRB binds its coenzyme 500-fold more tightly than its substrate. While the active site of apo BLVRB is highly dynamic on multiple timescales, active site dynamics are largely quenched within holo BLVRB, in which dynamics are redistributed to other regions of the enzyme. We show that a single point mutation of Arg78➔Ala leads to both an increase in active site micro-millisecond motions and an increase in the microscopic rate constants of coenzyme binding. This demonstrates that altering BLVRB active site dynamics can directly cause a change in functional characteristics. Our studies thus address the solution behavior of apo and holo BLVRB and identify a role of enzyme dynamics in coenzyme binding.

In silico and crystallographic studies identify key structural features of biliverdin IXβ reductase inhibitors having nanomolar potency

Heme cytotoxicity is minimized by a two-step catabolic reaction that generates biliverdin (BV) and bilirubin (BR) tetrapyrroles. The second step is regulated by two non-redundant biliverdin reductases (IXα (BLVRA) and IXβ (BLVRB)), which retain isomeric specificity and NAD(P)H-dependent redox coupling linked to BR's antioxidant function. Defective BLVRB enzymatic activity with antioxidant mishandling has been implicated in metabolic consequences of hematopoietic lineage fate and enhanced platelet counts in humans. We now outline an integrated platform of in silico and crystallographic studies for the identification of an initial class of compounds inhibiting BLVRB with potencies in the nanomolar range. We found that the most potent BLVRB inhibitors contain a tricyclic hydrocarbon core structure similar to the isoalloxazine ring of flavin mononucleotide and that both xanthene- and acridine-based compounds inhibit BLVRB's flavin and dichlorophenolindophenol (DCPIP) reductase functions. Crystallographic studies of ternary complexes with BLVRB-NADP⁺-xanthene-based compounds confirmed inhibitor binding adjacent to the cofactor nicotinamide and interactions with the Ser-111 side chain. This residue previously has been identified as critical for maintaining the enzymatic active site and cellular reductase functions in hematopoietic cells. Both acridine- and xanthene-based compounds caused selective and concentration-dependent loss of redox coupling in BLVRB-overexpressing promyelocytic HL-60 cells. These results provide promising chemical scaffolds for the development of enhanced BLVRB inhibitors and identify chemical probes to better dissect the role of biliverdins, alternative substrates, and BLVRB function in physiologically relevant cellular contexts.

Absence of the biliverdin reductase-a gene is associated with increased endogenous oxidative stress

Bilirubin, a byproduct of heme catabolism, has been shown to be an effective lipid-soluble antioxidant in vitro. Bilirubin is able to inhibit free radical chain reactions and protects against oxidant-induced damage in vitro and ex vivo. However, direct evidence for bilirubin's antioxidant effects in vivo remains limited. As bilirubin is formed from biliverdin by biliverdin reductase, we generated global biliverdin reductase-a gene knockout (Bvra^-/-) mice to assess the contribution of bilirubin as an endogenous antioxidant. Bvra^-/- mice appear normal and are born at the expected Mendelian ratio from Bvra^+/- x Bvra^+/- matings. Compared with corresponding littermate Bvra^+/+ and Bvra^+/- animals, Bvra^-/- mice have green gall bladders and their plasma concentrations of biliverdin and bilirubin are approximately 25-fold higher and 100-fold lower, respectively. Naïve Bvra^-/- and Bvra^+/+ mice have comparable plasma lipid profiles and low-molecular weight antioxidants, i.e., ascorbic acid, α-tocopherol and ubiquinol-9. Compared with wild-type littermates, however, plasma from Bvra^-/- mice contains higher concentrations of cholesteryl ester hydroperoxides (CE-OOH), and their peroxiredoxin 2 (Prx2) in erythrocytes is more oxidized as assessed by the extent of Prx2 dimerization. These data show that Bvra^-/- mice experience higher oxidative stress in blood, implying that plasma bilirubin attenuates endogenous oxidative stress.

Characterization of heme oxygenase and biliverdin reductase gene expression in zebrafish (Danio rerio): Basal expression and response to pro-oxidant exposures

While heme is an important cofactor for numerous proteins, it is highly toxic in its unbound form and can perpetuate the formation of reactive oxygen species. Heme oxygenase enzymes (HMOX1 and HMOX2) degrade heme into biliverdin and carbon monoxide, with biliverdin subsequently being converted to bilirubin by biliverdin reductase (BVRa or BVRb). As a result of the teleost-specific genome duplication event, zebrafish have paralogs of hmox1 (hmox1a and hmox1b) and hmox2 (hmox2a and hmox2b). Expression of all four hmox paralogs and two bvr isoforms were measured in adult tissues (gill, brain and liver) and sexually dimorphic differences were observed, most notably in the basal expression of hmox1a, hmox2a, hmox2b and bvrb in liver samples. hmox1a, hmox2a and hmox2b were significantly induced in male liver tissues in response to 96h cadmium exposure (20μM). hmox2a and hmox2b were significantly induced in male brain samples, but only hmox2a was significantly reduced in male gill samples in response to the 96h cadmium exposure. hmox paralogs displayed significantly different levels of basal expression in most adult tissues, as well as during zebrafish development (24 to 120hpf). Furthermore, hmox1a, hmox1b and bvrb were significantly induced in zebrafish eleutheroembryos in response to multiple pro-oxidants (cadmium, hemin and tert-butylhydroquinone). Knockdown of Nrf2a, a transcriptional regulator of hmox1a, was demonstrated to inhibit the Cd-mediated induction of hmox1b and bvrb. These results demonstrate distinct mechanisms of hmox and bvr transcriptional regulation in zebrafish, providing initial evidence of the partitioning of function of the hmox paralogs.

Rv2074 is a novel F420 H2 -dependent biliverdin reductase in Mycobacterium tuberculosis

Bilirubin is a potent antioxidant that is produced from the reduction of the heme degradation product biliverdin. In mammalian cells and Cyanobacteria, NADH/NADPH-dependent biliverdin reductases (BVRs) of the Rossmann-fold have been shown to catalyze this reaction. Here, we describe the characterization of Rv2074 from Mycobacterium tuberculosis, which belongs to a structurally and mechanistically distinct family of F420 H2 -dependent BVRs (F-BVRs) that are exclusively found in Actinobacteria. We have solved the crystal structure of Rv2074 bound to its cofactor, F420 , and used this alongside molecular dynamics simulations, site-directed mutagenesis and NMR spectroscopy to elucidate its catalytic mechanism. The production of bilirubin by Rv2074 could exploit the anti-oxidative properties of bilirubin and contribute to the range of immuno-evasive mechanisms that have evolved in M. tuberculosis to allow persistent infection.

BLVRB redox mutation defines heme degradation in a metabolic pathway of enhanced thrombopoiesis in humans

Human blood cell counts are tightly maintained within narrow physiologic ranges, largely controlled by cytokine-integrated signaling and transcriptional circuits that regulate multilineage hematopoietic specification. Known genetic loci influencing blood cell production account for <10% of platelet and red blood cell variability, and thrombopoietin/cellular myeloproliferative leukemia virus liganding is dispensable for definitive thrombopoiesis, establishing that fundamentally important modifier loci remain unelucidated. In this study, platelet transcriptome sequencing and extended thrombocytosis cohort analyses identified a single loss-of-function mutation (BLVRB(S111L)) causally associated with clonal and nonclonal disorders of enhanced platelet production. BLVRB(S111L) encompassed within the substrate/cofactor [α/β dinucleotide NAD(P)H] binding fold is a functionally defective redox coupler using flavin and biliverdin (BV) IXβ tetrapyrrole(s) and results in exaggerated reactive oxygen species accumulation as a putative metabolic signal leading to differential hematopoietic lineage commitment and enhanced thrombopoiesis. These data define the first physiologically relevant function of BLVRB and implicate its activity and/or heme-regulated BV tetrapyrrole(s) in a unique redox-regulated bioenergetic pathway governing terminal megakaryocytopoiesis; these observations also define a mechanistically restricted drug target retaining potential for enhancing human platelet counts.

Physiology, Biochemistry, and Applications of F420- and Fo-Dependent Redox Reactions

5-Deazaflavin cofactors enhance the metabolic flexibility of microorganisms by catalyzing a wide range of challenging enzymatic redox reactions. While structurally similar to riboflavin, 5-deazaflavins have distinctive and biologically useful electrochemical and photochemical properties as a result of the substitution of N-5 of the isoalloxazine ring for a carbon. 8-Hydroxy-5-deazaflavin (Fo) appears to be used for a single function: as a light-harvesting chromophore for DNA photolyases across the three domains of life. In contrast, its oligoglutamyl derivative F420 is a taxonomically restricted but functionally versatile cofactor that facilitates many low-potential two-electron redox reactions. It serves as an essential catabolic cofactor in methanogenic, sulfate-reducing, and likely methanotrophic archaea. It also transforms a wide range of exogenous substrates and endogenous metabolites in aerobic actinobacteria, for example mycobacteria and streptomycetes. In this review, we discuss the physiological roles of F420 in microorganisms and the biochemistry of the various oxidoreductases that mediate these roles. Particular focus is placed on the central roles of F420 in methanogenic archaea in processes such as substrate oxidation, C1 pathways, respiration, and oxygen detoxification. We also describe how two F420-dependent oxidoreductase superfamilies mediate many environmentally and medically important reactions in bacteria, including biosynthesis of tetracycline and pyrrolobenzodiazepine antibiotics by streptomycetes, activation of the prodrugs pretomanid and delamanid by Mycobacterium tuberculosis, and degradation of environmental contaminants such as picrate, aflatoxin, and malachite green. The biosynthesis pathways of Fo and F420 are also detailed. We conclude by considering opportunities to exploit deazaflavin-dependent processes in tuberculosis treatment, methane mitigation, bioremediation, and industrial biocatalysis.

Sequence-Structure-Function Classification of a Catalytically Diverse Oxidoreductase Superfamily in Mycobacteria

The deazaflavin cofactor F420 enhances the persistence of mycobacteria during hypoxia, oxidative stress, and antibiotic treatment. However, the identities and functions of the mycobacterial enzymes that utilize F420 under these conditions have yet to be resolved. In this work, we used sequence similarity networks to analyze the distribution of the largest F420-dependent protein family in mycobacteria. We show that these enzymes are part of a larger split β-barrel enzyme superfamily (flavin/deazaflavin oxidoreductases, FDORs) that include previously characterized pyridoxamine/pyridoxine-5'-phosphate oxidases and heme oxygenases. We show that these proteins variously utilize F420, flavin mononucleotide, flavin adenine dinucleotide, and heme cofactors. Functional annotation using phylogenetic, structural, and spectroscopic methods revealed their involvement in heme degradation, biliverdin reduction, fatty acid modification, and quinone reduction. Four novel crystal structures show that plasticity in substrate binding pockets and modifications to cofactor binding motifs enabled FDORs to carry out a variety of functions. This systematic classification and analysis provides a framework for further functional analysis of the roles of FDORs in mycobacterial pathogenesis and persistence.

Biliverdin reductase isozymes in metabolism

The biliverdin reductase (BVR) isozymes BVRA and BVRB are cell surface membrane receptors with pleiotropic functions. This review compares, for the first time, the structural and functional differences between the isozymes. They reduce biliverdin, a byproduct of heme catabolism, to bilirubin, display kinase activity, and BVRA, but not BVRB, can act as a transcription factor. The binding motifs present in the BVR isozymes allow a wide range of interactions with components of metabolically important signaling pathways such as the insulin receptor kinase cascades, protein kinases (PKs), and inflammatory mediators. In addition, serum bilirubin levels have been negatively associated with abdominal obesity and hypertriglyceridemia. We discuss the roles of the BVR isozymes in metabolism and their potential as therapeutic targets.

Developmental changes in the transcriptome of the rat choroid plexus in relation to neuroprotection

Background

The choroid plexuses are the interface between the blood and the cerebrospinal fluid (CSF) contained within the ventricular spaces of the central nervous system. The tight junctions linking adjacent cells of the choroidal epithelium create a physical barrier to paracellular movement of molecules. Multispecific efflux transporters as well as drug-metabolizing and antioxidant enzymes functioning in these cells contribute to a metabolic barrier. These barrier properties reflect a neuroprotective function of the choroid plexus. The choroid plexuses develop early during embryogenesis and provide pivotal control of the internal environment throughout development when the brain is especially vulnerable to toxic insults. Perinatal injuries like hypoxia and trauma, and exposure to drugs or toxic xenobiotics can have serious consequences on neurogenesis and long-term development. The present study describes the developmental expression pattern of genes involved in the neuroprotective functions of the blood–CSF barrier.

Methods

The transcriptome of rat lateral ventricular choroid plexuses isolated from fifteen-day-old embryos, nineteen-day old fetuses, two-day old pups, and adults was analyzed by a combination of Affymetrix microarrays, Illumina RNA-Sequencing, and quantitative RT-PCR.

Results

Genes coding for proteins involved in junction formation are expressed early during development. Overall perinatal expression levels of genes involved in drug metabolism and antioxidant mechanisms are similar to, or higher than levels measured in adults. A similar developmental pattern was observed for multispecific efflux transporter genes of the Abc and Slc superfamilies. Expression of all these genes was more variable in choroid plexus from fifteen-day-old embryos. A large panel of transcription factors involved in the xenobiotic- or cell stress-mediated induction of detoxifying enzymes and transporters is also expressed throughout development.

Conclusions

This transcriptomic analysis suggests relatively well–established neuroprotective mechanisms at the blood-CSF barrier throughout development of the rat. The expression of many transcription factors early in development raises the possibility of additional protection for the vulnerable developing brain, should the fetus or newborn be exposed to drugs or other xenobiotics.

Expression of Biliverdin Reductase A in peripheral blood leukocytes is associated with treatment response in HCV-infected patients

BACKGROUND AND AIMS

Hepatitis C virus (HCV) infection is associated with systemic oxidative stress. Since the heme catabolic pathway plays an important role in antioxidant protection, we attempted to assess the gene expression of key enzymes of heme catabolism, heme oxygenase 1 (HMOX1), heme oxygenase 2 (HMOX2), and biliverdin reductase A (BLVRA) in the liver and peripheral blood leukocytes (PBL) of patients chronically infected with HCV.

METHODS

Gene expressions (HMOX1, HMOX2, BLVRA) and HCV RNA were analyzed in PBL of HCV treatment naïve patients (n = 58) and controls (n = 55), with a subset of HCV patients having data on hepatic gene expression (n = 35). Based upon the therapeutic outcome, HCV patients were classified as either responders (n = 38) or treatment-failure patients (n = 20). Blood samples in HCV patients were collected at day 0, and week 12, 24, 36, and 48 after the initiation of standard antiviral therapy.

RESULTS

Compared to the controls, substantially increased BLVRA expression was detected in PBL (p<0.001) of therapeutically naïve HCV patients. mRNA levels of BLVRA in PBL closely correlated with those in liver tissue (r2 = 0.347,p = 0.03). A marked difference in BLVRA expression in PBL between the sustained responders and patients with treatment failure was detected at week 0 and during the follow-up (p<0.001). Multivariate analysis revealed that BLVRA basal expression in PBL was an independent predictor for sustained virological response (OR 15; 95% CI 1.05-214.2; P = 0.046). HMOX1/2 expression did not have any effect on the treatment outcome.

CONCLUSION

Our results suggest that patients with chronic HCV infection significantly upregulate BLVRA expression in PBL. The lack of BLVRA overexpression is associated with non-responsiveness to standard antiviral therapy; whereas, HMOX1/2 does not seem to have any predictive potential.

Crystallographic studies of heme oxygenase complexed with an unstable reaction intermediate, verdoheme

This article discusses the accuracy of X-ray structural studies of heme oxygenase (HO) in complex with an unstable intermediate, verdoheme. Heme degradation by HO proceeds through three successive steps of O(2) activation. The mechanism of the third step, the ring opening of verdoheme, has been the least understood. Recent structural studies of the verdoheme-HO complex provide detailed information concerning this mechanism. Due to X-ray-induced photoreduction and the instability of verdoheme, it has been difficult to obtain an accurate structure for the ferrous verdoheme-HO complex. Therefore, accurate structural studies, including analysis of the electronic state of the verdoheme-HO complex, are needed to elucidate the proper reaction mechanism.

The use of synthetic linear tetrapyrroles to probe the verdin sites of human biliverdin-IXalpha reductase and human biliverdin-IXbeta reductase

Many vertebrate species express two enzymes that are capable of catalysing the reduction of various isomers of biliverdin. Biliverdin-IXalpha reductase (BVR-A) is most active with its physiological substrate biliverdin-IXalpha, but can also reduce the three other biliverdin isomers IXbeta, IXdelta and IXgamma. Biliverdin-IXbeta reductase (BVR-B) catalyses the reduction of only the IXbeta, IXdelta and IXgamma isomers of biliverdin. Therefore, the activity of BVR-A can be measured using biliverdin-IXalpha as a specific substrate. We now show that the dimethyl esters of biliverdin-IXbeta and biliverdin-IXdelta are substrates for BVR-B, but not for BVR-A. This provides a useful method for specifically assaying the activity of both BVR-A and BVR-B in crude mixtures, using biliverdin-IXalpha for BVR-A and the dimethyl ester of either biliverdin-IXbeta or biliverdin-IXdelta for BVR-B. Human BVR-A has been suggested as a pharmacological target for neonatal jaundice. Because of the absence of a crystal structure with biliverdin bound to BVR-A, we have investigated indirect ways of examining tetrapyrrole binding. In the present study, we report that a number of sterically locked conformers of 18-ethylbiliverdin-IXalpha are substrates for human BVR-A, and discuss the implications for the biliverdin binding site. The oxidation of bilirubin-IXalpha ditaurate to biliverdin-IXalpha ditaurate is also described. We show that biliverdin-IXalpha ditaurate is a substrate for human BVR-A and discuss the possibility of using a competing substrate, which is reduced to a water soluble and excretable rubin, as a prototypic inhibitor of BVR-A.

Increased urinary excretion of biopyrrins, oxidative metabolites of bilirubin, in patients with schizophrenia

During periods of psychological stress, excess amounts of free radicals are produced, and they play an important role in the pathophysiological process. Bilirubin oxidative metabolites, biopyrrins, are generated from bilirubin as a result of this scavenging action against free radicals. We investigated whether the urinary excretion of biopyrrin is altered during the psychotic state in patients with schizophrenia. Biopyrrin concentrations in urine of 15 patients with schizophrenia and 100 age-matched healthy subjects were measured by enzyme-linked immunosorbent assay with an anti-bilirubin antibody. Urine samples were obtained from the patients on first admission (acute state), 1 month after admission (sub-acute state), and on discharge (remission state). Urinary concentrations of biopyrrins in patients with schizophrenia on admission were significantly higher than those in the controls. Response to treatment was associated with a significant decrease in the concentrations of biopyrrins. Moreover, urinary concentrations of biopyrrins were still significantly higher in patients with schizophrenia in the sub-acute and remission states than in the controls. These results demonstrated an increase in urinary biopyrrins in patients with schizophrenia and a decrease with recovery from the psychotic state. These findings indicate that the urinary biopyrrin level is a possible indicator that can be useful in the continuous monitoring of psychotic states in clinical practice.

Activation of biliverdin-IXalpha reductase by inorganic phosphate and related anions

The effect of pH on the initial-rate kinetic behaviour of BVR-A (biliverdin-IXalpha reductase) exhibits an alkaline optimum with NADPH as cofactor, but a neutral optimum with NADH as cofactor. This has been described as dual cofactor and dual pH dependent behaviour; however, no mechanism has been described to explain this phenomenon. We present evidence that the apparent peak of activity observed at neutral pH with phosphate buffer and NADH as cofactor is an anion-dependent activation, where inorganic phosphate apparently mimics the role played by the 2'-phosphate of NADPH in stabilizing the interaction between NADH and the enzyme. The enzymes from mouse, rat and human all exhibit this behaviour. This behaviour is not seen with BVR-A from Xenopus tropicalis or the ancient cyanobacterial enzyme from Synechocystis PCC 6803, which, in addition to being refractory to activation by inorganic phosphate, are also differentiated by an acid pH optimum with both nicotinamide nucleotides.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Urinary excretion of biopyrrins, oxidative metabolites of bilirubin, increases in patients with psychiatric disorders

Several authors have suggested that psychological stress induces the production of reactive oxygen species (ROS). Several studies have supported the idea that bilirubin exerts antioxidative effects in vivo, and it was reported psychological stress provokes bilirubin oxidation in vivo [Yamaguchi T., Shioji I., Sugimoto A., Yamaoka M., 2002. Psychological stress increases bilirubin metabolites in human urine. Biochem. and Biophys. Res. Commun. 293, 517-520]. We investigated whether the concentration of bilirubin oxidative metabolites (biopyrrins) is increased in urine from patients with psychiatric disorders. The concentration of biopyrrins in urine of 25 patients with psychiatric disorders (schizophrenia, 15; depression, 10) was compared with 96 healthy volunteers. The concentrations of biopyrrins, as measured by enzyme-linked immunosorbent assay, were normalized to the urinary concentration of creatinine. The concentration of biopyrrins in patients with psychiatric disorders (schizophrenia and depression) was significantly higher than that of healthy volunteers. In schizophrenia, biopyrrins levels correlated with scores of the Brief Psychiatric Rating Scale (BPRS), and in depression, biopyrrins levels correlated with scores of the Hamilton Depression Rating Scale (HAM-D). These finding suggest that psychotic states are associated with an increase in the oxidative metabolites of bilirubin in human urine.

Small interference RNA-mediated gene silencing of human biliverdin reductase, but not that of heme oxygenase-1, attenuates arsenite-mediated induction of the oxygenase and increases apoptosis in 293A kidney cells

BVR reduces biliverdin, the HO-1 and HO-2 product, to bilirubin. Human biliverdin (BVR) is a serine/threonine kinase activated by free radicals. It is a leucine zipper (bZip) DNA-binding protein and a regulatory factor for 8/7-bp AP-1-regulated genes, including HO-1 and ATF-2/CREB. Presently, small interference (si) RNA constructs were used to investigate the role of human BVR in sodium arsenite (As)-mediated induction of HO-1 and in cytoprotection against apoptosis. Activation of BVR involved increased serine/threonine phosphorylation but not its protein or transcript levels. The peak activity at 1 h (4-5-fold) after treatment of 293A cells with 5 mum As preceded induction of HO-1 expression by 3 h. The following suggests BVR involvement in regulating oxidative stress response of HO-1: siBVR attenuated As-mediated increase in HO-1 expression; siBVR, but not siHO-1, inhibited As-dependent increased c-jun promoter activity; treatment of cells with As increased AP-1 binding of nuclear proteins; BVR was identified in the DNA-protein complex; and AP-1 binding of the in vitro translated BVR was phosphorylation-dependent and was attenuated by biliverdin. Most unexpectedly, cells transfected with siBVR, but not siHO-1, displayed a 4-fold increase in apoptotic cells when treated with 10 mum As as detected by flow cytometry. The presence of BVR small interference RNA augmented the effect of As on levels of cytochrome c, TRAIL, and DR-5 mRNA and cleavage of poly(ADP-ribose) polymerase. The findings describe the function of BVR in HO-1 oxidative response and, demonstrate, for the first time, not only that BVR advances the role of HO-1 in cytoprotection but also affords cytoprotection independent of heme degradation.

Marked developmental changes in heme oxygenase-1 (HO-1) expression in the mouse placenta: correlation between HO-1 expression and placental development

Heme catabolism during embryonic period is not well understood. It has been suggested that placental heme oxygenase (HO)-1, which is an inducible isoform of the rate-limiting enzyme in the heme degradation pathway, may be involved in supporting normal fetal development. In this study, we examined the distribution of HO-1 protein in the developing mouse embryo, as well as developmental changes of ho-1 gene expression, and the enzyme activity of HO and biliverdin IXalpha reductase (BVR-A) in placenta. Ectoplacental cone in embryonic day (E) 6.5 embryo already showed HO-1 protein expression, which became restricted only to trophoblastic cells after placenta formation was completed on day E14.5. The placenta of E13.5-E14.5 embryos expressed high levels of HO-1 mRNA, which was decreased significantly towards the end of pregnancy. However, HO-1 expression in placenta was significantly higher than uterus throughout the gestational period. In contrast to HO-1, the placental level of BVR-A activity remained low and did not show changes throughout the gestational period. The correlation between HO-1 expression and placental development suggests that HO-1 might be essential for normal embryonic development.

The heme oxygenase dilemma in cellular homeostasis: new insights for the feedback regulation of heme catabolism

Heme must be synthesized and degraded within an individual nucleated cell. Heme degradation is catalyzed by the two isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, eventually yielding biliverdin/bilirubin, CO, and iron. These products possess important physiological roles but are potentially toxic to cells. Characteristically, human HO-1 contains no Cys residues, whereas HO-2 contains the potential heme-binding motifs of the Cys-Pro dipeptide. Expression of HO-1 is inducible or repressible, depending on cell types or cellular microenvironments, but expression levels of HO-2 are fairly constant. Thus, the main regulation of heme catabolism is a problem of the balance between induction and repression of HO-1. Notably, HO-1 expression is induced by heme in all mammalian cells examined, but is repressed by hypoxia in certain types of cultured human cells. The recent discovery of Bach1 as a heme-regulated and hypoxia-inducible repressor for transcription of the HO-1 gene has provided a missing link in the feedback control of heme catabolism. On the other hand, the human HO-1 gene promoter contains the (GT)n repeat polymorphism and a single nucleotide polymorphism (-427A --> T), both of which may contribute to fine-tuning of the transcription. Importantly, long (GT)n alleles are associated with susceptibility to smoking-induced emphysema or coronary artery disease, but may provide with resistance to cerebral malaria. The latter finding suggests a novel therapeutic strategy with inhibitors of HO-1 for the treatment of cerebral malaria. We discuss the potential regulatory role of Bach1 and HO-2 in heme catabolism and update the understanding of the regulation of HO-1 expression.

Unique features of recombinant heme oxygenase of Drosophila melanogaster compared with those of other heme oxygenases studied

We cloned a cDNA for a Drosophila melanogaster homologue of mammalian heme oxygenase (HO) and constructed a bacterial expression system of a truncated, soluble form of D. melanogaster HO (DmDeltaHO). The purified DmDeltaHO degraded hemin to biliverdin, CO and iron in the presence of reducing systems such as NADPH/cytochrome P450 reductase and sodium ascorbate, although the reaction rate was slower than that of mammalian HOs. Some properties of DmHO, however, are quite different from other known HOs. Thus DmDeltaHO bound hemin stoichiometrically to form a hemin-enzyme complex like other HOs, but this complex did not show an absorption spectrum of hexa-coordinated heme protein. The absorption spectrum of the ferric complex was not influenced by changing the pH of the solution. Interestingly, an EPR study revealed that the iron of heme was not involved in binding heme to the enzyme. Hydrogen peroxide failed to convert it into verdoheme. A spectrum of the ferrous-CO form of verdoheme was not detected during the reaction from hemin under oxygen and CO. Degradation of hemin catalyzed by DmDeltaHO yielded three isomers of biliverdin, of which biliverdin IXalpha and two other isomers (IXbeta and IXdelta) accounted for 75% and 25%, respectively. Taken together, we conclude that, although DmHO acts as a real HO in D. melanogaster, its active-site structure is quite different from those of other known HOs.

Biliverdin reductase: a major physiologic cytoprotectant

Bilirubin, an abundant pigment that causes jaundice, has long lacked any clear physiologic role. It arises from enzymatic reduction by biliverdin reductase of biliverdin, a product of heme oxygenase activity. Bilirubin is a potent antioxidant that we show can protect cells from a 10,000-fold excess of H2O2. We report that bilirubin is a major physiologic antioxidant cytoprotectant. Thus, cellular depletion of bilirubin by RNA interference markedly augments tissue levels of reactive oxygen species and causes apoptotic cell death. Depletion of glutathione, generally regarded as a physiologic antioxidant cytoprotectant, elicits lesser increases in reactive oxygen species and cell death. The potent physiologic antioxidant actions of bilirubin reflect an amplification cycle whereby bilirubin, acting as an antioxidant, is itself oxidized to biliverdin and then recycled by biliverdin reductase back to bilirubin. This redox cycle may constitute the principal physiologic function of bilirubin.

Psychological stress increases bilirubin metabolites in human urine

Some authors have suggested that psychological stress induces the production of reactive oxygen species (ROS). Some studies have supported that bilirubin exerts anti-oxidative effects in vivo. However, it is not known whether ROS induced by psychological stress provoke bilirubin oxidation in vivo. We investigated if the concentration of bilirubin oxidative metabolite (BOM), a bilirubin oxidative metabolite, increased in urine from subjects exposed to psychological stress. Sixty healthy male volunteers working in a pharmaceutical company were divided into a Group I which did not attend a conference, a Group II which attended a conference but did not deliver a speech, and a Group III which attended a conference and delivered speeches in the presence of the company executives. Subjective stress was scored (self-rating score) after subjects in Group III delivered their speeches at the conference. Urine was collected on the next day. The BOM concentrations, as measured by enzyme-linked immunosorbent assay, were normalized to the urinary concentration of creatinine. The concentration of BOM in Group III was significantly higher compared to that in Groups I and II (p<0.01 and p<0.05, respectively). Furthermore, in Group III, the concentration of BOM correlated with the self-rating stress score (r=0.53, p<0.01). These findings suggest that emotional stimuli are associated with an increase in the oxidative metabolites of bilirubin in human urine, and that BOMs could be useful markers of psychological stress.

Bilirubin-IXbeta is a marker of meconium, like zinc coproporphyrin

BACKGROUND

Because meconium accumulates continuously in the fetal intestine, analysis of the postnatally excreted material could yield important information of intrauterine metabolism and maturation. Therefore, a study of the bilirubin pigments in meconium and in the first neonatal stools was carried out.

METHODS

Meconium and stools from 37 neonates of various gestational ages were collected carefully, and stored at -20 degrees C, protected by aluminium foil. Samples were defrosted, vortex mixed with an equal amount of dimethyl sulfoxide, centrifuged, and submitted to analysis by high-pressure liquid chromatography using newly developed methods to identify and to quantitate the bilirubin-IXalpha, -IXbeta, -IXgamma, and -IXdelta isomers. In addition, samples were also submitted to diazo coupling with ethyl anthranilate. Total coproporphyrins and zinc coproporphyrin were assayed for comparison.

RESULTS

Unconjugated bilirubin-IXalpha and -IXbeta were detected in meconium but not the -IXgamma or the -IXbeta isomer. Bilirubin-IXbeta was the predominant pigment and comprised 63% to 96% of the unconjugated bilirubins in the first sample of meconium excreted. Its amount decreased rapidly during the first 5 days in full-term newborns, but this occurred more slowly in preterm neonates, especially in those with a gestational age less than 30 weeks. The decrease of bilirubin-IXbeta over time correlated with that of coproporphyrin.

CONCLUSIONS

Bilirubin-IXbeta is the prevailing bile pigment in the first excreted sample of meconium. It gradually decreases after birth and can be considered a biochemical marker of meconium, like zinc coproporphyrin.

Association of Serum Bilirubin Concentration with Risk of Coronary Artery Disease

Background: Lipid oxidation and formation of oxygen radicals are important elements of arterial plaque formation and atherosclerosis, and are involved in the pathophysiology of coronary artery disease (CAD). Because bilirubin has antioxidant properties, it has been suggested that it may have a protective role in the atherosclerotic process.

Approach: This review examines in vitro and in vivo studies indicating that bilirubin inhibits lipid oxidation and oxygen radical formation. Experimental and epidemiological evidence is presented that suggests that bilirubin may serve as a physiological antioxidant providing protection against atherosclerosis and CAD. Special attention is focused on studies that noted an inverse relationship between plasma bilirubin concentration and cardiovascular morbidity.

Content: Serum bilirubin concentrations in the upper portion of the reference interval reportedly reduce atherogenic risk and provide protection against CAD. In contrast, serum bilirubin concentrations in the lower portion of the reference interval may be associated with increased risk of ischemic heart disease.

Summary: Taken together, the evidence presented in this review supports the concept that bilirubin, via its antioxidant potential, has antiatherogenic properties and that an inverse relationship exists between circulating bilirubin concentrations and risk of CAD.

Studies on the specificity of the tetrapyrrole substrate for human biliverdin-IXalpha reductase and biliverdin-IXbeta reductase. Structure-activity relationships define models for both active sites

A comparison of the initial rate kinetics for human biliverdin-IXalpha reductase and biliverdin-IXbeta reductase with a series of synthetic biliverdins with propionate side chains "moving" from a bridging position across the central methene bridge (alpha isomers) to a "gamma-configuration" reveals characteristic behavior that allows us to propose distinct models for the two active sites. For human biliverdin-IXalpha reductase, as previously discussed for the rat and ox enzymes, it appears that at least one "bridging propionate" is necessary for optimal binding and catalytic activity, whereas two are preferred. All other configurations studied were substrates for human biliverdin-IXalpha reductase, albeit poor ones. In the case of mesobiliverdin-XIIIalpha, extending the propionate side chains to hexanoate resulted in a significant loss of activity, whereas the butyrate derivative retained high activity. For human biliverdin-IXalpha reductase, we suggest that a pair of positively charged side chains play a key role in optimally binding the IXalpha isomers. In the case of human biliverdin-IXbeta reductase, the enzyme cannot tolerate even one propionate in the bridging position, suggesting that two negatively charged residues on the enzyme surface may preclude productive binding in this case. The flavin reductase activity of biliverdin-IXbeta reductase is potently inhibited by mesobiliverdin-XIIIalpha and protohemin, which is consistent with the hypothesis that the tetrapyrrole and flavin substrate bind at a common site.

Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin, the oxidative product of heme in mammals, is excreted into the bile after its esterification with glucuronic acid to polar mono- and diconjugated derivatives. The accumulation of unconjugated and conjugated bilirubin in the serum is caused by several types of hereditary disorder. The Crigler-Najjar syndrome is caused by a defect in the gene which encodes bilirubin UDP-glucuronosyltransferase (UGT), whereas the Dubin-Johnson syndrome is characterized by a defect in the gene which encodes the canalicular bilirubin conjugate export pump of hepatocytes. Animal models such as the unconjugated hyperbilirubinemic Gunn rat, the conjugated hyperbilirubinemic GY/TR-, and the Eisai hyperbilirubinemic rat, have contributed to the understanding of the molecular basis of hyperbilirubinemia in humans. Elucidation of both the structure of the UGT1 gene complex, and the Mrp2 (cMoat) gene which encodes the canalicular conjugate export pump, has led to a greater understanding of the genetic basis of hyperbilirubinemia.

Bilirubin oxidation provoked by endotoxin treatment is suppressed by feeding ascorbic acid in a rat mutant unable to synthesize ascorbic acid

We examined the possibility that bilirubin oxidation is provoked in vivo by using scurvy-prone ODS-od/od rats treated with endotoxin (lipopolysaccharide). Recently, bilirubin oxidative metabolites were isolated from human urine and named biotripyrrin-a and biotripyrrin-b. In ODS-od/od rats fed an ascorbic-acid-free diet, the concentration of bilirubin metabolites in urine was increased 7.0-fold at 3 h after injection of lipopolysaccharide and 4.4-fold at 10 h compared to the control rats injected with saline. The dietary supplement of ascorbic acid, the physiological antioxidant, suppressed the increase in bilirubin metabolites in urine after lipopolysaccharide injection: concentrations of biotripyrrin-a and biotripyrrin-b in urine collected 6.5-10 h after the injection were lower in rats fed an ascorbic-acid-supplemented diet than in rats fed an ascorbic-acid-free diet. Moreover, feeding of ascorbic acid suppressed the hepatic mRNA level of heme oxygenase-1, the rate-limiting enzyme of bilirubin biosynthesis, in rats injected with lipopolysaccharide. These findings indicate that bilirubin oxidation is markedly stimulated in lipopolysaccharide-treated rats and suggest that bilirubin and ascorbic acid have physiologically protective effects against oxidative stress.

Cloning and characterization of the cDNA encoding human biliverdin-IX alpha reductase

Biliverdin reductase is classified into two isoforms in substrate specificity; biliverdin-IX alpha reductase and biliverdin-IX beta reductase with a molecular mass of 22 kDa and 34-42 kDa, respectively. We have cloned the cDNA encoding human biliverdin-IX alpha reductase from MOLT4 cDNA library. The cDNA of 1146 bp in nucleotide length contained an entire reading frame coding 296 amino acid residues. The NADH/NADPH binding consensus sequence was found in the amino-terminal region. Comparison between human and rat biliverdin-IX alpha reductases showed 82.8% identity in amino acid sequences and 80.3% identity in the coding nucleotides. The amino acid sequence of human biliverdin-IX alpha reductase showed no significant homology to that of human biliverdin-IX beta reductase. Northern blot analysis of poly(A) RNA from eight different human tissues revealed that the reductase mRNA was abundant in the brain, lung and pancreas but not in the liver. The distribution pattern of biliverdin-IX alpha message was different from that of heme oxygenase activity which is known to be high in the liver and to be low in the heart and lung.