Blue light and bilirubin excretion

A Platform for the Synthesis of Oxidation Products of Bilirubin

Bilirubin is the principal product of heme catabolism. High concentrations of the pigment are neurotoxic, yet slightly elevated levels are beneficial. Being a potent antioxidant, oxidative transformations of bilirubin occur in vivo and lead to various oxidized fragments. The mechanisms of their formation, intrinsic biological activities, and potential roles in human pathophysiology are poorly understood. Degradation methods have been used to obtain samples of bilirubin oxidation products for research. Here, we report a complementary, fully synthetic method of preparation. Our strategy leverages repeating substitution patterns in the parent tetracyclic pigment. Functionalized ready-to-couple γ-lactone, γ-lactam, and pyrrole monocyclic building blocks were designed and efficiently synthesized. Subsequent modular combinations, supported by metal-catalyzed borylation and cross-coupling chemistries, translated into the concise assembly of the structurally diverse bilirubin oxidation products (BOXes, propentdyopents, and biopyrrins). The discovery of a new photoisomer of biopyrrin A named lumipyrrin is reported. Synthetic bilirubin oxidation products made available in sufficient purity and quantity will support future in vitro and in vivo investigations.

Noninvasive, continuous fluorescence monitoring of bilirubin photodegradation

Nowadays phototherapy is widely used for treatment of various diseases. However, efficient application of phototherapy requires an understanding of light interactions with main endogenous chromophores (e.g., hemoglobin, bilirubin, and water) in tissue. In particular, bilirubin is the target chromophore in the treatment of neonatal jaundice, which is the most common disease affecting up to 80% of preterm infants. The most frequently recommended treatment technique for this disease is phototherapy with blue light in combination with conventional drug therapy. To follow threshold total serum bilirubin (TSB) concentration guidelines, it is essential to estimate TSB concentration accurately. The gold standard biochemical analysis is invasive and bulky. Moreover, noninvasive methods do not provide sufficient reproducibility and accuracy. In this research, the fluorescence sensing of bilirubin with human serum albumin complexes was studied. The fluorescence time course during light irradiation (central wavelength: 467 nm and power density: 12.13 mW cm^-2) was demonstrated to depend on the initial concentration. Specifically, for the bilirubin concentration C = 18.65 μM, an insignificant fluorescence signal increase was observed during the first 30 minutes of light irradiation, while for bilirubin concentration C = 373 μM, the fluorescence signal did not reach maximum during 2.5 hours of light irradiation. Thus, fluorescence sensing might show increased accuracy when used with other noninvasive bilirubin sensing methods.

Investigation of Ultrafast Configurational Photoisomerization of Bilirubin Using Femtosecond Stimulated Raman Spectroscopy

Phototherapy is an efficient and safe way to reduce high levels of free 4Z,15Z-bilirubin (ZZ-BR) in the serum of newborns. The success of BR phototherapy lies in photoinduced configurational and structural isomerization processes that form excretable isomers. However, the physical picture of photoinduced photoisomerization of ZZ-BR is still unclear. Here, we strategically implement tunable femtosecond stimulated Raman spectroscopy and several time-resolved electronic spectroscopies, assisted by quantum chemical calculations, to dissect the detailed primary configurational isomerization dynamics of free ZZ-BR in organic solvents. The results of this study demonstrate that upon photoexcitation, ultrafast configurational isomerization proceeds by a volume-conserving "hula twist", followed by intramolecular hydrogen-bond distortion and large-scale rotation of the two dipyrrinone halves of the ZZ-BR isomer in a few picoseconds. After that, most of the population recovers back to ZZ-BR, and a very small amount is converted into stable BR isomers via structural isomerization.

Understanding the Logistics for the Distribution of Heme in Cells

Heme is essential for the survival of virtually all living systems-from bacteria, fungi, and yeast, through plants to animals. No eukaryote has been identified that can survive without heme. There are thousands of different proteins that require heme in order to function properly, and these are responsible for processes such as oxygen transport, electron transfer, oxidative stress response, respiration, and catalysis. Further to this, in the past few years, heme has been shown to have an important regulatory role in cells, in processes such as transcription, regulation of the circadian clock, and the gating of ion channels. To act in a regulatory capacity, heme needs to move from its place of synthesis (in mitochondria) to other locations in cells. But while there is detailed information on how the heme lifecycle begins (heme synthesis), and how it ends (heme degradation), what happens in between is largely a mystery. Here we summarize recent information on the quantification of heme in cells, and we present a discussion of a mechanistic framework that could meet the logistical challenge of heme distribution.

Highly Emissive Biological Bilirubin Molecules: Shedding New Light on the Phototherapy Scheme

Bilirubin (BR) is the main end-product of the hemoglobin catabolism. For decades, its photophysics has been mainly discussed in terms of ultrafast deactivation of the excited state in solution, where, indeed, BR shows a very low green emission quantum yield (EQY), 0.03%, resulting from an efficient nonradiative isomerization process. Herein, we present, for the first time, unique and exceptional photophysical properties of solid-state BR, which amend by changing the type of crystal, from a closely packed α crystal to an amorphous loosely packed β crystal. BR α crystals show a very bright red emission with an EQY of ca. 24%, whereas β crystals present, in addition, a low green EQY of ca. 0.5%. By combining density functional theory (DFT) calculations and time-resolved emission spectroscopy, we trace back this dual emission to the presence of two types of BR molecules in the crystal: a "stiff" monomer, M1, distorted by particularly strong internal H-bonds and a "floppy" monomer, M2, having a structure close to that of BR in solution. We assign the red strong emission of BR crystals to M1 present in both the α and β crystals, while the low green emission, only present in the amorphous (β) crystal, is interpreted as M2 emission. Efficient energy-transfer processes from M2 to M1 in the closely packed α crystal are invoked to explain the absence of the green component in its emission spectrum. Interestingly, these unique photophysical properties of BR remain in polar solvents such as water. Based on these unprecedented findings, we propose a new model for the phototherapy scheme of BR inside the human body and highlight the usefulness of BR as a strong biological fluorescent probe.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Unconjugated bilirubin is associated with protection from early-life wheeze and childhood asthma

BACKGROUND

Wheeze and allergic sensitization are the strongest early-life predictors of childhood asthma development; the molecular origins of these early-life phenotypes are poorly understood.

OBJECTIVES

We sought to identify metabolites associated with early-life wheeze, allergic sensitization, and childhood asthma.

METHODS

We conducted a nested case-control study using Environmental influences on Child Health Outcomes Program cohorts for discovery and independent replication. Wheeze and allergic sensitization were defined by number of wheeze episodes and positive specific IgE at age 1 year, respectively. Asthma was defined as physician diagnosis of asthma at age 5 or 6 years. We used untargeted metabolomics, controlling for observed and latent confounding factors, to assess associations between the plasma metabolome and early-life wheeze, allergy, and childhood asthma.

RESULTS

Eighteen plasma metabolites were associated with first-year wheeze in the discovery cohort (n = 338). Z,Z unconjugated bilirubin (UCB) and its related metabolites exhibited a dose-response relationship with wheeze frequency; UCB levels were 13% (β = 0.87; 95% CI, 0.74-1.02) and 22% (β = 0.78; 95% CI, 0.68-0.91) lower in children with 1 to 3 and 4+ wheeze episodes compared with those who never wheezed, respectively. UCB levels were also associated with childhood asthma (β = 0.82; 95% CI, 0.68-0.98). Similar trends were observed in 2 independent cohorts. UCB was significantly negatively correlated with eicosanoid- and oxidative stress-related metabolites. There were no significant associations between metabolites and allergic sensitization.

CONCLUSIONS

We identified a novel inverse, dose-dependent association between UCB and recurrent wheeze and childhood asthma. Inflammatory lipid mediators and oxidative stress byproducts inversely correlated with UCB, suggesting that UCB modulates pathways critical to the development of early-life recurrent wheeze and childhood asthma.

Conformational Control of the Photodynamics of a Bilirubin Dipyrrinone Subunit: Femtosecond Spectroscopy Combined with Nonadiabatic Simulations

The photochemistry of bilirubin has been extensively studied due to its importance in the phototherapy of hyperbilirubinemia. In the present work, we investigated the ultrafast photodynamics of a bilirubin dipyrrinone subunit, vinylneoxanthobilirubic acid methyl ester. The photoisomerization and photocyclization reactions of its (E) and (Z) isomers were studied using femtosecond transient absorption spectroscopy and by multireference electronic structure theory, where the nonadiabatic dynamics was modeled with a Landau-Zener surface hopping technique. The following picture has emerged from the combined theoretical and experimental approach. Upon excitation, dipyrrinone undergoes a very fast vibrational relaxation, followed by an internal conversion on a picosecond time scale. The internal conversion leads either to photoisomerization or regeneration of the starting material. Further relaxation dynamics on the order of tens of picoseconds was observed in the ground state. The nonadiabatic simulations revealed a strong conformational control of the photodynamics. The ultrafast formation of a cyclic photochemical product from a less-populated conformer of the studied subunit was predicted by our calculations. We discuss the relevance of the present finding for the photochemistry of native bilirubin. The work has also pointed to the limits of semiclassical nonadiabatic simulations for simulating longer photochemical processes, probably due to the zero-point leakage issue.

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.

Recent Advances in the Understanding of the Reaction Chemistries of the Heme Catabolizing Enzymes HO and BVR Based on High Resolution Protein Structures

In mammals, catabolism of the heme group is indispensable for life. Heme is first cleaved by the enzyme Heme Oxygenase (HO) to the linear tetrapyrrole Biliverdin IXα (BV), and BV is then converted into bilirubin by Biliverdin Reductase (BVR). HO utilizes three Oxygen molecules (O2) and seven electrons supplied by NADPH-cytochrome P450 oxidoreductase (CPR) to open the heme ring and BVR reduces BV through the use of NAD(P)H. Structural studies of HOs, including substrate-bound, reaction intermediate-bound, and several specific inhibitor-bound forms, reveal details explaining substrate binding to HO and mechanisms underlying-specific HO reaction progression. Cryo-trapped structures and a time-resolved spectroscopic study examining photolysis of the bond between the distal ligand and heme iron demonstrate how CO, produced during the HO reaction, dissociates from the reaction site with a corresponding conformational change in HO. The complex structure containing HO and CPR provides details of how electrons are transferred to the heme-HO complex. Although the tertiary structure of BVR and its complex with NAD+ was determined more than 10 years ago, the catalytic residues and the reaction mechanism of BVR remain unknown. A recent crystallographic study examining cyanobacterial BVR in complex with NADP+ and substrate BV provided some clarification regarding these issues. Two BV molecules are bound to BVR in a stacked manner, and one BV may assist in the reductive catalysis of the other BV. In this review, recent advances illustrated by biochemical, spectroscopic, and crystallographic studies detailing the chemistry underlying the molecular mechanism of HO and BVR reactions are presented.

Light-induced morphological transition between unconjugated bilirubin photoisomers

Morphology switching by an external stimulus creates the possibility to detect and control the activity and functionality of bio-molecules. Unconjugated bilirubin (UCB), a waste product resulting from heme catabolism, is highly sensitive towards blue light induced configurational conversion from (4Z,15Z) to (4Z,15E)-bilirubin. UCB has a distinct elongated nanostructure which is readily photoswitchable to spherical by external blue light (470 nm) irradiation. Herein, the morphology alteration by blue light was nicely correlated with the photoisomerisation of UCB, using different microscopic and spectroscopic techniques. To restrict the other photo-incidents during phototreatment on UCB, a suitable time frame was calibrated by monitoring the absorption, HPLC, lifetime distribution and ¹H NMR studies. Furthermore, by the help of this morphological transition as a marker, UCB early stage photoisomerisation has also been triggered by two-photon irradiation (940 nm).

Monitoring cellular redox state under hypoxia using a fluorescent sensor based on eel fluorescent protein

Genetically encoded fluorescent sensors are widely used to visualize secondary messengers, metabolites and dynamic events in living cells. However, almost all of these sensors are based on Aequorea GFPs or GFP-like proteins, which do not correctly maturate and fluoresce under hypoxia or anoxic conditions, greatly limiting their application in biomedical research. Herein, we provide a novel strategy for design of sensors and report a series of thiol redox-sensitive sensor based on a recently discovered oxygen-independent fluorescent protein UnaG from Japanese eel. These redox sensors have large dynamic range, rapid responsiveness, a flexible "switch", and pH-independence, are particularly compatible with hypoxia conditions, and therefore represent a substantial improvement for live-cell redox measurement. We further demonstrated the versatility of these redox sensors, by simultaneously monitoring redox changes and hypoxia state in living cells, thereby proving its capability as a powerful and flexible tool for indexing multidimensional metabolism data in the context of physiological stressors and pathological states. These redox sensors are not only the first case of UnaG-based functional sensors, but also the first case of functional sensors based on non GFP-like proteins. Based on this strategy, more oxygen-independent biosensors could be developed, hence, provide new opportunities for bioimaging.

A substrate-bound structure of cyanobacterial biliverdin reductase identifies stacked substrates as critical for activity

Biliverdin reductase catalyses the last step in haem degradation and produces the major lipophilic antioxidant bilirubin via reduction of biliverdin, using NAD(P)H as a cofactor. Despite the importance of biliverdin reductase in maintaining the redox balance, the molecular details of the reaction it catalyses remain unknown. Here we present the crystal structure of biliverdin reductase in complex with biliverdin and NADP+. Unexpectedly, two biliverdin molecules, which we designated the proximal and distal biliverdins, bind with stacked geometry in the active site. The nicotinamide ring of the NADP+ is located close to the reaction site on the proximal biliverdin, supporting that the hydride directly attacks this position of the proximal biliverdin. The results of mutagenesis studies suggest that a conserved Arg185 is essential for the catalysis. The distal biliverdin probably acts as a conduit to deliver the proton from Arg185 to the proximal biliverdin, thus yielding bilirubin.

Study of Model Systems for Bilirubin and Bilin Chromophores: Determination and Modification of Thermal and Photochemical Properties

Bilin chromophores and bilirubin are involved in relevant biological functions such as light perception in plants and as protective agents against Alzheimer and other diseases. Despite their extensive use, a deep rationalization of the main factors controlling the thermal and photochemical properties has not been performed yet, which in turn hampers further applications of these versatile molecules. In an effort to understand those factors and allow control of the relevant properties, a combined experimental and computational study has been carried out for diverse model systems to understand the interconversion between Z and E isomers. In this study, we have demonstrated the crucial role of steric hindrance and hydrogen-bond interactions in thermal stability and the ability to control them by designing novel compounds. We also determined several photochemical properties and studied the photodynamics of two model systems in more detail, observing a fast relaxation of the excited state shorter than 2 ps in both cases. Finally, the computational study allowed us to rationalize the experimental evidence.

Neonatal Blue Light Phototherapy and Melanocytic Nevus Count in Children: A Systematic Review and Meta-Analysis of Observational Studies

BACKGROUND

Neonatal blue light phototherapy (NBLP) is an established method of managing neonatal hyperbilirubinemia. Approximately 5% of newborns are exposed to NBLP. Evidence of whether NBLP predisposes to the development of melanocytic nevi later in life has been conflicting.

OBJECTIVES

The goal of the current study was to conduct a systematic review and meta-analysis to quantitatively assess the effect of NBLP on melanocytic nevus count.

METHODS

We searched for observational studies in Medline, EMBASE, and the Cochrane Central Register from their inception to April 15, 2015. Meta-analysis of Observational Studies in Epidemiology guidelines were followed. DerSimonian and Laird random-effects models were used to calculate the weighted mean difference (WMD). Publication bias was assessed using a funnel plot and the Egger's test.

RESULTS

Five studies with a total of 2,921 subjects were included, of whom 642 underwent NBLP. With random-effects modeling, those who had previous NBLP did not have a significantly higher mean number of melanocytic nevi (WMD = 0.32 [95% confidence interval -0.67, 1.31], p = 0.53). Visual inspection of the funnel plot suggested potential publication bias, although the Egger's test (p = 0.09) indicated no small-study effect.

CONCLUSION

There was no evidence that prior NBLP exposure significantly increased the number of melanocytic nevi. Available evidence has not revealed any cause for major concern for NBLP. Other risk factors such as exposure to sunlight, childhood history of sunburn, and fair skin complexion might play a greater role in the development of melanocytic nevi in childhood.

Ultrafast deactivation of bilirubin: dark intermediates and two-photon isomerization

The Franck-Condon state A couples to the dark intermediate B, which shows tight molecular skeleton and distorted hydrogen bonding. B deactivates with nearly 100% efficiency. 2-Photon excitation at 400 nm triggers Z,E isomerization efficiently.

Continuous de novo biosynthesis of haem and its rapid turnover to bilirubin are necessary for cytoprotection against cell damage

It is well known that haem serves as the prosthetic group of various haemoproteins that function in oxygen transport, respiratory chain, and drug metabolism. However, much less is known about the functions of the catabolites of haem in mammalian cells. Haem is enzymatically degraded to iron, carbon monoxide (CO), and biliverdin, which is then converted to bilirubin. Owing to difficulties in measuring bilirubin, however, the generation and transport of this end product remain unclear despite its clinical importance. Here, we used UnaG, the recently identified bilirubin-binding fluorescent protein, to analyse bilirubin production in a variety of human cell lines. We detected a significant amount of bilirubin with many non-blood cell types, which was sensitive to inhibitors of haem metabolism. These results suggest that there is a basal level of haem synthesis and its conversion into bilirubin. Remarkably, substantial changes were observed in the bilirubin generation when cells were exposed to stress insults. Since the stress-induced cell damage was exacerbated by the pharmacological blockade of haem metabolism but was ameliorated by the addition of biliverdin and bilirubin, it is likely that the de novo synthesis of haem and subsequent conversion to bilirubin play indispensable cytoprotective roles against cell damage.

Post-transcriptional regulation by miR-137 underlies the low abundance of CAR and low rate of bilirubin clearance in neonatal mice

AIM

Jaundice, potentially fatal encephalopathy, is common in approximately two-thirds of all well term infants. It is largely due to low expression of constitutive androstane receptor (CAR) in newborns; however, the mechanisms for this low expression were poorly understood.

MATERIALS AND METHODS

Expression of miR-137 and CAR was compared between neonatal and adult mice and between healthy and a mouse model of obstructive jaundice (OJ) using real-time RT-PCR and Western blot methods. Rate of bilirubin clearance was measured. DNA methylation of miR-137 was analyzed.

KEY FINDINGS

Inverse expressions of miR-137 and CAR were consistently observed between newborn and adult mice, with a significantly higher miR-137 level and lower CAR protein and mRNA levels in neonatal liver than in adult liver. Similar reciprocal relationship was found existing between adult OJ mice and healthy control animals with a higher miR-137 level and lower CAR protein and mRNA levels in OJ than in healthy mice. Forced expression of miR-137 in primary hepatocytes repressed CAR protein levels, which was prevented by the inhibitor of miR-137. Knockdown of endogenous miR-137 by its inhibitor increased the rate of bilirubin clearance in OJ mice. Finally, we found that miR-137 was epigenetically over-activated due to hypomethylation in neonatal mice and in adult OJ mice, relative to adult healthy animals.

SIGNIFICANCE

Our findings indicate that miR-137 is a repressor of CAR and thus a critical determinant of bilirubin clearance and may be considered a molecular target for the treatment of neonatal hyperbilirubinemia.

A possible molecular mechanism of immunomodulatory activity of bilirubin

Bilirubin is an endogenous product of heme degradation in mammals. Bilirubin has long been considered as a cytotoxic waste product that needs to be excreted. However, increasing evidence suggests that bilirubin possesses multiple biological activities. In particular, recent studies have shown that bilirubin should be a protective factor for several autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus. Since these autoimmune diseases are closely associated with specific types of human leukocyte antigens (HLAs), we have hypothesized that bilirubin might bind to the antigenic peptide-binding groove of the HLA molecules and exert its immunosuppressive actions. In order to evaluate the hypothesis, theoretical docking studies between bilirubin and the relevant HLA molecules have been undertaken. The in silico studies have clearly shown that bilirubin may bind to the antigenic peptide-binding groove of the HLA molecules relevant to the autoimmune diseases with significant affinity. The bound bilirubin may block the binding of antigenic peptides to be presented to T cell receptors and lead to suppression of the autoimmune responses. Based on this hypothesis new drug discovery research for autoimmune diseases will be conducted.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

A second conserved GAF domain cysteine is required for the blue/green photoreversibility of cyanobacteriochrome Tlr0924 from Thermosynechococcus elongatus

Phytochromes are widely occurring red/far-red photoreceptors that utilize a linear tetrapyrrole (bilin) chromophore covalently bound within a knotted PAS-GAF domain pair. Cyanobacteria also contain more distant relatives of phytochromes that lack this knot, such as the phytochrome-related cyanobacteriochromes implicated to function as blue/green switchable photoreceptors. In this study, we characterize the cyanobacteriochrome Tlr0924 from the thermophilic cyanobacterium Thermosynechococcus elongatus. Full-length Tlr0924 exhibits blue/green photoconversion across a broad range of temperatures, including physiologically relevant temperatures for this organism. Spectroscopic characterization of Tlr0924 demonstrates that its green-absorbing state is in equilibrium with a labile, spectrally distinct blue-absorbing species. The photochemically generated blue-absorbing state is in equilibrium with another species absorbing at longer wavelengths, giving a total of 4 states. Cys499 is essential for this behavior, because mutagenesis of this residue results in red-absorbing mutant biliproteins. Characterization of the C 499D mutant protein by absorbance and CD spectroscopy supports the conclusion that its bilin chromophore adopts a similar conformation to the red-light-absorbing P r form of phytochrome. We propose a model photocycle in which Z/ E photoisomerization of the 15/16 bond modulates formation of a reversible thioether linkage between Cys499 and C10 of the chromophore, providing the basis for the blue/green switching of cyanobacteriochromes.

Crystallographic analysis of human serum albumin complexed with 4Z,15E-bilirubin-IXalpha

Bilirubin, an insoluble yellow-orange pigment derived from heme catabolism, accumulates to toxic levels in individuals with impaired or immature liver function. The resulting jaundice may be managed with phototherapy to isomerize the biosynthetic 4Z,15Z-bilirubin-IXalpha to more soluble and excretable isomers, such as 4Z,15E-bilirubin. Bilirubin and its configurational isomers are transported to the liver by human serum albumin (HSA) but their precise binding location(s) on the protein have yet to be determined. To investigate the molecular details of their interaction, we co-crystallised bilirubin with HSA. Strikingly, the crystal structure--determined to 2.42 A resolution--revealed the 4Z,15E-bilirubin-IXalpha isomer bound to an L-shaped pocket in sub-domain IB. We also determined the co-crystal structure of HSA complexed with fusidic acid, an antibiotic that competitively displaces bilirubin from the protein, and showed that it binds to the same pocket. These results provide the first crystal structure of a natural bilirubin pigment bound to serum albumin, challenge some of the present conceptions about HSA-bilirubin interactions, and provide a sound structural framework for finally resolving the long-standing question of where 4Z,15Z-bilirubin-IXalpha binds to the protein.

Initial photochemistry of bilirubin probed by femtosecond spectroscopy

Bilirubin is a breakdown product from heme catabolism, and reduced excretion of bilirubin can lead to jaundice. Phototherapy is the most common treatment for neonatal jaundice, a condition frequently encountered in newborn infants. Knowledge of the photochemistry of bilirubin, which is dominated by (ultra)fast components, is necessary for the profound understanding of the processes in phototherapy. Here, we report results from femtosecond fluorescence upconversion measurements on bilirubin and half-bilirubin model compounds, as well as pump-probe absorption measurements on bilirubin. A fast component of ca. 120 fs in the multiexponential fluorescence decay, being only visible in the bilirubin molecule, is interpreted as exciton localization within the molecular halves. The slower components of several hundreds of femtoseconds and a few picoseconds, occurring in bilirubin and the half-bilirubin model, are interpreted as relaxation to a (twisted) intermediate, which decays further with ca. 15 ps to the ground state.

In situ circular dichroic electrochemical study of bilirubin and bovine serum albumin complex

The electrooxidation of bilirubin (BR) and bovine serum albumin (BSA) complexes was studied by in situ circular dichroism (CD) spectroelectrochemistry. The result showed that the mechanism of the whole electrooxidation process of this complex corresponded to electrochemical processes (EE mechanism) in aqueous solution. Some parameters of the process were obtained by double logarithm method, differential method and nonlinear regression method. In visible region, CD spectra of the two enantiomeric components of the complex and their fraction distribution against applied potentials were obtained by singular value decomposition least-square (SVDLS) method. Meanwhile, the distribution of the five components of secondary structure was also obtained by the same method in far-UV region. The peak potential gotten from EE mechanism corresponds to a turning point for the component transition, beyond which the whole reaction reaches a new equilibrium. Under applied positive potentials, the enantiomeric equilibrium between M and P form is broken and M form transfers to its enantiomer of P, while the fraction of alpha-helix increases and that improves the transition to P form.

Effect of position changing on bilirubin levels during phototherapy

OBJECTIVE

Turning of infants during phototherapy for hyperbilirubinemia is practiced in many nurseries. However, there is little research evidence in support of this practice. This study examined the effect of turning on serum total bilirubin concentration and on the duration of phototherapy.

STUDY DESIGN

We first conducted a pilot study in term infants requiring phototherapy using transcutaneous bilirubinometry in order to determine the time required to clear the skin of bilirubin. This "blanching time" was found to be approximately 150 minutes. We then conducted a randomized study comparing turning the baby during phototherapy with care in the supine position only.

RESULTS

Thirty term infants were enrolled in the study (turned - 14; supine - 16). No differences were found between the groups in baseline data, such as birth weight, gestational age, age at start of phototherapy, or type of feeds. Infants in the supine group showed a significantly larger drop in serum total bilirubin concentration and required a shorter duration of phototherapy.

CONCLUSION

We conclude that infants should be nursed supine during phototherapy. Based on these results, we propose a modification to the traditional model of bilirubin kinetics during phototherapy.

Affinity of human serum albumin for bilirubin varies with albumin concentration and buffer composition: results of a novel ultrafiltration method

Albumin binding is a crucial determinant of bilirubin clearance in health and bilirubin toxicity in certain disease states. However, prior attempts to measure the affinity of albumin for bilirubin have yielded highly variable results, reflecting both differing conditions and the confounding influence of impurities. We therefore have devised a method based on serial ultrafiltration that successively removes impurities in [(14)C]bilirubin until a stable binding affinity is achieved, and then we used it to assess the effect of albumin concentration and buffer composition on binding. The apparent binding affinity of human serum albumin for [(14)C]bilirubin was strongly dependent on assay conditions, falling from (5.09 +/- 0.24) x 10(7) liters/mol at lower albumin concentrations (15 microm) to (0.54 +/- 0.05) x 10(7) liters/mol at higher albumin concentrations (300 microm). To determine whether radioactive impurities were responsible for this change, we estimated impurities in the stock bilirubin using a novel modeling approach and found them to be 0.11-0.13%. Formation of new impurities during the study and their affinity for albumin were also estimated. After correction for impurities, the binding affinity remained heavily dependent on the albumin concentration (range (5.37 +/- 0.26) x 10(7) liters/mol to (0.65 +/- 0.03) x 10(7) liters/mol). Affinities decreased by about half in the presence of chloride (50 mm). Thus, the affinity of human albumin for bilirubin is not constant, but varies with both albumin concentration and buffer composition. Binding may be considerably less avid at physiological albumin concentrations than previously believed.

Threshold concentration of unbound bilirubin to induce neurological deficits in a patient with type I Crigler-Najjar syndrome

Based on the clinical course of a 16-year-old boy with type I Crigler-Najjar syndrome, we estimated the threshold concentration of unbound bilirubin, as assayed by the horseradish peroxidase method, that apparently induces toxicity to the brain. Before the age of 15, the patient did not manifest any neurological or behavioural dysfunction despite increased bilirubin in serum. The binding affinity and the binding capacity of the patient's serum albumin for bilirubin determined when he was about 14 years old were 10(8)(mol/L)-1 and 1.01 to 1.04 mol/L, respectively. These values were nearly the same as those of normal controls reported in the literature. The total bilirubin binding capacity was greater than the patient's total bilirubin concentration, showing that his serum albumin was not saturated with bilirubin. The reserve bilirubin binding capacity (RBBC) was estimated to be 158 mumol/L and the unbound bilirubin concentration to be 15.1 nmol/L. Concentration of unbound bilirubin peaked at 21.7 nmol/L at the age of 15 years and 11 months, i.e. 2 months before the onset of difficulties in walking and speaking. At this time, the RBBC was estimated as -64 mumol/L. A peak concentration of total bilirubin, 811 mumol/L, was observed during the period of rapid loss of the ability to walk or speak. At the age of 16 years and 1 month the RBBC decreased to -98 mumol/L and the unbound bilirubin concentration to 18.8 nmol/L. Following phototherapy, the patient's neurological state returned to normal; he could speak and walk normally. At the age of 16 years and 2 months the RBBC returned to 105 mumol/L and unbound bilirubin decreased to 16.6 nmol/L. These results suggest that maintaining the concentration of unbound bilirubin at < 20 nmol/L and the total bilirubin concentration at lower than the binding capacity of serum albumin is important for prevention of neurological deficits in Crigler-Najjar syndrome. The upper limit of unbound bilirubin in such an older patient was nearly the same as that reported for newborns.

Cloning and overexpression of rat kidney biliverdin IX alpha reductase as a fusion protein with glutathione S-transferase: stereochemistry of NADH oxidation and evidence that the presence of the glutathione S-transferase domain does not effect BVR-A activity

Native biliverdin IX alpha reductase (BVR-A) is a monomer of molecular mass 34 kDa. We have developed an expression vector that allows the isolation of 40 mg of a glutathione S-transferase (GST)-BVR-A fusion protein from 1 litre of culture. The fusion protein (60 kDa) behaves as a dimer on gel filtration (120 kDa), so that we have artificially created a BVR-A dimer. The recombinant rat kidney enzyme exhibits pre-steady-state 'burst' kinetics that show a pH dependence similar to that already described for ox kidney BVR-A. Similar behaviour was obtained in the presence and absence of the GST domain both for the burst kinetics and during initial-rate studies in the presence and absence of albumin. The stereospecificity of the BVR-A-catalysed oxidation of [4-3H]NADH, labelled at the A and B faces, was shown to occur exclusively via the B face.

Bilirubin bound to cells does not form photoisomers

Cultured cells from one human and one murine cell line were incubated with bilirubin by different methods that allowed bilirubin to be bound to cells. The cells were irradiated with visible light of different wavelengths. Bilirubin bound to human serum albumin was also irradiated with light. After irradiation, bilirubin and its photoisomers were extracted and analyzed by HPLC. No photoisomers were found in samples of irradiated cells, while the types and amounts of photoisomers that were expected from the literature were found in samples of irradiated bilirubin/albumin mixtures. We conclude that the formation of therapeutically active photoisomers during phototherapy most probably does not take place in skin cells, but most likely in bilirubin bound to albumin in the vessels or in the interstitial space.

Ultraviolet light burn: a cutaneous complication of visible light phototherapy of neonatal jaundice

Visible light phototherapy is an easily administered and effective treatment for neonatal indirect hyperbilirubinemia. Reported cutaneous side effects include transient rashes and the uncommon bronze baby syndrome. A more hazardous side effect is ultraviolet burn. Two premature infants developed phototherapy-induced erythema, one associated with a second-degree burn, after exposure to fluorescent daylight bulbs inadvertently used without Plexiglass shields, thus allowing prolonged ultraviolet A (UVA) exposure. Premature infants, especially during the first two weeks of life, may be significantly susceptible to UVA-induced erythema. Plexiglass shields should always be in place during visible light phototherapy, and nursery staff should be made aware of their purpose.

Oral treatment for jaundice using immobilized bilirubin oxidase

Jaundice is characterized by an excessive accumulation of bilirubin in the blood and tissues. A novel approach to reduce plasma levels of bilirubin by blocking its enterohepatic circulation was investigated. The treatment consisted of oral administration of immobilized bilirubin oxidase, which could oxidize bilirubin in the intestine to less toxic and more water-soluble products. In vivo administration of 0.1 to 2.0 mg/day of immobilized enzyme over a four-day period to chronically jaundiced Gunn rats effectively lowered plasma bilirubin levels, but only when the molar ratio of total serum bilirubin to rat serum albumin (B/RSA) was larger than 0.35. Plasma bilirubin concentration decreased in that group from an initial value of 11.3 to 6.3 mg/dl (-40%, n = 5) after eight days. This decrease was statistically significant (p < 0.05 by Student's t test). However, administration of bilirubin oxidase to rats with a B/RSA ratio less than 0.35 (n = 10) resulted in no statistically significant change in plasma bilirubin concentration.

Neonatal exposure to protoporphyrin-activating lighting as a contributing cause of childhood acute lymphocytic leukemia

While being a relatively rare disease, acute lymphocytic leukemia (ALL) is the leading form of cancer in children in the developed world today. ALL sharply peaks in incidence at ages three to four years. In the United States there have been persistent, unexplained increases in incidence of ALL in the past two decades. We hypothesize that exposure to photosensitizing lighting immediately after birth may be a contributing cause of ALL. Fluorescent lamps and other light sources with strong illumination, around 400 nanometers, are protoporphyrin-activating. Activation of protoporphyrin produces superoxides and free radicals that can induce breaks in DNA. In newborn nurseries in the US, the intensity of lighting has increased five- to 10-fold over the past two decades. Thus, protoporphyrin-activating light may be a contributing cause of childhood ALL. Additional retrospective and prospective studies should be undertaken of the relationship between exposure of newborns to protoporphyrin-activating illumination and the development of childhood ALL, along with in vitro studies of the hematologic effects of fluorescent lighting. Protoporphyrin-activating lighting is clearly not the sole determinant of ALL, but it could be a completely preventable cause. Inexpensive plastic filters could reduce these exposures substantially.

Hepatic handling of photoirradiated bilirubin. A study in isolated perfused Wistar rat liver

Conjugation has been considered the rate-limiting step for bilirubin hepatic transport, and bypass of this metabolic step could explain why photobilirubins can be rapidly cleared by the liver. In this paper we assessed whether photoirradiation may enhance the bilirubin overall hepatic transport in the isolated perfused Wistar rat liver, a model possessing intact transport and conjugating systems. Bilirubin was administered as a bolus so as to reach a perfusate concentration of approximately 10 microM (bilirubin/albumin molar ratio 1:17). Perfusate light exposure (0.56.10(15) quanta s-1 cm-2) yielded 7-10% of configurational photoisomers, which were further identified as (4Z,15E/4E,15Z)-bilirubin IX alpha. Under such conditions, the perfusate removal rate was increased by 39% over that from dark conditions. Likewise, biliary excretion, estimated as total bilirubin recovery at 60 min, was also increased (+48%). This later improvement was mainly produced at the expense of unconjugated bilirubin, which most likely derived from its configurational photoisomers that, once excreted into bile, readily re-isomerized to the parent compound. In addition, this increment was partially due to a delayed improvement of monoglucuronide pigment excretion. The calculated hepatic pigment content was significantly higher under light conditions. A direct assessment of hepatic content of different bilirubin moieties at 20 min after bilirubin administration confirmed that such an increment was fully accounted for by unconjugated pigment. Our finding that hepatic pigment content rose (despite a higher biliary excretion) when the bilirubin was irradiated suggests a higher net uptake of photoisomers than native pigment. This observation, and the finding that bilirubin photoisomers were usually excreted without undergoing conjugation even if the metabolic system is active, contribute to explain the greater appearance of unconjugated bilirubin in Wistar rat bile under light exposure.

The influence of gestational age on bilirubin conjugation in newborns

Unconjugated, mono- and diconjugated bilirubin levels were determined in serum soon after birth, and followed up for several days. Fourteen preterm neonates were studied with a gestational age below 33 weeks (n = 7) or between 34 and 37 weeks (n = 7), respectively, as well as 19 full-term newborns either untreated (n = 9) or treated by phototherapy (n = 10). Bilirubin and its derivatives were analysed by alkaline methanolysis and spectrometry after separation by thin-layer chromatography. In normal full-term neonates total and unconjugated bilirubin reached peak levels at days 2-4. Thereafter, a decline of 11% per day was detectable. Monoconjugates in serum amounted to 3.1 +/- 1.1% of total pigment and remained at that level. The relative amount of diconjugates increased from 0.55 +/- 0.25% (2-4th postnatal day) to 1.62 +/- 0.99% (9-13th day of life). The rapid decline of unconjugated bilirubin paralleled by an increase of diconjugates are an expression of the maturation process for bilirubin conjugation. The premature neonates with less than 33 weeks gestation exhibited an increase of unconjugated serum bilirubin up to the 4-5th postnatal day, the decline thereafter amounted 2% per day. The fraction of 2.3 +/- 1.1% monoconjugates was small and exhibited only a moderate increase in the follow up. In contrast diconjugates were undetectable or very low and remained at this level. These results suggest the presence of a more severe immaturity as well as a slower maturation process of bilirubin conjugation in preterm newborns.(ABSTRACT TRUNCATED AT 250 WORDS)