Photoreversible Aggregation of the Biliprotein Containing the First and Second GAF Domains of a Cyanobacteriochrome All2699 in Nostoc sp. PCC7120

As plant photoreceptors, phytochromes are capable of detecting red light and far-red light, thereby governing plant growth. All2699 is a photoreceptor found in Nostoc sp. PCC7120 that specifically responds to red light and far-red light. All2699g1g2 is a truncated protein carrying the first and second GAF (cGMP phosphodiesterase/adenylyl cyclase/FhlA) domains of All2699. In this study, we found that, upon exposure to red light, the protein underwent aggregation, resulting in the formation of protein aggregates. Conversely, under far-red light irradiation, these protein aggregates dissociated. We delved into the factors that impact the aggregation of All2699g1g2, focusing on the protein structure. Our findings showed that the GAF2 domain contains a low-complexity (LC) loop region, which plays a crucial role in mediating protein aggregation. Specifically, phenylalanine at position 239 within the LC loop region was identified as a key site for the aggregation process. Furthermore, our research revealed that various factors, including irradiation time, temperature, concentration, NaCl concentration, and pH value, can impact the aggregation of All2699g1g2. The aggregation led to variations in Pfr concentration depending on temperature, NaCl concentration, and pH value. In contrast, ΔLC did not aggregate and therefore lacked responses to these factors. Consequently, the LC loop region of All2699g1g2 extended and enhanced sensory properties.

Two-photon Absorption and Photoionization of a Bacterial Phytochrome

Phytochromes constitute a family of photosensory proteins that are utilized by various organisms to regulate several physiological processes. Phytochromes bind a bilin pigment that switches its isomeric state upon absorption of red or far-red photons, resulting in protein conformational changes that are sensed by the organism. Previously, the ultrafast dynamics in bacterial phytochrome was resolved to atomic resolution by time-resolved serial femtosecond X-ray diffraction (TR-SFX), showing extensive changes in its molecular conformation at 1 picosecond delay time. However, the large excitation fluence of mJ/mm2 used in TR-SFX questions the validity of the observed dynamics. In this work, we present an excitation-dependent ultrafast transient absorption study to test the response of a related bacterial phytochrome to excitation fluence. We observe excitation power-dependent sub-picosecond dynamics, assigned to the population of high-lying excited state Sn through resonantly enhanced two-photon absorption, followed by rapid internal conversion to the low-lying S1 state. Inspection of the long-lived spectrum under high fluence shows that in addition to the primary intermediate Lumi-R, spectroscopic signatures of solvated electrons and ionized chromophore radicals are observed. Supported by numerical modelling, we propose that under excitation fluences of tens of μJ/mm2 and higher, bacterial phytochrome partly undergoes photoionization from the Sn state in competition with internal conversion to the S1 state in 300 fs. We suggest that the extensive structural changes of related, shorter bacterial phytochrome, lacking the PHY domain, resolved from TR-SFX may have been affected by the ionized species. We propose approaches to minimize the two-photon absorption process by tuning the excitation spectrum away from the S1 absorption or using phytochromes exhibiting minimized or shifted S1 absorption.

The Red Edge: Bilin-Binding Photoreceptors as Optogenetic Tools and Fluorescence Reporters

This review adds the bilin-binding phytochromes to the Chemical Reviews thematic issue "Optogenetics and Photopharmacology". The work is structured into two parts. We first outline the photochemistry of the covalently bound tetrapyrrole chromophore and summarize relevant spectroscopic, kinetic, biochemical, and physiological properties of the different families of phytochromes. Based on this knowledge, we then describe the engineering of phytochromes to further improve these chromoproteins as photoswitches and review their employment in an ever-growing number of different optogenetic applications. Most applications rely on the light-controlled complex formation between the plant photoreceptor PhyB and phytochrome-interacting factors (PIFs) or C-terminal light-regulated domains with enzymatic functions present in many bacterial and algal phytochromes. Phytochrome-based optogenetic tools are currently implemented in bacteria, yeast, plants, and animals to achieve light control of a wide range of biological activities. These cover the regulation of gene expression, protein transport into cell organelles, and the recruitment of phytochrome- or PIF-tagged proteins to membranes and other cellular compartments. This compilation illustrates the intrinsic advantages of phytochromes compared to other photoreceptor classes, e.g., their bidirectional dual-wavelength control enabling instant ON and OFF regulation. In particular, the long wavelength range of absorption and fluorescence within the "transparent window" makes phytochromes attractive for complex applications requiring deep tissue penetration or dual-wavelength control in combination with blue and UV light-sensing photoreceptors. In addition to the wide variability of applications employing natural and engineered phytochromes, we also discuss recent progress in the development of bilin-based fluorescent proteins.

Crystal structure of a far-red-sensing cyanobacteriochrome reveals an atypical bilin conformation and spectral tuning mechanism

Cyanobacteriochromes (CBCRs) are small, linear tetrapyrrole (bilin)-binding photoreceptors in the phytochrome superfamily that regulate diverse light-mediated adaptive processes in cyanobacteria. More spectrally diverse than canonical red/far-red-sensing phytochromes, CBCRs were thought to be restricted to sensing visible and near UV light until recently when several subfamilies with far-red-sensing representatives (frCBCRs) were discovered. Two of these frCBCRs subfamilies have been shown to incorporate bilin precursors with larger pi-conjugated chromophores, while the third frCBCR subfamily uses the same phycocyanobilin precursor found in the bulk of the known CBCRs. To elucidate the molecular basis of far-red light perception by this third frCBCR subfamily, we determined the crystal structure of the far-red-absorbing dark state of one such frCBCR Anacy_2551g3 from Anabaena cylindrica PCC 7122 which exhibits a reversible far-red/orange photocycle. Determined by room temperature serial crystallography and cryocrystallography, the refined 2.7-Å structure reveals an unusual all-Z,syn configuration of the phycocyanobilin (PCB) chromophore that is considerably less extended than those of previously characterized red-light sensors in the phytochrome superfamily. Based on structural and spectroscopic comparisons with other bilin-binding proteins together with site-directed mutagenesis data, our studies reveal protein-chromophore interactions that are critical for the atypical bathochromic shift. Based on these analyses, we propose that far-red absorption in Anacy_2551g3 is the result of the additive effect of two distinct red-shift mechanisms involving cationic bilin lactim tautomers stabilized by a constrained all-Z,syn conformation and specific interactions with a highly conserved anionic residue.

Characterization of the pheophorbide a oxygenase/phyllobilin pathway of chlorophyll breakdown in grasses

Although the PAO/phyllobilin pathway of chlorophyll breakdown is active in grass leaf senescence, the abundance of phyllobilins is far below the amount of degraded chlorophyll. The yellowing of fully developed leaves is the most prominent visual symptom of plant senescence. Thereby, chlorophyll is degraded via the so-called pheophorbide a oxygenase (PAO)/phyllobilin pathway to a species-specific set of phyllobilins, linear tetrapyrrolic products of chlorophyll breakdown. Here, we investigated the diversity and abundance of phyllobilins in cereal and forage crops, i.e. barley, rice, ryegrass, sorghum and wheat, using liquid chromatography-mass spectrometry. A total of thirteen phyllobilins were identified, among them four novel, not yet described ones, pointing to a rather high diversity of phyllobilin-modifying activities present in the Gramineae. Along with these phyllobilins, barley orthologs of known Arabidopsis thaliana chlorophyll catabolic enzymes were demonstrated to localize in the chloroplast, and two of them, i.e. PAO and pheophytin pheophorbide hydrolase, complemented respective Arabidopsis mutants. These data confirm functionality of the PAO/phyllobilin pathway in grasses. Interestingly, when comparing phyllobilin abundance with amounts of degraded chlorophyll in senescent leaves, in most analyzed grass species only minor fractions of chlorophyll were recovered as phyllobilins, opposite to A. thaliana where phyllobilin quantities match degraded chlorophyll rather well. These data show that, despite the presence and activity of the PAO/phyllobilin pathway in barley (and other cereals), phyllobilins do not accumulate stoichiometrically, implying possible degradation of chlorophyll beyond the phyllobilin level.

Stable (18) O-labeling method for stercobilin and other bilins for metabolomics

RATIONALE

Bilin tetrapyrroles including stercobilin are unique to mammalian waste; they have been used as markers of source water contamination and may have important diagnostic value in human health conditions. Unfortunately, commercial isotopomers for bilins are not available. Thus, there is a need for isotopomer standards of stercobilin and other bilins for quantification in environmental and clinical diagnostic applications.

METHODS

A procedure is described here using H2 (18) O to label the carboxylic acid groups of bilin tetrapyrroles. Reaction conditions as a function of temperature and reagent volume were found to produce a mixture of isotopomers, as assessed by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). Stability as a function of storage time and temperature and in conjunction with solid-phase extraction (SPE) was assessed.

RESULTS

The highest labeling efficiency was achieved at 70 °C for 8 h, while a stable ratio of the isotopmers could be produced at 60 °C for 4 h. The stability of the isotopic distribution was maintained under storage (room temperature or frozen) for 20 days. It was also stable throughout SPE. The high mass accuracy and resolving power of FTICRMS enables clear distinction between (18) O-labeled bilins from other unlabeled bilins present, avoiding a potential interference in quantitation.

CONCLUSIONS

A procedure was developed to label bilins with (18) O. The final ratio of the (18) O-labeled bilin isotopomers was reproducible and highly stable for at least 20 days under storage. This ratio was not changed in any statistically significant way even after SPE. Thus a reliable method for producing stable isotopomer ratios for bilins has been achieved. Copyright © 2016 John Wiley & Sons, Ltd.

Examination of the ligand-binding and enzymatic properties of a bilin-binding protein from the poisonous caterpillar Lonomia obliqua

The bilin-binding proteins (BBP) from lepidopteran insects are members of the lipocalin family of proteins and play a special role in pigmentation through the binding of biliverdin IXγ. Lopap, a BBP-like protein from the venom of the toxic caterpillar Lonomia obliqua has been reported to act as a serine protease that activates the coagulation proenzyme prothrombin. Here we show that BBPLo, a variant of lopap from the same organism binds biliverdin IXγ, forming a complex that is spectrally identical with previously described BBP proteins. Although BBPLo is nearly identical in sequence to lopap, no prothrombinase activity was detected in our recombinant preparations using reconstituted systems containing coagulation factors Xa and Va, as well as anionic phospholipids. In addition to biliverdin, BBPLo was found to form a 1∶1 complex with heme prompting us to examine whether the unusual biliverdin IXγ ligand of BBPs forms as a result of oxidation of bound heme in situ rather than by a conventional heme oxygenase. Using ascorbate or a NADPH⁺-ferredoxin reductase-ferredoxin system as a source of reducing equivalents, spectral changes are seen that suggest an initial reduction of heme to the Fe(II) state and formation of an oxyferrous complex. The complex then disappears and a product identified as a 5-coordinate carbonyl complex of verdoheme, an intermediate in the biosynthesis of biliverdin, is formed. However, further reaction to form biliverdin was not observed, making it unlikely that biliverdin IXγ is formed by this pathway.

Tandem mass spectrometry of bilin tetrapyrroles by electrospray ionization and collision-induced dissociation

RATIONALE

Bilins are metabolic products of hosts and bacteria on porphyrins, and are markers of health state and human waste contamination. Although bilin tandem mass spectrometry reports exist, their fragmentation behavior as a function of structure has not been compared, nor has fragmentation been examined as a function of collision energy.

METHODS

The fragmentation of bilins generated by positive ion mode electrospray ionization is examined by collision-induced dissociation (CID). CID on a quadrupole ion trap and on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer as a function of collision energy is compared. Methyl esterification was used to deduce which product ions contain the inner pyrrole rings. FT-ICR high mass accuracy measurements were used to determine the formulas of the resultant product ions.

RESULTS

The central carbon's bonding to the inner pyrrole rings influences fragmentation. Bilirubin is unique because fragmentation adjacent to the central methylene group between innermost rings predominates, and loss of a terminal pyrrole is observed only with helium collision gas. The other bilins lose the terminal pyrroles first; as CID energy is increased, additional fragmentation due to neutral losses of small molecules such as H(2)O, CO, CO(2), and methanol occurs.

CONCLUSIONS

Based on these observations, fragmentation schemes for the bilins are proposed that are strongly dependent on the molecular structure and collision energy; only bilirubin fragmentation is influenced significantly by the collision gas used. This report should have value in identification of this class of molecules for biomarker detection.

The anti-mutagenic properties of bile pigments

Bile pigments, including bilirubin and biliverdin, are endogenous compounds belonging to the porphyrin family of molecules. In the past, bile pigments and bilirubin in particular were thought of as useless by-products of heme catabolism that can be toxic if they accumulate. However, in the past 20 years, research probing the physiological relevance of bile pigments has been mounting, with evidence to suggest bile pigments possess significant antioxidant and anti-mutagenic properties. More specifically, bile pigments are potent peroxyl radical scavengers and inhibit the mutagenic effects of a number of classes of mutagens (polycyclic aromatic hydrocarbons, heterocyclic amines, oxidants). Coincidentally, persons with elevated circulating bilirubin concentrations have a reduced prevalence of cancer and cardio-vascular disease. Despite the encouraging in vitro anti-mutagenic effects of bile pigments, relatively little research has been conducted on their inhibitory capacity in bacterial and cultured cell assays of mutation, which might link the existing in vitro and in vivo observations. This is the first review to summarise the published data and it is our hope it will stimulate further research on these potentially preventative compounds.

Pathogenesis of hepatolithiasis based on the analysis of components of intrahepatic stones

BACKGROUND/AIMS

It has been thought that intrahepatic stones are brown pigment stones (bilirubin carbonate stones) but we analyzed a chemical compound to reveal that intrahepatic stones have unique components, and studied their pathogenesis.

METHODOLOGY

A total of 45 gallbladder stones (15 cholesterol stones, 15 black pigment stones, and 15 brown pigment stones) and 15 intrahepatic stones were analyzed about amounts of fatty acids, bile acids and trace elements in the gallstones. Thus we established the characteristic components of the intrahepatic stones and studied their pathogenesis.

RESULTS

Concerning the amounts of free fatty acids contained in the gallstones, comparing the 247.2 +/- 116.3 mg/g in the brown pigment stones to the 382.8 +/- 176.3 mg/g in the intrahepatic stones, demonstrates a significant difference between the two groups (p = 0.0191). The ratio of free saturated fatty acids/free unsaturated fatty acids was 1.3 +/- 0.5 in the cholesterol stones, 1.3 +/- 0.6 in the black pigment stones, 5.2 +/- 2.8 in the brown pigment stones and 8.3 +/- 3.5 in the intrahepatic stones (p = 0.0086).

CONCLUSIONS

It became clear that the intrahepatic stones contained high levels of free bile acids and that bacterial infection, which deconjugates the glycine and taurine conjugations, is involved in the pathogenesis of gallstones. The fatty acid analysis demonstrated high levels of free saturated fatty acids in the gallstones as well as the involvement of phospholipases, which break down phospholipids in bile, particularly phospholipase A1.

Biosynthesis of open-chain tetrapyrroles inProchlorococcus marinus

Members of the genus Prochlorococcus belong to the most abundant phytoplankton on earth. In contrast to other cyanobacteria, Prochlorococcus is characterized by divinyl-chlorophyll containing light-harvesting complexes and the lack of phycobilisomes. Despite the lack of phycobilisomes, all sequenced genomes of Prochlorococcus possess genes that putatively encode enzymes involved in the biosynthesis of open-chain tetrapyrrole molecules. Here, biochemical evidence is presented indicating that high-light- and low-light-adapted Prochlorococcus ecotypes possess genes encoding functional enzymes for the biosynthesis of open-chain tetrapyrrole molecules. Experiments on recombinant protein as well as through complementation studies of a cyanobacterial insertion mutant revealed the functionality of the bilin reductases investigated.

Biglutaminyl-biliverdin IX alpha as a heme degradation product in the dengue fever insect-vector Aedes aegypti

Hemoglobin digestion in the midgut of hematophagous animals results in the release of its prosthetic group, heme, which is a pro-oxidant molecule. Heme enzymatic degradation is a protective mechanism that has been described in several organisms, including plants, bacteria, and mammals. This reaction is catalyzed by heme oxygenase and results in formation of carbon monoxide, ferrous ion, and biliverdin IXalpha. During digestion, a large amount of a green pigment is produced and secreted into the intestinal lumen of Aedes aegypti adult females. In the case of another blood-sucking insect, the kissing-bug Rhodnius prolixus, we have recently shown that heme degradation involves a complex pathway that generates dicysteinyl-biliverdin IX gamma. The light absorption spectrum of the Aedes purified pigment was similar to that of biliverdin, but its mobility on a reverse-phase chromatography column suggested a compound less hydrophobic than biliverdin IXalpha. Structural characterization by ESI-MS revealed that the mosquito pigment is the alpha isomer of biliverdin bound to two glutamine residues by an amide bond. This biglutaminyl-biliverdin is formed by oxidative cleavage of the heme porphyrin ring followed by two subsequent additions of glutamine residues to the biliverdin IXalpha. The role of this pathway in the adaptation of this insect vector to a blood-feeding habit is discussed.

The anti-mutagenic and antioxidant effects of bile pigments in the Ames Salmonella test

The aim of this study was to explore the potential pro- and anti-mutagenic effects of endogenous bile pigments unconjugated bilirubin (BR), biliverdin (BV) and a synthetic, water soluble conjugate, bilirubin ditaurate (BRT) in the Ames Salmonella test. The bile pigments were tested over a wide concentration range (0.01-2 micromol/plate) in the presence of three bacterial strains (TA98, TA100, TA102). A variety of mutagens including benzo[alpha]pyrene (B[alpha]P), 2,4,7 trinitrofluorenone (TNFone), 2-aminofluorene (2-AF), sodium azide (NaN(3)) and tertiary-butyl hydroperoxide (t-BuOOH), were used to promote the formation of mutant revertants. Tests were conducted with (B[alpha]P, 2-AF, t-BuOOH) and without (TNFone, NaN(3), t-BuOOH) metabolic activation incorporating the addition of the microsomal liver preparation, S9. The bile pigments alone did not induce mutagenicity in any of the strains tested (p>0.05). Anti-mutagenic effects of the bile pigments were observed in the presence of all mutagens except for NaN(3) and the anti-mutagenic effects appeared independent of the strain tested. For TNFone induced genotoxicity, the order of effectiveness was BR> or =BRT>BV. However, the order was BV> or =BRT> or =BR for 2-AF. Antioxidant testing in the TA102 strain revealed bile pigments could effectively inhibit the genotoxic effect of t-BuOOH induced oxidative stress. The apparent antioxidant and anti-mutagenic behaviour of bile pigments further suggests their presence in biological systems is of possible physiological importance.

A Conserved Histidine-Aspartate Pair Is Required for Exovinyl Reduction of Biliverdin by a Cyanobacterial Phycocyanobilin:Ferredoxin Oxidoreductase

Phycocyanobilin:ferredoxin oxidoreductase is a member of the ferredoxin-dependent bilin reductase family and catalyzes two vinyl reductions of biliverdin IXalpha to produce phycocyanobilin, the pigment precursor of both phytochrome and phycobiliprotein chromophores in cyanobacteria. Atypically for ferredoxin-dependent enzymes, phycocyanobilin:ferredoxin oxidoreductase mediates direct electron transfers from reduced ferredoxin to its tetrapyrrole substrate without metal ion or organic cofactors. We previously showed that bound bilin radical intermediates could be detected by low temperature electron paramagnetic resonance and absorption spectroscopies (Tu, S., Gunn, A., Toney, M. D., Britt, R. D., and Lagarias, J. C. (2004) J. Am. Chem. Soc. 126, 8682-8693). On the basis of these studies, a mechanism involving sequential electron-coupled proton transfers to protonated bilin substrates buried within the phycocyanobilin:ferredoxin oxidoreductase protein scaffold was proposed. The present investigation was undertaken to identify catalytic residues in phycocyanobilin:ferredoxin oxidoreductase from the cyanobacterium Nostoc sp. PCC7120 through site-specific chemical modification and mutagenesis of candidate proton-donating residues. These studies identified conserved histidine and aspartate residues essential for the catalytic activity of phycocyanobilin:ferredoxin oxidoreductase. Spectroscopic evidence for the formation of stable enzyme-bound biliverdin radicals for the H85Q and D102N mutants support their role as a "coupled" proton-donating pair during the reduction of the biliverdin exovinyl group.

Sterically locked synthetic bilin derivatives and phytochrome Agp1 from Agrobacterium tumefaciens form photoinsensitive Pr- and Pfr-like adducts

Phytochrome photoreceptors undergo reversible photoconversion between the red-absorbing form, Pr, and the far-red-absorbing form, Pfr. The first step in the conversion from Pr to Pfr is a Z to E isomerization around the C15=C16 double bond of the bilin chromophore. We prepared four synthetic biliverdin (BV) derivatives in which rings C and D are sterically locked by cyclizing with an additional carbon chain. In these chromophores, which are termed 15Za, 15Zs, 15Ea, and 15Es, the C15=C16 double bond is in either the Z or E configuration and the C14-C15 single bond in either the syn or anti conformation. The chromophores were assembled with Agrobacterium phytochrome Agp1, which incorporates BV as natural chromophore. All locked BV derivatives bound covalently to the protein and formed adducts with characteristic spectral properties. The 15Za adduct was spectrally similar to the Pr form and the 15Ea adduct similar to the Pfr form of the BV adduct. Thus, the chromophore of Agp1 adopts a C15=C16 Z configuration and a C14-C15 anti conformation in the Pr form and a C15=C16 E configuration and a C14-C15 anti conformation in the Pfr form. Both the 15Zs and the 15Es adducts absorbed only in the blue region of the visible spectra. All chromophore adducts were analyzed by size exclusion chromatography and histidine kinase activity to probe for protein conformation. In either case, the 15Za adduct behaved like the Pr and the 15Ea adduct like the Pfr form of Agp1. Replacing the natural chromophore by a locked 15Ea derivative can thus bring phytochrome holoprotein in the Pfr form in darkness. In this way, physiological action of Pfr can be studied in vivo and separated from Pr/Pfr cycling and other light effects.

The mobility of phytochrome within protonemal tip cells of the moss Ceratodon purpureus, monitored by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a versatile tool for investigating the mobilities of fluorescent molecules in cells. In this article, we show that it is possible to distinguish between freely diffusing and membrane-bound forms of biomolecules involved in signal transduction in living cells. Fluorescence correlation spectroscopy was used to measure the mobility of phytochrome, which plays a role in phototropism and polarotropism in protonemal tip cells of the moss Ceratodon purpureus. The phytochrome was loaded with phycoerythrobilin, which is fluorescent only in the phytochrome-bound state. Confocal laser scanning microscopy was used for imaging and selecting the xy measuring position in the apical zone of the tip cell. Fluorescence correlation was measured at ancient z-positions in the cell. Analysis of the diffusion coefficients by nonlinear least-square fits showed a subcellular fraction of phytochrome at the cell periphery with a sixfold higher diffusion coefficient than in the core fraction. This phytochrome is apparently bound to the membrane and probably controls the phototropic and polarotropic response.

Green colouration of cocoons in Antheraea yamamai (Lepidoptera: Saturniidae): light-induced production of blue bilin in the larval haemolymph

When the larvae of a saturniid silkmoth, Antheraea yamamai, are maintained under high intensity light (5000 lux), they produce green cocoons whereas the cocoons produced under light of low intensity (e.g., 50 lux) or in darkness are yellow. The green colour of the cocoon is due to the presence of a blue bilin pigment in combination with yellow pigment, and light stimulates the accumulation of blue bilin. In the present study, we show that two blue bilins, with similar characteristics to the sarpedobilin in the green cocoon, can be induced in larval haemolymph both in vivo and in vitro. In both conditions, the amount of these bilins increased with increasing intensity or duration of light exposure. Induction also occurred at 0 degrees C. In contrast, the chromophore of the constitutive biliprotein of the haemolymph did not change depending on light conditions. Size fractionation of the haemolymph indicates that the precursor of the blue bilins induced by light is bound to a protein with a molecular mass of 5000 Da or more. Thus, in these insects, the blue bilin responsible for green colouration is facultative under photochemical stimulation.

Biliverdin binds covalently to agrobacterium phytochrome Agp1 via its ring A vinyl side chain

The widely distributed phytochrome photoreceptors carry a bilin chromophore, which is covalently attached to the protein during a lyase reaction. In plant phytochromes, the natural chromophore is coupled by a thioether bond between its ring A ethylidene side chain and a conserved cysteine residue within the so-called GAF domain of the protein. Many bacterial phytochromes carry biliverdin as natural chromophore, which is coupled in a different manner to the protein. In phytochrome Agp1 of Agrobacterium tumefaciens, biliverdin is covalently attached to a cysteine residue close to the N terminus (position 20). By testing different natural and synthetic biliverdin derivatives, it was found that the ring A vinyl side chain is used for chromophore attachment. Only those bilins that have ring A vinyl side chain were covalently attached, whereas bilins with an ethylidene or ethyl side chain were bound in a noncovalent manner. Phycocyanobilin, which belongs to the latter group, was however covalently attached to a mutant in which a cysteine was introduced into the GAF domain of Agp1 (position 249). It is proposed that the regions around positions 20 and 249 are in close contact and contribute both to the chromophore pocket. In competition experiments it was found that phycocyanobilin and biliverdin bind with similar strength to the wild type protein. However, in the V249C mutant, phycocyanobilin bound much more strongly than biliverdin. This finding could explain why during phytochrome evolution in cyanobacteria, the chromophore-binding site swapped from the N terminus into the GAF domain.

Inhibition of cholesterol crystallization under bilirubin deconjugation: partial characterization of mechanisms whereby infected bile accelerates pigment stone formation

Pigment gallstones have been reported to be closely associated with biliary tract infection. We previously reported that addition of unconjugated bilirubin (UCB), which is deconjugated by beta-glucuronidase in infected bile, could enhance cholesterol crystal formation in supersaturated model bile (MB). The present study evaluated the effect of beta-glucuronidase on the processes of pigment gallstone formation and cholesterol crystallization. Supersaturated MB (taurocholate/lecithin/cholesterol at 71:18:11, a total lipid concentration of 10.0 g/dl and a cholesterol saturation index (CSI) of 2.0) and native rat bile were mixed at a ratio of 3:1. Then, mixed bile was incubated with or without beta-glucuronidase and changes of the following parameters were investigated over time: (1) the UCB/total bilirubin ratio; (2) cholesterol crystal formation; (3) the precipitate weight and the cholesterol concentration in the precipitate and supernatant; and (4) the lipid distribution of vesicles in the supernatant. Compared with beta-glucuronidase-free bile, (1) beta-glucuronidase-containing bile showed a significant increase of the UCB/total bilirubin ratio, (2) as well as a significantly longer nucleation time (96+/-17.0 vs. 114+/-20.0) and fewer cholesterol crystals. (3) The precipitate weight and the cholesterol concentration in the precipitate were significantly increased, while the cholesterol concentration in supernatant was decreased. (4) When mixed bile was incubated with beta-glucuronidase, the cholesterol concentration in the vesicles was lower than in bile without beta-glucuronidase. The precipitate weight and the cholesterol concentration in the precipitate was increased by incubation with beta-glucuronidase, while cholesterol concentration was decreased in the supernatant (especially in the vesicles). This means that bile vesicles were more stable and it was more difficult for cholesterol crystals to form. Thus, the presence of beta-glucuronidase may inhibit the formation of pure cholesterol stones even in the presence of cholesterol supersaturation.

Phycocyanobilin:ferredoxin oxidoreductase of Anabaena sp. PCC 7120. Biochemical and spectroscopic

In cyanobacteria, the biosynthesis of the phycobiliprotein and phytochrome chromophore precursor phycocyanobilin is catalyzed by the ferredoxin-dependent enzyme phycocyanobilin:ferredoxin oxidoreductase (PcyA), which mediates an atypical four-electron reduction of biliverdin IXalpha. Here we describe the expression, affinity purification, and biochemical characterization of recombinant PcyA from Anabaena sp. PCC 7120. A monomeric protein with a native M(r) of 30,400 +/- 5,000, recombinant PcyA forms a tight and stable stoichiometric complex with its substrate biliverdin IXalpha. The enzyme exhibits a strong preference for plant type [2Fe-2S] ferredoxins; however, flavodoxin can also serve as an electron donor. HPLC analyses establish that catalysis proceeds via the two electron-reduced intermediate 18(1),18(2)-dihydrobiliverdin, indicating that exovinyl reduction precedes A-ring (endovinyl) reduction. Substrate specificity studies indicate that the arrangement of the A- and D-ring substituents alters the positioning of the bilin substrate within the enzyme, profoundly influencing the course of catalysis. Based on these observations and the apparent lack of a metal or small molecule cofactor, a radical mechanism for biliverdin IXalpha reduction by phycocyanobilin:ferredoxin oxidoreductase is envisaged.

Bilirubin/biliverdin-Cu(II) induced DNA breakage; reaction mechanism and biological significance

Bilirubin and its metabolic precursor biliverdin are heme degradation products but have been proposed as physiological antioxidants. Reports from another laboratory as well as from ours have shown bilirubin to form a complex with the transition metal ion-Cu(II). Such a complex was shown by us to cause oxidative DNA damage. Further, biliverdin was also shown to be capable of causing similar DNA damage. In the present studies we have aimed to elucidate the mechanism of DNA breakage reaction by these bile pigments. Absorption and fluorescence studies indicate binding of bile pigments to DNA and copper ions. Cu(II) is reduced by these compounds to Cu(I) which is an essential intermediate in the DNA breakage reaction. Redox recycling of Cu(II) leads to generation of reactive oxygen species. Strand scission by the bile pigments-Cu(II) system is found to be biologically significant as assayed by bacteriophage inactivation. Our results, therefore are suggestive of one of the mechanisms through which endogenous DNA damage may occur.

Regioselective oxidative liberation of aryl-substituted tripyrrinone metal complexes from N-confused porphyrin

[reaction: see text] A Cu(II) complex of the first aryl-substituted tripyrrinone derivative, 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrinato copper(II) (3-Cu) was obtained in the reaction of N-confused tetraphenylporphyrin (NCTPP) and Cu(OAc)(2) in refluxing toluene, and subsequent treatment with acid afforded a free tripyrrolic ligand that could bind a variety of transition metals.

Does bilirubin play a role in the pathogenesis of both cholesterol and pigment gallstone formation? Direct and indirect influences of bilirubin on bile lithogenicity

Bilirubin is found in the center of cholesterol gallstones, but its pathogenic role in their formation is unknown. Bilirubin causes a disproportionate reduction of biliary lipid secretion without affecting bile salt secretion in association with a change of biliary lecithin species, which modulates the cholesterol crystallization process. Therefore, the present study investigated whether bilirubin can influence the cholesterol crystallization procedure, and the mechanism(s) of any such action. Supersaturated model bile was prepared (taurocholate/lecithin/cholesterol at 71:18:11, a total lipid concentration of 9.0 g/dl, and cholesterol saturation index of 1.8), and cholesterol crystallization was monitored over time using a spectrophotometer and video-enhanced differential contrast microscopy in the absence or presence of bilirubin (at a final concentration of 10 microM, 20 microM, 40 microM, and 100 microM). Bilirubin enhanced the onset of cholesterol crystallization by 50%, whereas the crystal growth rate and final crystal mass were reduced at a high concentration of bilirubin. Taken together, these results suggest that bilirubin influences the cholesterol crystallization process, by either a direct interaction with biliary lipids that alters metastability, an indirect alteration of the bile salt-micellar lipid holding capacity, or both. Thus, bilirubin may play a role in the pathogenesis of both cholesterol and pigment gallstones.

A remarkable skeletal rearrangement of a coordinated tetrapyrrole: chemical consequences of palladium pi-coordination to a bilindione

Pd4(OEB)2, in which a [Pd2]2+ unit is bound in pi-fashion to olefinic sites that are exocyclic to pyrrole rings of the octaethylbilindione ligand, undergoes an unprecedented sequence of reactions that results in the rearrangement of the framework of the bilindione ligand and the formation of trans-Pd(py)2I2. This process of bilindione rearrangement and oxidation occurs as a direct consequence of the pi-coordination of the palladium. The reaction results in the migration of a nitrogen atom from a pyrrole carbon atom to what was formerly a meso carbon atom to transform a former pyrrole ring into a six-membered ring. This process also involves cleavage of the Pd-Pd and Pd-C bonds, oxidation of palladium, and introduction of an oxygen atom (from water) not necessarily in this particular sequence.

Triplet states as non-radiative traps in multichromophoric entities: single molecule spectroscopy of an artificial and natural antenna system

Energy transfer in antenna systems, ordered arrays of chromophores, is one of the key steps in the photosynthetic process. The photophysical processes taking place in such multichromophoric systems, even at the single molecule level, are complicated and not yet fully understood. Instead of directly studying individual antenna systems, we have chosen to focus first on systems for which the amount of chromophores and the interactions among the chromophores can be varied in a systematic way. Dendrimers with a controlled number of chromophores at the rim fulfill those requirements perfectly. A detailed photophysical study of a second-generation dendrimer, containing eight peryleneimide chromophores at the rim, was performed 'J. Am. Chem. Soc., 122 (2000) 9278'. One of the most intriguing findings was the presence of collective on/off jumps in the fluorescence intensity traces of the dendrimers. This phenomenon can be explained by assuming a simultaneous presence of both a radiative trap (energetically lowest chromophoric site) and a non-radiative trap (triplet state of one chromophore) within one individual dendrimer. It was shown that an analogue scheme could explain the collective on/off jumps in the fluorescence intensity traces of the photosynthetic pigment B-phycoerythrin (B-PE) (Porphyridium cruentum). The different values of the triplet lifetime that could be recovered for a fluorescence intensity trace of B-PE were correlated with different intensity levels in the trace, suggesting different chromophores acting as a trap as function of time.

An improved method for isolation of unconjugated bilirubin from rat and dog bile

Radiolabeled unconjugated bilirubin (UCB) is currently prepared by biosynthetic labeling of bilirubin in fistula bile from precursor-labeled delta-aminolevulinic acid (ALA) in rats or dogs. With existing methods, yields of labeled UCB from the bile are generally less than 50%. We here report modifications of the original method of Ostrow et al (Ostrow JD, Hammaker L, Schmid R. The preparation of crystalline bilirubin-C(14). J Clin Invest 1961;40:1442-52) that result in improvement of yields to 72% from both dog and rat bile. The modifications include the initial deproteination of bile with a reverse-phase C18 cartridge, removal of ethanol before alkaline hydro-lysis to avoid esterification of UCB, and adjustments for the high proportion of non-glucuronide UCB conjugates in dog bile not precipitated as lead salts. These improvements should save significantly on both costs and animal usage.

Sodium etiobilirubin-IVgamma-C10-sulfonate: a highly solvated bile pigment structure containing two different non-ridge-tile conformers in the unit cell

The crystal structure of the title compound is the first example of a bilirubin existing in both extended and cyclic conformations and the first bile pigment structure showing two markedly different conformations in the unit cell. In contrast to previous rubin structures the dipyrrinone rings are twisted out of planarity in both conformers. Because of numerous hydrogen-bonding and ionic interactions a highly complex tetrameric structure is observed in which each extended conformer is held pincer-like by another.

In vitro assembly of phytochrome B apoprotein with synthetic analogs of the phytochrome chromophore

Phytochrome B (PhyB), one of the major photosensory chromoproteins in plants, mediates a variety of light-responsive developmental processes in a photoreversible manner. To analyze the structural requirements of the chromophore for the spectral properties of PhyB, we have designed and chemically synthesized 20 analogs of the linear tetrapyrrole (bilin) chromophore and reconstituted them with PhyB apoprotein (PHYB). The A-ring acts mainly as the anchor for ligation to PHYB, because the modification of the side chains at the C2 and C3 positions did not significantly influence the formation or difference spectra of adducts. In contrast, the side chains of the B- and C-rings are crucial to position the chromophore properly in the chromophore pocket of PHYB and for photoreversible spectral changes. The side-chain structure of the D-ring is required for the photoreversible spectral change of the adducts. When methyl and ethyl groups at the C17 and C18 positions are replaced with an n-propyl, n-pentyl, or n-octyl group, respectively, the photoreversible spectral change of the adducts depends on the length of the side chains. From these studies, we conclude that each pyrrole ring of the linear tetrapyrrole chromophore plays a different role in chromophore assembly and the photochromic properties of PhyB.

Defining the bilin lyase domain: lessons from the extended phytochrome superfamily

Through pattern searches of genomic databases, new members of the growing family of phytochrome-related genes were identified and used to construct a 130-180 amino acid motif that delimits the bilin lyase domain, a subdomain of the extended phytochrome family that is sufficient for covalent attachment of linear tetrapyrroles (bilins). To test this hypothesis, portions of locus sll0821, a novel phytochrome-related gene from Synechocystis sp. PCC6803 that encodes a large protein with two potential bilin binding sites, were amplified, and the recombinant apoproteins were tested for bilin binding and phytochrome photoactivity. Our experiments indicated that both sites of this protein, termed Cph2 for cyanobacterial phytochrome 2, possessed bilin lyase activity, revealing two distinct classes of bilin lyase domains--those whose bilin adducts are red, far-red reversible and a second class whose bilin adducts are nonphotochromic. Spectroscopic analysis of photochromic phycocyanobilin and fluorescent phycoerythrobilin adducts of a 24-kDa fragment of Cph2 definitively established that the motif identified by pattern searches represents a bona fide bilin lyase domain. Site-directed mutagenesis of highly conserved charged residues within bilin lyase domains of nearly all members of the extended phytochrome superfamily has identified a glutamate residue critical for bilin binding.

Kinetics of c-phycocyanin reaction with hypochlorite

Hydrochlorous acid bleaches c-phycocyanin visible absorbance with a second-order rate constant (pH 7.4) of 1.3x10(3) M(-1) s(-1). In excess of protein, ca. 0.16 bilin moieties are disrupted by each reacted HOCl molecule. This indicates that the main reaction takes place at the apoprotein level, with a total rate constant (in monomeric units concentration) of 2.5x10(4) M(-1) s(-1). This rate constant is too low to provide protection to other biomolecules under physiological conditions. The reported antiinflammatory properties of phycocyanin are not then related to the removal of HOCl. On the other hand, the rather slow reaction rate with HOCI could be beneficial to its role as antiinflammatory agent since it will allow the protein to maintain its integrity at the inflammation locus.

Fluorescence polarization studies on four biliproteins and a bilin model for phycoerythrin 545

Fluorescence (excitation) polarization spectroscopy in the wavelength region of the bilin chromophores was applied to phycoerythrocyanin (CV-phycocyanin), phycocyanins 645 and 612, and phycoerythrin 545. The cryptomonad biliproteins - phycoerythrin 545 and phycocyanins 612 and 645 - were studied as both protein dimers having an alpha(2)beta(2) polypeptide structure and as alphabeta monomers. The cyanobacterial phycoerythrocyanin (CV-phycocyanin) was a trimeric oligomer. The changes in polarization across the spectrum were attributed to transfers of energy between bilins. Cryptomonad biliproteins are isolated as dimers. The similarities between their steady-state fluorescence polarization spectra and those of the corresponding monomers suggested that the monomers' conformations were analogous to the dimers. This supports the use of monomers in the study of dimer bilin organization. The unusual polarization spectrum of phycoerythrin 545 was explained using a model for the topography of its bilins. Obtaining the emission spectra of phycoerythrin 545 at several temperatures and a deconvolution of the dimer circular dichroism spectrum also successfully tested the bilin model. Circular dichroism spectroscopy was used to determine which polarization changes are formed by Förster resonance energy transfers and which may be produced by internal conversions between high- and low-energy states of pairs of exciton-coupled bilins. Attempts were made to assign energy transfer events to the corresponding changes in fluorescence polarization for each of the four biliproteins.

Phycoerythrin 545: monomers, energy migration, bilin topography, and monomer/dimer equilibrium

Phycoerythrin 545 was isolated having an alpha2beta2 (dimer) protein structure at pH 6.0 and 2 g/L protein concentration with eight bilin chromophores. Monomers (alphabeta) were produced by lowering the protein concentration to 0.15 g/L and the pH to 4.5. Dimer dissociation was monitored by dynamic light scattering and gel-filtration column chromatography. Monomers were stable and had bilin optical spectra different from the alpha2beta2 dimers, although they have very similar protein secondary structures. The optical spectra of phycoerythrin 545 showed four types of behavior with temperature: 10-20 degrees C, dimers; 40-50 degrees C, dimers/monomers; 60 degrees C, nearly fully disordered; 70 degrees C, disordered alpha and beta polypeptides. At 40 degrees C, the protein dissociated partially to monomer, which could be totally reversed to dimers at 20-25 degrees C. The visible circular dichroism difference spectrum for the protein dimers minus monomers exhibited positive and negative bands--such spectra may indicate exciton splitting between closely-spaced bilins. Circular dichroism also revealed a spectrum suggesting exciton coupling for the second excited state of the bilins. Ultrafast fluorescence using a two-photon method showed the fastest time for protein dimers to be 2. 4 ps and monomers had a 39-ps lifetime. Phycocyanin 645 was found to have a 550-fs lifetime.

Aqueous dissociation constants of bile pigments and sparingly soluble carboxylic acids by 13C NMR in aqueous dimethyl sulfoxide: effects of hydrogen bonding

pKas for the acid dissociation of the carboxyl groups of bilirubin in water have been reported recently to be 8.1-8.4, or higher. These high values were attributed to intramolecular hydrogen bonding. They have led to suggestions that monoanions of bilirubin predominate at physiologic pH and are the species transported most readily into hepatocytes by carriers. Such high aqueous pKas are inconsistent with recent 13C nuclear magnetic resonance (NMR) measurements on mesobilirubin XIII alpha, done on aqueous solutions containing dimethyl sulfoxide. To investigate whether the presence of dimethyl sulfoxide leads to unreliable values when using 13C NMR spectroscopy to determine pKas of carboxylic acids that can undergo intramolecular hydrogen bonding, we measured the pKas of 13C-labeled fumaric, maleic, and phthalic acids in solutions containing up to 27 vol% dimethyl sulfoxide. In addition, we used 13C NMR to estimate the pKas of 2,2'-methylenebis[5-carbomethoxy-4-methylpyrrole-3-[1-13C] propanoic acid], a model for the two central rings of bilirubin. Our results show that 13C NMR of aqueous dimethyl sulfoxide solutions can be used with confidence to measure pKas of intramolecularly hydrogen-bonded carboxylic acids. They support our previous estimates for the pKas of bilirubin and confirm that intramolecular hydrogen bonding has little effect on the acidity of bilirubins in water. Together with previous studies and chemical arguments they strongly suggest that reported aqueous pKas of > 8, or even > 6, for the carboxyl groups of bilirubin are incorrect and that arguments used to rationalize them are questionable.

Synthesis of water-soluble bile pigments bound to amine-ended monomethoxypolyethyleneglycol: thiol addition and attempted enzymatic reduction of a bilindione derivative

The water-soluble amide to an NH2-ended monomethoxypolyethyleneglycol (MPEG-NH2, molecular mass of about 2000) of the dipyrrinone xanthobilirubic acid (XBR, 1) and the bis-amides of mesobiliverdin-XIII alpha (MBV, 2) and mesobilirubin-XIII alpha (MBR, 3) have been prepared with high yields. Contrary to what is observed with biliverdin-IX alpha, 4, the enzymatic reduction of the mesobiliverdin derivative 2-MPEGA to the corresponding mesobilirubin 3-MPEGA by the soluble biliverdin reductase/NADPH system in pH 7.4 aqueous phosphate does not occur. In contrast, thiol addition to 2-MPEGA and to 4 under similar conditions is immediate, although this equilibrium is slightly less favourable for 2-MPEGA. These results enable us to discount the intrinsically low reactivity of 2-MPEGA towards thiols as the reason for its lack of enzymatic reduction, and suggest instead that this particular mesobiliverdin cannot fit properly into the enzyme binding site, either because of steric hindrance or the lack of the two propionic acid groups.

Bilin chromophores as reporters of unique protein conformations of phycocyanin 645

At 45 degrees C, phycocyanin 645 maximally undergoes a reversible and stable conformational change. The change is observed in the visible (chromophore) region of the absorption and circular dichroism (CD) spectra. In the absorption spectrum, the absorbance is lower at 45 degrees C but remains much closer to the normal spectrum than to a strongly denatured spectrum. In the CD, a similar situation exists except that a negative band on the blue edge of the spectrum is much more strongly affected at 45 degrees C than the other bands. On returning to 20 degrees C, all these changes are restored to the original states. The protein is an alpha 2 beta 2 dimer at both 20 and 45 degrees C, and CD in the far-UV shows the identical protein secondary structures at both 20 and 45 degrees C. Fluorescence studies show that energy transfer occurs at both temperatures. At 50 degrees C the results are saliently different as the secondary structure changes and the spectral changes are mostly irreversible. At 50 degrees C, some monomers (alpha beta) are produced, and these monomers are very unstable at that temperature, resulting in the formation of some fully denatured polypeptides. Stable monomers can be produced at 20 degrees C and have visible absorption and CD spectra identical to the dimer at 45 degrees C. Therefore, the chromophores are reporting a tertiary conformational change at 45 degrees C, in which the two halves of the dimer each assume a monomer-like conformation prior to dissociating. These results are compared with a hypothesis for the chromophore topography, and the CD change at the blue edge of the spectra may result from the separation at 45 degrees C of a chromophore pair.

Nonenzymatic bilin addition to the alpha subunit of an apophycoerythrin

C-Phycoerythrin is a light-harvesting protein whose alpha and beta subunits carry thioether-linked phycoerythrobilin (PEB) at cysteine residues alpha-82, alpha-139, beta-48,59 (doubly-linked), beta-80, and beta-165. The two subunits of Calothrix sp. PCC 7601 C-phycoerythrin, overexpressed together as apopolypeptides in Escherichia coli, formed inclusion bodies. Purified apo-alpha was soluble in the absence of urea, whereas the apo-beta subunit was only soluble at high urea concentrations. Products of nonenzymatic addition of PEB and phycocyanobilin (PCB) to apo-alpha were characterized by isolation of bilin peptides and spectroscopy. Reaction of PEB with the apo-alpha subunit led primarily to 15,16-dihydrobiliverdin (Cys-82) or urobilin (Cys-139) adducts, and small amounts of the natural PEB adducts at both Cys-82 and Cys-139. PCB reacted primarily with Cys-82 to form phycocyanobilin and mesobiliverdin adducts. Both PEB and PCB also formed relatively small amounts of adducts with Cys-59, which is not a bilin attachment residue in natural phycoerythrin. Sodium azide was found to promote the addition of PEB to simple thiols but not to apo-alpha phycoerythrin.

Chemical structure of a new family of bile pigments from human urine

Seven bilirubin metabolites negative to the diazo reaction were identified in the urine of healthy persons by enzyme-linked immunosorbent assay (ELISA) using the anti-bilirubin monoclonal antibody 24G7. Two of the seven metabolites were isolated and their chemical structures were determined using fast-atom bombardment-mass spectroscopy (FAB-MS) and 1H-NMR. The two metabolites are 1,14,15,17-tetrahydro-2,7,13-trimethyl-1,14- dioxo-3-vinyl-16H-tripyrrin-8,12-dipropionic acid and 1,14,15,17-tetrahydro-3,7,13-trimethyl-1,14-dioxo-2-vinyl-16H- tripyrrin-8,12-dipropionic acid. They are regioisomers of each other. The two bilirubin metabolites are novel tripyrrole biocompounds and belong to a third group of bile pigments following biliverdin and bilirubin. We gave these compounds the generic names biotripyrrin-a and biotripyrrin-b, respectively.

The antioxidant role of bile pigments evaluated by chemical tests

Bilirubin, biliverdin and their serum albumin complexes were tested as oxyradical scavengers (superoxide generated by the xanthine/xanthine oxidase system and peroxyl radical-trapping antioxidant ability). As superoxide scavengers the free bile pigments showed activities near to that of serum albumin, higher than the water soluble vitamin E analog Trolox and lower than ascorbic acid. The peroxyl radical-trapping antioxidant abilities of the tested bile pigments were much higher than those of the serum albumin and of the same order as their serum albumin complexes. This interaction with peroxyl radicals showed different stoichiometric factors for bilirubin (approximately 2) and biliverdin (approximately 4).

Mechanism of conversion of verdomyoglobin to ferric complex of biliverdin

The reaction of conversion of verdoheme to Fe(III)biliverdin was studied with different hemeproteins (Mb, HRP, and BrCN modified Mb). It was shown that the heme iron of Mb is not involved in this reaction. It was also shown that the extra oxygen atom in biliverdin does not come from H2O2 or water molecules. Based on these results a mechanism for this reaction is proposed.

[Interaction of human serum albumin with bilirubin in solution and in suspensions containing bile pigment of membranous particles]

Spectrophotometrically measured in homogeneous aqueous medium association constants for the formation of bilirubin--human human serum albumin complex did not correlate with the intensity of the same complex formation as a result of bilidiene release from nerve cell plasma membrane particles containing biliary pigment in aqueous medium during changed hydrogen parameter and ionic strength of the medium (pH22 degrees 6.8, 7.2, or 7.5 and NaCl concentration in the medium 0.135, 0.150, or 0.165 M), such changes being compatible with normal mammalian cellular and tissue vital activity, the rest conditions being compatible. Study of the time course of the said complex formation in membranous particle suspension seems to be more adequate and therefore more reliable for organic anions (e.g. bilirubin) transport related assessment of detoxifying activity of infused human serum albumin preparations than measurement of the said association constants in a homogeneous aqueous medium.

Anaerobic breakdown of uroporphyrins I and III to bile pigments by extracts of Clostridium tetanomorphum

Two blue bile pigments were formed under anaerobic conditions from the tetrapyrrole precursor delta-aminolevulinate by cells and cell extracts of Clostridium tetanomorphum. These compounds were also formed by cell extracts from the octacarboxylic tetrapyrrole, uroporphyrin III. Bactobilin, the first bacterial bile pigment to be discovered, is related to uroporphyrin I. The present results hence increase the number of bile pigments related to bactobilin. Bactobilin and its isomers differ markedly from the eukaryotic bile pigments which are all related to the dicarboxylic compound, protoporphyrin IX. The enzyme participating in the formation of the bacterial bile pigments was obligatorily anaerobic, in decided contrast to the only other known bile pigment-forming enzyme, the eukaryotic oxygen-requiring heme oxygenase.

Phycobilins of cryptophycean algae. Novel linkage of dihydrobiliverdin in a phycoerythrin 555 and a phycocyanin 645

Cryptomonad strain IVF2 phycoerythrin 555 carries phycoerythrobilins attached through single thioether bonds at alpha-Cys-18, beta-Cys-82, and beta-Cys-158 and a doubly linked 15,16-dihydrobiliverdin (DBV) at beta-DiCys-50,61 (for sequence numbering, see Sidler, W., Nutt, H., Kumpf, B., Frank, G., Suter, F., Brenzel, A., Wehrmeyer, W., and Zuber, H. (1990) Biol. Chem. Hoppe-Seyler 371, 573-547). Analysis of the beta-DiCys-50,61-linked DBV by 1H homonuclear and 1H-13C heteronuclear NMR spectroscopy establishes that the thioether bond from Cys-50 is to the 3"-carbon of the DBV ring A and that from Cys-61 is to the 18'-carbon of ring D, i.e. the peptide-linked bilin is an 8,12-bis(2-carboxyethyl)-3-(2-(cysteinyl-S)-ethyl)-18-(1-(cysteinyl-S)-e thyl)- 2,7,13,17-tetramethylbiladiene-ab-1,19(16H,21H)-dione. DBV is also present at beta-DiCys-50,61 in cryptomonad strain UW374 phycocyanin 645 (Wedemayer, G. J., Kidd, D. G., Wemmer, D. E., and Glazer, A. N. (1992) J. Biol. Chem. 267, 7315-7331). NMR spectroscopy shows that the thioether bonds to this DBV are also at 3" and 18'. Linkage of tetrapyrroles to polypeptides through the 3"-carbon has not hitherto been reported.

Phytochrome assembly. Defining chromophore structural requirements for covalent attachment and photoreversibility

Assembly of holophytochrome in the plant cell requires covalent attachment of the linear tetrapyrrole chromophore precursor, phytochromobilin, to a unique cysteine in the nascent apoprotein. In this investigation we compare chromophore analogs with the natural chromophore precursor for their ability to attach covalently to recombinant oat apophytochrome and to form photoactive holoproteins. Ethylidene-containing analogs readily form covalent adducts with apophytochrome, whereas chromophores lacking this double bond are poor substrates for attachment. Kinetic measurements establish that although the chromophore binding site on apophytochrome is best tailored to phytochromobilin, apophytochrome will accommodate the two analogs with modified D-rings, phycocyanobilin and phycoerythrobilin. The phycocyanobilin-apophytochrome adduct is photoactive and undergoes a light-induced protein conformational change similar to the native holoprotein. By contrast, the phycoerythrobilin adduct is locked into a photochemically inactive protein conformation that is similar to the red light-absorbing Pr form of phytochrome. These results support the hypothesis that the photoconversion from Pr to Pfr, the far red light- absorbing form of phytochrome, involves the photoisomerization of the C15 double bond. Knowledge gained from these studies provides impetus for rational design of chromophore analogs whose insertion into apophytochrome should elicit profound changes in light-mediated plant growth and development.

High-performance liquid chromatographic analysis of bile pigments as their native tetrapyrroles and as their dipyrrolic azosulfanilate derivatives

A reversed-phase high-performance liquid chromatographic (HPLC) analysis of bile pigments is described that provides baseline separation of the major bilirubin conjugates found in bile. The advantage of the technique is that the bile pigments can be analyzed directly as their native tetrapyrroles without prior solvent extractions or derivatization. The use of ammonium acetate in place of sodium salts permits preparative isolation and lyophilization of the pigments for mass spectroscopy. The derivatization of the pigments as their dipyrrolic azosulfanilates with subsequent HPLC analysis demonstrates baseline separation of the endo- and exovinyl azodipyrroles and allows identification of that half of the tetrapyrrole which contains the conjugate in the instances of monoglycosides.