Bilin-Dependent Photoacclimation in Chlamydomonas reinhardtii

In land plants, linear tetrapyrrole (bilin)-based phytochrome photosensors optimize photosynthetic light capture by mediating massive reprogramming of gene expression. But, surprisingly, many green algal genomes lack phytochrome genes. Studies of the heme oxygenase mutant (hmox1) of the green alga Chlamydomonas reinhardtii suggest that bilin biosynthesis in plastids is essential for proper regulation of a nuclear gene network implicated in oxygen detoxification during dark-to-light transitions. hmox1 cannot grow photoautotrophically and photoacclimates poorly to increased illumination. We show that these phenotypes are due to reduced accumulation of photosystem I (PSI) reaction centers, the PSI electron acceptors 5'-monohydroxyphylloquinone and phylloquinone, and the loss of PSI and photosystem II antennae complexes during photoacclimation. The hmox1 mutant resembles chlorophyll biosynthesis mutants phenotypically, but can be rescued by exogenous biliverdin IXα, the bilin produced by HMOX1. This rescue is independent of photosynthesis and is strongly dependent on blue light. RNA-seq comparisons of hmox1, genetically complemented hmox1, and chemically rescued hmox1 reveal that tetrapyrrole biosynthesis and known photoreceptor and photosynthesis-related genes are not impacted in the hmox1 mutant at the transcript level. We propose that a bilin-based, blue-light-sensing system within plastids evolved together with a bilin-based retrograde signaling pathway to ensure that a robust photosynthetic apparatus is sustained in light-grown Chlamydomonas.

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.

Metabolic engineering to produce phytochromes with phytochromobilin, phycocyanobilin, or phycoerythrobilin chromophore inEscherichia coli

By co-expression of heme oxygenase and various bilin reductase(s) in a single operon in conjunction with apophytochrome using two compatible plasmids, we developed a system to produce phytochromes with various chromophores in Escherichia coli. Through the selection of different bilin reductases, apophytochromes were assembled with phytochromobilin, phycocyanobilin, and phycoerythrobilin. The blue-shifted difference spectra of truncated phytochromes were observed with a phycocyanobilin chromophore compared to a phytochromobilin chromophore. When the phycoerythrobilin biosynthetic enzymes were co-expressed, E. coli cells accumulated orange-fluorescent phytochrome. The metabolic engineering of bacteria for the production of various bilins for assembly into phytochromes will facilitate the molecular analysis of photoreceptors.

Agrobacterium phytochrome as an enzyme for the production of ZZE bilins

Photoconversion of phytochrome from the red-absorbing form Pr to the far-red-absorbing form Pfr is initiated by a Z to E isomerization around the ring C-ring D connecting double bond; the chromophore undergoes a ZZZ to ZZE isomerization. In vivo, phytochrome chromophores are covalently bound to the protein, but several examples of noncovalent in vitro adducts have been reported which also undergo Pr to Pfr photoconversion. We show that free biliverdin or phycocyanobilin, highly enriched in the ZZE isomer, can easily be obtained from chromophores bound in a noncovalent manner to Agrobacterium phytochrome Agp1, and used for spectral assays. Photoconversion of free biliverdin in a methanol/HCl solution from ZZE to ZZZ proceeded with a quantum yield of 1.8%, but was negligible in neutral methanol solution, indicating that this process is proton-dependent. The ZZE form of biliverdin and phycocyanobilin were tested for their ability to assemble with Agp1 and cyanobacterial phytochrome Cph1, respectively. In both cases, a Pfr-like adduct was formed but the chromophore was bound in a noncovalent manner to the protein. Agp1 Pfr undergoes dark reversion to Pr; the same feature was found for the noncovalent ZZE adduct. After dark reversion, the chromophore became covalently bound to the protein. In analogy, the PCB chromophore became covalently bound to Cph1 upon irradiation with strong far-red light which initiated ZZE to ZZZ isomerization. Agrobacterium Agp2 belongs to a yet small group of phytochromes which also assemble in the Pr form but convert from Pr to Pfr in darkness. When the Agp2 apoprotein was assembled with the ZZE form of biliverdin, the formation of the final adduct was accelerated compared to the formation of the ZZZ control, indicating that the ZZE chromophore fits directly into the chromophore pocket of Agp2.

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.

Recombinant holophytochrome in Escherichia coli

We have successfully co-expressed two genes from the bilin biosynthetic pathway of Synechocystis together with cyanobacterial phytochrome 1 (Cph1) from the same organism to produce holophytochrome in Escherichia coli. Heme oxygenase was used to convert host heme to biliverdin IXalpha which was then reduced to phycocyanobilin via phycocyanobilin:ferredoxin oxidoreductase, presumably with the aid of host ferredoxin. In this host environment Cph1 apophytochrome was able to autoassemble with the phycocyanobilin in vivo to form fully photoreversible holophytochrome. The system can be used as a tool for further genetic studies of phytochrome function and signal transduction as well as providing an excellent source of holophytochrome for physicochemical studies.

Gallbladder mucin and cholesterol and pigment gallstone formation in hamsters

We measured gallbladder mucin production by hamsters fed diets lithogenic for either cholesterol or pigment gallstones. In hamsters on the cholesterol stone diet, gallbladder production of 3H-glucosamine-labeled mucin was elevated two- and seven-fold after 1 and 3 weeks, respectively. After 1 week cholesterol crystals were seen in a mucus gel on the gallbladder surface. In hamsters on the pigment stone diet, gallbladder mucin production was significantly elevated after 1 and 3 weeks. The first precipitation of pigment crystals was in mucus in bile or on the gallbladder surface. Black pigment stones grew by agglomeration of pigment crystals enmeshed in mucus. In conclusion, gallbladder mucin production is increased before cholesterol or pigment stone formation, and the earliest deposition of crystals is in mucus in bile or on the gallbladder surface.

Oral calcium promotes pigment gallstone formation

Dietary calcium supplementation has been recommended for prevention of osteoporosis and has become a standard component of most "health food" diets. Biliary calcium has been recognized to play a central role in the formation of pigment gallstones. We have recently demonstrated that 5 days of oral calcium supplementation significantly increases biliary calcium in the prairie dog (K. D. Lillemoe, T. H. Magnuson, G. E. Peoples, et al., Gastroenterology 94: A563, 1988). We hypothesized, therefore, that long-term oral calcium supplementation would promote pigment gallstone formation. Sixteen adult male prairie dogs were maintained on a standard nonlithogenic diet. Eight animals received calcium supplementation (2.5 x control levels) in their water, while the remaining eight animals served as controls. After 8 weeks, cholecystectomy was performed, and the common bile duct was cannulated. Bile was examined microscopically and analyzed for ionized calcium, bilirubin, glycoprotein, and biliary lipids. The cholesterol saturation index (CSI) was calculated. Pigment stones and calcium bilirubinate sludge were present in all animals receiving calcium supplementation. Only one control animal had evidence of pigment stones (P less than 0.001). Biochemical analysis of gallbladder bile demonstrated a significant increase in total bilirubin and bilirubin monoglucuronide (P less than 0.01) as well as bile glycoprotein content (P less than 0.05) after oral calcium supplementation. Gallbladder bile ionized calcium was also increased although not significantly. These data suggest that oral calcium supplementation promotes gallbladder sludge and pigment gallstone formation in the prairie dog. This observation raises concern that oral calcium supplementation, especially in the older female population, may enhance gallstone formation.

A two-molecule mechanism of haem degradation

Coupled oxidation of octaethylhaemin and phenylhydrazine hydrochloride with 16,16O2 and 18,18O2 produced octaethyl[16O]verdohaemochrome and octaethyl[18O]-verdohaemochrome respectively. Reactions of these products with 16,16O2 in the presence of phenylhydrazine hydrochloride yielded octaethyl[16O, 16O]biliverdin and octaethyl[18O, 16O]biliverdin. The same reactions with 18,18O2 yielded octaethyl[16O, 18O]biliverdin and octaethyl[18O, 18O]biliverdin. Accordingly, the two oxygen atoms of biliverdin are incorporated from different O2 molecules in separate reactions, namely the formation of verdohaemochrome and the conversion of verdohaemochrome into biliverdin. These reactions account for a "two-molecule mechanism' of biliverdin formation from haem with verdohaemochrome participating as an intermediate product.

Formation and disposition of newly synthesized heme in adult rat hepatocytes in primary culture

Studies with the intact liver have suggested that newly synthesized heme exists transiently in a small pool before its incorporation into tissue heme proteins. The same or a closely related pool may regulate synthesis of heme and serve as the precursor of "early peak" bilirubin. To delineate this postulated pool by a direct approach, we have utilized primary cultures of adult rat hepatocytes. Cultures pulse-labeled with delta-amino[3H]levulinic acid at various time points were fractionated into 105,000 X g supernatant and pellet. Labeled heme appeared within 1 to 2 min in the cytosol fraction, followed by transfer to the pellet. The kinetics of heme formation and transfer and of labeled bilirubin production were analyzed by computer simulation utilizing the least squares method. The experimental findings conformed best to a four-compartment model that includes a second cytosolic heme compartment exchanging with the initially labeled compartment but not serving as a direct precursor of bilirubin. Calculation of apparent rate coefficients indicated that, in cultured hepatocytes, 20% of newly formed heme is converted directly to bile pigment, whereas 80% is utilized for formation of cellular heme proteins (64% in the pellet, 16% in the second cytosol compartment). This experimental approach has provided direct evidence for a rapidly formed cytosolic heme fraction which appears to be identical with the previously postulated regulatory or "unassigned" heme pool of the liver.

Bile pigment synthesis in plants. Incorporation of haem into phycocyanobilin and phycobiliproteins in Cyanidium caldarium

A procedure was developed whereby haem was taken up by dark-grown cells of the unicellular rhodophyte Cyanidium caldarium. These cells were subsequently incubated either in the dark with 5-aminolaevulinate, which results in excretion of phycocyanobilin into the suspending medium or incubated in the light, which results in synthesis and accumulation of phycocyanin and chlorophyll a within the cells. Phycocyanobilin was isolated from phycocyanin by cleavage from apoprotein in methanol. Phycocyanobilin prepared from phycocyanin or excreted from cells given 5-aminolaevulinate was methylated and purified by t.l.c. By using 14C labelling either in the haem or in 5-aminolaevulinate administered, haem incorporation into phycocyanobilin was demonstrated in both dark and light systems. Since chlorophyll a synthesized in the light in the presence of labelled haem contained no radioactivity, it was clear that haem was directly incorporated into phycocyanobilin and not first converted into protoporphyrin IX. These results clearly demonstrate phycocyanobilin synthesis via haem and not via magnesium protoporphyrin IX as has also been postulated.

Mechanism of bile-pigment synthesis in algae. 18O incorporation into phycocyanobilin in the unicellular rhodophyte, Cyanidium caldarium

The origin of the lactam oxygen atoms of phycocyanobilin from Cyanidium caldarium was studied using 18O labelling. By inhibiting photosynthesis, a high 18O enrichment was maintained in the gas phase and the resulting incorporation of label showed that the lactam oxygen atoms were derived from two oxygen molecules. Slow exchange of these oxygen atoms with water was demonstrated directly by using H218O.

Comparative aspects of bile pigment formation and excretion

Breakdown of haem which is of key importance in most organisms, involves oxidative CO-evolving cleavage of the macrocyclic ring with formation of biliverdin-IX. In two major pathways established so far formation of biliverdin-IX alpha is followed by (a) biliary secretion or (b) reduction to bilirubin-IX alpha, formation of more hydrophilic derivatives (usually glycosidic conjugates) and biliary secretion. The scattered comparative information available indicates marked species variation with regard to the methin-bridge carbon atom removed from haem and the metabolic site of cleavage, the nature of bilirubin conjugates and the developmental sequence of maturation of enzyme activities and transport proteins involved in the chain of events leading from breakdown of haem to the excretion of the final end products.

Development of germfree animal characteristics in conventional rats in antibiotics

Conventional (CONV) rats were fed by stomach tube for five days with either benzylpenicillin, ampicillin, tetracykline, oxitetracykline, neomycin, bacitracin + neomycin, kanamycin or colistin. On the 2nd-3rd day all the animals developed one or several of the following symptoms or characteristics typical for germfree (GF) rats: no coprostanol formation, no stercobilin production, a GF pattern after gel electrophoresis of fecal supernatant and proteolytic activity in the feces. Under the same conditions succinylsulfathiazole or metronidazole had much less pronounced effects than the antibiotics. When clofibrate, acetylsalicylic acid or ferrous sulphate were administered the effects were none or negligible. The GF characteristics persisted for several weeks after the end of the administration of the drugs. In some instances this was the case up to 7 weeks, when the animals were contaminated by anal route with a suspension of the cecum contents from intact CONV animals. On the 2nd day after this treatment the GF characteristics had disappeared.

[Biosynthesis of phorcabiline, blue-green biliary pigment of Actias selene (Lepidoptera, Attacidae)]

The blue-green bile pigments of Actias selene (Attacidae) have been investigated at different stages of its development. Coproporphyrinogen-14-C, protoporphyrin-IX3-H, and pterobilin-14-C, injected to larvae are metabolised into phorcabilin I, the main neopterobilin in this animal. It is concluded that phorcabilin I is a bile pigment of the IX gamma series and that pterobilin is its direct precursor. A method for the preparation of labelled protoporphyrin from quail egg-shell is reported.

Isotopic studies of the erythropoietic and hepatic components of congenital porphyria and 'erythropoietic' protoporphyria

1. Labelled glycine and/or δ-aminolaevulinic acid (ALA) were administered to a child with congenital erythropoietic porphyria (Günther's disease), to three normal children and to three patients with erythropoietic protoporphyria.

2. The utilization of [15N]ALA for the synthesis of faecal ‘urobilin’ in the congenital erythropoietic patient was normal.

3. This suggests there is no significant increase of hepatic bile-pigment formation in congenital erythropoietic porphyria.

4. The utilization of glycine for synthesis of faecal ‘urobilin’ and protoporphyrin in all three patients with erythropoietic protoporphyria was increased. There was a similarly high utilization of [4-14C]ALA administered either orally or intravenously to one of the patients.

5. The utilization of [4-14C]ALA was not affected by phlebotomy.

6. Utilizations of both ALA and glycine for free erythrocyte and plasma protoporphyrins were low.

7. This study provides further evidence that in erythropoietic protoporphyria there is a greatly increased hepatic contribution to the early labelled fraction of bile pigment and that in this disease the excessive protoporphyrin is formed mainly in the liver.

Relationship between age of culture and occurrence of the pigments of photosystem II of photosynthesis in heterocysts of a blue-green alga

Microspectrophotometric examination of the pigments in vivo of heterocysts of Anabaena sp. L-31 has shown that most heterocysts of 2-day-old cultures possess only very small amounts, if any, of c-phycocyanin, allo-phycocyanin, and c-phycoerythrin, the main pigments comprising photosystem II of photosynthesis. The quantities of these pigments, however, increase with age of cultures, and by the end of 5 days the majority of heterocysts contain comparatively large amounts. The culmination of this sequential development is observed in most heterocysts of 7- to 15-day-old cultures when the full complement of photosystem II pigments is present. The spectral characteristics at this stage are similar to those of vegetative cells and suggest a dedifferentiation of heterocysts.

Heme binding and transport--a spectrophotometric study of plasma glycoglobulin hemochromogens

A hitherto unreported phenomenon is the immediate production of the spectrum of ferrohemochromogens (in the presence of sodium dithionite) upon the addition in vitro of hydroxyhemin (pH 7.6-7.8) to the plasmas or sera, as well as to certain Cohn plasma protein fractions, of all mammalian species thus far examined. This distinctive reaction is characteristic of a coordination complex with heme iron, and is ascribed to a remarkable affinity for heme of certain plasma glycoglobulins, which include hemopexin. Spectrophotometry has permitted estimations of the specific heme-binding capacity (as ferrohemochromogen) of the plasmas, the rate of removal from plasma of injected heme, and the production of bile pigments therefrom. The study leads to a new proposal of the cooperative roles of the glycoglobulin hemochromogens and hematin-albumin in heme transport and bile pigment production.

Isotopic studies of the conversion of oxophlorins and their ferrihaems into bile pigments in the rat

1. Tritiated oxymesoporphyrins and their ferrihaems were tested as possible intermediates in the catabolism of haemoglobin. The tritiated compounds were injected into rats with biliary fistulae and the incorporation of the isotope into bile, bile pigment, urine, faeces, liver, kidney and spleen was measured. 2. alpha-Oxymesoferrihaem was extensively converted into bile pigment and specifically to the expected mesobilirubin. 3. beta-Oxymesoferrihaem was poorly converted into bile pigment and was not converted into mesobiliverdin IXbeta. The latter was independently shown to be excreted rapidly in bile. 4. The free oxyporphyrins were also poor precursors of bile pigment, and alpha-oxymesoporphyrin competed with bilirubin for excretion by the liver. 5. By analogy with the results obtained with alpha-oxymesoferrihaem it is concluded that alpha-oxyprotoferrihaem is an intermediate in the catabolism of haemoglobin, undergoing further oxidation to bile pigment under the catalysis of an enzyme of definite specificity.

Bile pigment formation in plants

The unicellular alga Cyanidium caldarium evolves carbon monoxide during the syntheis of the bile pigment, phycocyanobilin. Carbon monoxide and phycocyanobilin were produced in stoichiometric amounts at comparable rates. Therefore, the mechanism of bile pigment formation in this plant parallels that in mammals.

Carbon monoxide production associated with ineffective erythropoiesis

The rate of endogenous carbon monoxide production ( Vco), determined by the closed rebreathing system technique, was elevated above the normal range in four of five patients studied with ineffective erythropoiesis (four patients with primary refractory anemia, one with thalassemia). The mean molar ratio of Vco to Vheme (rate of circulating heme catabolism, determined from (51)Cr red cell survival curves) was 3.0 +/- 0.6 (SE), indicating that most of the CO originated from sources other than circulating erythrocyte hemoglobin, in contrast to previous findings in patients with hemolytic anemia, where Vco paralleled Vheme closely.After administration of glycine-2-(14)C to these patients, endogenous CO was isolated by washout of body CO stores at high pO(2) or by reacting peripheral venous blood samples with ferricyanide. The CO was then oxidized to CO(2) by palladium chloride and trapped for counting in a liquid scintillation spectrometer. "Early labeled" peaks of (14)CO were demonstrated which paralleled "early labeled" peaks of stercobilin and preceded maximal labeling of circulating heme. Production of "early labeled" (14)CO in patients with ineffective erythropoiesis was greatly increased, up to 14 times that found in a normal subject. The increased Vco and "early (14)CO" production shown by these patients are presumably related mainly to heme catabolism in the marrow. The possibility exists that hepatic heme and porphyrin compounds may also contribute significantly to Vco, as suggested by the finding of a high Vco in an additional patient with porphyria cutanea tarda.

Studies on haem alpha-methenyl oxygenase. Isomeric structure of formylbiliverdin, a possible precursor of biliverdin

1. The enzyme from guinea-pig liver that degrades pyridine-haemochromogen into formylbiliverdin, a possible precursor of biliverdin, breaks the porphyrin ring exclusively at the alpha-methene bridge. The enzyme is therefore appropriately named haem alpha-methenyl oxygenase. 2. It is deduced that this enzyme plays an essential role in haemoglobin catabolism in vivo.