Rod structure of a phycoerythrin II-containing phycobilisome. II. Complete sequence and bilin attachment site of a phycoerythrin gamma subunit

The major phycoerythrins of marine unicellular cyanobacteria Synechococcus spp. and those of red algae are isolated as stable complexes with the composition (alpha beta)6 gamma. The gamma subunits carry bilins and in this respect are unique among phycobilisome rod linker polypeptides. There has been no complete amino acid sequence data on any gamma subunit. Synechococcus sp. WH8020 phycoerythrin II (PE II) gamma subunit was isolated from purified PE II hexamers. Partial amino acid sequence determination showed that the gamma subunit was encoded by the mpeC gene, an open reading frame 275 base pairs 3' of mpeA and mpeB, which encode the alpha and beta subunits of PE II, respectively (Wilbanks, S. M., and Glazer, A. N. (1993) J. Biol. Chem. 268, 1226-1235). A single phycourobilin is attached through a thioether bond to gamma-Cys-49. Derivatization with 4-vinylpyridine showed that the only other cysteinyl residue, gamma-Cys-64, is unsubstituted. MpeC encodes a polypeptide of 293 residues with a predicted molecular weight of 32,100 and a pI of 8.9. These properties are like those of non-bilin-bearing linker polypeptides associated with C-phycoerythrin and hence the gamma subunit is designated gamma (LR32). Alignment of the sequence of the PE II-gamma with those of the latter polypeptides shows that PE II-gamma has a 49-residue extension at the N terminus, that encompasses the phycourobilin attachment site, and is shorter by a similar number of residues at the C terminus. These differences in linker polypeptide structure offer a possible explanation for the observed much higher stability of PE II hexamers relative to those of C-phycoerythrins.

Genes of the R-phycocyanin II locus of marine Synechococcus spp., and comparison of protein-chromophore interactions in phycocyanins differing in bilin composition

R-phycocyanin II (RPCII) is a recently discovered member of the phycocyanin family of photosynthetic light-harvesting proteins. Genes encoding the alpha and beta subunits of RPCII were cloned and sequenced from marine Synechococcus sp. strains WH8020 and WH8103. The deduced amino acid sequences of RPCII were compared to two other types of phycocyanin, C-phycocyanin (CPC) and phycoerythrocyanin (PEC). These three types vary in the composition of their covalently bound bilin prosthetic groups. In terms of amino acid sequence identity RPCII is highly homologous to CPC and PEC, suggesting that the known three-dimensional structures of the latter two are representative of RPCII. Thus the amino acid residues contacting the three bilins of RPCII could be inferred and compared to those in CPC and PEC. Certain residues were identified among the three phycocyanins as possibly correlating with specific bilin isomers. In overall sequence RPCII and CPC are more homologous to one another than either is to PEC. This probably reflects functional homology in the roles of RPCII and CPC in the transfer of light energy to the core of the phycobilisome, a function not attributed to PEC. The genomes of Synechococcus sp. strains WH8020, WH8103 and WH7803 share homologous open reading frames in the vicinity of RPCII genes. The nucleotide sequence extending 3' from RPCII genes in strain WH8020 revealed two open reading frames homologous to components of an alpha CPC phycocyanobilin lyase. These open reading frames may encode a lyase specific for the attachment of phycoerythrobilin to alpha RPCII.

High vesicular cholesterol and protein in bile are associated with formation of cholesterol but not pigment gallstones

To examine the differentiating parameters between cholesterol and pigment gallstones, we compared the nucleation times, concentrations of biliary lipid and protein, and the distribution of vesicular cholesterol in gallbladder bile of 16 patients with cholesterol, eight patients with black pigment gallstones, and nine gallstone-free control patients. Cholesterol monohydrate crystals were present in the fresh bile of only the cholesterol gallstone group. The nucleation time was significantly faster in the cholesterol stone group (3.3 +/- 3.2 days) than in the other two groups (pigment stone: 15.8 +/- 6.6, control: 16.9 +/- 5.7). The cholesterol saturation indices and the distribution of vesicular cholesterol were significantly higher in the cholesterol gallstone group than those in the other two groups. The total biliary protein concentration was significantly (P < 0.01) higher in the cholesterol gallstone group [2.57 +/- 1.91 (SD) mg/ml] than that in the black pigment stone group (1.09 +/- 0.59). All parameters in patients with black pigment gallstone were essentially similar to the controls. We conclude that the presence of cholesterol crystals, rapid nucleation time, high vesicular cholesterol distribution, elevated cholesterol saturation index, and high protein concentration are associated with cholesterol gallstones but not with black pigment gallstones.

Induction of kidney heme oxygenase-1 (HSP32) mRNA and protein by ischemia/reperfusion: possible role of heme as both promotor of tissue damage and regulator of HSP32

Presently we describe, for the first time, induction of microsomal heme oxygenase-1 (HO-1) mRNA and protein in response to ischemia/reperfusion and therefore define HO-1 as stress protein in the kidney. Specifically, Northern blot analysis of kidneys of rats subjected to bilateral ischemia for 30 min revealed an increase of 8- to 10-fold in the level of 1.8 Kb HO-1 mRNA 6 hr after reperfusion. The increase in transcript level was maintained when assessed after 24 hr. The levels of 1.3 and 1.9 Kb transcripts for the second isozyme of HO, HO-2, were decreased at both time points. The increase in HO-1 mRNA was reflected in HO-1 protein level, as judged by Western blot analysis and at the level of activity as judged by the rate of bilirubin formation. An absence of change in adrenal HO-1 mRNA level subsequent to renal ischemia/reperfusion suggested that the induction of kidney HO-1 did not reflect a generalized response of the rat organs to stress; rather, it was a target organ specific response. Moreover, in kidneys subjected to ischemia 6 and 24 hr after reperfusion, significant increases in the cellular content of heme were observed; heme is a known inducer of HO-1 synthesis. Ischemia/reperfusion also adversely affected concentration of cytochrome P-450 in both mitochondrial and the microsomal fractions of the kidney. We suggest that increase in tissue heme levels may be a significant factor in damage caused by ischemia/reperfusion to renal tissue, whereby the metalloporphyrin promotes oxygen-free radical formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion of biliary calcium is increased in dogs with pigment gallstones

We have previously demonstrated that gallbladder bile is supersaturated with calcium bilirubinate in a canine dietary model of pigment gallstones. Supersaturation resulted from combined increases in the concentrations of both biliary calcium and unconjugated bilirubin. The elevations in biliary calcium and unconjugated bilirubin concentrations remain unexplained but could possibly be due to increases in hepatic or ductular secretion, alterations in bile composition with respect to calcium-or bilirubin-binding affinity, decreases in absorption from the gallbladder lumen, or, in the case of unconjugated bilirubin, production within the lumen by hydrolysis of conjugated bilirubin. Here, we study a single possible cause for the observed increase in biliary calcium concentration during pigment gallstone formation in dogs. Secretion of calcium into bile in dogs with pigment gallstones before and after infusion of the bile salt, taurocholate, was compared to normal dogs. A significant increase in bile acid-independent bile flow and calcium output (CaO) was observed at any given bile acid output. Thus, plots of bile flow and CaO versus bile acid output yielded two separate functions in normal dogs and dogs with pigment gallstones. The slopes of these functions were similar, but intercepts extrapolated to zero bile acid output were markedly different, indicating that bile acid-independent, but not bile acid-dependent, bile flow and CaO was increased. The increase in CaO was not due to secretion of bile with increased concentrations of calcium but rather to the increases in the rate of bile flow. These findings might, in part, explain elevated calcium concentrations since increased amounts of calcium would be presented to the gallbladder in these animals during gallstone formation.

Bilirubin production and conjugation from newly formed heme in isolated rat hepatocytes

1. Heme synthesis from delta-aminolevulinic acid (delta-ALA) in freshly isolated rat hepatocytes was maximal at 100 microM with a rate of approx. 7 nmol being synthesized per g wet weight cells. 2. Approximately 8% of synthesized heme was converted to bilirubin and 50% of the newly synthesized bilirubin was conjugated. 3. The ratio of di to monoconjugate was approx. 2.5. Incorporation of delta-ALA into bilirubin was increased by additional delta-ALA, heme and was also doubled in cells isolated from animals treated with CoCl2. 4. Bilirubin formation was inhibited approx. 90% by in vitro treatment with heme oxygenase inhibitors zinc and tin protoporphyrin.

Characterization of phycocyanin produced by cpcE and cpcF mutants and identification of an intergenic suppressor of the defect in bilin attachment

Mutants of the cyanobacterium Synechococcus sp. PCC 7002 constructed by the insertional inactivation of either the cpcE or cpcF gene produce low levels of spectroscopically detectable phycocyanin. The majority of the phycocyanin produced in these strains appears to lack the alpha subunit phycocyanobilin (PCB) chromophore (Zhou, J., Gasparich, G. E., Stirewalt, V. L., de Lorimier, R., and Bryant, D. A. (1992) J. Biol. Chem. 267, 16138-16145). Purification of the phycocyanin produced in the mutants revealed two fractions each with an aberrant absorption spectrum. Tryptic peptide maps of the major fraction showed that the alpha-84 PCB peptide was absent. The two PCB peptides derived from the beta subunit were normal. Tryptic digests of the less abundant phycocyanin fraction contained a family of bilin peptides derived from the alpha subunit. Several distinct bilin adducts were present. A major component was a mesobiliverdin adduct, a previously described product of the in vitro reaction of PCB and apophycocyanin. The same results were obtained with both the cpcE mutant and the cpcF mutant. In vitro reactions with PCB and the fractions containing apo alpha subunit showed that the alpha-84 bilin attachment site was unmodified and competent for adduct formation. Pseudo-revertants of both strains were observed to arise at high frequency. Analysis of the phycocyanin from a cpcE pseudo-revertant, which produced a near wild-type level of phycocyanin with alpha subunit carrying PCB, revealed a single amino acid substitution, alpha-Tyr129----Cys. This residue, which is conserved in all phycocyanins sequenced to date, forms part of the alpha-84 bilin binding site and lies within 5 A of alpha-Cys84. A mutated cpcA gene containing this substitution was constructed by site-directed mutagenesis and transformed, along with cpcB, into a cpcBAC deletion strain containing an insertionally inactivated cpcE. This strain produces high levels of phycocyanin and the majority of the alpha subunit carries PCB at alpha-Cys84.

Phycocyanin alpha-subunit phycocyanobilin lyase

Phycobiliproteins, unlike other light-harvesting proteins involved in photosynthesis, bear covalently attached chromophores. The bilin chromophores are attached through thioether bonds to cysteine residues. The cyanobacterium Synechococcus sp. PCC 7002 has eight distinct bilin attachment sites on seven polypeptides, all of which carry the same chromophore, phycocyanobilin. When two genes in the phycocyanin operon of this organism, cpcE and cpcF, are inactivated by insertion, together or separately, the surprising result is elimination of correct bilin attachment at only one site, that on the alpha subunit of phycocyanin. We have overproduced CpcE and CpcF in Escherichia coli. In vitro, these proteins catalyze the attachment of phycocyanobilin to the alpha subunit of apophycocyanin at the appropriate site, alpha-Cys-84, to form the correct adduct. CpcE and CpcF also efficiently catalyze the reverse reaction, in which the bilin from holo-alpha subunit is transferred either to the apo-alpha subunit of the same C-phycocyanin or to the apo-alpha subunit of a heterologous C-phycocyanin. The forward and reverse reactions each require both CpcE and CpcF and are specific for the alpha-Cys-84 position. Phycocyanobilin is the immediate precursor of the protein-bound bilin.

Separation and sensitive determination of i-urobilin and 1-stercobilin by high-performance liquid chromatography with fluorimetric detection

i-Urobilin and 1-stercobilin were separated by high-performance liquid chromatography on a reversed-phase octadecylsilane-bonded column and detected fluorimetrically through formation of phosphor with zinc ions in the eluent. The separation and the intensity of the fluorescence response were affected by concentrations of zinc acetate and sodium borate buffer, pH and methanol content in the eluent. The optimal eluent used consisted of 0.1% zinc acetate in 75 mM boric acid buffer (pH 6.0)-methanol (25:75). The detection limit was 0.2 microgram/l for both i-urobilin and 1-stercobilin (signal-to-noise ratio 2), which makes the method 250-2500 times more sensitive than conventional methods.

Intravenous bilirubin infusion causes vacuolization of the cytoplasm of hepatocytes and canalicular cholestasis

To examine whether intravenous bilirubin infusion causes cholestasis and impairs liver metabolism, bile secretion and ethanol clearance were measured in 34 anaesthetized pigs before and after intravenous infusion of 0.5 mumol kg-1 min-1 bilirubin for 4.5 hours. Bilirubin infusion increased plasma bilirubin to 556 +/- 76 mumol l-1 and hepatic tissue bilirubin to 3.5 +/- 1.3 mmol kg tissue weight-1. Bilirubin infusion depressed bilirubin secretion and net hepatic uptake of cholate and taurocholate, and caused a 86 +/- 6% reduction of cholate-induced bile secretion. Bilirubin caused formation of large cytoplasmic vacuoles in hepatocytes and dilatation of bile canaliculi. Ethanol clearance and secretin-dependent ductular bile secretion were unaffected by bilirubin. We conclude that intravenous infusion of unconjugated bilirubin causes accumulation of bilirubin in the liver, vacuolization of the hepatocyte cytoplasm and canalicular but not ductular cholestasis. The canalicular cholestasis is not due to impaired hepatic mitochondrial energy metabolism, but may be due to inhibition of a common pathway for lipid, bilirubin and bile salt secretion from hepatocytes.

Identification of glutathione conjugates of the dimethyl ester of bilirubin in the bile of Gunn rats

The Gunn rat, which is deficient in the UDP-glucuronosyltransferase for bilirubin, promptly excreted polar conjugates of the dimethyl ester of bilirubin in bile after intravenous infusion of this ester. The conjugates proved to be monoglutathione thioether adducts of the vinyl groups of the parent tetrapyrrole. High performance liquid chromatographic analysis of the conjugates as their dipyrrolic azosulfanilates demonstrated that only one of the dipyrroles of each tetrapyrrole was conjugated. The nonconjugated dipyrrole eluted as either the methyl endo- or exovinyl azodipyrrole. The amino acid composition of the pigments was consistent with that of a monoglutathione conjugate. NMR spectroscopy of the two major pigments demonstrated the loss of the proton signals of the C-18 vinyl group, indicating it to be the site of conjugation. Cation fast atomic bombardment tandem mass spectrometry demonstrated a molecular ion, [M + H]+, of m/z 937, which fragmented with a loss of 307 atomic mass units, consistent with glutathione. A molecular ion of m/z 807 was observed for the conjugate treated with gamma-glutamyltranspeptidase, consistent with the loss of glutamate. The mass spectrometry data indicated that the conjugates also contained a functional group whose mass was equivalent to hydroxyl, suggesting initial formation of an epoxide, which then reacts with glutathione. Pretreatment of the rat with 2,3,7,8-tetrachlorodibenzo-p-dioxin to induce cytochrome P-450 resulted in a 6-fold increase of the biliary excretion of the glutathione conjugates. Such induction also resulted in the excretion of a glutathione conjugate of bilirubin itself.

Phycoerythrins of marine unicellular cyanobacteria. I. Bilin types and locations and energy transfer pathways in Synechococcus spp. phycoerythrins

Marine Synechococcus strains WH8103, WH8020, and WH7803 each possess two different phycoerythrins, PE(II) and PE(I), in a weight ratio of 2-4:1. PE(II) and PE(I) differ in amino acid sequence and in bilin composition and content. Studies with strain WH7803 indicated that both PE(II) and PE(I) were present in the same phycobilisome rod substructures and that energy absorbed by PE(II) was transferred to PE(I). Strain WH8103 and WH8020 PE(I)s carried five bilin chromophores thioether-linked to cysteine residues in sequences homologous to those previously characterized in C-, B-, and R-PEs. In contrast, six bilins were attached to strain WH8103 and WH8020 PE(II)s. Five of these were at positions homologous to bilin attachment sites in other phycoerythrins. The additional bilin attachment site was on the alpha subunit. The locations and bilin types in these PE(s) and in the marine Synechocystis strain WH8501 PE(I) (Swanson, R. V., Ong, L. J., Wilbanks, S. M., and Glazer, A. N. (1991) J. Biol. Chem. 266, 9528-9534) are: (table; see text) Since phycourobilin (PUB) (lambda max approximately 495 nm) transfers energy to phycoerythrobilin (PEB) (lambda max approximately 550 nm), inspection of these data shows that the invariant PEB group at beta-82 is the terminal energy acceptor in phycoerythrins. The adaptations to blue-green light, high PUB content and the presence of an additional bilin on the alpha subunit, increase the efficiency of light absorption by PE(II)s at approximately 500 nm.

Gallstones in the golden hamster. XXXVI. Pigment cholelithiasis produced by retinoic acid

Previously we have reported on the pigmentary lithogenic action of vitamin A in the form of retinol acetate. In the present work the possible lithogenic action of retinoic acid was tested, since this differs from retinol in several metabolic aspects, which can contribute to the understanding of the pathogenesis of the pigment cholelithiasis produced by vitamin A. Two experiments were performed in which the lithogenicity of retinol acetate added to a colony chow at the level of 25,000 IU%, was compared with that of 3 dietetic levels of all-trans retinoic acid. In the first experiment seric triglycerides were determined in order to establish whether there is a relation between the hypertriglyceridemic effect of retinoids and their lithogenicity; in the second experiment GPT and GOT were determined as indicators of hepatotoxicity. The results showed that the retinoic acid at levels of 24,000 and 35,000 IU% of diet, produced a cholelithiasis incidence similar to that of 25,000 IU% of retinol acetate, whereas the retinoic acid level of 12,000 IU% was not lithogenic. The dietetic retinoic acid produced a reduction of hepatic vitamin A, that was directly proportional to the level supplied. There was no relation between the hypertriglyceridemic effect of retinoids and its lithogenicity. The retinoids produced a light increase in GPT, which was higher with retinol acetate, whereas GOT had not significative changes. It is concluded that all-trans retinoic acid produces pigment gallstones in the hamster, with an incidence similar to that produced by retinol acetate.

[Computerized tomography differentiation of pigment and cholesterol bile duct calculi]

Successful oral litholytic and other non-operative therapies of gallstones require exact determination of the stone components. Since computed tomography (CT) provides highly sensitive measurement of density, we performed a study to evaluate whether CT measurement of stone density allows to predict the composition of radiolucent gallstones. 28 patients presenting with 29 radiolucent gallbladder (n = 17) or common bile duct stones (n = 12) were included. Prior to operative or endoscopic therapy the attenuation values (Hounsfield Units/HU) were assessed in vivo by CT under standardised conditions (Somatom II; 125 KV; 130 mAs). After surgical or endoscopic stone removal the concrements were dehydrated, homogenised and then analysed by infrared spectroscopy. 18 cholesterol and 11 pigment stones could be identified. The attenuation values (Hounsfield Units) of cholesterol stones amounting to 28-98 HU (48.7 +/- 4.4 HU) differed significantly (p less than 0.001) from pigment stones (90-120 HU/105.5 +/- 2.8 HU). We conclude that computed tomography provides exact discrimination between cholesterol and pigment stones in vivo. Since only cholesterol stones can be dissolved by cheno/ursodeoxycholic acid we recommend to measure the radiodensity of gallstones by CT prior to any litholytic therapy.

Physical-chemical basis of gallstone formation

The broad outlines of the physical chemistry of cholesterol solubilization are well understood, but minor components of native biles probably alter the equilibrium solubility of cholesterol in bile and, more importantly, profoundly affect the kinetics of cholesterol crystal formation. The physical chemistry of UCB solubilization is much less clear; two decades after the limits of cholesterol solubilization in bile were reported, we still cannot provide accurate information on the limits of UCB solubility in bile. Although UCB is predominantly solubilized by the same biliary aggregates as cholesterol, the ionic nature of UCB implies that other, quantitatively minor biliary components profoundly alter UCB solubilization. Understanding UCB solubilization in bile and the prevention of black pigment stone formation await further studies. An understanding of the physical chemistry of cholesterol and UCB, both as solubilized in bile and as precipitated in gallstones, is basic to an understanding of the pathogenesis of gallstone disease, as well as current and potential chemical methods of gallstone dissolution and prevention.

Diagnosis of atretic prolonged obstructive jaundice; technetium 99m hepatolite excretion study

Since 1982 we have developed a diagnostic method for differentiation between atretic prolonged obstructive jaundice (biliary atresia) and nonatretic prolonged obstructive jaundice in a neonate. The first report was presented at the Fukuoka Meeting in 1983. Analysis of duodenal juice (excretion test of intravenous injected hepatolite into the duodenum) has proved to be extremely reliable to differentiate biliary atresia from nonatretic prolonged obstructive jaundice. The major diagnostic features of the study are (1) biliary tract to bowel transit of radiotracer, and (2) observation on color of duodenal juice. Diagnostic accuracy was 98.4%. It is time-saving, requiring only 6 hours, noninvasive, and simple.

Effects of ursodeoxycholate on maximal biliary secretion of bilirubin in the rat

The effect of sodium ursodeoxycholate (0.5 and 1.0 mumol/min/100 g) on the maximal biliary secretion (Tm) of bilirubin and on the concentration of bilirubin in liver and plasma at the end of a bilirubin load was studied in Wistar rats. Administration of ursodeoxycholate at 0.5 mumol/min/100 g caused a 0.8-fold increase in bile flow and a significant increase in the bilirubin Tm (+24%). This was associated with a significant reduction of liver and plasma bilirubin concentrations (-16% and -17%, respectively). Bilirubin UDP-glucuronosyltransferase activity was not significantly enhanced. There was a significant increase in the biliary excretion of bilirubin conjugates (+30%) and in the diconjugates/monoconjugates ratio in bile (+31%). When ursodeoxycholate was given at 1.0 mumol/min/100 g, it produced a 1.7-fold increase in bile flow, but the bilirubin Tm was significantly reduced (-21%). Liver bilirubin concentrations were decreased (-20%) and there was a significant enhancement in total pigment concentration in plasma (+19%). Both the excretion of unconjugated bilirubin and that of bilirubin conjugates were significantly reduced (-60% and -18%, respectively). There was a significant decrease in the bilirubin-UDP glucuronosyltransferase activity and the diconjugates/monoconjugates ratio in bile (-27% and -27%, respectively). These results indicate that ursodeoxycholate is able to increase maximal bilirubin secretion only when administered at low doses and that infusion at higher rates can significantly interfere with different steps in the hepatobiliary transport of the pigment.

Spontaneous formation of pigmentary precipitates in bile salt-depleted rat bile and its prevention by micelle-forming bile salts

During studies on the effect of bile salt-pool depletion in the bile-fistula rat (adult male Sprague-Dawley), the spontaneous formation of an orange-brown precipitate was noted. The nature of this phenomenon and its relationship to BS and calcium concentration was investigated in depth. Bile from 18 animals was collected in the dark into transparent tubes containing sodium azide, ascorbic acid, and glucaro-1,4-lactone. The tubes were flushed with nitrogen, sealed, and incubated at 37 degrees C. The pigmentary precipitate formed in all the bile salt-depleted (less than 3-5 mM) bile samples (i.e., those collected after 5-7 h of external biliary drainage), but not in bile salt-rich biles. It appeared within 30-240 min after collection, both in bile samples collected at room temperature and at 37 degrees C, initially as a pale flocculation and then slowly sedimenting to form, after centrifugation, a solid, dark-orange pellet. There were no pH changes during incubation, and bile cultures were negative. Under polarizing microscopy, the precipitate appeared amorphous, and there was no evidence of birefringence. High-performance liquid chromatography showed that unconjugated bilirubin was the prevalent pigmentary component, but significant amounts of monoconjugated bilirubin also coprecipitated. Lipid chemistry showed the presence of lecithin (80.1% of total lipids), which was rich in palmitoyl and linoleoyl fatty acids, and of fatty acids (predominantly palmitic and oleic). Infrared spectroscopy and x-ray diffraction showed the presence of calcium bilirubinate and palmitate. In-vivo replenishment of the bile salt pool by intravenous infusion of either taurocholate or taurochenodeoxycholate (1 mumol/min) completely prevented the pigmentary precipitation. In vitro experiments showed inhibition of the precipitate formation by the addition of individual bile salt in concentrations approximating their critical micellar concentration. Precipitate formation was hastened by the addition of calcium chloride (4-12 mM), but only in bile salt-depleted biles. As the composition of the precipitate closely resembles that of human brown-pigment stones and sludge, these findings may provide new insights into an understanding of the pathogenesis of pigment gallstone disease.

Proton-induced dismutation of superoxide in aprotic media by bile pigments

Cyclic voltammetry of molecular oxygen in aprotic media (dimethylformamide) and in the presence of bilirubin and other bile pigments shows that superoxide anion (O2-.) undergoes proton-induced dismutation. Lactam hydrogens of bile pigments are sufficiently acid to induce (O2-.) disproportionation to O2 and H2O2. Because of its characteristic lipophilic behavior, a biological role for natural bilirubin similar to that of other non-enzymatic lipophilic scavengers of (O2-.) is suggested.

Posttranslational modifications of the beta subunit of a cryptomonad phycoerythrin. Sites of bilin attachment and asparagine methylation

Determination of the partial amino acid sequence of the beta subunit of cryptomonad strain CBD phycoerythrin 566 established the nature, locations, and modes of attachment of the three bilin prosthetic groups and revealed a site of posttranslational methylation. Isolation of peptides cross-linked by a phycobiliviolin led to an unambiguous assignment of two thioether linkages, from residues beta-Cys-50 and beta-Cys-61 to this bilin. Two bilins were attached through single thioether linkages, a phycobiliviolin at beta-Cys-158 and a phycoerythrobilin at beta-Cys-82 (the residue numbering is that for B-phycoerythrin; Sidler, W., Kumpf, B., Suter, F., Morisset, W., Wehrmeyer, W., and Zuber, H. (1985) Biol. Chem. Hoppe-Seyler 366, 233-244). The partial sequences (99 residues) established for phycoerythrin 566 beta subunit showed a 79% identity with that of the red algal Porphyridium cruentum B-phycoerythrin beta subunit. A particularly remarkable finding is that the unique methylasparagine residue at position beta-72, highly conserved in cyanobacterial and red algal phycobiliproteins (Klotz, A. V., and Glazer, A. N. (1987) J. Biol. Chem. 262, 17350-17355), is also present at beta-72 in the cryptomonad phycoerythrin. Comparison of the locations of donor and acceptor bilins in cryptomonad phycoerythrin with those found for cyanobacterial and red algal phycobiliproteins showed different favored pathways of energy migration in the cryptomonad protein.

Pigment gallstone formation and altered ion transport

Feeding corn and alfalfa to young prairie dogs resulted in formation of gallstones composed of 45 percent cholesterol, 30 percent bile pigments, and 25 percent calcium bilirubinate in half of the animals. This diet also resulted in increased gallbladder bile concentrations of calcium, phospholipids, and cholesterol. Although the cholesterol saturation index was significantly increased compared with control subjects, it remained less than 1. In addition to the changes in biliary lipid composition, the corn and alfalfa-fed animals had significantly decreased transepithelial potential difference and short-circuit current when compared with control animals. These are changes in gallbladder mucosal function similar to those that have been reported in humans with gallstones. This model may therefore prove to be of great value in studying the pathogenesis of noncholesterol gallstones.