C-phycocyanin: a potent peroxyl radical scavenger in vivo and in vitro

C-Phycocyanin (from Spirulina platensis) effectively inhibited CCl(4)-induced lipid peroxidation in rat liver in vivo. Both native and reduced phycocyanin significantly inhibited peroxyl radical-induced lipid peroxidation in rat liver microsomes and the inhibition was concentration dependent with an IC(50) of 11.35 and 12.7 microM, respectively. The radical scavenging property of phycocyanin was established by studying its reactivity with peroxyl and hydroxyl radicals and also by competition kinetics of crocin bleaching. These studies have demonstrated that phycocyanin is a potent peroxyl radical scavenger with an IC(50) of 5.0 microM and the rate constant ratios obtained for phycocyanin and uric acid (a known peroxyl radical scavenger) were 1.54 and 3.5, respectively. These studies clearly suggest that the covalently linked chromophore, phycocyanobilin, is involved in the antioxidant and radical scavenging activity of phycocyanin.

On the role of the N-terminal group in the allosteric function of glucosamine-6-phosphate deaminase from Escherichia coli

Glucosamine-6-phosphate deaminase (EC 3.5.99.6) from Escherichia coli is an allosteric enzyme of the K-type, activated by N-acetylglucosamine 6-phosphate. It is a homohexamer and has six allosteric sites located in clefts between the subunits. The amino acid side-chains in the allosteric site involved in phosphate binding are Arg158, Lys160 and Ser151 from one subunit and the N-terminal amino group from the facing polypeptide chain. To study the functional role of the terminal amino group, we utilized a specific non-enzymic transamination reaction, and we further reduced the product with borohydride, to obtain the corresponding enzyme with a terminal hydroxy group. Several experimental controls were performed to assess the procedure, including reconditioning of the enzyme samples by refolding chromatography. Allosteric activation by N-acetylglucosamine 6-phosphate became of the K-V mixed type in the transaminated protein. Its kinetic study suggests that the allosteric equilibrium for this modified enzyme is displaced to the R state, with the consequent loss of co-operativity. The deaminase with a terminal hydroxy acid, obtained by reducing the transaminated enzyme, showed significant recovery of the catalytic activity and its allosteric activation pattern became similar to that found for the unmodified enzyme. It had lost, however, the pH-dependence of homotropic co-operativity shown by the unmodified deaminase in the pH range 6-8. These results show that the terminal amino group plays a part in the co-operativity of the enzyme and, more importantly, indicate that the loss of this co- operativity at low pH is due to the hydronation of this amino group.

A GDP-fucose-protected, pyridoxal-5'-Phosphate/NaBH(4)-sensitive lys residue common to human alpha1-->3Fucosyltransferases corresponds to Lys(300) in FucT-IV

Human alpha1-->3/4fucosyltransferases (FucTs) contain a common essential pyridoxal-5'-phosphate(PLP)/NaBH(4) reactive, GDP-fucose-protectable Lys. For identification, site-directed mutants at lysines of FucT-IV and -VII were prepared and tested. Non conserved lysine mutants K119Y and K394Q were similar to wild-type FucT-IV. However, mutants of conserved lysines K228R and K300R were distinct. The specific activity of K228R was 2- to 3-fold lower but retained K(m) values for donor and acceptor substrates as wild-type FucT-IV. The specific activity of K300R was reduced over 400-fold with an apparent K(m) for GDP-fucose over 200 microM. FucT-VII mutants K169R and K240R (equivalent to K228R and K300R for FucT-IV, respectively) were inactive. No change in PLP/NaBH(4) sensitivity occurred with K119Y, K228R, and K394Q compared to wild-type FucT-IV. These and previous results (A. L. Sherwood, A. T. Nguyen, J. M. Whitaker, B. A. Macher, M. R. Stroud, and E. H. Holmes, J. Biol. Chem. 273, 25256-25260, 1998) demonstrate that of three conserved lysines in FucT-IV, two (Lys(228) and Lys(283)) are not involved in substrate binding but perhaps in catalysis. The third site, Lys(300), is involved in GDP-fucose binding and PLP/NaBH(4) inactivation.

Uptake and washout of borocaptate sodium and borono-phenylalanine in cultured melanoma cells: a multi-nuclear NMR study

The cellular uptake and washout of the two principal boron neutron capture therapy (BNCT) agents, borocaptate sodium (BSH) and borono-phenylalanine (BPA), were monitored on-line, noninvasively, using nuclear magnetic resonance (NMR) spectroscopy. The uptake and washout of inorganic borate (B(i)) was also followed for comparison. M2R mouse melanoma cells grown on polystyrene microspheres were perfused inside the NMR sample tube. (11)B NMR was used to detect the presence of B(i), BSH and BPA, and (19)F NMR was applied to detect fluorinated BPA ((19)F-BPA). The results revealed chemical modifications of BSH due to spontaneous formation of the borocaptate dimer, BSSB, in the culture medium. BPA readily formed a complex with glucose contained in the culture medium but was also converted in the cells to a yet unidentified compound in a reaction that probably involves the hydrolysis of BPA and the release of B(i). The cellular accumulation ratio for BPA was significantly higher than 1 and was also significantly higher than that for BSH. On the other hand, the cellular retention time observed for BSH was much longer than for BPA, indicating a strong trapping of BSH in cells.

Spectromicroscopy of boron in human glioblastomas following administration of Na2B12H11SH

Boron neutron capture therapy (BNCT) is an experimental, binary treatment for brain cancer which requires as the first step that tumor tissue is targeted with a boron-10 containing compound. Subsequent exposure to a thermal neutron flux results in destructive, short range nuclear reaction within 10 microm of the boron compound. The success of the therapy requires than the BNCT agents be well localized in tumor, rather than healthy tissue. The MEPHISTO spectromicroscope, which performs microchemical analysis by x-ray absorption near edge structure (XANES) spectroscopy from microscopic areas, has been used to study the distribution of trace quantities of boron in human brain cancer tissues surgically removed from patients first administered with the compound Na2B12H11SH (BSH). The interpretation of XANES spectra is complicated by interference from physiologically present sulfur and phosphorus, which contribute structure in the same energy range as boron. We addressed this problem with the present extensive set of spectra from S, B, and P in relevant compounds. We demonstrate that a linear combination of sulfate, phosphate and BSH XANES can be used to reproduce the spectra acquired on boron-treated human brain tumor tissues. We analyzed human glioblastoma tissue from two patients administered and one not administered with BSH. As well as weak signals attributed to BSH, x-ray absorption spectra acquired from tissue samples detected boron in a reduced chemical state with respect to boron in BSH. This chemical state was characterized by a sharp absorption peak at 188.3 eV. Complementary studies on BSH reference samples were not able to reproduce this chemical state of boron, indicating that it is not an artifact produced during sample preparation or x-ray exposure. These data demonstrate that the chemical state of BSH may be altered by in vivo metabolism.

[The degradation of glycoproteins with lithium borohydride. Isolation and analysis of O-glycopeptides with reduced C-terminal amino acid residue]

By the example of fetuin and a blood-group-specific mucin from porcine stomach, we showed that, under conditions of reductive degradation of glycoproteins with LiBH4-LiOH in 70% aqueous tert-butyl alcohol, the reduction and cleavage of amide bonds occur much faster than the simultaneous beta-elimination of carbohydrate chains O-linked with Ser and Thr residues of the peptide chain. The major degradation products containing the O-linked glycans are the O-glycosylated derivatives of 2-aminopropane-1,3-diol and 2-aminobutane-1,3-diol (the products of reduction of glycosylated Ser and Thr) and the glycopeptides containing 2-4 amino acid residues with reduced C-terminal amino acid. Seventeen homogeneous O-glycopeptides were isolated from the fetuin degradation products by ion-exchange and reversed-phase HPLC. Their structures were determined by MALDI-TOF mass spectrometry and by analyses for amino acids, amino alcohols, and carbohydrates. The application of the reaction for characterization of O-glycans and localization of O-glycosylation sites in O- and N,O-glycoproteins is discussed.

Boron-enriched streptavidin potentially useful as a component of boron carriers for neutron capture therapy of cancer

A boron-enriched streptavidin has been prepared by chemical conjugation of a boron-rich compound, B(12)H(11)SH(2)(-) (BSH), to a genetically engineered streptavidin variant. The streptavidin variant used has 20 cysteine residues per molecule, derived from a C-terminal cysteine stretch consisting of five cysteine residues per subunit. Because natural streptavidin has no cysteine residues, the reactive sulfhydryl groups of the cysteine stretch serve as unique conjugation sites for sulfhydryl chemistry. BSH was conjugated irreversibly to the sulfhydryl groups of the streptavidin variant via a sulfhydryl-specific homobifunctional chemical cross-linker. Quantitative boron analysis indicates that the resulting streptavidin-BSH conjugate carries approximately 230 boron atoms/molecule. This indicates that the chemical conjugation of BSH to the streptavidin variant was highly specific and efficient because this method should allow the conjugation of a maximum of 240 boron atoms/streptavidin molecule. This boron-enriched streptavidin retained both full biotin-binding ability and tetrameric structure, suggesting that the conjugation of BSH has little, if any, effect on the fundamental properties of streptavidin. This boron-enriched streptavidin should be very useful as a component of targetable boron carriers for neutron capture therapy of cancer. For example, a monoclonal antibody against a tumor-associated antigen can be attached tightly to the boron-enriched streptavidin upon simple biotinylation, and the resulting conjugate could be used to target boron to tumor cells on which the tumor-associated antigen is overexpressed.

The two types of 3-dehydroquinase have distinct structures but catalyze the same overall reaction

The structures of enzymes catalyzing the reactions in central metabolic pathways are generally well conserved as are their catalytic mechanisms. The two types of 3-dehydroquinate dehydratase (DHQase) are therefore most unusual since they are unrelated at the sequence level and they utilize completely different mechanisms to catalyze the same overall reaction. The type I enzymes catalyze a cis-dehydration of 3-dehydroquinate via a covalent imine intermediate, while the type II enzymes catalyze a trans-dehydration via an enolate intermediate. Here we report the three-dimensional structures of a representative member of each type of biosynthetic DHQase. Both enzymes function as part of the shikimate pathway, which is essential in microorganisms and plants for the biosynthesis of aromatic compounds including folate, ubiquinone and the aromatic amino acids. An explanation for the presence of two different enzymes catalyzing the same reaction is presented. The absence of the shikimate pathway in animals makes it an attractive target for antimicrobial agents. The availability of these two structures opens the way for the design of highly specific enzyme inhibitors with potential importance as selective therapeutic agents.

Neutral core oligosaccharides of bovine submaxillary mucin--use of lead tetraacetate in the cold for establishing branch positions

Oligosaccharide alditols were released from bovine submaxillary mucin by alkaline borohydride treatment. The fractions containing smaller neutral oligosaccharides were separated by HPLC, to give, in the mono-, di- and trisaccharide-alditol sizes, 19 different structures, in addition to three fucosylated tetrasaccharide alditols. Molecules were identified by NMR spectroscopy, electrospray-mass-spectrometry-mass-spectrometry, permethylation analyses, and highly selective Pb(OAc)4 oxidation at -30 degrees C, followed by borohydride reduction. Pb(OAc)4 oxidation was found to be generally applicable in identifying the branch position(s) of substitutions for all core structures. Among the isolated oligosaccharide alditols were structures not previously reported, including 12 structures not reported from bovine submaxillary mucin, and four structures (three core structures) not found in the Carbbank database.

Posttranslational formation of formylglycine in prokaryotic sulfatases by modification of either cysteine or serine

Eukaryotic sulfatases carry an alpha-formylglycine residue that is essential for activity and is located within the catalytic site. This formylglycine is generated by posttranslational modification of a conserved cysteine residue. The arylsulfatase gene of Pseudomonas aeruginosa also encodes a cysteine at the critical position. This protein could be expressed in active form in a sulfatase-deficient strain of P. aeruginosa, thereby restoring growth on aromatic sulfates as sole sulfur source, and in Escherichia coli. Analysis of the mature protein expressed in E. coli revealed the presence of formylglycine at the expected position, showing that the cysteine is also converted to formylglycine in a prokaryotic sulfatase. Substituting the relevant cysteine by a serine codon in the P. aeruginosa gene led to expression of inactive sulfatase protein, lacking the formylglycine. The machinery catalyzing the modification of the Pseudomonas sulfatase in E. coli therefore resembles the eukaryotic machinery, accepting cysteine but not serine as a modification substrate. By contrast, in the arylsulfatase of Klebsiella pneumoniae a formylglycine is found generated by modification of a serine residue. The expression of both the Klebsiella and the Pseudomonas sulfatases as active enzymes in E. coli suggests that two modification systems are present, or that a common modification system is modulated by a cofactor.

Distribution and excretion of boron after intravenous administration of disodium mercaptoundecahydro-closo-dodecaborate to rats

Disodium mercaptoundecahydro-closo-dodecaborate (BSH) is an important compound for boron neutron capture therapy. The pharmacokinetics of boron by BSH were studied in normal rats after rapid intravenous injection at three doses (30, 100, 300 mg/kg) or continuous infusion (100 mg/kg/30 min). The boron concentration in biological samples was measured by inductively coupled plasma atomic emission spectroscopy. The blood half-lives of boron in the elimination phase (t1/2 beta) after rapid injection of BSH at doses of 30, 100 and 300 mg/kg were 1.7, 17 and 19 hr, respectively. AUC (32, 219 and 4030 micrograms.hr/ml) increased with the dose, but there was no proportionality among the values. Total clearance decreased drastically from 233 ml/hr/kg (100 mg/kg) to 38 ml/hr/kg (300 mg/kg). As boron was excreted mainly into urine, these results suggest that renal function failure might occur with dosing of 300 mg/kg. In the case of continuous infusion of 100 mg/kg of BSH for 30 min, the pharmacokinetic parameters were similar to those of rapid injection of 100 mg/kg. The highest boron concentration was observed in the kidney and the lowest in the brain. After multiple dosing of BSH at 100 mg/kg/day x 14 days, the boron concentrations in blood, liver, lung and kidney at 24 hr after the last dosing were higher than those after single dosing and were similar to those of simulated values calculated from the single dosing parameters. These results clearly indicated that boron does not accumulate unexpectedly in any tissue with multiple dosing of 100 mg/kg of BSH for two weeks.

MutY DNA glycosylase: base release and intermediate complex formation

MutY protein, a DNA glycosylase found in Escherichia coli, recognizes dA:dG, dA:8-oxodG, and dA:dC mismatches in duplex DNA, excising the adenine moiety. We have investigated the mechanism of action of MutY, addressing several points of disagreement raised by previous studies of this enzyme. MutY forms a covalent intermediate with its DNA substrate but does not catalyze strand cleavage. The covalent intermediate has a half-life of approximately 2.6 h, 2 orders of magnitude greater than the half-life of Schiff bases formed when E. coli formamidopyrimidine-DNA glycosylase (Fpg) and endonuclease III react with their respective substrates. The covalent complex between MutY and its DNA substrate involves Lys-142; however, the position of this residue in the presumptive active site differs from that of catalytic residues involved in Schiff base formation associated with endonuclease III and related DNA glycosylases/AP lyases. MutY converts DNA duplexes containing the dA:8-oxodG mispair to a product containing an abasic site; heat-induced cleavage of this product may account for the several reports in the literature that ascribe AP lyase activity to MutY. The MutY-DNA intermediate complex is highly stable and hinders access by Fpg to DNA, thereby avoiding a double-strand break. Cross-linking of MutY to DNA may play an important role in the regulation of base excision repair.

A new class of phosphotransferases phosphorylated on an aspartate residue in an amino-terminal DXDX(T/V) motif

When incubated with their substrates, human phosphomannomutase and L-3-phosphoserine phosphatase are known to form phosphoenzymes with chemical characteristics of an acyl-phosphate. The phosphorylated residue in phosphomannomutase has now been identified by mass spectrometry after reduction of the phosphoenzyme with tritiated borohydride and trypsin digestion. It is the first aspartate in a conserved DVDGT motif. Replacement of either aspartate of this motif by asparagine or glutamate resulted in complete inactivation of the enzyme. The same mutations performed in the DXDST motif of L-3-phosphoserine phosphatase also resulted in complete inactivation of the enzyme, except for the replacement of the second aspartate by glutamate, which reduced the activity by only about 40%. This suggests that the first aspartate of the motif is also the phosphorylated residue in L-3-phosphoserine phosphatase. Data banks contained seven other phosphomutases or phosphatases sharing a similar, totally conserved DXDX(T/V) motif at their amino terminus. One of these (beta-phosphoglucomutase) is shown to form a phosphoenzyme with the characteristics of an acyl-phosphate. In conclusion, phosphomannomutase and L-3-phosphoserine phosphatase belong to a new phosphotransferase family with an amino-terminal DXDX(T/V) motif that serves as an intermediate phosphoryl acceptor.

The reductive modulation of chloroplast fructose-1,6-bisphosphatase by tributylphosphine and sodium borohydride

The cleavage of disulfide bonds is the major modification of chloroplast fructose-1,6-bisphosphatase when the light-mediated ferredoxin-thioredoxin system enhances the activity of the enzyme. In vitro, only thiol-bearing compounds are functional in the stimulation of fructose 1,6-bisphosphate hydrolysis. This investigation was undertaken to determine the effectivity of other reductants for enhancing the catalytic capacity. In the presence of 1 mM fructose 1,6-bisphosphate and 0.1 mM Ca2+, the five-fold activation triggered by 3.5 mM tributylphosphine is further potentiated by 15% (v/v) 2-propanol. When the enzyme is incubated in the presence of 0.15 M sodium trichloroacetate in place of the cosolvent, NaH4B initially stimulates the activity but subsequently causes the inactivation of the enzyme. A model developed to analyze this dual effect suggests that the concerted action of fructose 1,6-bisphosphate, Ca2+ and trichloroacetate yields an enzyme form that is slightly activable by reduction (t0.5 = 28 min.). However, chloroplast fructose-1,6-bisphosphatase becomes highly sensitive to trichloroacetate inactivation (t0.5 = 5 min.) when NaH4B reduces fructose 1,6-bisphosphate. Hence, the thiol/disulfide exchange constitutes a particular case of reductive mechanisms that stimulate the activity of chloroplast fructose-1,6-bisphosphatase.

Interaction of mercaptoundecahydrododecaborate (BSH) with phosphatidylcholine: relevance to boron neutron capture therapy

The interaction of mercaptoundecahydrododecaborate (B12H11SH2-, BSH) with phosphatidylcholine was investigated in this study in order to illuminate possible uptake mechanisms of BSH in tumor cells. BSH has been used clinically in Japan as a boron containing agent in patients with malignant brain tumors for boron neutron capture therapy (BNCT). After infusion, BSH accumulates selectively in tumor tissue. Little is known for the mechanism of boron uptake to tumor cells. Fourier transform infrared (FTIR) spectrometry was used to quantify BSH (at wavenumber 2490 cm-1) and phosphatidylcholine (at wavenumber 2850-2970 cm-1). After extraction into carbon tetrachloride (CCl4), we could find an absorbance maximum at 2490 cm-1 as a B-H band in the mixture of BSH with phosphatidylcholine, which is attributed to a BSH-phosphatidylcholine complex, which could dissolve well in CCl4. The molar ratio of BSH to phosphatidylcholine in the CCl4 solution was at most one mole of BSH to two moles of phosphatidylcholine independent of the excess BSH. The doubly negatively charged BSH can interact with two phosphatidylcholine molecules through their singly positively charged choline residues. These ion pairs could be responsible for membrane binding and penetration, and for cell internalization.

Structural analysis of oligosaccharide-alditols released by reductive beta-elimination from the jelly coats of the anuran Bufo arenarum

Amphibian egg jelly coats are formed by components secreted along the oviduct. These secretion products overlay the oocytes as they are transported toward the cloaca. Mucin type glycoproteins are the major constituents of the egg jelly coats. In this study, the O-linked carbohydrate chains of the jelly coat surrounding the eggs of Bufo arenarum were released by alkaline borohydride treatment. Fractionation of the mixture of O-linked oligosaccharide-alditols was achieved by a combination of chromatographic techniques comprising gel-permeation chromatography, ion-exchange chromatography and high-performance liquid chromatography using an amino-bonded silica column, afforded 11 fractions. The primary structures of these O-glycans were determined by one-dimensional and two-dimensional 1H-NMR spectroscopy in conjunction with matrix-assisted laser-desorption-ionization--time-of-flight mass spectrometry. 11 oligosaccharide structures, possessing a core consisting of Galbeta1-->3GalNAc-ol with or without branching through a GlcNAc residue linked beta1-->6 to the GalNAc residue (core type 2 or core type 1, respectively) are described. These oligosaccharide-alditols with these types of cores have been identified previously in mammalian mucins or in oviducal amphibian jellies. These glycans contain blood group determinants such as H, A or Cad antigens.

Mechanisms for covalent binding of benoxaprofen glucuronide to human serum albumin. Studies By tandem mass spectrometry

Tandem MS has been used to establish the structure and specific binding sites of covalent protein adducts formed upon incubation of the acyl glucuronide of the propionic acid nonsteroidal anti-inflammatory drug benoxaprofen with human serum albumin in vitro. Benoxaprofen 1-O-beta-glucuronide was enzymatically synthesized in vitro and incubated with human serum albumin both in the presence and in the absence of NaCNBH3. The modified human serum albumins were digested with trypsin and separated by HPLC. The modified peptides were detected using HPLC-electrospray MS (with selected-ion monitoring) and were structurally characterized by tandem MS using matrix-assisted laser desorption ionization in both the post-source decay and high-energy collision-induced dissociation modes. These studies established that benoxaprofen glucuronide forms covalent adducts with protein nucleophiles both by nucleophilic displacement of glucuronic acid at the anomeric center and by condensation of the rearranged acyl glucuronic acid isomers with epsilon-amino functions of lysine residues after acyl migration of the aglycone from the anomeric center. Lys-159 was identified as the major binding site. Thus, we have established that members of the less reactive propionic acid class of acyl glucuronides, such as the glucuronide of benoxaprofen, are also capable of reacting with protein nucleophiles to form covalent adducts analogous to those of tolmetin glucuronide (tolmetin is an acetic acid nonsteroidal anti-inflammatory drug), via the mechanisms previously reported from this laboratory, and that the specific covalent binding site profile appears to be drug dependent.

A substrate-induced change in the stereospecificity of the serine-hydroxymethyltransferase-catalysed exchange of the alpha-protons of amino acids--evidence for a second catalytic site

NMR has been used to study the catalysis of the hydrogen-deuterium exchange of the alpha-protons of amino acids by serine hydroxymethyltransferase (EC 2.1.2.1) from Escherichia coli. 13C-NMR was used to follow the exchange of the alpha-protons of [2-13C]glycine. The enzyme-catalysed first-order exchange rate of the pro-2S proton of glycine was approximately 7000 times more efficient than that of the pro-2R proton of glycine at both pH 7.0 and 7.8. 1H-NMR was used to follow the hydrogen-deuterium exchange rates of the alpha-protons of L- and D-2-amino derivatives of butyric, pentanoic and hexanoic acids at pH 7.8. Increasing the size of the R-group leads to a progressive change in the stereospecificity of the exchange reaction from the pro-2S proton of glycine to the 2R proton of L-amino acids. The stereospecificity for the alpha-protons of L-amino acids increased as the size of the R-group increased. With glycine, removal of tetrahydrofolate led to a large decrease in the stereospecificity of the exchange reaction but did not affect the exchange rates of the alpha-protons of any of the larger amino acids studied. We show that the Schiff base formed between L-2-aminohexanoic acid (L-norleucine) and pyridoxal 5'-phosphate binds at a different site from the Schiff base between glycine and pyridoxal 5'-phosphate. The molecular basis of these results is discussed.

Structural analysis of oligosaccharide-alditols released by reductive beta-elimination from oviducal mucins of Rana dalmatina

The O-linked oligosaccharides of the jelly coat surrounding the eggs of Rana dalmantina were released by alkaline borohydride treatment. Low-molecular-mass, monosialyl oligosaccharide-alditols were isolated by anion-exchange chromatography and fractionated by consecutive normal-phase high-performance liquid chromatography on a silica-based alkylamine column. The structures of the oligosaccharide-alditols were determined by 400-MHz 1H-NMR spectroscopy in combination with matrix assisted laser desorption ionization-time of flight analysis. The five structures were identified range in size from trisaccharides to hexasaccharides, possessing a core consisting of Gal(beta 1-3)GalNAc-ol (core type 1). Novel oligosaccharide-alditols are: [formula: see text] The carbohydrate chains isolated from Rana dalmatina are different from those found in other amphibian species, in which the presence of species-specific material has been characterized. Since the role of carbohydrates appears more and more apparent during the fertilization process, the biodiversity of the O-linked oligosaccharides could support such a biological role.

Spectral and photochemical properties of borohydride-treated D1-D2-cytochrome b-559 complex of photosystem II

The D1-D2-cytochrome b-559 reaction center complex of photosystem II with an altered pigment composition was prepared from the original complex by treatment with sodium borohydride (BH4-). The absorption spectra of the modified and original complexes were compared to each other and to the spectra of purified chlorophyll a and pheophytin a (Pheo a) treated with BH4- in methanolic solution. The results of these comparisons are consistent with the presence in the modified complex of an irreversibly reduced Pheo a molecule, most likely 13(1)-deoxo-13(1)-hydroxy-Pheo a, replacing one of the two native Pheo a molecules present in the original complex. Similar to the original preparation, the modified complex was capable of a steady-state photoaccumulation of Pheo- and P680+. It is concluded that the pheophytin a molecule which undergoes borohydride reduction is not involved in the primary charge separation and seems to represent a previously postulated photochemically inactive Pheo a molecule. The Qy and Qx transitions of this molecule were determined to be located at 5 degrees C at 679.5-680 nm and 542 nm, respectively.

Preparation of Schiff base adducts of phosphatidylcholine core aldehydes and aminophospholipids, amino acids, and myoglobin

We have prepared Schiff base adducts of the core aldehydes of phosphatidylcholine and aminophospholipids, free amino acids, and myoglobin. The Schiff bases of the ethanolamine and serine glycerophospholipids were obtained by reacting sn-1-palmitoyl(stearoyl)-2-[9-oxo]nonanoyl-glycerophosphocholine (PC-Ald) with a twofold excess of the aminophospholipid in chloroform/methanol 2:1 (vol/vol) for 18 h at room temperature. The Schiff bases of the amino acids and myoglobin were obtained by reacting the aldehyde with an excess of isoleucine, valine, lysine, methyl ester lysine and myoglobin in aqueous methanol for 18 h at room temperature. Prior to isolation, the Schiff bases were reduced with sodium cyanoborohydride in methanol for 30 min at 4 degrees C. The reaction products were characterized by normal-phase high-performance liquid chromatography and on-line mass spectrometry with electrospray ionization. The amino acids and aminophospholipids yielded single adducts. A double adduct was obtained for myoglobin, which theoretically could have accepted up to 23 PC-Ald groups. The yields of the products ranged from 12 to 44% for the aminophospholipids and from 15-57% for the amino acids, while the Schiff base of the myoglobin was estimated at 5% level. The new compounds are used as reference standards for the detection of high molecular weight Schiff bases in lipid extracts of natural products.

Oxidation of cholesterol in synaptosomes and mitochondria isolated from rat brains

Cholesterol and alpha-tocopherol oxidations were studied in brain subcellular fractions isolated from cerebral hemispheres of 4-month-old, male Fischer 344 rats. The fractions were suspended in buffered media (pH 7.4, 37 degrees C0 and oxidized by adding (i) ferrous iron (Fe2+) with or without ascorbate or (ii) peroxynitrite (an endogenous oxidant produced by the reaction of superoxide and nitric oxide). Treatment of subcellular fractions with Fe2+ in the presence or absence of ascorbate produced primarily 7-keto- and 7-hydroxy-cholesterols and small amounts of 5 alpha, 6 alpha-epoxycholesterol. Since brain contains high levels of ascorbate, and release of iron could result in oxysterol formation. Peroxynitrite oxidized alpha-tocopherol but not cholesterol. Hence, the toxicity of peroxynitrite or nitric oxide could not be due to cytotoxic oxysterols. When synaptosomes were incubated for 5 min in the presence of 0.5 to 2 microM Fe2+ and ascorbate, alpha-tocopherol was oxidized while cholesterol remained unchanged. Thus, alpha-tocopherol is functioning as an antioxidant, protecting cholesterol. Diethylenetriaminepentaacetic acid blocked production of oxysterols, whereas citrate, ADP and EDTA did not. A significant percentage of mitochondrial cholesterol was oxidized by treatment with Fe2+ and ascorbate. Hence, mitochondrial membrane properties dependent on cholesterol could be particularly susceptible to oxidation. The oxysterols formed were retained within the membranes of synaptosomes and mitochondria. The 7-oxysterols produced are known to be inhibitors of membrane enzymes and also can modify membrane permeability. Hence, oxysterols may plan an important role in brain tissue damage during oxidative stress.

Novel insights into the chemical mechanism of ATP synthase. Evidence that in the transition state the gamma-phosphate of ATP is near the conserved alanine within the P-loop of the beta-subunit

The chemical mechanism by which the F1 moiety of ATP synthase hydrolyzes and synthesizes ATP remains unknown. For this reason, we have carried out studies with orthovanadate (Vi), a phosphate analog which has the potential of "locking" an ATPase, in its transition state by forming a MgADP.Vi complex, and also the potential, in a photochemical reaction resulting in peptide bond cleavage, of identifying an amino acid very near the gamma-phosphate of ATP. Upon incubating purified rat liver F1 with MgADP and Vi for 2 h to promote formation of a MgADP.Vi-F1 complex, the ATPase activity of the enzyme was markedly inhibited in a reversible manner. When the resultant complex was formed in the presence of ultraviolet light inhibition could not be reversed, and SDS-polyacrylamide gel electrophoresis revealed, in addition to the five known subunit bands characteristic of F1 (i.e. alpha, beta, gamma, delta, and epsilon), two new electrophoretic species of 17 and 34 kDa. Western blot and N-terminal sequencing analyses identified both bands as arising from the beta subunit with the site of peptide bond cleavage occurring at alanine 158, a conserved residue within F1-ATPases and the third residue within the nucleotide binding consensus GX4GK(T/S) (P-loop). Quantification of the amount of ADP bound within the MgADP. Vi-F1 complex revealed about 1.0 mol/mol F1, while quantification of the peptide cleavage products revealed that no more than one beta subunit had been cleaved. Consistent with the cleavage reaction involving oxidation of the methyl group of alanine was the finding that [3H] from NaB[3H]4 incorporates into MgADP.Vi-F1 complex following treatment with ultraviolet light. These novel findings provide information about the transition state involved in the hydrolysis of ATP by a single beta subunit within F1-ATPases and implicate alanine 158 as residing very near the gamma-phosphate of ATP during catalysis. When considered with earlier studies on myosin and adenylate kinase, these studies also implicate a special role for the third residue within the GX4GK(T/S) sequence of many other nucleotide-binding proteins.

Structural identification by mass spectrometry of oxidized phospholipids in minimally oxidized low density lipoprotein that induce monocyte/endothelial interactions and evidence for their presence in vivo

Entry of monocytes into the vessel wall is an important event in atherogenesis. Previous studies from our laboratory suggest that oxidized arachidonic acid-containing phospholipids present in mildly oxidized low density lipoproteins (MM-LDL) can activate endothelial cells to bind monocytes. In this study, biologically active oxidized arachidonic acid-containing phospholipids were produced by autoxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (Ox-PAPC) and analyzed by liquid chromatography and electrospray ionization mass spectrometry in conjuction with biochemical derivatization techniques. We have now determined the molecular structure of two of three molecules present in MM-LDL and Ox-PAPC that induce monocyte-endothelial interactions. These lipids were identified as 1-palmitoyl-2-(5-oxovaleryl)-sn-glycero-3-phosphocholine (m/z 594.3) and 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (m/z 610.2). These two molecules were produced by unambiguous total synthesis and found to be identical by analytical techniques and bioactivity assays to those present in MM-LDL and Ox-PAPC. Evidence for the importance of all three oxidized phospholipids in vivo was suggested by their presence in fatty streak lesions from cholesterol-fed rabbits and by their immunoreactivity with natural antibodies present in ApoE null mice. Overall, these studies suggest that specific oxidized derivatives of arachidonic acid-containing phospholipids may be important initiators of atherogenesis.

Identification of the pH sensor for nucleotide binding in the uncoupling protein from brown adipose tissue

The transport inhibiting nucleotide binding to the uncoupling protein (UCP) has a unique pH dependence and has been postulated to be controlled by the dissociation state of a carboxyl group in UCP with pK 4.5 and, in addition only for the nucleoside triphosphate, by a group with pK 7.2. To prove this assumption and to identify the carboxyl group, Woodward reagent K (WRK) was applied to UCP. In mitochondria, WRK was found to inhibit binding of GTP in a noncompetitive manner using WRK in the millimolar range. In isolated UCP, GTP binding is inhibited by WRK at a 1 to 2 ratio to UCP, suggesting that WRK primarily reacts with only one carboxyl group. Prebound GTP protects against WRK reaction as monitored by GTP binding. The protection decreases from pH 5 to 7 due to better reactivity of WRK and less tight GTP binding. WRK does not inhibit H+ transport by UCP but prevents GTP inhibition of H+ transport. For elucidating the WRK target residue, the WRK derivatized group was labeled with [3H] by reduction with [3H]NaBH4. Both GTP and GDP largely protected against WRK-dependent [3H] labeling. CNBr fragmentation identified the region T121-M197 as the [3H] incorporation site. Combined CNBr and tryptophane cleavage by the reagent 3-bromo-3-methyl-2-((2-nitrophenyl) thio)-3H-indole (BNPS) allowed to further delimit the 2.8 kDa peptide W173-M197 as the [3H] label carrier which contains two acid residues E190 and D195. To further identify the residue, limited tryptic digestion in sarcosyl-treated UCP was performed, and a tryptic fragment enclosing E190 and D195 was isolated which carried most of the [3H] label. Edman degradation showed the major [3H] label at the eighth position corresponding to E190 and no peak at D195. Thus, the original postulate of the pH-sensing carboxyl group regulating both the nucleoside di- and triphosphate binding has been verified. It is identified as E190 situated in the fourth transmembrane helix. In total, now four residues close to the nucleotide binding sites in UCP have been determined.