Purification, characterization, and cDNA cloning of a novel acidic endoglycoceramidase from the jellyfish, Cyanea nozakii

Endoglycoceramidase (EC ) is an enzyme capable of cleaving the glycosidic linkage between oligosaccharides and ceramides in various glycosphingolipids. We report here the purification, characterization, and cDNA cloning of a novel endoglycoceramidase from the jellyfish, Cyanea nozakii. The purified enzyme showed a single protein band estimated to be 51 kDa on SDS-polyacrylamide gel electrophoresis. The enzyme showed a pH optimum of 3.0 and was activated by Triton X-100 and Lubrol PX but not by sodium taurodeoxycholate. This enzyme preferentially hydrolyzed gangliosides, especially GT1b and GQ1b, whereas neutral glycosphingolipids were somewhat resistant to hydrolysis by the enzyme. A full-length cDNA encoding the enzyme was cloned by 5'- and 3'-rapid amplification of cDNA ends using a partial amino acid sequence of the purified enzyme. The open reading frame of 1509 nucleotides encoded a polypeptide of 503 amino acids including a signal sequence of 25 residues and six potential N-glycosylation sites. Interestingly, the Asn-Glu-Pro sequence, which is the putative active site of Rhodococcus endoglycoceramidase, was conserved in the deduced amino acid sequences. This is the first report of the cloning of an endoglycoceramidase from a eukaryote.

Human sphingosine kinase: purification, molecular cloning and characterization of the native and recombinant enzymes

Sphingosine 1-phosphate (S1P) is a novel lipid messenger that has important roles in a wide variety of mammalian cellular processes including growth, differentiation and death. Basal levels of S1P in mammalian cells are generally low, but can increase rapidly and transiently when cells are exposed to mitogenic agents and other stimuli. This increase is largely due to increased activity of sphingosine kinase (SK), the enzyme that catalyses its formation. In the current study we have purified, cloned and characterized the first human SK to obtain a better understanding of its biochemical activity and possible activation mechanisms. The enzyme was purified to homogeneity from human placenta using ammonium sulphate precipitation, anion-exchange chromatography, calmodulin-affinity chromatography and gel-filtration chromatography. This resulted in a purification of over 10(6)-fold from the original placenta extract. The enzyme was cloned and expressed in active form in both HEK-293T cells and Escherichia coli, and the recombinant E. coli-derived SK purified to homogeneity. To establish whether post-translational modifications lead to activation of human SK activity we characterized both the purified placental enzyme and the purified recombinant SK produced in E. coli, where such modifications would not occur. The premise for this study was that post-translational modifications are likely to cause conformational changes in the structure of SK, which may result in detectable changes in the physico-chemical or catalytic properties of the enzyme. Thus the enzymes were characterized with respect to substrate specificity and kinetics, inhibition kinetics and various other physico-chemical properties. In all cases, both the native and recombinant SKs displayed remarkably similar properties, indicating that post-translational modifications are not required for basal activity of human SK.

Compartmental modeling with nitrogen-15 to determine effects of degree of fat saturation on intraruminal N recycling

Two- and three-compartment models were developed to describe N kinetics within the rumen using three Holstein heifers and one nonlactating Holstein cow fitted with ruminal and duodenal cannulas. A 4 x 4 Latin square design included a control diet containing no supplemental fat and diets containing 4.85% of diet dry matter as partially hydrogenated tallow (iodine value = 13), tallow (iodine value = 51), or animal-vegetable fat (iodine value = 110). Effects of fat on intraruminal N recycling and relationships between intraruminal N recycling and ruminal protozoa concentration or the efficiency of microbial protein synthesis were determined. A pulse dose of 15(NH4)2SO4 was introduced into the ruminal NH3 N pool, and samples were taken over time from the ruminal NH3 N and nonammonia N pools. For the three-compartment model, precipitates of nonammonia N after trichloroacetic acid and ethanol extraction were defined as slowly turning over nonammonia N; rapidly turning over nonammonia N was determined by difference. Curves of 15N enrichment were fit to models with two (NH3 N and nonammonia N) or three (NH3 N, rapidly turning over nonammonia N, and slowly turning over nonammonia N) compartments using the software SAAM II. Because the three-compartment model did not remove a small systematic bias or improve the fit of the data, the two-compartment model was used to provide measurements of intraruminal N recycling. Intraruminal NH3 N recycling (45% for control) decreased linearly as fat unsaturation increased (50.2, 43.0, and 41.7% for partially hydrogenated tallow, tallow, and animal-vegetable fat, respectively). Intraruminal nitrogen recycling was not correlated with efficiency of microbial protein synthesis or ruminal protozoa counts.

Generation of catalytically active 6-phosphofructokinase from Saccharomyces cerevisiae in a cell-free system

PFK1 and PFK2 coding for the subunits of 6-phosphofructokinase from Saccharomyces cerevisiae were cloned into plasmids suitable for runoff transcription. In vitro translation products of both kinds of subunit were obtained using rabbit reticulocyte lysate as the synthesis and folding system. They were monitored by chemiluminescent Western-blot analysis. Folding and assembly of the alpha-subunit and beta-subunit of 6-phosphofructokinase were found to occur in the cell-free system resulting in an enzymatically active protein. The in vitro generated enzyme exhibits a folding state that is similar to that of the heterooctameric form of 6-phosphofructokinase in the presence of fructose 6-phosphate, ATP and ammonium sulfate, as demonstrated by size-exclusion HPLC followed by ELISA.

Allantoate amidinohydrolase (Allantoicase) from Chlamydomonas reinhardtii: its purification and catalytic and molecular characterization

An allantoate-degrading enzyme has been purified to electrophoretic homogeneity for the first time from a photosynthetic organism, the unicellular green algae Chlamydomonas reinhardtii. The purification procedure included a differential protein extraction followed by conventional steps such as ammonium sulfate fractionation, gel filtration, anion exchange chromatography, and preparative electrophoresis. Under the routine assay conditions (7 mM allantoate), specific activity for the purified enzyme was 185 U/mg, which rose to 225 U/mg under kinetic considerations (saturating substrate). Therefore, a turnover number of 4.5 x 10(4) min(-1) can be deduced for the 200-kDa protein. The enzyme is a true allantoicase (EC 3.5.3.4) that catalyzes the degradation of allantoate to (-)ureidoglycolate and (+)ureidoglycolate to glyoxylate. The enzyme exhibited hyperbolic kinetic for allantoate and ureidoglycolate with K(m) values of 2 and 0.7 mM, respectively. V(max) of the reaction with allantoate as substrate was nine times higher than that with ureidoglycolate. The native enzyme has a molecular weight of 200 kDa and consists of six identical or similar-sized subunits of 34 kDa each, organized in two trimers of 100 kDa. Each subunit has five cysteine residues, four of which are involved in disulfide bonds, with a total of 12 disulfide bonds in the 200-kDa protein. Allantoate inhibits competitively the reaction with ureidoglycolate as substrate. In addition, buffers and group-specific reagents affect the activity in the same manner irrespective of the substrate used. Those results suggest that both substrates use the same active site. The effect of group-specific reagents suggest that the amino acids histidine, tyrosine, and cysteine are essentials for the allantoicase activity with both substrates.

Use of 15N reverse gradient two-dimensional nuclear magnetic resonance spectroscopy to follow metabolic activity in Nicotiana plumbaginifolia cell-suspension cultures

Nitrogen metabolism was monitored in suspension cultured cells of Nicotiana plumbaginifolia Viv. using nuclear magnetic resonance (NMR) spectroscopy following the feeding of (15NH4)2SO4 and K15NO3. By using two-dimensional 15N-1H NMR with heteronuclear single-quantum-coherence spectroscopy and heteronuclear multiple-bond-coherence spectroscopy sequences, an enhanced resolution of the incorporation of 15N label into a range of compounds could be detected. Thus, in addition to the amino acids normally observed in one-dimensional 15N NMR (glutamine, aspartate, alanine), several other amino acids could be resolved, notably serine, glycine and proline. Furthermore, it was found that the peak normally assigned to the non-protein amino-acid gamma-aminobutyric acid in the one-dimensional 15N NMR spectrum was resolved into a several components. A peak of N-acetylated compounds was resolved, probably composed of the intermediates in arginine biosynthesis, N-acetylglutamate and N-acetylornithine and, possibly, the intermediate of putrescine degradation into gamma-aminobutyric acid, N-acetylputrescine. The occurrence of 15N-label in agmatine and the low detection of labelled putrescine indicate that crucial intermediates of the pathway from glutamate to polyamines and/or the tobacco alkaloids could be monitored. For the first time, labelling of the peptide glutathione and of the nucleotide uridine could be seen.

Asparagine as a nitrogen source for improving the secretion of mouse alpha-amylase in Saccharomyces cerevisiae protease A-deficient strains

A modified chemically defined medium was achieved by using asparagine as a nitrogen source to increase the production of secreted mouse alpha-amylase in several Saccharomyces cerevisiae protease A-deficient (pep4) strains. The specific productivity (quantity) and the 53 kDa non-glycosylated active form (quality) of mouse salivary alpha-amylase in liquid medium containing asparagine was remarkably improved compared to media containing other nitrogen sources, including ammonium sulphate, glutamic acid, arginine, casamino acids, yeast extract and peptone. Similar improvement was also observed on starch solid agar regarding the clarity and size of the halo zone formed by alpha-amylase activity. Compared with ammonium sulphate, advantages of using asparagine as the nitrogen source in liquid or solid medium included increasing the cell mass of test strains, recovering the viability of protease-deficient strains to levels similar to the wild-type strain, and increasing the copy number of the mouse alpha-amylase expression vector in test strains. In turn, these advantages apparently contributed to the increase of secretion of mouse alpha-amylase in several test strains and especially in the protease A-deficient strains. In addition to demonstrating the use of modified chemically defined medium to improve the quality and quantity of secreted mouse alpha-amylase, this study also provides a new strategy to improve the secretion of heterologous proteins in protease A deficient strains.

Effect of nitrogen source on pullulan production by Aureobasidium pullulans grown in a batch bioreactor

Pullulan production by Aureobasidium pullulans ATCC 201253 using selected nitrogen sources was studied in a medium using corn syrup as a carbon source. Independent of the corn syrup concentration present, the use of corn steep liquor or hydrolysed soy protein as a nitrogen source instead of ammonium sulphate did not elevate polysaccharide production by ATCC 201253 cells grown in an aerated, batch bioreactor containing 4 litres of medium. Pullulan production on corn steep liquor or hydrolysed soy protein as a nitrogen source became more comparable as the concentration of corn syrup was increased. Cell weights after 7 days of growth on any of the nitrogen sources were similar. The viscosity of the polysaccharide on day 7 was highest for cells grown on ammonium sulphate and 12.5% corn syrup. The pullulan content of the polysaccharide elaborated by ammonium sulphate-grown cells on day 7 decreased as the corn syrup level rose in the medium while the pullulan content of polysaccharide produced by cells grown on corn steep liquor or soytone generally increased.

Stimulation of peroxidase activity by decamerization related to ionic strength: AhpC protein from Amphibacillus xylanus

AhpC protein, purified from Amphibacillus xylanus with a molecular mass of 20.8 kDa, protects cells against oxidation damage. The enzyme catalyses the reduction of hydroperoxides in cooperation with the 55 kDa flavoprotein, A. xylanus NADH oxidase (NADH oxidase-AhpC system). A. xylanus AhpC has two disulfide linkages between monomers and can act in the homodimer form. Gel-filtration column chromatography and dynamic light scattering (DLS) suggest that A. xylanus AhpC also forms a large oligomeric assembly (10-12 mers). A. xylanus AhpC was crystallized and X-ray diffraction data were collected to 3.0 A. The self-rotation function revealed fivefold and twofold axes located perpendicularly to each other, suggesting that the molecular assembly of A. xylanus AhpC is composed of ten monomers. The oligomerization of A. xylanus AhpC is affected by ionic strength in the DLS measurements. The H(2)O(2) reductase activity of the A. xylanus NADH oxidase-AhpC system is also affected by ionic strength, and it was found that the decamerization of AhpC might be required for the activation of the NADH oxidase-AhpC system.

Purification of xyloglucan endotransglycosylases (XETs): a generally applicable and simple method based on reversible formation of an enzyme-substrate complex

We describe a novel and general, mechanism-based, method for purification of xyloglucan endotransglycosylases (XETs) from crude plant extracts. Putative isoforms, obtained by step-wise precipitation with (NH4)2SO4, were incubated with tamarind xyloglucan (approximately 1 MDa) to form stable xyloglucan-XET complexes with apparent molecular masses >500 kDa on gel-permeation chromatography (GPC). Subsequent addition of xyloglucan-derived oligosaccharides (a mixture of XET acceptor substrates) caused a shift in the GPC elution volume of the activity back to that expected of a approximately 32 kDa protein, presumably by completing the transglycosylation reaction and so freeing the enzyme from the xyloglucan (donor substrate). This simple two-step method enabled the isolation of each XET activity attempted [various (NH4)2SO4 cuts from extracts of cauliflower florets and mung bean seedlings], in pure form as judged by SDS/PAGE.

Selective peroxisome degradation in Yarrowia lipolytica after a shift of cells from acetate/oleate/ethylamine into glucose/ammonium sulfate-containing media

We have shown that peroxisomes of the yeast Yarrowia lipolytica are subject to specific degradation after exposure of acetate/oleate-grown cells to glucose excess conditions. Electron microscopic analysis has revealed that the peroxisomes were degraded by uptake in the vacuole. Our results suggest that peroxisomes are taken up by macroautophagic processes, because sequestration of individual peroxisomes, which occurs typically at the beginning of microautophagy, was never observed. The observation that a peroxisomal amine oxidase activity is specifically induced by ethylamine was used for the development of a plate assay screening procedure to isolate peroxisome degradation-defective mutants.

Production of large quantities of isotopically labeled protein in Pichia pastoris by fermentation

Heterologous expression in Pichia pastoris has many of the advantages of eukaryotic expression, proper folding and disulfide bond formation, glycosylation, and secretion. Contrary to other eukaryotic systems, protein production from P. pastoris occurs in simple minimal defined media making this system attractive for production of labeled proteins for NMR analysis. P. pastoris is therefore the expression system of choice for NMR of proteins that cannot be refolded from inclusion bodies or that require post-translational modifications for proper folding or function. The yield of expressed proteins from P. pastoris depends critically on growth conditions, and attainment of high cell densities by fermentation has been shown to improve protein yields by 10-100-fold. Unfortunately, the cost of the isotopically enriched fermentation media components, particularly 15NH4OH, is prohibitively high. We report fermentation methods that allow for both 15N-labeling from (15NH4)2SO4 and 13C-labeling from 13C-glucose or 13C-glycerol of proteins produced in Pichia pastoris. Expression of an 83 amino acid fragment of thrombomodulin with two N-linked glycosylation sites shows that fermentation is more cost effective than shake flask growth for isotopic enrichment.

Use of soybean oil and ammonium sulfate additions to optimize secondary metabolite production

A valine-overproducing mutant (MA7040, Streptomyces hygroscopicus) was found to produce 1.5 to 2.0 g/L of the immunoregulant, L-683,590, at the 0.6 m3 fermentation scale in a simple batch process using soybean oil and ammonium sulfate-based GYG5 medium. Levels of both lower (L-683,795) and higher (HH1 and HH2) undesirable homolog levels were controlled adequately. This batch process was utilized to produce broth economically at the 19 m3 fermentation scale. Material of acceptable purity was obtained without the multiple pure crystallizations previously required for an earlier culture, MA6678, requiring valine supplementation for impurity control. Investigations at the 0.6 m3 fermentation scale were conducted, varying agitation, pH, initial soybean oil/ammonium sulfate charges, and initial aeration rate to further improve growth and productivity. Mid-cycle ammonia levels and lipase activity appeared to have an important role. Using mid-cycle soybean oil additions, a titer of 2.3 g/L of L-683,590 was obtained, while titers reached 2.7 g/L using mid-cycle soybean oil and ammonium sulfate additions. Both higher and lower homolog levels remained acceptable during this fed-batch process. Optimal timing of mid-cycle oil and ammonium sulfate additions was considered a critical factor to further titer improvements.

Decolourization and biodegradation of N,N'-dimethyl-p-phenylenediamine by Klebsiella pneumoniae RS-13 and Acetobacter liquefaciens S-1

Klebsiella pneumoniae RS-13 and Acetobacter liquefaciens S-1, both methyl red (MR)-degrading bacterial strains, degraded N,N'-dimethyl-p-phenylenediamine (DMPD) under aerobic conditions. DMPD, a toxic and mutagenic aromatic amine, is formed during the reductive cleavage of azo dyes such as MR. The effects of physical parameters, such as temperature and aeration, and chemical parameters, such as pH and concentrations of glucose, ethanol and ammonium sulphate in the culture medium, on the degradation of DMPD by these bacteria were determined. Klebsiella pneumoniae RS-13 degraded DMPD more efficiently than A. liquefaciens S-1 under all physicochemical conditions, except in the presence of ethanol as carbon and energy sources. In addition, Kl. pneumoniae RS-13 degraded DMPD at low levels of carbon and nitrogen at pH 6-8. These results indicate that it is feasible to use Kl. pneumoniae RS-13 to completely degrade the detoxify MR under aerobic conditions.

Growth requirements and nitrogen metabolism of Malassezia furfur

Investigations on the nutrient requirements of Malassezia yeasts led to the preparation of a new minimal medium consisting of an amino nitrogen and a lipid source, which only allowed the growth of the species M. furfur. Carbohydrates, electrolytes, vitamins and trace elements were not required. Using the basal medium, a nitrogen auxanogram for M. furfur was developed, which allowed investigation of the assimilation of 22 amino acids and 9 further nitrogen sources. With the exception of cysteine, all amino acids were metabolized, as were ammonium salts, urea, creatine, creatinine, uric acid and allantoin. KNO3, however, failed to support growth. Depending on the source used, the yeast cells changed micromorphologically: both oval and round forms were induced. Assimilation of several amino acids resulted in dimorphism, especially in the case of glycine and serine. The cell yield differed significantly depending on the nitrogen source; short-chain unbranched amino acids were utilized most readily. Thus, M. furfur can be characterized as a relatively undemanding yeast species which is optimally adapted to the superficial skin environment. All other lipid-dependent Malassezia species seem to require more complex media.

In vivo mutational analysis of highly conserved amino acid residues of the small subunit Cpa1p of the carbamylphosphate synthetase of Saccharomyces cerevisiae

The role of selected amino acid residues located in the putative catalytic domain and of two conserved histidine residues within the small subunit of the carbamylphosphate synthetase (CPS) specific to the arginine biosynthesis pathway of the yeast Saccharomyces cerevisiae was studied using site-directed mutagenesis to change all residues to aspartic acid. Carbamylphosphate synthesis catalysed by modified CPS was tested in vivo. The C264D, H307D and H349D mutants were unable to grow on minimal medium, indicating the importance of these three residues for efficient CPS activity, whereas, four other mutated residues located in the catalytic site (including a proline residue) do not affect the growth rate. These results in comparison to those obtained with the CPS of Escherichia coli, implicate residues Cys 264 and His 349 in the glutaminase catalytic activity, and His 307 in the binding of glutamine to the active site. Using these three defective mutants, we investigated the in vivo utilization of ammonia by CPS. C264D and H307D mutants are able to use ammonia as a substrate when provided in sufficiently high concentrations (up to 200 mM). The H349D mutant, however, did not grow even at ammonium sulfate concentrations above 400 mM, suggesting that this substitution is critical to NH3-dependent CPS activity although the ammonia binding site is presumably located within the large subunit of the enzyme.

A new variety of Aureobasidium pullulans characterized by exopolysaccharide structure, nutritional physiology and molecular features

The black yeast Aureobasidium pullulans (de Bary) Arnaud is known to synthesize the exopolysaccharide pullulan, a poly-alpha-1,6-maltotriose. Nine strains were found to produce additional aubasidan-like EPS, i.e. glucans with alpha-1,4-D-, beta-1,6-D- and beta-1,3-D-glycosidic bonds. These strains had previously been found to deviate in genotypic characters. Additional physiological differences were found: the optimal nitrogen source for exopolysaccharide production in liquid medium was NaNO3 for aubasidan-producing strains, and (NH4)2SO4 for the remaining strains. A new variety, A. pullulans var. aubasidani Yurlova, is described for the strains producing aubasidan-like components. The new variety can be distinguished from A. pullulans var. pullulans by the absence of assimilation of methyl-alpha-D-glucoside and lactose.

Effect of nitrogen source on biosynthesis of rapamycin by Streptomyces hygroscopicus

Six non-amino acid nitrogen compounds were examined as nitrogen source for growth of Streptomyces hygroscopicus and biosynthesis of rapamycin. Of the nitrogen sources studied, ammonium sulfate was the best with respect to formation of rapamycin, and supported cell growth comparable to the organic nitrogen sources used in the control chemically defined medium, i.e., aspartate, arginine plus histidine. In the new chemically defined medium, which is buffered with 200 mM 2-(N-morpholino)ethanesulfonic acid to prevent decline of pH during fermentation, an ammonium sulfate concentration of 40 mM was optimal for biosynthesis of rapamycin. Rapamycin production increased by more than 30% on both volumetric and specific bases as compared to the previous medium containing the three amino acids as nitrogen source.

Crystal structure of Escherichia coli inorganic pyrophosphatase complexed with SO4(2-). Ligand-induced molecular asymmetry

The three-dimensional structure of inorganic pyrophosphatase from Escherichia coli complexed with sulfate was determined at 2.2 A resolution using Patterson's search technique and refmed to an R-factor of 19.2%. Sulfate may be regarded as a structural analog of phosphate, the product of the enzyme reaction, and as a structural analog of methyl phosphate, the irreversible inhibitor. Sulfate binds to the pyrophosphatase active site cavity as does phosphate and this diminishes molecular symmetry, converting the homohexamer structure form (alpha3)2 into alpha3'alpha3". The asymmetry of the molecule is manifested in displacements of protein functional groups and some parts of the polypeptide chain and reflects the interaction of subunits and their cooperation. The significance of re-arrangements for pyrophosphatase function is discussed.

Production of chitinase from shellfish waste by Pseudomonas aeruginosa K-187

The production of chitinolytic enzyme by Pseudomonas aeruginosa K-187, using shrimp and crab shell powder (SCSP) as the carbon source, was studied. It was observed that chemically treated SCSP induced a significant increase of enzyme production, as compared with untreated SCSP. Spent HCl and NaOH from the chitin production industry was used to process SCSP. Various strategies of SCSP processing are examined and compared in terms of chitinolytic enzyme production. A three-and-one-half-fold increase of enzyme production (0.68 U/ml to 2.4 U/ml) was attained using HCl/NaOH treated SCSP. The microorganism (K-187) was isolated from soil in Taiwan and has been characterized and reported in a previous paper.

Influence of cultural conditions on lipid production by mutant strain of Rhodotorula glutinis MTCC 1151

Mutant of Rhodotorula glutinis MTCC 1151 was found to produce high level of lipid (63.6% of biomass) as compared to parent strain (56.7% biomass). The lipid synthesizing capacity of mutant of R. glutinis was evaluated with different glucose concentrations, nitrogen sources, incubation time, and aeration-agitation. Maximum lipid yield (63.6% of biomass) was found with 5% glucose using ammonium sulfate (0.2%) as a nitrogen source under shake-flask conditions (50 ml broth in 250 ml conical flask at 125 rpm) after 4 days of incubation at 28 degrees C. The ability of ammonium sulfate to replace comparatively very costly yeast extract is highly appreciable.

Production of beta-mannanase by B. subtilis from agro-industrial by-products: screening and optimization. Mannanase from wastes

We have demonstrated that a mixture of wheat bran (35 g l-1), as a main substrate, and palm seed powder (10 g l-1), as a co-substrate, is appropriate for beta-mannanase production by Bacillus subtilis. A 2n factorial experimental design was employed as a primary step for medium optimization. The enzyme activity titters obtained at the optimized growth condition were equivalent to about 319% of the beta-mannanase activity and 114% of the specific activity levels reached by a galactomannan-based culture.

Lactic acid bacteria: hydrophobicity and strength of attachment to meat surfaces

The hydrophobicity and strength of attachment of several lactic acid bacteria with antimicrobial activity were studied. Hydrophobicity was determined by bacterial adherence to hydrocarbons (BATH; octane or xylene), adhesion to nitrocellulose filters (NCF), salt aggregation test (SAT) and adherence to phenyl-Sepharose beads (PSB). The relative hydrophobicity of lactic acid bacteria depended markedly on the method used. No correlation between either SAT or BATH (octane) and strength of attachment (Sr value) existed. However, a significant relationship between strength of attachment and BATH (xylene), NCF and PSB, respectively, was observed, showing the highest correlation coefficient (r = 0.778) for BATH (xylene).

Studies on the biosynthesis of ovothiol A. Identification of 4-mercaptohistidine as an intermediate

The recent discovery of N1-methyl-4-mercaptohistidine (ovothiol A), a small aromatic thiol, in Crithidia fasciculata made it possible to study its biosynthesis in an organism which can be cultured in large quantities and under defined growth conditions. Radiolabeling experiments using intact cells indicated that the methyl group in ovothiol A is derived from methionine, while 35S was incorporated from either cysteine or methionine. Three lines of evidence suggested that transsulfuration preceded the methylation step: (a) Crithidia fasciculata failed to convert radiolabeled N pi-methylhistidine to ovothiol A. (b) Ovothiol A was poorly separated from a component which was labeled by [14C]histidine and by [35S]cysteine, but not by [methyl-3H] methionine. (c) Dialysed crude extracts of C. fasciculata catalysed the conversion of histidine to a thiolated species in the presence of pyridoxal phosphate, iron and cysteine in the absence of S-adenosylmethionine. The product of the in vitro reaction was isolated as the bimane derivative. Structural analysis using 1H and 13C-NMR spectroscopy confirmed its identity as the bimane derivative of 4-mercaptohistidine.

Interaction of ferric complexes with NADH-cytochrome b5 reductase and cytochrome b5: lipid peroxidation, H2O2 generation, and ferric reduction

NADH is reactive in interacting with iron and liver microsomes to catalyze the formation of reactive oxygen species. NADH-dependent microsomal electron transfer involves the enzymes NADH-cytochrome b5 reductase and cytochrome b5. Experiments were carried out to evaluate the ability of reconstituted systems containing purified reductase in the absence or presence of b5 to reduce several ferric complexes, to generate H2O2, and to catalyze lipid peroxidation. The reductase directly reduced ferric-EDTA; addition of b5 inhibited this reduction probably due to competition for the reductase. Cytochrome b5 was required for reduction of low (5 microM) and high (50 microM) concentrations of ferric-histidine and ferric-ammonium sulfate and low concentrations of ferric-ATP. The reductase could interact directly with high (50 microM) concentrations of ferric-ATP. Peroxidation of phospholipids extracted from liver microsomes by the reductase required b5. Molar ratios of b5 to reductase approximating those found in liver microsomes (e.g., 10) were effective in catalyzing lipid peroxidation and ferric reduction. The role of b5 in catalyzing lipid peroxidation appears to involve reduction of the ferric catalyst to help form an initiation complex and degradation of lipid hydroperoxides by the hemeprotein to catalyze propagation of the peroxidation cycle. In contrast to results with microsomes, lipid peroxidation by the complete reconstituted system was sensitive to super-oxide dismutase; this sensitivity was decreased if the reconstituted system was dialyzed overnight to form vesicular preparations, indicating that accessibility of enzymes to sites of peroxidation was important. High rates of H2O2 formation were observed in the presence of ferric-EDTA plus reductase; rates of H2O2 formation with the other ferric complexes were low even in the presence of b5. These results indicate that the ability of NADH reductase and cytochrome b5 to interact with various ferric complexes depends on the nature of the chelating agent used to complex the iron and on the concentration of the iron.