Chromatographic determination of flavin derivatives in baker's yeast

A Reduced F420-Dependent Nitrite Reductase in an Anaerobic Methanotrophic Archaeon

Anaerobic methanotrophic archaea (ANME), which oxidize methane in marine sediments through syntrophic associations with sulfate-reducing bacteria, carry homologs of coenzyme F₄₂₀-dependent sulfite reductase (Fsr) of Methanocaldococcus jannaschii, a hyperthermophilic methanogen from deep-sea hydrothermal vents. M. jannaschii Fsr (MjFsr) and ANME-Fsr belong to two phylogenetically distinct groups, FsrI and FsrII, respectively. MjFsrI reduces sulfite to sulfide with reduced F₄₂₀ (F₄₂₀H₂), protecting methyl coenzyme M reductase (Mcr), an essential enzyme for methanogens, from sulfite inhibition. However, the function of FsrIIs in ANME, which also rely on Mcr and live in sulfidic environments, is unknown. We have determined the catalytic properties of FsrII from a member of ANME-2c. Since ANME remain to be isolated, we expressed ANME2c-FsrII in a closely related methanogen, Methanosarcina acetivorans. Purified recombinant FsrII contained siroheme, indicating that the methanogen, which lacks a native sulfite reductase, produced this coenzyme. Unexpectedly, FsrII could not reduce sulfite or thiosulfate with F₄₂₀H₂. Instead, it acted as an F₄₂₀H₂-dependent nitrite reductase (FNiR) with physiologically relevant K_m values (nitrite, 5 μM; F₄₂₀H_2, 14 μM). From kinetic, thermodynamic, and structural analyses, we hypothesize that in FNiR, F₄₂₀H₂-derived electrons are delivered at the oxyanion reduction site at a redox potential that is suitable for reducing nitrite (E⁰' [standard potential], +440 mV) but not sulfite (E⁰', -116 mV). These findings and the known nitrite sensitivity of Mcr suggest that FNiR may protect nondenitrifying ANME from nitrite toxicity. Remarkably, by reorganizing the reductant processing system, Fsr transforms two analogous oxyanions in two distinct archaeal lineages with different physiologies and ecologies. IMPORTANCE Coenzyme F₄₂₀-dependent sulfite reductase (Fsr) protects methanogenic archaea inhabiting deep-sea hydrothermal vents from the inactivation of methyl coenzyme M reductase (Mcr), one of their essential energy production enzymes. Anaerobic methanotrophic archaea (ANME) that oxidize methane and rely on Mcr, carry Fsr homologs that form a distinct clade. We show that a member of this clade from ANME-2c functions as F₄₂₀-dependent nitrite reductase (FNiR) and lacks Fsr activity. This specialization arose from a distinct feature of the reductant processing system and not the substrate recognition element. We hypothesize FNiR may protect ANME Mcr from inactivation by nitrite. This is an example of functional specialization within a protein family that is induced by changes in electron transfer modules to fit an ecological need.

Quantification of riboflavin, flavin mononucleotide, and flavin adenine dinucleotide in mammalian model cells by CE with LED-induced fluorescence detection

Cultured mammalian cells essential are model systems in basic biology research, production platforms of proteins for medical use, and testbeds in synthetic biology. Flavin cofactors, in particular flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are critical for cellular redox reactions and sense light in naturally occurring photoreceptors and optogenetic tools. Here, we quantified flavin contents of commonly used mammalian cell lines. We first compared three procedures for extraction of free and noncovalently protein-bound flavins and verified extraction using fluorescence spectroscopy. For separation, two CE methods with different BGEs were established, and detection was performed by LED-induced fluorescence with limit of detections (LODs 0.5-3.8 nM). We found that riboflavin (RF), FMN, and FAD contents varied significantly between cell lines. RF (3.1-14 amol/cell) and FAD (2.2-17.0 amol/cell) were the predominant flavins, while FMN (0.46-3.4 amol/cell) was found at markedly lower levels. Observed flavin contents agree with those previously extracted from mammalian tissues, yet reduced forms of RF were detected that were not described previously. Quantification of flavins in mammalian cell lines will allow a better understanding of cellular redox reactions and optogenetic tools.

Anaerobic α-amylase production and secretion with fumarate as the final electron acceptor in Saccharomyces cerevisiae

In this study, we focus on production of heterologous α-amylase in the yeast Saccharomyces cerevisiae under anaerobic conditions. We compare the metabolic fluxes and transcriptional regulation under aerobic and anaerobic conditions, with the objective of identifying the final electron acceptor for protein folding under anaerobic conditions. We find that yeast produces more amylase under anaerobic conditions than under aerobic conditions, and we propose a model for electron transfer under anaerobic conditions. According to our model, during protein folding the electrons from the endoplasmic reticulum are transferred to fumarate as the final electron acceptor. This model is supported by findings that the addition of fumarate under anaerobic (but not aerobic) conditions improves cell growth, specifically in the α-amylase-producing strain, in which it is not used as a carbon source. Our results provide a model for the molecular mechanism of anaerobic protein secretion using fumarate as the final electron acceptor, which may allow for further engineering of yeast for improved protein secretion under anaerobic growth conditions.

Genetic control of biosynthesis and transport of riboflavin and flavin nucleotides and construction of robust biotechnological producers

Riboflavin [7,8-dimethyl-10-(1'-d-ribityl)isoalloxazine, vitamin B₂] is an obligatory component of human and animal diets, as it serves as the precursor of flavin coenzymes, flavin mononucleotide, and flavin adenine dinucleotide, which are involved in oxidative metabolism and other processes. Commercially produced riboflavin is used in agriculture, medicine, and the food industry. Riboflavin synthesis starts from GTP and ribulose-5-phosphate and proceeds through pyrimidine and pteridine intermediates. Flavin nucleotides are synthesized in two consecutive reactions from riboflavin. Some microorganisms and all animal cells are capable of riboflavin uptake, whereas many microorganisms have distinct systems for riboflavin excretion to the medium. Regulation of riboflavin synthesis in bacteria occurs by repression at the transcriptional level by flavin mononucleotide, which binds to nascent noncoding mRNA and blocks further transcription (named the riboswitch). In flavinogenic molds, riboflavin overproduction starts at the stationary phase and is accompanied by derepression of enzymes involved in riboflavin synthesis, sporulation, and mycelial lysis. In flavinogenic yeasts, transcriptional repression of riboflavin synthesis is exerted by iron ions and not by flavins. The putative transcription factor encoded by SEF1 is somehow involved in this regulation. Most commercial riboflavin is currently produced or was produced earlier by microbial synthesis using special selected strains of Bacillus subtilis, Ashbya gossypii, and Candida famata. Whereas earlier RF overproducers were isolated by classical selection, current producers of riboflavin and flavin nucleotides have been developed using modern approaches of metabolic engineering that involve overexpression of structural and regulatory genes of the RF biosynthetic pathway as well as genes involved in the overproduction of the purine precursor of riboflavin, GTP.

Cloning and characterization of two flavohemoglobins from Aspergillus oryzae

Two flavohemoglobin (FHb) genes, fhb1 and fhb2, were cloned from Aspergillus oryzae. The amino acid sequences of the deduced FHb1 and FHb2 showed high identity to other FHbs except for the predicted mitochondrial targeting signal in the N-terminus of FHb2. The recombinant proteins displayed absorption spectra similar to those of other FHbs. FHb1 and FHb2 were estimated to be a monomer and a dimer in solution, respectively. Both of the isozymes exhibit high NO dioxygenase (NOD) activity. FHb1 utilizes either NADH or NADPH as an electron donor, whereas FHb2 can only use NADH. These results suggest that FHb1 and FHb2 are fungal counterparts of bacterial FHbs and act as NO detoxification enzymes in the cytosol and mitochondria, respectively. This study is the first to show that a microorganism contains two isozymes of FHb and that intracellular localization of the isozymes could differ.

Supercritical fluid extraction as an on-line clean-up technique for determination of riboflavin vitamins in food samples by capillary electrophoresis with fluorimetric detection

An automatic method for the separation and determination of riboflavin (RF) vitamins (RF, flavin mononucleotide and flavin adenine dinucleotide) in food samples (chicken liver, tablet and powder milk) is proposed. The method is based on the on-line coupling of a supercritical fluid extractor (SFE) with a continuous flow-CE system with guided optical fiber fluorimetric detection (CF-CE-FD). The whole SFE-CF-CE-FD arrangement allowed the automatic treatment of food samples (clean-up of the sample followed by the extraction of the analytes), and the direct introduction of a small volume of the extracted plug to the CE-FD system for the determination of RF vitamins. Fluorescence detection introduced an appropriated sensitivity and contributed to avoid interferences of nonfluorescent polar compounds coming from the matrix samples in the extracted plug. Electrophoretic responses were linear within the 0.05-1 microg/g range, whereas the detection limits of RF vitamins were in the 0.036-0.042 microg/g range. The proposed arrangement opens up interesting prospects for the direct determination of polar analytes in complex samples with a good throughput and high level of automation.

Sequence-structure-function analysis of the bifunctional enzyme MnmC that catalyses the last two steps in the biosynthesis of hypermodified nucleoside mnm5s2U in tRNA

MnmC catalyses the last two steps in the biosynthesis of 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U) in tRNA. Previously, we reported that this bifunctional enzyme is encoded by the yfcK open reading frame in the Escherichia coli K12 genome. However, the mechanism of its activity, in particular the potential structural and functional dependence of the domains responsible for catalyzing the two modification reactions, remains unknown. With the aid of the protein fold-recognition method, we constructed a structural model of MnmC in complex with the ligands and target nucleosides and studied the role of individual amino acids and entire domains by site-directed and deletion mutagenesis, respectively. We found out that the N-terminal domain contains residues responsible for binding of the S-adenosylmethionine cofactor and catalyzing the methylation of nm(5)s(2)U to form mnm(5)s(2)U, while the C-terminal domain contains residues responsible for binding of the FAD cofactor. Further, point mutants with compromised activity of either domain can complement each other to restore a fully functional enzyme. Thus, in the conserved fusion protein MnmC, the individual domains retain independence as enzymes. Interestingly, the N-terminal domain is capable of independent folding, while the isolated C-terminal domain is incapable of folding on its own, a situation similar to the one reported recently for the rRNA modification enzyme RsmC.

Biochemical fluorometric method for the determination of riboflavin in milk

Methods of analysis of vitamin B2 in foods generally consist of the extraction of the sample, followed by enzymatic hydrolysis and quantitative measurement of the analyte, typically through RP-HPLC. The scope of our work here is to present a soft method to measure the free riboflavin content of a nontransparent and nonhomogeneous matrix such as milk, avoiding any extraction and separation of phases that are required in any published method for determination of the free RBF content in foods. We combine the front-face (FF) measurement of the light emission of milk with the ability of the apo-form of the riboflavin-binding protein (RBP) from chicken egg white to quench the riboflavin fluorescence. Thus, we titrate the RBF present in milk by gradually adding a solution of RBP to the milk sample and measuring, upon each addition, the FF residual emission due to uncomplexed RBF. The RBP binding capability has been measured in the same matrix of the analyte. Our results indicate a concentration of free RBF practically co-incident with the certified value for total B2 vitamin content in reference milk CRM 421.

KEYWORDS

Front-face fluorescence; riboflavin; apo-riboflavin-binding protein; milk fluorescence.

Cloning, expression, and characterization of a self-sufficient cytochrome P450 monooxygenase from Rhodococcus ruber DSM 44319

A new member of class IV of cytochrome P450 monooxygenases was identified in Rhodococcus ruber strain DSM 44319. As the genome of R. ruber has not been sequenced, a P450-like gene fragment was amplified using degenerated primers. The flanking regions of the P450-like DNA fragment were identified by directional genome walking using polymerase chain reaction. The primary protein structure suggests a natural self-sufficient fusion protein consisting of ferredoxin, flavin-containing reductase, and P450 monooxygenase. The only flavin found within the enzyme was riboflavin 5'-monophosphate. The enzyme was successfully expressed in Escherichia coli, purified and characterized. In the presence of NADPH, the P450 monooxygenase showed hydroxylation activity towards polycyclic aromatic hydrocarbons naphthalene, indene, acenaphthene, toluene, fluorene, m-xylene, and ethyl benzene. The conversion of naphthalene, acenaphthene, and fluorene resulted in respective ring monohydroxylated metabolites. Alkyl aromatics like toluene, m-xylene, and ethyl benzene were hydroxylated exclusively at the side chains. The new enzyme's ability to oxidize such compounds makes it a potential candidate for biodegradation of pollutants and an attractive biocatalyst for synthesis.

Identification of human and rat FAD-AMP lyase (cyclic FMN forming) as ATP-dependent dihydroxyacetone kinases

Rat liver FAD-AMP lyase or FMN cyclase is the only known enzymatic source of the unusual flavin nucleotide riboflavin 4',5'-cyclic phosphate. To determine its molecular identity, a peptide-mass fingerprint of the purified rat enzyme was obtained. It pointed to highly related, mammalian hypothetical proteins putatively classified as dihydroxyacetone (Dha) kinases due to weaker homologies to biochemically proven Dha kinases of plants, yeasts, and bacteria. The human protein LOC26007 cDNA was used to design PCR primers. The product amplified from human brain cDNA was cloned, sequenced (GenBank Accession No. ), and found to differ from protein LOC26007 cDNA by three SNPs. Its heterologous expression yielded a protein active both as FMN cyclase and ATP-dependent Dha kinase, each activity being inhibited by the substrate(s) of the other. Cyclase and kinase activities copurified from rat liver extracts. Evidence supports that a single protein sustains both activities, probably in a single active center. Putative Dha kinases from other mammals are likely to be FMN cyclases too. Future work will profit from the availability of the structure of Citrobacter freundii Dha kinase, which contains substrate-interacting residues conserved in human Dha kinase/FMN cyclase.

Fluorimetric HPLC detection of endogenous riboflavin 4',5'-cyclic phosphate in rat liver at nanomolar concentrations

We have investigated the biological occurrence of riboflavin 4',5'-cyclic phosphate (cyclic FMN or cFMN), the flavin product known to be formed in vitro from FAD by the rat liver enzyme FAD-AMP lyase (cyclizing) or FMN cyclase (EC 4.6.1.15). The expected difficulties were the low concentration of the compound, the tendency of the more abundant FAD to decompose chemically to cFMN, and the acid lability of cFMN itself. A protocol was devised to prepare deproteinized rat liver extracts, avoiding conditions prone to the chemical formation of cFMN and making exposure to conditions of cFMN degradation as short as possible. In these extracts, cFMN was assayed by HPLC with fluorimetric detection. The identity of liver cFMN was confirmed by its HPLC separation from other known flavins, its coelution with authentic cFMN, and its susceptibility to acid degradation, yielding a mixture of 5'-FMN and 4'-FMN. The amount of total cFMN recovered in the liver extracts was 22+/-11 pmol/g fresh tissue. Careful control experiments were performed to rule out the possibility that this could be a chemical product of FAD degradation during extract preparation. These controls indicated that, on average, 97% of the measured extract concentration of cFMN, i.e., about 21+/-10 pmol/g, was already present in the liver at the beginning of the process and was extracted from the tissue. This figure is likely to be an underestimation of the hepatic content, as indicated by control experiments.

Oxidative protein folding in eukaryotes: mechanisms and consequences

The endoplasmic reticulum (ER) provides an environment that is highly optimized for oxidative protein folding. Rather than relying on small molecule oxidants like glutathione, it is now clear that disulfide formation is driven by a protein relay involving Ero1, a novel conserved FAD-dependent enzyme, and protein disulfide isomerase (PDI); Ero1 is oxidized by molecular oxygen and in turn acts as a specific oxidant of PDI, which then directly oxidizes disulfide bonds in folding proteins. While providing a robust driving force for disulfide formation, the use of molecular oxygen as the terminal electron acceptor can lead to oxidative stress through the production of reactive oxygen species and oxidized glutathione. How Ero1p distinguishes between the many different PDI-related proteins and how the cell minimizes the effects of oxidative damage from Ero1 remain important open questions.

Liquid chromatographic analysis of riboflavin vitamers in foods using fluorescence detection

The analysis of free riboflavin (RF) and its two coenzymes, flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD), is optimized using reversed phase liquid chromatography with fluorescence detection. The stationary phase was amide-based and endcapped with trimethylsilyl, and the isocratic mobile phase consisted of a 10:90 v/v acetonitrile/phosphate buffer (pH 5). Peaks were identified by the retention characteristics and fluorescence spectra. Detection limits were 0.03, 0.05, and 0.24 ng for RF, FMN, and FAD, respectively. The vitamins were extracted using acetonitrile and the phosphate buffer. The procedure was applied to the determination of B2 vitamers in different types of food such as milk and soy-based infant formulas, beer, fruit juices, and honey of different types. Most B2 vitamin appeared as RF, while the coenzymes were present in lower amounts. The method was validated using two certified reference materials, and results within the certified range were obtained.

Assay of riboflavin in sample wines by capillary zone electrophoresis and laser-induced fluorescence detection

To routinely assay the concentration of riboflavin (RF) in wines, a rapid and sensitive method was developed and evaluated. The method is based on a simple sample preparation, capillary zone electrophoretic separation and laser-induced fluorescence detection (CZE-LIF). Sample preparation required only dilution and filtration. Under optimized conditions, the limit of detection of riboflavin was 0.5 micro g/L, using a hydrodynamic sample introduction of 10 s at 54 mbar. The method was fully validated: the recovery of RF in wines was >95%. The concentrations of RF within the three sample types of Italian wines investigated here ranged from 69 to 151 micro g/L with a mean value (+/-SD) of 112 +/- 25 micro g/L, from 74 to 193 micro g/L with a mean value of 115 +/- 45 micro g/L, and from 156 to 292 micro g/L with a mean value of 226 +/- 40 micro g/L, for white, rosé, and red wines, respectively. Such an accurate and highly sensitive CZE-LIF method represents a powerful improvement over previous methods in terms of sensitivity, simplicity, and efficiency. It is well suited to satisfy the demands for accurate and sensitive detection with minimal sample preparation and cleanup.

The FAD- and O(2)-dependent reaction cycle of Ero1-mediated oxidative protein folding in the endoplasmic reticulum

The endoplasmic reticulum (ER) supports disulfide formation through an essential protein relay involving Ero1p and protein disulfide isomerase (PDI). We find that in addition to having a tightly associated flavin adenine dinucleotide (FAD) moiety, yeast Ero1p is highly responsive to small changes in physiological levels of free FAD. This sensitivity underlies the dependence of oxidative protein folding on cellular FAD levels. FAD is synthesized in the cytosol but can readily enter the ER lumen and promote Ero1p-catalyzed oxidation. Ero1p then uses molecular oxygen as its preferred terminal electron acceptor. Thus Ero1p directly couples disulfide formation to the consumption of molecular oxygen, but its activity is modulated by free lumenal FAD levels, potentially linking disulfide formation to a cell's nutritional or metabolic status.

Formation and transfer of disulphide bonds in living cells

Protein disulphide bonds are formed in the endoplasmic reticulum of eukaryotic cells and the periplasmic space of prokaryotic cells. The main pathways that catalyse the formation of protein disulphide bonds in prokaryotes and eukaryotes are remarkably similar, and they share several mechanistic features. The recent identification of new redox-active proteins in humans and yeast that mechanistically parallel the more established redox-active enzymes indicates that there might be further uncharacterized redox pathways throughout the cell.

Optimizing separation conditions for riboflavin, flavin mononucleotide and flavin adenine dinucleotide in capillary zone electrophoresis with laser-induced fluorescence detection

A method was developed for the quantitative determination of riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), using free solution capillary zone electrophoresis in uncoated fused-silica capillaries with laser-induced fluorescence (LIF) detection. Various factors influencing the separation and detection of flavin vitamers were investigated, including pH (5.5-10.5), concentration and nature of the run buffer (phosphate, borate and carbonate), applied voltage (15-30 kV), temperature (15-30 degrees C) and injection time. Optimal resolution and detection were obtained with a pH 9.8, 30 mM aqueous phosphate buffer at 15 degrees C and 30 kV of applied voltage. LIF detection was obtained with a He-Cd laser source using an excitation wavelength at 442 nm and lambda(em) > or = 515 nm. Riboflavin could be determined in the concentration ranges 0.5-350 microg/l with a rather low detection limit (LOD) down to 50 amol. The LODs of FAD and FMN were slightly higher, 300 and 350 amol, respectively. Combined with a simple clean-up procedure, the practical utility of this method is illustrated by the measurements of flavin derivates in foods and beverages, such as wines, milk, yoghurt and raw eggs.

Purification, characterization, and substrate and inhibitor structure-activity studies of rat liver FAD-AMP lyase (cyclizing): preference for FAD and specificity for splitting ribonucleoside diphosphate-X into ribonucleotide and a five-atom cyclic phosphodiester of X, either a monocyclic compound or a cis-bicyclic phosphodiester-pyranose fusion

An enzyme with FAD-AMP lyase (cyclizing) activity, splitting FAD to AMP and riboflavin 4',5'-phosphate (cFMN), was recently identified [Fraiz, F., et al. (1998) Biochem. J. 330, 881-888]. Now, it has been purified to apparent homogeneity from a rat liver supernatant, by a procedure that includes affinity for ADP-agarose (adsorption required the activating cation Mn2+ and desorption required its removal), to a final activity of 2.2 units/mg after a 240-fold purification with a 15% yield. By SDS-PAGE, only one protein band was observed (Mr = 59 000). The correspondence between protein and enzyme activity was demonstrated by renaturation after SDS-PAGE, by gradient ultracentrifugation followed by analytical SDS-PAGE, and by native PAGE with visualization of enzyme activity by fluorescence. A native Mr of 100 000 (ultracentrifugation) or 140 000 (gel filtration) indicated that FAD-AMP lyase could be a dimer. The enzyme required millimolar concentrations of Mn2+ or Co2+, exhibited different optimum pH values with these cations (pH 8.5 or 7.3, respectively), and was strongly inhibited by ADP or ATP, but not by dADP, dATP, or the reaction products AMP and cFMN. A specificity study was conducted with 35 compounds related to FAD, mostly nucleoside diphosphate-X (NDP-X) derivatives. Besides FAD, the enzyme split 11 of these compounds with the pattern NDP-X --> NMP + P=X. Structure-activity correlations of substrates, nonsubstrates, and inhibitors, and the comparison of the enzymic reactivities of NDP-X compounds with their susceptibilities to metal-dependent chemical degradation, pinpointed the following specificity pattern. FAD-AMP lyase splits ribonucleoside diphosphate-X compounds in which X is an acyclic or cyclic monosaccharide or derivative bearing an X-OH group that is able to attack internally the proximal phosphorus with the geometry necessary to form a P=X product, either a five-atom monocyclic phosphodiester or a cis-bicyclic phosphodiester-pyranose fusion. For instance, NDP-glucose and GDP-alpha-L-fucose were substrates, but dTDP-glucose, NDP-mannose, and GDP-beta-L-fucose were not. Judging from kcat/Km ratios, we found the best substrate to be FAD, followed closely by ADP-glucose (kcat/Km only 2-fold lower, but not a physiological compound in mammals), whereas other substrates exhibited 50-500-fold lower kcat/Km values. However, there was no evidence for specific flavin recognition. Instead, what seems to be recognized is the NDP moiety of NDP-X, with a strong preference for ADP-X. Splitting would then depend on the presence of an adequate X-OH group. The possibility that, besides FAD, there could be in mammals other ADP-X substrates of FAD-AMP lyase is discussed, with emphasis placed on some ADP-ribose derivatives.

A flavoprotein oxidase defines a new endoplasmic reticulum pathway for biosynthetic disulphide bond formation

Ero1 and Pdi1 are essential elements of the pathway for the formation of disulphide bonds within the endoplasmic reticulum (ER). By screening for alternative oxidation pathways in Saccharomyces cerevisiae, we identified ERV2 as a gene that when overexpressed can restore viability and disulphide bond formation to an ero1-1 mutant strain. ERV2 encodes a luminal ER protein of relative molecular mass 22,000. Purified recombinant Erv2p is a flavoenzyme that can catalyse O2-dependent formation of disulphide bonds. Erv2p transfers oxidizing equivalents to Pdi1p by a dithiol-disulphide exchange reaction, indicating that the Erv2p-dependent pathway for disulphide bond formation closely parallels that of the previously identified Ero1p-dependent pathway.

Chromatographic determination of riboflavin and its derivatives in food

Three elution methods on two different reversed-phase C18 columns were developed to determine flavin derivatives in raw egg white, raw egg yolk, egg powder, pasteurised milk, fermented milk products and liver (chicken, calf and pig). Additionally, 11 thin-layer chromatography solvent systems were used to confirm presence of flavins detected in assessed products. It was found that an Alphabond C18 column was not as effective as a Symmetry C18 column. Method A (mobile phase gradient of methanol-0.05 M ammonium acetate, pH 6.0 applied on an Alphabond C18 column) can be used for determination of flavin adenine dinucleotide, flavin mononucleotide, riboflavin 4',5'-cyclic phosphate, riboflavin, 10-formylmethylflavin and 10-hydroxyethylflavin in products that do not contain 7alpha-hydroxyriboflavin. Method B (mobile phase gradient of methanol-demineralized water, on an Alphabond C18 column) can be useful to separate flavin coenzymes from other flavin compounds or to confirm the presence of 7alpha-hydroxyriboflavin and 10-hydroxyethylflavin in analysed samples. Method C (mobile phase gradient of methanol-0.05 M ammonium acetate, pH 6.0, on a Symmetry C18 column) allows separation of all flavins detected in tested products: flavin adenine dinucleotide, flavin mononucleotide, riboflavin 4',5'-cyclic phosphate, riboflavin, 10-formylmethylflavin, 10-hydroxyethylflavin, 7alpha-hydroxyriboflavin, riboflavin-beta-D-galactoside and riboflavin-alpha-D-glucoside.

Chromatographic identification of a new flavin derivative in plain yogurt

The presence of flavin derivatives in plain yogurt was assessed by high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC). The total amount of flavins in yogurts produced by different companies was variable and oscillated between 150.0 and 218.8 microg/100 g. Riboflavin (RF) was the predominant flavin. Besides RF, flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), 7alpha-hydroxyriboflavin (7alpha-HRF), 4'- or 5'-D-riboflavin-beta-D-galactoside (RFgal), and traces of 10-formylmethylflavin (10-FMF) and 10-hydroxyethylflavin (10-HEF) have been found. It is known that RFgal may be obtained using enzymes or cultures of different microorganisms, but its presence in foodstuffs has not been demonstrated, yet.