Characterization of a new flavin metabolite from human urine

Transcriptional regulation of the albumin gene depends on the removal of histone methylation marks by the FAD-dependent monoamine oxidase lysine-specific demethylase 1 in HepG2 human hepatocarcinoma cells

Lysine-specific demethylase (LSD) 1 is an FAD-dependent demethylase that catalyzes the removal of methyl groups from lysine-4 in histone H3, thereby mediating gene repression. Here we tested the hypothesis that riboflavin deficiency causes a loss of LSD1 activity in HepG2 human hepatocarcinoma cells, leading to an accumulation of lysine-4-dimethylated histone H3 (H3K4me2) marks in the albumin promoter and aberrant upregulation of albumin expression. Cells were cultured in riboflavin-defined media providing riboflavin at concentrations representing moderately deficient (3.1 nmol/L), sufficient (12.6 nmol/L), and supplemented (301 nmol/L) cells in humans for 7 d. The efficacy of treatment was confirmed by assessing glutathione reductase activity and concentrations of reduced glutathione as markers of riboflavin status. LSD activity was 21% greater in riboflavin-supplemented cells compared with riboflavin-deficient and -sufficient cells. The loss of LSD activity was associated with a gain in the abundance of H3K4me2 marks in the albumin promoter; the abundance of H3K4me2 marks was ∼170% higher in riboflavin-deficient cells compared with sufficient and supplemented cells. The abundance of the repression mark, K9-trimethylated histone H3, was 38% lower in the albumin promoter of riboflavin-deficient cells compared with the other treatment groups. The expression of albumin mRNA was aberrantly increased by 200% in riboflavin-deficient cells compared with sufficient and supplemented cells. In conclusion, riboflavin deficiency impairs gene regulation by epigenetic mechanisms, mediated by a loss of LSD1 activity.

Micronutrient cofactor research with extensions to applications

Following identification of essential micronutrients, there has been a continuum of research aimed at revealing their absorption, transport, utilization as cofactors, and excretion and secretion. Among those cases that have received our attention are vitamin B6, riboflavin, biotin, lipoate, ascorbate, and certain metal ions. Circulatory transport and cellular uptake of the water-soluble vitamins exhibit relative specificity and facilitated mechanisms at physiological concentrations. Isolation of enzymes and metabolites from micro–organisms and mammals has provided information on pathways involved in cofactor formation and metabolism. Kinases catalysing phosphorylation of B6and riboflavin have a preference for Zn2+in stereospecific chelates with adenosine triphosphate. The synthetase for flavin adenine dinucleotide prefers Mg2+. The flavin mononucleotide-dependent oxidase that converts the 5′–phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between B6and riboflavin and is a primary control point for conversion of B6to its coenzyme. Sequencing and cloning of a side–chain oxidase for riboflavin was achieved. Details on binding and function have been delineated for some cofactor systems, especially in several flavoproteins. There is both photochemical oxidation and oxidative catabolism of B6and riboflavin. Both biotin and lipoate undergo oxidation of their acid side chains with redox cleavage of the rings. Applications from our findings include the development of affinity absorbents, enhanced drug delivery, delineation of residues in biopolymer modification, pathogen photoinactivation in blood components, and input into human dietary recommendations. Ongoing and future research in the cofactor arena can be expected to add to this panoply. At the molecular level, the way in which the same cofactor can participate in diverse catalytic reactions resides in interactions with surrounding enzyme structures that must be determined case by case. At the level of human intake, more knowledge is desirable for making micronutrient recommendations based on biochemical indicators, especially for the span between infancy and adulthood.

Proliferation of peripheral blood mononuclear cells increases riboflavin influx

Previously we demonstrated that proliferation of peripheral blood mononuclear cells (PBMC) causes a five-fold increase in cellular uptake of biotin; this increase is mediated by an increased number of biotin transporters on the PBMC surface. In the present study, we investigated the specificity of this phenomenon by determining whether the cellular uptake of riboflavin also increases in proliferating PBMC and whether the increase is also mediated by an increased number of transporters per cell. We characterized [3H]riboflavin uptake in both quiescent and proliferating PBMC. In quiescent PBMC, [3H]riboflavin uptake exhibited saturation kinetics and was reduced by addition of unlabeled riboflavin (P < 0.05) or lumichrome (P < 0.01). These observations are consistent with transporter-mediated uptake. [3H]Riboflavin uptake was reduced at 4 degrees C compared with 37 degrees C (P < 0.01) and by 2, 4-dinitrophenol (P < 0.05) but not by ouabain or incubation in sodium-free medium. These data provide evidence for an energy-dependent but sodium-independent transporter. Proliferating PBMC accumulated approximately four times more [3H]riboflavin than quiescent PBMC (P < 0.05). Because both transporter affinity and transporter number per cell (as judged by maximal transport rate) were similar in quiescent and proliferating PBMC, we hypothesize that the increased riboflavin uptake by proliferating PBMC reflects only increased cellular volume. To test this hypothesis, PBMC volume was reduced using hyperosmolar medium; [3H]riboflavin uptake decreased to about 50% of isotonic controls (P < 0.01). Thus we conclude that proliferating PBMC increase cellular content of riboflavin and biotin by two different mechanisms.

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.

A trail of research on cofactors: an odyssey with friends

Over the span of 40 y and with the participation of over 60 students and postdoctoral colleagues, my laboratory has been able to elucidate numerous aspects of cofactor metabolism and function. Findings have been on the absorption, transport, utilization and excretion of vitamin B-6, riboflavin, biotin, lipoate and ascorbate. Specificity studies on those trace but essential enzymes that catalyze conversion of such vitamins as B-6 and riboflavin to their functional coenzymes led to our development of "biochemically specific absorbents" that prototypically exemplified what later was called "affinity chromatography." Characterization of the purified kinases for B-6 and riboflavin revealed preference for Zn2+ with the eucaryotic enzymes and delimited effects of inhibitors that relate to drug action. Flavin adenine dinucleotide synthetase, separable from flavokinase in mammals, prefers Mg2+. Specifics for binding and function of flavocoenzymes were delineated for several flavoproteins. The flavin mononucleotide-dependent oxidase that converts the 5'-phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between riboflavin and B-6 that we characterized in mechanistic detail and found to be the primary control point for conversion of B-6 to its coenzyme. Sequencing and cloning of a side-chain oxidase for riboflavin was achieved. Isolation and identification of metabolites of biotin and of lipoic acid, first from bacteria obtained by enrichment culture and then from mammals, provided seminal information on catabolic pathways involved, as have our other studies with flavin catabolites isolated from milk and urine.