Effect of riboflavin on red-cell metabolism of vitamin B6

Molecular Mechanisms Provide a Landscape for Biomarker Selection for Schizophrenia and Schizoaffective Psychosis

Research evaluating the role of the 5,10-methylenetetrahydrofolate reductase (MTHFR C677T) gene in schizophrenia has not yet provided an extended understanding of the proximal pathways contributing to the 5-10-methylenetetrahydrofolate reductase (MTHFR) enzyme's activity and the distal pathways being affected by its activity. This review investigates these pathways, describing mechanisms relevant to riboflavin availability, trace mineral interactions, and the 5-methyltetrahydrofolate (5-MTHF) product of the MTHFR enzyme. These factors remotely influence vitamin cofactor activation, histamine metabolism, catecholamine metabolism, serotonin metabolism, the oxidative stress response, DNA methylation, and nicotinamide synthesis. These biochemical components form a broad interactive landscape from which candidate markers can be drawn for research inquiry into schizophrenia and other forms of mental illness. Candidate markers drawn from this functional biochemical background have been found to have biomarker status with greater than 90% specificity and sensitivity for achieving diagnostic certainty in schizophrenia and schizoaffective psychosis. This has implications for achieving targeted treatments for serious mental illness.

Sequential hydrolysis of FAD by ecto-5' nucleotidase CD73 and alkaline phosphatase is required for uptake of vitamin B2 into cells

Extracellular hydrolysis of flavin-adenine dinucleotide (FAD) and flavin mononucleotide (FMN) to riboflavin is thought to be important for cellular uptake of vitamin B₂ because FAD and FMN are hydrophilic and do not pass the plasma membrane. However, it is not clear whether FAD and FMN are hydrolyzed by cell surface enzymes for vitamin B₂ uptake. Here, we show that in human cells, FAD, a major form of vitamin B₂ in plasma, is hydrolyzed by CD73 (also called ecto-5' nucleotidase) to FMN. Then, FMN is hydrolyzed by alkaline phosphatase to riboflavin, which is efficiently imported into cells. We determined that this two-step hydrolysis process is impaired on the surface of glycosylphosphatidylinositol (GPI)-deficient cells due to the lack of these GPI-anchored enzymes. During culture of GPI-deficient cells with FAD or FMN, we found that hydrolysis of these forms of vitamin B₂ was impaired, and intracellular levels of vitamin B₂ were significantly decreased compared with those in GPI-restored cells, leading to decreased formation of vitamin B₂-dependent pyridoxal 5'-phosphate and mitochondrial dysfunction. Collectively, these results suggest that inefficient uptake of vitamin B₂ might account for mitochondrial dysfunction seen in some cases of inherited GPI deficiency.

Plasma vitamins B2, B6, and B12, and related genetic variants as predictors of colorectal cancer risk

BACKGROUND

B-vitamins are essential for one-carbon metabolism and have been linked to colorectal cancer. Although associations with folate have frequently been studied, studies on other plasma vitamins B2, B6, and B12 and colorectal cancer are scarce or inconclusive.

METHODS

We carried out a nested case-control study within the European Prospective Investigation into Cancer and Nutrition, including 1,365 incident colorectal cancer cases and 2,319 controls matched for study center, age, and sex. We measured the sum of B2 species riboflavin and flavin mononucleotide, and the sum of B6 species pyridoxal 5'-phosphate, pyridoxal, and 4-pyridoxic acid as indicators for vitamin B2 and B6 status, as well as vitamin B12 in plasma samples collected at baseline. In addition, we determined eight polymorphisms related to one-carbon metabolism. Relative risks for colorectal cancer were estimated using conditional logistic regression, adjusted for smoking, education, physical activity, body mass index, alcohol consumption, and intakes of fiber and red and processed meat.

RESULTS

The relative risks comparing highest to lowest quintile were 0.71 [95% confidence interval (95% CI), 0.56-0.91; P(trend) = 0.02] for vitamin B2, 0.68 (95% CI, 0.53-0.87; P(trend) <0.001) for vitamin B6, and 1.02 (95% CI, 0.80-1.29; P(trend) = 0.19) for vitamin B12. The associations for vitamin B6 were stronger in males who consumed ≥30 g alcohol/day. The polymorphisms were not associated with colorectal cancer.

CONCLUSIONS

Higher plasma concentrations of vitamins B2 and B6 are associated with a lower colorectal cancer risk.

IMPACT

This European population-based study is the first to indicate that vitamin B2 is inversely associated with colorectal cancer, and is in agreement with previously suggested inverse associations of vitamin B6 with colorectal cancer.

Effect of a nutritional supplement on premenstrual symptomatology in women with premenstrual syndrome: a double-blind longitudinal study

To assess the effectiveness of a vitamin/mineral supplement in controlling symptoms of premenstrual syndrome (PMS), we conducted a double-blind randomized study on 44 women with PMS. Subjects were carefully screened and excluded if underlying physical or psychopathological conditions were noted. Follicular and luteal testing with a menstrual symptom questionnaire, subdividing PMS into four subgroups, was completed for 1 month prior to treatment and for three menstrual cycles during treatment. Subjects were randomly assigned to receive either placebo or six or 12 tablets of the supplement a day for three menstrual cycles. All subjects had significant differences in severity of symptoms between the follicular and luteal phase of the control cycle. Comparing pre- vs posttreatment luteal phase scores, significant placebo effects were noted for two PMS subgroups. Significant treatment effects were noted in three subgroups for the six-tablet group and in all four subgroups for the 12-tablet group. These results suggest that this nutritional supplement may play a role in the management of women with PMS.

Mechanisms underlying the differential effects of ethanol on the bioavailability of riboflavin and flavin adenine dinucleotide

Chronic alcoholism is associated with a high prevalence of riboflavin deficiency. Experiments were designed in an animal model to determine whether ethanol alters selectively the absorption of riboflavin and flavin adenine dinucleotide (FAD), the predominant dietary form of the vitamin. Rats received by gavage a liver homogenate to which either [14C]riboflavin or [14C]FAD was added with either ethanol or isocaloric sucrose solutions. Ethanol markedly diminished the bioavailability of [14C]FAD to a greater degree than that of [14C]riboflavin. Corroboration of an ethanol-impaired intraluminal hydrolysis of FAD was provided by using everted jejunal segments and measuring mucosal uptake of [14C]riboflavin together with nonradiolabeled FAD. In subsequent studies with mucosal cell extracts, ethanol markedly inhibited activities of FAD pyrophosphatase and flavin mononucleotide (FMN) phosphatase. These findings suggest that dietary sources of riboflavin (FMN and FAD) are not absorbed as well in the presence of ethanol than are vitamin preparations containing riboflavin, which is utilized more readily.

Impaired collagen maturity in vitamins B2 and B6 deficiency--probable molecular basis of skin lesions

Solubility of collagen was increased and the proportion of insoluble collagen was reduced in the skin of both riboflavin as well as pyridoxine-deficient rats. Collagen content of the skin, and aldehyde concentration of salt-soluble collagen were also lower in the deficient groups. The alpha:beta subunit ratio of salt-soluble collagen was higher in riboflavin deficiency. In food-restricted weight-matched control groups, similar changes in collagen solubility, but of lesser magnitude were observed. Both food restriction and riboflavin deficiency decreased plasma PLP concentration. Increase in the solubility of collagen, decrease in the aldehyde content of soluble collagen and increase in the alpha:beta subunit ratio of soluble collagen, suggest that the maturation of collagen may be affected in pyridoxine or riboflavin deficiency. These molecular events may be etiologically related to the pathogenesis of the skin lesions in vitamin B2 or B6 deficiency.

Interactions of thiamin, riboflavin, and other B-vitamins

Interactions of the B-complex vitamins are essential in the performance of metabolic and catabolic reactions in the body. Even vitamin C and the fat-soluble vitamins may be involved in these interactions. Clinical and biochemical aberrations associated with various disease states can often be explained on the basis of these vitamin interrelationships. Health and nutritional well-being are dependent upon the maintenance and proper functioning of these vitamin-dependent metabolic pathways.

Glutathione reductase activity and pyridoxine (pyridoxamine) phosphate oxidase activity in the red cell

The red-cell enzymes, glutathione reductase (FAD-dependent) and pyridoxine (pyridoxamine) phosphate oxidase (FMN-dependent), were studied in control subjects. The wide range in the glutathione reductase activity correlated inversely with the percentage stimulation by FAD added in vitro, and with pyridoxine (pyridoxamine) phosphate oxidase activity. Both enzymes were stimulated after ingestion of riboflavin. The results support the suggestion that the rate of metabolism of riboflavin in the red cell controls the activity of both enzymes, and the rate of red-cell metabolism of vitamin B-6.

Pyridoxine (pyridoxamine) phosphate oxidase activity in the red cell

A method has been developed for the measurement of red cell pyridoxine (pyridoxamine) phosphate oxidase activity. The more stable substrate, pyridoxamine phosphate, was incubated with a red cell haemolysate and the product pyridoxal phosphate was measured by Lactobacillus casei microbiological assay. A wide range of oxidase activities was found in control subjects (6-136 ng pyridoxal phosphate/g Hb x 10(-2)). There is a close correlation between the rate of conversion of pyridoxine to pyridoxal phosphate in whole blood and the oxidase activity. There was a marked increase in both after oral riboflavin. These results suggest that the oxidase plays a large part in the regulation of vitamin B-6 metabolism in red cells.

Abnormal red-cell metabolism of pyridoxine associated with beta-thalassaemia

Red-cell conversion of pyridoxine to pyridoxal phosphate was studied in control subjects, and patients with heterozygous and homozygous beta-thalassaemia. In 7% of control subjects the rate of pyridoxine conversion was well below the range found in the other control subjects (5.0-8.6%, mean 6.5%/g Hb x 10(-2)) but in heterozygous beta-thalassaemia was below that range in 63% of the patients. The conversion rate was also slow or borderline in the majority of patients with severe transfusion-dependent homozygous beta-thalassaemia, in spite of the presence of some donor cells; but was normal, or fast as in other anaemias, in all but one patient with mild homozygous thalassaemia. There was a much higher incidence of a slow conversion rate in the parents of the severe homozygotes than in parents of the mild homozygotes, illustrating the familial pattern. This supports our view that the red-cell conversion rate of pyridoxine is an inherited characteristic, independent of thalassaemia. The cause of a reduced rate of pyridoxine conversion was investigated. The increase to a normal rate following riboflavin ingestion suggests a defect in the activity of the flavin mononucleotide (FMN)-dependent pyridoxine phosphate oxidase.