Effects of riboflavin deficiency on oxidative phosphorylation, flavin enzymes and coenzymes in rat liver

Systemic Regulation of Host Energy and Oogenesis by Microbiome-Derived Mitochondrial Coenzymes

Gut microbiota have been shown to promote oogenesis and fecundity, but the mechanistic basis of remote influence on oogenesis remained unknown. Here, we report a systemic mechanism of influence mediated by bacterial-derived supply of mitochondrial coenzymes. Removal of microbiota decreased mitochondrial activity and ATP levels in the whole-body and ovary, resulting in repressed oogenesis. Similar repression was caused by RNA-based knockdown of mitochondrial function in ovarian follicle cells. Reduced mitochondrial function in germ-free (GF) females was reversed by bacterial recolonization or supplementation of riboflavin, a precursor of FAD and FMN. Metabolomics analysis of GF females revealed a decrease in oxidative phosphorylation and FAD levels and an increase in metabolites that are degraded by FAD-dependent enzymes (e.g., amino and fatty acids). Riboflavin supplementation opposed this effect, elevating mitochondrial function, ATP, and oogenesis. These findings uncover a bacterial-mitochondrial axis of influence, linking gut bacteria with systemic regulation of host energy and reproduction.

Flavins Act as a Critical Liaison Between Metabolic Homeostasis and Oxidative Stress in the Retina

Derivatives of the vitamin riboflavin, FAD and FMN, are essential cofactors in a multitude of bio-energetic reactions, indispensable for lipid metabolism and also are requisites in mitigating oxidative stress. Given that a balance between all these processes contributes to the maintenance of retinal homeostasis, effective regulation of riboflavin levels in the retina is paramount. However, various genetic and dietary factors have brought to fore pathological conditions that co-occur with a suboptimal level of flavins in the retina. Our focus in this review is to, comprehensively summarize all the possible metabolic and oxidative reactions which have been implicated in various retinal pathologies and to highlight the contribution flavins may have played in these. Recent research has found a sensitive method of measuring flavins in both diseased and healthy retina, presence of a novel flavin binding protein exclusively expressed in the retina, and the presence of flavin specific transporters in both the inner and outer blood-retina barriers. In light of these exciting findings, it is even more imperative to shift our focus on how the retina regulates its flavin homeostasis and what happens when this is disrupted.

Riboflavin in development and cell fate

Riboflavin (7,8-dimethyl-10-ribitylisoalloxazine; vitamin B2) is a water-soluble vitamin, cofactor derivatives of which (FAD, FMN) act as electron acceptors in the oxidative metabolism of carbohydrate, amino acids and fatty acids and which in the reduced state can donate electrons to complex II of the electron transport chain. This means that riboflavin is essential for energy generation in the aerobic cell, through oxidative phosphorylation. The classic effects of riboflavin deficiency on growth and development have generally been explained in terms of these functions. However, research also suggests that riboflavin may have specific functions associated with cell fate determination, which would have implications for growth and development. In particular, riboflavin depletion interferes with the normal progression of the cell cycle, probably through effects on the expression of regulatory genes, exerted at both the transcriptional and proteomic level.

Supplementation with iron and riboflavin enhances dark adaptation response to vitamin A-fortified rice in iron-deficient, pregnant, nightblind Nepali women

BACKGROUND

Nightblindness affects 16-52% of pregnant women in areas of Nepal and in some cases persists after vitamin A treatment. Iron and riboflavin affect vitamin A utilization and photoreceptor function, respectively, and pilot data in the study population showed a high prevalence of iron and riboflavin deficiencies.

OBJECTIVE

The objective was to assess the effect of supplemental iron and riboflavin on pupillary threshold (PT) and plasma retinol in nightblind, pregnant Nepali women given vitamin A-fortified rice.

DESIGN

Nightblind pregnant women were randomly assigned to receive, 6 d/wk under supervision for 6 wk, a vitamin A-fortified rice curry dish providing 850 microg retinal activity equivalents/d with either a 30-mg Fe and 6-mg riboflavin (FeR + VA) capsule or a placebo control (VA only) capsule. Hemoglobin, erythrocyte riboflavin, and plasma ferritin and retinol were measured before and after the intervention. Dark adaptation was assessed by PT score.

RESULTS

Women who were iron deficient at baseline (n=38) had significantly greater improvement in PT score with iron and riboflavin supplementation than without (P=0.05). Iron and riboflavin supplements significantly reduced the prevalences of riboflavin deficiency (from 60% to 6%; P<0.0001), iron deficiency anemia (from 35% to 15%; P<0.007), and abnormal PT (from 87% to 30%; P<0.05) from baseline. Mean increases in erythrocyte riboflavin (P<0.0001) and plasma ferritin (P=0.01) were greater in the FeR + VA group than in the VA only group.

CONCLUSIONS

Iron deficiency may limit the efficacy of vitamin A to normalize dark adaptation in pregnant Nepali women. Further studies are needed to assess the effect of simultaneous delivery of iron and vitamin A for the treatment of nightblindness.

Mitochondrial function and toxicity: role of the B vitamin family on mitochondrial energy metabolism

The B vitamins are water-soluble vitamins required as coenzymes for enzymes essential for cell function. This review focuses on their essential role in maintaining mitochondrial function and on how mitochondria are compromised by a deficiency of any B vitamin. Thiamin (B1) is essential for the oxidative decarboxylation of the multienzyme branched-chain ketoacid dehydrogenase complexes of the citric acid cycle. Riboflavin (B2) is required for the flavoenzymes of the respiratory chain, while NADH is synthesized from niacin (B3) and is required to supply protons for oxidative phosphorylation. Pantothenic acid (B5) is required for coenzyme A formation and is also essential for alpha-ketoglutarate and pyruvate dehydrogenase complexes as well as fatty acid oxidation. Biotin (B7) is the coenzyme of decarboxylases required for gluconeogenesis and fatty acid oxidation. Pyridoxal (B6), folate and cobalamin (B12) properties are reviewed elsewhere in this issue. The experimental animal and clinical evidence that vitamin B therapy alleviates B deficiency symptoms and prevents mitochondrial toxicity is also reviewed. The effectiveness of B vitamins as antioxidants preventing oxidative stress toxicity is also reviewed.

A biochemical evaluation of the erythrocyte glutathione reductase (EC 1.6.4.2) test for riboflavin status. 1. Rate and specificity of response in acute deficiency

1. Acute riboflavin deficiency was produced in weanling rats by feeding a deficient diet and using tailcups to prevent refection. Animals were killed at weekly interval for 7 weeks, by the end of which they had become severely deficient, and mortality was high.

2. Growth of the deficient animals virtually ceased in the early stages of deficiency; food intake was severely and progressively depressed. Liver: body-weight increased markedly; packed cell volume fell at a late stage only. Pathological signs accumulated throughout the deficiency but were not closely related to the biochemical changes within the deficient group.

3. The activation coefficient (stimulated: basal activity; AC) of glutathione oxidoreductase (EC 1.6.4.2; glutathione reductase; GR) in erythrocytes rose to a mean value of 3–8 after 3 weeks, and subsequently remained almost constant: this change was not seen in pair-fed or ud lib.-fed controls. Both deficient and pair-fed animals exhibited a twofold reduction in FAD-stimulated erythrocyteGR activity at an early stage. In liver, both deficient and pair-fed groups showed a major progressive fall in FAD-stimulated GR activity, but only the deficient group showed an increase in AC, which occurred towards the later stages of the experiment. In skin, too, the deficient group showed an increase in AC during the terminal stages.

4. Hepatic, intestinal and brain succinate: (acceptor) oxidoreductase (EC 1.3.99,1; succinate dehydrogenase) activity fell relatively early during deficiency; in liver and intestine this was at least partly shared by the pair-fed group, and therefore attributable to inanition. Changes in hepatic NADH:(acceptor) oxidoreductase (EC I.6.99.3; NADH dehydrogenase) activity appeared to be entidy attributable to inanition.

5. An early reduction was observed in hepatic ATP: riboflavin 5-phosphotransferase (EC2.7.1.26; flavo- kinase) activity in the deficient group, values falling by nearly half within 1 week, and then remaining almost constant. Similar but smaller changes were seen in renal flavokinase activity. Hepatic ATP: FMN adenylyltrans- ferase (EC2.7.7.2; FAD pyrophosphorylase) was unchanged until the third week, at which point it rose sharply to a new plateau in the deficient group; in kidney it did not respond. These changes were not observed in pair-fed or ad-lib.-fed controls.

6. Hepatic flavin levels fell dramatically during the first 2 weeks of deficiency, FAD being conserved at the expense of FMN. Smaller changes were observed in kidney.

7. Of the processes which are affected by riboflavin deficiency, AC of erythrocyte GR (EGRAC) responds earlier, more dramatically and more specifically than most others, with the possible exception of hepatic flavin levels and flavokinase. Potentially, it is therefore a good index of over-all body riboflavin status, but in acute deficiency the rate of response of many variables is not related to the final extent of response; consequently the correlation between EGRAC and other riboflavin-sensitive processes is less satisfactory than it would be in an equilibrium situation.

Ultrastructural changes of renal tubules in the riboflavin deficient mouse

Ultrastructural changes of the tubular epithelium in the mouse kidney produced by dietary riboflavin deficiency were studied by electron microscopy and cytochemistry. In riboflavin deficient mouse kidney, the ultrastructural changes are localized to the pars recta of the proximal tubule. They comprise so called vacuolar degeneration on light microscopy, which consists of the formation of giant mitochondria and vacuoles. During the development of riboflavin deficiency, mitochondria decrease in number and enlarge in size through fusion. Sometimes they are larger than nuclei in size. The vacuoles observed in tubular epithelia are divided into two different groups according to their morphological characteristics and origins. One is derived from proliferated peroxisomes, and another from increased cytoplasmic bodies termed cytosomes and cytosegresomes. These increased vacuoles occupy almost all of cytoplasm. Cytochemical studies also reveal that these vacuoles are peroxisomes and lysosomes. These changes are reversible on supplementation with riboflavin.