Riboflavin-5-Phosphate (Active B2).

MTHFR carries an FAD cofactor and makes the 5-methyl form of folate. Riboflavin status shapes how much methylfolate the body can generate for itself.

Methionine synthase reductase uses FAD to restore the oxidised cobalt centre of cobalamin. Riboflavin keeps the B12 enzyme catalytically alive.

Pyridoxine 5-phosphate oxidase needs FMN to produce pyridoxal-5-phosphate. Riboflavin sits directly upstream of active B6.

Kynurenine monooxygenase is an FAD enzyme on the route from tryptophan to niacin. Thin riboflavin narrows endogenous niacin production.

Flavins are required for reductive release of iron from ferritin and for normal iron handling in the gut. Riboflavin status changes how well an iron dose is used.

Glutathione reductase is an FAD enzyme that returns oxidised glutathione to its active thiol form. Riboflavin is what keeps that recycling loop running.

Xanthine oxidoreductase and aldehyde oxidase each carry both FAD and the molybdenum cofactor in one protein. Neither metal nor flavin works there without the other.

Acyl-CoA dehydrogenases and electron transfer flavoprotein are all FAD enzymes on the fatty acid oxidation route that carnitine feeds. Carnitine delivers the substrate and riboflavin runs the steps.

Electron transfer flavoprotein dehydrogenase hands electrons from FAD directly to the coenzyme Q pool. The two work on either side of one junction in the respiratory chain.

Pyruvate and alpha-ketoglutarate dehydrogenase complexes use thiamine pyrophosphate at one subunit and FAD at another. Both vitamins have to be present for the complex to turn over.

Riboflavin 5-phosphate feeds the FAD pool that glutathione reductase needs to regenerate reduced glutathione. Selenium sits one step away, as the element built into glutathione peroxidase, which spends that same reduced glutathione. The two nutrients therefore occupy opposite ends of one recycling loop rather than acting on each other directly.

Methylenetetrahydrofolate reductase carries FAD as its prosthetic group, and FAD is made from riboflavin. Low riboflavin status lowers the activity of that enzyme, which is the step that produces the circulating methyl folate form. Supplying the active B2 form supports the normal working of a folate-handling enzyme, which is a mechanism statement rather than a measured outcome.

Homocysteine is handled by two routes: the folate and B12 route, whose flavin-dependent enzyme depends on riboflavin, and the betaine route. Trimethylglycine supplies the second. Riboflavin status has been described as a modifier of the first route, so the pair supports normal homocysteine turnover from two directions.

The kynurenine route that converts tryptophan toward niacin runs through kynurenine 3-monooxygenase, an FAD enzyme, and kynureninase, which is B6-dependent. Riboflavin 5-phosphate keeps the flavin side of that route supplied. This is textbook pathway wiring, not a claim about how much niacin any person makes.

Choline dehydrogenase, the mitochondrial enzyme that converts choline to betaine, is a flavoprotein. Its cofactor comes from the riboflavin pool. Pairing the two supports the normal conversion step that feeds betaine into one-carbon metabolism.

Dihydrolipoamide dehydrogenase, the shared E3 subunit of the pyruvate and alpha-ketoglutarate dehydrogenase complexes, is an FAD enzyme that re-oxidises the lipoamide arm. Riboflavin supplies that FAD. The relationship is one of enzyme and cofactor rather than an additive effect on any endpoint.

Reduced glutathione, regenerated by the FAD enzyme glutathione reductase, is one of the systems that returns oxidised ascorbate to its active form. Riboflavin sits upstream of that regeneration step. The link is a recycling chain measured in cells and biochemical systems, not a demonstrated clinical benefit of taking the two together.

Tocopherol radicals are returned to tocopherol partly by ascorbate, which is itself recycled by glutathione, which depends on a flavin enzyme. Riboflavin therefore sits three steps back in the same chain. The further back a nutrient sits in a chain, the weaker the case for a combined effect, and that is where this one sits.

Pyridoxine 5-phosphate oxidase, which converts pyridoxine and pyridoxamine phosphates to the active pyridoxal 5-phosphate, is an FMN enzyme. Riboflavin 5-phosphate is that FMN. Supplying P5P directly bypasses the step, so the two forms interact at the activation point rather than duplicating each other.

Every acyl-CoA dehydrogenase in mitochondrial fatty acid oxidation is a flavoprotein, and the electrons they pull off pass to electron transfer flavoprotein, another FAD carrier. Medium-chain fats enter that pathway without needing carnitine transport. Riboflavin supports the flavin side of the same route.

Pantothenic acid becomes coenzyme A and riboflavin becomes FAD, and the two cofactors work in the same mitochondrial reactions of fatty acid and carbohydrate oxidation. Neither substitutes for the other. The pairing supports normal energy-yielding metabolism at the cofactor level.

Riboflavin is a photosensitiser: in solution and under light it generates reactive species that degrade folates, which is why liquid multivitamins carrying both are protected from light. The interaction is one of formulation stability, not of absorption or physiology. In a dry tablet or capsule it is far less of an issue.