Vitamin B2 (Riboflavin).

Riboflavin's coenzyme form FMN powers the enzyme that converts vitamin B6 into its active form, pyridoxal 5'-phosphate. Without enough riboflavin the body cannot fully activate B6, so steady riboflavin keeps B6 ready for its part in amino acid and neurotransmitter metabolism.

As its coenzyme FAD, riboflavin is the cofactor for MTHFR, the enzyme that produces the active circulating form of folate, 5-methyltetrahydrofolate. Steady riboflavin keeps that conversion running, so folate can carry out its part in one-carbon and methylation metabolism.

Flavoenzymes that depend on riboflavin support the normal absorption of iron and the release of stored iron from ferritin. With adequate riboflavin, the body makes fuller use of the iron it takes in from food and supplements.

Riboflavin, as FAD, is required by the enzyme that carries tryptophan along the route the body uses to make its own niacin and NAD. Adequate riboflavin supports that internal niacin production alongside what the diet provides.

The FAD built from riboflavin drives methionine synthase reductase, which restores the oxidised cobalamin cofactor. Riboflavin status therefore limits how much of the cobalamin present stays usable.

Glutathione reductase is a flavoprotein that uses FAD to return oxidised glutathione to its reduced form. Riboflavin supply sets how fast the glutathione pool is recycled.

Riboflavin has to be phosphorylated to FMN and then adenylylated to FAD, and both steps are magnesium-dependent ATP reactions. Without magnesium the vitamin stays in its inactive form.

Riboflavin kinase, the enzyme that makes FMN, is a zinc-dependent kinase. Zinc availability is part of converting dietary riboflavin into its coenzyme.

Complex I and complex II are flavoproteins that depend on FMN and FAD, and both hand their electrons to the coenzyme Q pool. Riboflavin loads the chain and CoQ10 carries the electrons onward.

Carnitine carries long-chain acyl groups into the mitochondrion and the acyl-CoA dehydrogenases that then shorten them are FAD enzymes. Riboflavin status limits the step immediately after transport.

Dihydrolipoamide dehydrogenase, the shared E3 subunit of the dehydrogenase complexes, is an FAD flavoprotein that re-oxidises the lipoamide arm. Riboflavin keeps the lipoate cycle turning.

Glutathione peroxidase is a selenoenzyme that consumes reduced glutathione, and the enzyme that regenerates that glutathione, glutathione reductase, is FAD dependent and so depends on riboflavin. Without flavin supply the selenium enzyme runs out of reduced substrate. This is one of the tightest cofactor links in antioxidant biochemistry.

Xanthine oxidase and aldehyde oxidase each hold a molybdenum centre and an FAD in the same protein, so both trace nutrients are structural requirements of a single enzyme. Neither cofactor substitutes for the other. That makes the pairing a requirement statement rather than an optimisation claim.

Converting pyridoxine or pyridoxamine phosphate to the active pyridoxal 5-phosphate is done by an oxidase that carries FMN, a riboflavin derived cofactor. Low riboflavin status therefore limits activation of vitamin B6 regardless of how much B6 is supplied. Supplying B6 already in the 5-phosphate form is how a formula sidesteps that step.

The body can make some of its niacin from tryptophan, and one of the committed steps in that route is carried out by an FAD dependent monooxygenase. When riboflavin is short, that conversion slows and more tryptophan leaves the pathway as other metabolites. It is a good illustration of how one B vitamin gates the use of another.

Oxidising choline to betaine aldehyde, the first step toward using choline as a methyl donor, is carried out by an FAD dependent dehydrogenase. Riboflavin therefore sits upstream of choline's methyl donor role. Choline's separate role in phospholipid synthesis does not depend on flavins.

Every round of mitochondrial fatty acid beta oxidation begins with an acyl-CoA dehydrogenase that uses FAD and passes electrons on through electron transfer flavoprotein, which is also flavin dependent. Long chain fatty acids from fish oil are handled by that machinery. Riboflavin supply is therefore part of what determines how well an ingested fat is oxidised rather than stored.

Alpha tocopherol is regenerated from its radical form with help from ascorbate and glutathione, and glutathione is returned to its reduced state by the FAD dependent glutathione reductase. Riboflavin thus supports the loop that keeps vitamin E working in the membrane. The measured endpoints here are redox markers.

N-acetylcysteine supplies cysteine for glutathione synthesis while riboflavin, as FAD, powers the reductase that keeps that glutathione in its reduced form. One provides the material and the other maintains its state. Together they act on glutathione status, which is a marker rather than an outcome.

Homocysteine can be remethylated either by the folate route, whose MTHFR step needs FAD and so depends on riboflavin, or by the betaine dependent route, which does not. Supplying both means neither route is the sole path. The endpoint here is a plasma homocysteine measurement, a marker.

MTHFR, the enzyme that makes the methyl folate used to regenerate methionine, is an FAD dependent flavoenzyme, which puts riboflavin upstream of the methionine that becomes S-adenosylmethionine. Supplemental SAM-e enters below that step. The relationship is pathway position, not an additive effect.

Thiamine and riboflavin both feed mitochondrial energy production, thiamine as the pyrophosphate cofactor of dehydrogenase complexes and riboflavin as FAD within the same complexes and in the respiratory chain. A dietary intake analysis reported associations between intake of these vitamins and blood pressure measures, and an association is not a cause. The mechanistic overlap is the sounder half of this pairing.

Pantothenate becomes coenzyme A, which carries the acyl groups that FAD dependent dehydrogenases then oxidise, so the two vitamins handle consecutive parts of fat and carbohydrate oxidation. Neither compensates for a shortage of the other. This is why B complex products carry the set rather than single members.

Several lactic acid bacteria and other commensals synthesise riboflavin, and some strains are selected specifically for that capacity, while riboflavin also acts as an extracellular electron shuttle for certain gut species. So the relationship runs in both directions and the size of the contribution to a person's intake is not established. Colonic synthesis is also absorbed less efficiently than dietary riboflavin from the small intestine.

Riboflavin absorbs visible light and generates reactive oxygen species when it does, which degrades ascorbate, folate and tryptophan in the same solution; this is the same chemistry behind light induced losses in milk. In a liquid product that puts riboflavin and vitamin C in direct conflict unless the container blocks light. In a dry capsule the effect is negligible.