Interactions with drugs and other nutrients
The phenothiazines such as chlorpromazine, used in the treatment of schizophrenia, and the tricyclic anti-depressant drugs such as imipramine, are structural analogues of riboflavin, and inhibit flavokinase. In experimental animals, administration of these drugs at doses equivalent to those used clinically results in an increase in the EGR activation coefficient and increased urinary excretion of riboflavin, with reduced tissue concentrations of riboflavin phosphate and FAD, despite feeding diets providing more riboflavin than is needed to meet requirements. Although there is no evidence that patients treated with these drugs for a prolonged period develop clinical signs of ribo-flavin deficiency, long-term use of chlorpromazine is associated with a reduction in metabolic rate.
Riboflavin deficiency is sometimes associated with hypochromic anemia as a result of impaired iron absorption. A greater proportion of a test dose of iron is retained in the intestinal mucosal cells bound to ferritin, and hence lost in the feces, rather than being absorbed, because the mobilization of iron bound to ferritin in mucosal cells for transfer to transferrin requires oxidation by a flavin-dependent enzyme.
Riboflavin depletion decreases the oxidation of dietary vitamin B6 to pyridoxal; pyridoxine oxidase is a flavoprotein and is very sensitive to riboflavin depletion. It is not clear to what extent there is functional vitamin B6 deficiency in riboflavin deficiency. This is partly because vitamin B6 nutritional status is gener-ally assessed by the metabolism of a test dose of tryp-tophan, and kynurenine hydroxylase in the tryptophan oxidative pathway is a flavoprotein; riboflavin deficiency can therefore disturb tryptophan metabolism quite separately from its effects on vitamin B6 nutritional status.
The disturbance of tryptophan metabolism in riboflavin deficiency, due to impairment of kynurenine hydroxylase, can also result in reduced synthesis of NAD from tryptophan, and may therefore be a factor in the etiology of pellagra.