The majority of riboflavin found in food is in the form of FAD. Smaller amounts are present in the form of FMN and free riboflavin, which is an isoalloxazine ring bound to a ribitol side chain, with the absorption half-life of about 1.1 h.7 Flavins that are covalently bound do not appear to be available for absorption. However, FAD and FMN are predominantly in a non-covalently-bound form attached to an enzyme. FAD and FMN must be hydrolyzed to riboflavin before absorption can occur. Nonspecific phosphatases in the brush border membranes of enterocytes are responsible for this catalysis.8 Once in the enterocytes, free riboflavin undergoes ATP-dependent phosphorylation, which is catalyzed by cytosolic flavokinase to form FMN; it can now enter the plasma and small intestine. However, most of this is further converted to FAD by the FAD-dependent FAD synthetase.9 In the rare instance when riboflavin may be in excess, it is excreted in the urine as riboflavin, 7-hydroxymethylriboflavin (7-a-hydroxyriboflavin), or luminflavin.10
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