Riboflavin functions metabolically as the essential component of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These two coenzymes act as intermediaries in transfers of electrons in biologic oxidation-reduction reactions. More than 100 enzymes are known to bind FAD or FMN. These enzymes, called flavoproteins or flavoenzymes, are involved in oxidation and reduction. Flavoproteins are essential for the metabolism of carbohydrates, amino acid and lipids. Some are also essential for the activation of the vitamins pyridoxine and folate to their respective coenzyme forms.
In the channel catfish (Ictalurus punctatus) and rainbow trout (Oncorhynchus mykiss), intestinal absorption of riboflavin is by a carrier-mediated process (Casirola et al., 1995; Casirola and Ferraris, 1997). Riboflavin is transported in the plasma as both free riboflavin and FMN, both of which are bound in appreciable amounts to plasma proteins.
Dietary supplementation of riboflavin beyond that required for optimal growth was found not to enhance the immune system of chinook salmon (Oncorhynchus tshawytscha) (Leith et al., 1989).