metabolic pathways and in energy production (McCormick and Greene, 1994).
The redox reactions in which flavocoenzymes participate include flavoprotein-catalyzed dehydrogenations that are both pyridine nucleotide (niacin) dependent and independent, reactions with sulfur-containing compounds, hydroxylations, oxidative decarboxylations (involving thiamin as its pyrophosphate), dioxygenations, and reduction of oxygen to hydrogen peroxide (McCormick and Greene, 1994). There are obligatory roles of flavocoenzymes in the formation of some vitamins and their coenzymes. For example, the biosynthesis of two niacin-containing coenzymes from tryptophan occurs via FAD-dependent kynurenine hydroxylase, an FMN-dependent oxidase catalyzes the conversion of the 5'-phosphates of vitamin B6 to coenzymic pyridoxal 5'-phosphate, and an FAD-dependent dehydrogenase reduces 5,10-methylene-tetrahydrofolate to the 5'-methyl product that interfaces with the B12-dependent formation of methionine from homocysteine and thus with sulfur amino acid metabolism.
Most dietary riboflavin is consumed as a complex of food protein with FMN and FAD (Merrill et al., 1981; Nichoalds, 1981). In the stomach, gastric acidification releases most of the coenzyme forms of riboflavin (FAD and FMN) from the protein. The noncovalently bound coenzymes are then hydrolyzed to riboflavin by nonspecific pyrophosphatases and phosphatases in the upper gut (McCormick, 1994; Merrill et al., 1981). Primary absorption of riboflavin occurs in the proximal small intestine via a rapid, saturable transport system (McCormick, 1994; Merrill et al., 1981). The rate of absorption is proportional to intake, and it increases when riboflavin is ingested along with other foods (Jusko and Levy, 1967, 1975) and in the presence of bile salts (Jusko and Levy, 1975; Mayersohn et al., 1969). A small amount of riboflavin circulates via the enterohepatic system (McCormick, 1994).
At low intake levels most absorption of riboflavin is via an active or facilitated transport system. Although older studies in animals (Daniel et al., 1983; Meinen et al., 1977; Rivier, 1973) suggested