Riboflavin covalently bound to protein is released by proteolytic digestion. Phosphorylated forms (FAD, FMN) of riboflavin are hydrolyzed by phosphatases in the upper gastrointestinal tract to free the vitamin for absorption. Free riboflavin enters mucosal cells of the small intestine after apparently being absorbed in all parts of the small intestine. Cells from deficient animals have a greater maximal absorption uptake of riboflavin (Rose et al., 1986). At low concentrations, riboflavin absorption is an active carrier-mediated process. At high concentrations, however, riboflavin is absorbed by passive diffusion, proportional to concentration.
In mucosal cells, riboflavin is phosphorylated to FMN by the enzyme flavokinase (Cooperman and Lopez, 1984). The FMN then enters the portal system, where it is bound to plasma albumin, and is transported to the liver, where it is converted to FAD. Riboflavin-binding proteins have also been reported to be present in the serum and uterine secretions in the pig. Presumably the lack of the specific vitamin transport protein prevents adequate transfer of dietary riboflavin to the developing fetus, and riboflavin losses occur via maternal urine.
Animals do not appear to have the ability to store appreciable amounts of riboflavin, with liver, kidney and heart having the greatest concentrations. The liver, the major site of storage, contains about one-third of the total body riboflavin. Intakes of riboflavin above current needs are rapidly excreted in urine, primarily as free riboflavin. Minor quantities of absorbed riboflavin are excreted in feces, bile and sweat.
