Riboflavin covalently bound to protein is released by proteolytic digestion. Phosphorylated forms of riboflavin (FAD, FMN) are hydrolyzed by phosphatases in the upper gastrointestinal tract to free the vitamin for absorption. Free riboflavin is absorbed via an active, saturable transport system in all segments of the small intestine. Hepatic cells from deficient animals have an increased maximal absorption rate of riboflavin (Rose et al., 1986). Hepatic cells also absorb riboflavin via facilitated diffusion.
In mucosal cells, riboflavin is phosphorylated to FMN by the enzyme flavokinase (Cooperman and Lopez, 1991). The FMN then enters the portal system, where it is bound to plasma albumin and is then transported to the liver, where it is converted to FAD. Riboflavin-binding proteins have been reported to be present in the serum from pregnant cows.
Animals do not appear to have the ability to store appreciable amounts of riboflavin; the liver, kidney and heart have the greatest concentrations. 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.
