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 is absorbed by mucosal cells via an active saturable transport system in all parts of the small intestine. Transport of flavin by blood plasma is known to involve both loose association with albumin and tight associations with some globulins (McCormick, 1990). A genetically controlled riboflavin binding protein is present in serum and eggs. There is a hereditary recessive disorder in chickens, renal riboflavinuria, in which the riboflavin-binding protein is absent (White, 1996). Eggs become riboflavin-deficient and embryos generally do not survive beyond the fourteenth day of incubation (Clagett, 1971). Also, if riboflavin-binding protein is in excess it can diminish riboflavin availability to the chicken embryo (Lee and White, 1996). 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. In addition to poultry, specific binding proteins have been found in serum from pregnant cows and rats, human fetal blood, and uterine secretions in the pig.
Hepatic cells from deficient animals have a relatively greater maximal absorption uptake of riboflavin (Rose et al., 1986). Hepatic cell riboflavin absorption occurs via facilitated diffusion. 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, transported to the liver, and there converted to FAD. Rivlin (1984) suggested there may be physiological mechanisms in pregnancy that facilitate transfer of riboflavin from maternal stores to the fetus in a manner that is fundamentally similar to that in the laying hen. Riboflavin is efficiently transferred to the fetus.
Animals do not appear to have the ability to store appreciable amounts of riboflavin, with the liver, kidneys 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.
