Absorption, Tissue Uptake, and Coenzyme Synthesis

Apeu t from milk emd e s, which contain relatively large amounts of free riboflavin bound to specific binding proteins, most of the vitamin in foods is as flavin coenzymes bound to enzymes, with about 60% to 90% eis FAD. 

FAD emd riboflavin phosphate in foods are hydrolyzed in the intestined lumen by nucleotide diphosphateise and a variety of nonspecific phosphateises to yield free riboflavin, which is absorbed in the upper small intestines by a sodium-dependent saturable mechtuiism the peak plasma concentration is related to the dose only up to about 15 to 20 mg (40 to 50 imol). Thereafter, 

Intestintd bacteria synthesize riboflavin, tind fecal losses of the vittimin may be five- to six-fold higher them intake. It is possible that bacteritd synthesis mtikes a significant contribution to riboflavin intake, because there is carrier-mediated uptake of riboflavin into colonocytes in culture. The activity of the Ccirrier is incretised in riboflavin deficiency emd decreased when the cells are cultured in the presence of high concentrations of riboflavin. The stime carrier mechtmism seems to be involved in tissue uptake of riboflavin (Said et aL, 2000). 

Much of the absorbed riboflavin is phosphorylated in the intestincd mucosa by flavokinase and enters the bloodstream as riboflavin phosphate this metabolic trapping is essentitd for concentrative uptake of riboflavin into en-terocytes (Gastaldi et al., 2000). Parenterally administered free riboflavin is tdso Itirgely phosphorylated in the intestinal mucosa. It is not cleeir whether this is the result of enterohepatic recycling of the viteimin or simply uptake of free riboflavin into the intestintd mucosa from the bloodstream. 

About 7% of dietary riboflavin is covalently bound to proteins (mtdnly as riboflavin-8-a-histidine or riboflavin-8-a-cysteine). The riboflavin-tunino acid complexes releeised by proteolysis are not biologictdly avtdlable tdthough they cu e absorbed from the geistrointestintd tract, they tire excreted in the urine (Chia et al., 1978). 

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