Riboflavin (vitamin B2) is not synthesized by higher animals. Therefore, it is an absolute dietary requirement for the synthesis of certain essential coenzymes that are needed for intermediary metabolism in nearly all living cells. Riboflavin must be transported from the food sources within the gastrointestinal tract, across the gut wall into the circulatory system, and thence into the cells of each organ. This transport process occurs against a concentration gradient, in order to ensure the efficient retrieval of the very small amounts that occur in many foods, and from the low concentrations in plasma to higher concentrations inside living cells.
Gut riboflavin transport systems have been studied by partly isolated segments of the small intestine within an anesthetized animal; by an isolated everted gut segment, or by 'vesicles', prepared from the 'brush border.'
Studies with these model systems have shown that the transport of riboflavin at low (e.g., micromolar) concentrations is temperature- and energy-dependent (it is inhibited by inhibitors of ATP production from energy substrates), it becomes saturated as the concentration of riboflavin increases, and it is sodium ion dependent. These characteristics are shared with many other types of small molecules that are actively transported across the gut wall. More specifically for riboflavin, the active transport mechanism involves phosphorylation (to riboflavin phosphate, also known as flavin mononucleotide, or FMN) followed by dephosphorylation, both occurring within the intestinal cells (Figure 1). This latter process is not shared by several other B vitamins, but it is one of a number of common strategies which the gut may use to entrap essential nutrients, and then relocate them, in a controlled manner and direction. A similar strategy is employed at other sites in the body, to ensure entrapment of circulating riboflavin by cells whose nascent flavin-dependent enzymes need a supply of the vitamin from beyond their borders.
Although the active transport of riboflavin across the gut wall and across other cell membrane barriers within the animal is a saturable process, if large pharmacological amounts are present then the slower and less efficient but nonsaturable process of passive absorption predominates and contributes significantly to the total mass transfer. The active transport process is increased in ribo-flavin deficiency and decreased if the riboflavin content of the tissues is high. The transport pathway involves calcium and calmodulin but not sodium. Specific riboflavin receptors have recently been identified, as has a role for microtubules in transport.
Although some of the available riboflavin in natural foods may be present as the free vitamin, ready for intestinal transport, a larger fraction is present in the form of phosphorylated coenzymes: FMN and flavin adenine dinucleotide (FAD), and there may also be very small amounts of a gluco-side of the vitamin. These forms are all efficiently converted to free vitamin by enzymes secreted into the gut lumen, and they are therefore highly available for absorption. There are also small amounts of covalently bound forms of riboflavin, present in enzymes such as succinate dehydrogenase (succi-nate: ubiquinone oxidoreductase EC 18.104.22.168), which cannot be released by the hydrolytic enzymes in the gut and are therefore unavailable for absorption. Also unavailable (or very poorly available) in man is the riboflavin synthesized by the gut flora of the large bowel. Certain animal species such as rodents can utilize this riboflavin source by coprophagy.
A wide variety of artificial analogs of riboflavin have been prepared in order to explore the structural versus functional essentials of the molecule. Some of these analogs have riboflavin-like activity; others are inactive, while a number are antagonists, and can cause functional deficiency. These structural changes can affect absorption or the conversion of riboflavin to its coenzyme forms within the body. Certain drugs that are used for purposes unrelated to ribo-flavin function, such as the phenothiazines used as antipsychotic drugs, may have sufficient structural similarity to riboflavin to act as antagonists in some situations.
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