Riboflavin (vitamin B-2) is chemically known as 7,8-dimethyl-10
(1'-D-ribityl) isoalloxazine. In tissues there is a broad distribution of
flavin but little is present as free riboflavin. The majority is found in
flavocoenzymes [mainly flavin adenine dinucleotide (FAD)] and lesser
amounts are in a mononucleotide [riboflavin-5'-phosphate (FMN)]. Levels of
flavin range from a few µg/g wet weight in skeletal muscle and intestine,
and up to 30 to 35 µg/g in liver and kidney. Flavocoenzymes are largely
non-covalently associated within diverse flavoproteins, but some exist as
8-a-linked FAD.
As the catalytically operating moiety within bound flavocoenzymes,
riboflavin participates in oxidation-reduction reactions in numerous
metabolic pathways and in energy production via the respiratory chain.
Flavoproteins participate in both one- and two-electron transfers. They
operate in pyridine nucleotide-dependent and independent and independent
dehydrogenations, reactions with sulfur-containing compounds,
hydroxylation, oxidative decarboxylations, deoxygenations, and reduction
of oxygen to hydrogen peroxide following abstraction of hydrogen from
substrates.
Deficiencies: Ariboflavinosis is characterized by
weakness, sore throat, hyperemia and edema of the pharyngeal and oral
mucous membranes, cheilosis, angular stomatitis, glossitis, seborrheic
dermatitis, corneal vascularization, and normochromic, normocytic anemia
associated with pure red cell hypoplasia of the bone marrow.
Clinical uses: Riboflavin is used therapeutically to
ameliorate ariboflavinosis resulting from diverse causes such as
inadequate dietary intake, decreased assimilation, rare genetic defects in
the formation of specific flavoproteins, hormonal disorders and after use
of certain drugs. Also, riboflavin supplements are provided during
treatment of neonatal jaundice with phototherapy. The side chain of the
vitamin is photochemically destroyed as it is involved in the
photosensitized oxidation of bilirubin to more polar excretable compounds.
Diet recommendations: Suggested amounts range from an
Adequate Intake of 0.3 mg/day for early infants to RDAs of 1.1 mg/day for
women and 1/3 mg/day for men. An additional 0.3 mg/day is recommended
during pregnancy and 0.4 mg/day for lactation.
Food sources: Small amounts of riboflavin, occurring
largely as digestible coenzymes, are present in most plant and animal
tissues. Especially good sources are milk, eggs, enriched cereals and
grains, ice cream, liver, some lean meats, and green vegetables such as
broccoli. As much as a third of North American intake is attributed to
milk and other dairy products.
Toxicity: The limited capacity to absorb orally
administered riboflavin precludes its potential for harm. Riboflavin
intake of many times the RDA is without demonstrable toxicity.
Nevertheless, the photosensitizing properties of riboflavin raise the
possibility of some theoretical, potential risks.
Recent research: A comprehensive assessment of the
human biokinetics for riboflavin absorption, transport, metabolism, and
excretion has just been completed. Ongoing studies of a more biochemical
nature are leading to detailed understanding of structures and mechanisms
of numerous flavoproteins from diverse organisms.
