VITAMIN B2

VITAMIN B2

A. History. Riboflavin or vitamin B2 was first isolated in 1879 from milk whey which is an essential dietary factor
for rats. Since it was first isolated from milk, vitamin B2 is also known as lactoflavin. Originally, it was also known
as ovoflavin (from eggs) and hepatoflavin (from liver). Its synthesis was done by Richard Kuhn and Paul
Karrer. It is popularly called as the “yellow enzyme” because of its colour.
B. Occurrence. In nature, it occurs almost exclusively as a constituent of one of the two flavin coenzymes, namely, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Milk, cheese, eggs, liver, kidney, heart and brewer’s yeast are excellent sources of this vitamin. Cow’s milk contains about 5 times as much riboflavin as human milk. Leafy vegetables are good sources. They are usually richer in riboflavin than they are in thiamine. Fruits and most root vegetables contain moderate quantities. Whole grains, cereals and milled flour contain low riboflavin content. The riboflavin contents in cereals, however, increase strikingly during germination. The ordinary cooking processes do not affect the riboflavin content of the food. Roasted or boiled meat retains about 75% of the vitamin. It is only very rarely that vitamin B2 is present in free or uncombined state as in retina and spleen. Fermentation residues from alcohol manufacture probably offer the richest large supplies.
C. Structure. Riboflavin (C17H20N4O6) belongs to a class of water-soluble pigments called lyochromes. A molecule of thiamine consists of a sugar alcohol, D-ribitol, attached to a chromogenic dimethyl isoalloxazine ring atposition number9.
D. Properties. Riboflavin is a bright orange-yellow crystalline powder. It is soluble in water and ethanol but insoluble in ether and chloroform. It is stable to heat and acids but is easily decomposed by alkalies and exposure to light. The aqueous solution exhibits yellow-green fluorescence. It stands ordinary cooking and canning. On exposure to light, the ribityl residue splits off, forming a compound lumiflavin in alkaline solution and lumichrome in acidic or neutral
solution.
E. Metabolism. Flavokinase Riboflavin is synthesized by all green plants, most bacteria, yeasts and moulds. Ashbya gossypii, an yeast, produces it in such large amounts that riboflavin crystals are formed in the culture medium. Animals have, so far, not been shown to synthesize riboflavin. In man, the ingested riboflavin is largely passed out as such or as its coenzyme, the FMN. Experiments with plant tissues have suggested that riboflavin and flavoproteins may play a significant role in phototropic curvature of various plant organs (Galston, 1950). Riboflavin is essential for growth and tissue respiration; it may have a role in light adaptation and is required for conversion of pyridoxine to pyridoxal phosphate. When riboflavin is phosphorylated in the presence of an enzyme, flavokinase, it gets converted to FMN which is essential in the biosynthesis of fats.

Riboflavin + ATP l FMN + ADP

FMN may undergo a further reaction with ATP, in the presence of an enzyme found in yeast and animal tissues, to produce FAD. It is a chief constituent of electron transport system (ETS). A decrease in the amount of FAD, therefore, would severely hamper the efficiency of ETS.

FMN + AMP l FAD + PP

The coenzymes undergo reversible oxidation-reduction in the presence of their enzymes and a suitable substrate. The flavoenzymes play a key role in cell metabolism. They function in accepting hydrogen atoms from reduced pyridine nucleotides. They have been shown to participate in the enzymic oxidation of glucose, fatty acids, amino acids and purines.
F. Deficiency. Riboflavin deficiency is usually caused by inadequate intake. Faulty absorption may contribute in patients with biliary atresia or hepatitis or in those receiving probenecid, phenothiazine or oral contraceptives. Phototherapy destroys riboflavin content. It is interesting to note that riboflavin deficiency without deficiency of other member of the B complex is rare. Persons deficient in vitamin B2 show chelosis (fissuring at the corners of the mouth and lips), glossitis (inflammation of the tongue), keratitis, conjunctivitis photophobia, lacrimation, corneal vascularization (bloodshot eyes) and seborrheic dermatitis. But these symptoms are not specific to ariboflavinosis since similar symptoms may also develop in the absence of nicotinic acid and iron. Cheilosis (= perle′che) begins with pallor at the angles of the mouth, following by thinning and maceration of the epithelium. Superficial fissures often covered by yellow crusts develop in the angles of the mouth and extend radially into the skin for distances upto 2 cm. In glossitis, the tongue is smooth, and loss of papillary structure occurs. A normocytic and normochronic anemia with bone marrow hyperplasia is common. However, patients suffering from pellagra and beriberi are usually also deficient in riboflavin content.
G. Human requirements. The minimum daily requirement of riboflavin varies from 0.6 to 1.7 mg for children and adults. During pregnancy and lactation, the women require up to 2.0 mg
daily.
H. Treatment : Ariboflavinosis may be prevented by a diet that contains adequate amounts of milk, eggs, leafy vegetables, and lean meats. Treatment consists in oral administration of 3-10 mg of riboflavin daily. If no response occurs within a few days, intramuscular injections of 2 mg of riboflavin in saline solution may be administered 3 times in a day.