Riboflavinand and function of ribofalvin
The second member of the B complex-riboflavinwas recognized in the 1920s when it became evident that some growth-promoting properties of vitamin B were retained after heat had destroyed the antiberiberi activity. In 1932 the vitamin was identified as part of an enzyme and was synthesized in 1935.
Riboflavin functions as a part ofa group of enzymes, called jlavoproteins, which are involved in the metabolism of carbohydrates, fats, and proteins. The metabolically active forms of riboflavin include rib oflavi phosphate, also known as riboflavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The ring structure of the riboflavin part of the coenzymes can alternate behveen oxidize.(FMN, FAD) and reduced forms (FMNH2, FADH2) and is responsible for their role as hydrogen carriers in a variety of metabolic reactions, too numerous to mention. In general, flavoprotein dehydrogenases initiate the transfer of hydrogen from the oxidation of specific substrates to oxygen in the process of cellular respira tion that results in the formation of ATP. For example, FAD-dependent oxidations are involved in the decarboxylation of pyruvate and other alpha-ketoacids (reoxidation oflipoic acid), conversion of succina te to fumara te in the CAC, and B-oxidation of fatty acids. FMN is found in the flavopro·ein that transfers hydrogen from reduced niacin-coenzyme, NADH, to the next acceptor in the mitochondrial ectron transport chain. Examples of riboflavin enzymes not involved in energy metabolism include xanthine oxidase, L-amino acid oxidase, pyridoxine phosphate oxidase, and glutathione reductase. In vitro stimulation by FAD of the erythrocyte glutathione reductase activity has become a widely used measure of riboflavin status.
Riboflavin is essential for normal growth and tissue intenance. Deficiency of riboflavin damages some types of tissues more than others. Fissures on the lips and the corners of the mouth (cheilosis) and scaly, sometimes greasy dermatitis around the nose are characteristic of riboflavin deficiency in humans. Anemia has been ed in some studies, 51 and a reduction in erythrocyte surival time has been reported recently.
Riboflavin also plays an important role in the health the eye. Ocular symptoms appear on a low riboflavin and may precede all other manifestations. Eye strain and fatigue, itching and burning, sensitivity to light, and tal headaches are the most frequent complaints. Cataracts have been observed in rats, mice, chickens, and nkeys after prolonged deficiency of riboflavin. ,riboflavin deficiency has also been shown to lead to adrecortex dysfunction in humans. In humans, riboflavin deficiency is apt to occur along with a deficiency of other of the B complex. In experimental animals maternal riboflavin deficiency causes congenital malformations in the offspring
