AUTHOR Dr. Carlos Orozco (BSc, MSc, ND, MD, PhD, FPAMS).
Riboflavin is also known as vitamin B2. Riboflavin is the precursor for the coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).
The enzymes that require FMN or FAD as cofactors are termed flavoproteins. Several flavoproteins also contain metal ions and are termed metalloflavoproteins. Both classes of enzymes are involved in a wide range of redox reactions, e.g. succinate dehydrogenase and xanthine oxidase. During the course of the enzymatic reactions involving the flavoproteins the reduced forms of FMN and FAD are formed, FMNH2 and FADH2, respectively.
Biological Function of Vitamin B2:
Riboflavin (vitamin B2) is manufactured in the body by the intestinal flora and is easily absorbed, although very small quantities are stored, so there is a constant need for this vitamin. It activates vitamin B6 and folate, it acts as a coenzyme in the respiratory enzymatic system, as mentioned above, it is an important constituent of flavoproteins i.e. its role in the Krebs cycle from Fumarate to Oxaloacetate. It also plays a major role in the growth and development of the fetus as well as in the maintenance of the mucosal and epithelial tissues.
Oxidation-reduction (redox) reactions
Living organisms derive most of their energy from oxidation-reduction (redox) reactions, which are processes involving the transfer of electrons. Flavin coenzymes participate in
redox reactions in numerous metabolic pathways (3). Flavins are critical for the metabolism of carbohydrates, fats, and proteins. FAD is part of the electron transport (respiratory) chain, which is central to energy production. In conjunction with cytochrome P-450, flavins also participate in the metabolism of drugs and toxins (4).
Glutathione reductase is an FAD-dependent enzyme that participates in the redox cycle of glutathione. The glutathione redox cycle plays a major role in protecting organisms from reactive oxygen species, such as hydroperoxides.
Glutathione reductase requires FAD to regenerate two molecules of reduced glutathione from oxidized glutathione. Riboflavin deficiency has been associated with increased oxidative stress (4). Measurement of glutathione reductase activity in red blood cells is commonly used to assess riboflavin nutritional status (5).
Xanthine oxidase, another FAD-dependent enzyme, catalyzes the oxidation of hypoxanthine and xanthine to uric acid. Uric acid is one of the most effective water-soluble antioxidants in the blood. Riboflavin deficiency can result in decreased xanthine oxidase activity, reducing blood uric acid levels (6).
Glutathione peroxidase, a selenium-containing enzyme, requires two molecules of reduced glutathione to break down hydroperoxides (seediagram).
- Tanphaichitr V. Thiamin. In: Shils M, ed. Nutrition in Health and Disease. 9th ed. Baltimore: Williams & Wilkins; 1999:381-389.
- Rindi G. Thiamin. In: Ziegler EE, Filer LJ, eds. Present Knowledge in Nutrition. 7th ed. WashingtonD.C.: ILSI Press; 1996:160-166.
- Brody T. Nutritional Biochemistry. 2nd ed. San Diego: Academic Press; 1999.
- Bender DA. Optimum nutrition: thiamin, biotin and pantothenate. ProcNutr Soc. 1999;58(2):427-433. (PubMed).
- Todd K, Butterworth RF. Mechanisms of selective neuronal cell death due to thiamine deficiency. Ann N Y Acad Sci. 1999;893:404-411. (PubMed).
- Krishna S, Taylor AM, Supanaranond W, et al. Thiamine deficiency and malaria in adults from southeast Asia. Lancet. 1999;353(9152):546-549. (PubMed)