Riboflavin, designated vitamin B2, was first identified by Blyth in 1879 and originally called lactochrome because of its intense yellow colour. However, Blyth did not realise the significance of the vitamin to human nutrition. Riboflavin was originally designated vitamin G by American researchers, following the discovery of thiamine, which they designated vitamin F. Riboflavin is part of the B complex of vitamins and consists of a substituted isoalloxazine (flavin) ring bonded to the sugar alcohol, ribotol (an open chain version of the pentose sugar ribose). Being water soluble riboflavin is not stored in humans, and is excreted unmetabolised in the urine, which it can colour a bright yellow. Tissues can become saturated at low intakes that would be available in most supplements (>1 mg). Riboflavin deficiency is rare, but when manifest causes cheilosis, angular stomatitis and superficial interstitial keratosis of the cornea. Riboflavin is absorbed to enterocytes in its free form as may occur in cereals, milk and eggs. However, the riboflavin present in animal tissue bound to phosphates must first be hydrolysed before absorption.
Once absorbed riboflavin is transported to the liver where it is first phosphorylated to flavin mononucleotide (FMN; flavin phosphate). Flavin mononucleotide can then be further converted to flavin adenine dinucleotide (FAD) by addition of adenosine monophosphate. Both FNM and FAD are required as prosthetic groups in a number of oxidative enzymes, termed flavoproteins. Two successive electrons can be transferred to the isoalloxazine ring of FMN and FAD yielding FMNH2 and FADH2, respectively. FMN and FAD have similar coenzyme properties but both are stronger oxidising agents when compared to NAD+. However unlike NAD+, the flavoproteins do not transport electrons from one part of the cell to another because they remain bound to their enzymes. Both FAD and FMN have wide ranging roles in metabolism. However their most well know role is in energy metabolism pathways where FMN is required for the activity of NADH dehydrogenase (ubiquinone) in the electron transport chain, and FAD is required for the enzymes succinate dehydrogenase (citric acid cycle) and fatty acyl CoA dehydrogenase (β-oxidation of fatty acid).