Riboflavin is d-Ribitol in which the hydroxy group at position 5 is substituted by a 7,8-dimethyl-2,4-dioxo-3,4-dihydrobenzo[g]pteridin-10(2H)-yl moiety. It is a nutritional factor found in milk, eggs, malted barley, liver, kidney, heart, and leafy vegetables, but the richest natural source is yeast. The free form occurs only in the retina of the eye, in whey, and in urine; its principal forms in tissues and cells are as flavin mononucleotide and flavin-adenine dinucleotide. It has a role as a photosensitizing agent, a food colouring, an Escherichia coli metabolite, a mouse metabolite, a cofactor, a plant metabolite, a human urinary metabolite, an anti-inflammatory agent, a fundamental metabolite and an antioxidant. It is a vitamin B2 and a flavin. It is a conjugate acid of a riboflavin(1-).