Riboflavin-photosensitized thiamine oxidation in aqueous solutions on exposure to ultraviolet and visible light

Abstract

It was shown the irradiation of aqueous solutions of thiamine and riboflavin with ultraviolet UVA (315–400 nm) and visible light (λ > 400 nm) leads to oxidation of thiamine to thiochrome and oxodihydrothiochrome. In absence of riboflavin in aerobic conditions thiamine oxidation to thiochrome was mainly due to reaction with singlet oxygen. Under anaerobic conditions, thiamine in the presence of riboflavin was photostable and UV and visible light irradiation of their aqueous solutions did not result in thiamine oxidation. Under aerobic conditions, thiochrome was oxidized to form oxodihydrothiohrome in reactions with singlet oxygen and riboflavin in triplet states. Under anaerobic conditions, oxidation of thiochrome to oxodihydrothiochrome occurred due to reaction with riboflavin in triplet state. Possible mechanisms of the damage of cellular eye structures due to inactivation of thiamine-dependent enzymes on exposure to intensive UVA and visible light in the presence of endogenous photosensitizer riboflavin are discussed.