Riboflavin photodegradation and photosensitizing effects are highly dependent on oxygen and ascorbate concentrations

Riboflavin (RF) is a normal component of the eye lens which triggers a stro ng photosensitizing activity when exposed to light. Upon irradiation with s hort wavelength radiations below 400 nm, RF-photosensitized damage may occu r. However, vitamin C is present at high concentrations in the normal lens and plays an important role in inhibiting these photosensitization processe s. An in vitro simple model was used with the objective of understanding be tter the relationships between vitamin C and oxygen concentrations on the m echanisms of RF-mediated photodegradation of tryptophan (Trp), a target par ticularly sensitive to photo-oxidation, Under nitrogen, the RF decompositio n reached its maximal value, and vitamin C and Trp photo-oxidation was negl igible. When increasing oxygen pressure, RF photodegradation dropped and vi tamin C photo-oxidation strongly increased and was maximal at 100% O-2, RF- induced photodegradation of Trp first increased with oxygen concentration, up to 40 mum O-2, and then decreased. RF and Trp degradation were significa ntly protected by vitamin C so that no more than 20% of the substrates conc entration rc ere oxidized in the presence of vitamin C higher than 0.8 mM, From our results we conclude that in the specific conditions of the normal lens, the high vitamin C concentration (2 mM) is compatible with the WA rad iation hazard, despite the presence of RF, However, if lenticular vitamin C decreases below 0.8 mM photodegradation of RF may occur and Trp may theref ore be photo-oxidized by a Type-I mechanism.