Photoprotection in erythropoietic protoporphyria: mechanism of photoprotection by beta carotene.

Erythropoietic protoporphyria is a genetic disease caused by the accumulation of protoporphyrin IX. This molecule absorbs 400-nm light and its presence is at times associated with severe cutaneous photosensitivity. The only effective treatment for this disease is oral administration of beta carotene. Several possible mechanisms of photoprotection by beta carotene were investigated using the photohemolysis of red blood cells as an in vitro model. Additional studies were done in an in vivo model which involves lethal hematoporphyrin photosensitization of white mice. The photoprotective effects of beta carotene were compared with those of alpha tocopherol, an agent which possesses some but not all the properties that have been implicated in explaining the known effectiveness of beta carotene. In the photohemolysis model, both compounds demonstrated partial protection. In hematoporphyrin-photosensitized mice, tocopherol showed some protection at high doses, while beta carotene showed greater protection at lower concentrations. Although these results suggest that photoprotective was due to free radical scavenging or singlet oxygen quenching, properties common to both agents, they do not rule out the possible role of 400-nm light absorption, a property of beta carotene alone.