Superoxide formation by protoporphyrin as seen by spin trapping

The cutaneous photosensitivity of individuals with erythropoietic protoporphyria has been associated with the abnormally large amounts of protoporphyrin, found in the red blood cells [ 11. Patients with erythropoietic protoporphyria are sensitive to radiation near 400 nm which corresponds with the absorption maxima of protoporphyrin [2]. The photochemical damage is thought to be the result of light absorption by the porphyrins in the skin or superficial dermal vasculature. There is still much unknown about the molecular mechanism that produces the damage. Because P-carotene ameliorates many of the symptoms, singlet oxygen has been proposed as the causative agent [2]. Moreover, a characteristic singlet oxygen product of cholesterol has been observed [3] when red blood cell ghosts are irradiated in the presence of protoporphyrin. The formation of malonaldehyde, an indicator of lipid peroxidation, with the photohemolysis of erythropoietic protoporphyric red cells has been observed [4]. However, it was observed [5,6] that only negligible amounts of malonaldehyde or the cholesterol derivative are produced during irradiation prior to leakage of hemoglobin from the red cells. Photooxidation of amino acids and photoaggregation with evidence of crosslinking of membrane polypeptides has been noted [5,7], suggesting a free radical mechanism. Thus, the question of whether a singlet oxygen or a free radical mechanism is involved in the photodamage is still unsettled. We have observed the light-induced generation of superoxide by protoporphyrin using the technique of spin trapping. The superoxide spin adduct of