Mechanism of in vitro photohemiolysis in erythropoietic protoporphyria (EPP).

Four photosensitive patients with EPP showed markedly elevated RBC protoporphyrin levels. In vitro exposure of. a monolayer of their RBC in buffered saline to 3 x 10(6) ergs/mm2 of 4000 Å irradiation (= 45 minutes under a bank of fluorescent black lights) produced complete hemolysis within 24 hours; normal RBC showed negligible hemolysis after similar exposure. All of the cellular potassium was released from the patients' cells before the onset of significant hemolysis. Hemolysis was largely prevented by suspending the patients' cells in a 1:1 mixture of isotonic buffered NaCl and isotonic sucrose. These findings suggest that the initial photochemical lesion is of a "small hole" nature, creating channels through which small ions (but not sucrose) can diffuse. Since the effect of the intracellular colloid is offset by addition of sucrose to the external medium, the lesions appear to be less than 10 Å in diameter.