THE EFFECT OF ALA AND RADIATION ON PORPHYRIN HEME BIOSYNTHESIS IN ENDOTHELIAL-CELLS/

To study porphyrin biosynthesis in human microvascular endothelial cel ls, HMEC-1 cells, a transformed human microvascular endothelial cell l ine, were incubated with 5-aminolevulinic acid (ALA), the precursor of endogenous porphyrins, and porphyrin accumulation was measured spectr ofluorometrically. The HMEC-1 cells accumulated porphyrin in a concent ration-related and a time-dependent fashion. Protoporphyrin was the pr edominant porphyrin accumulated in the cells, The effect of light on p rotoporphyrin accumulation was evaluated by exposing the ALA-loaded HM EC-1 cells to ultraviolet-A (UVA) and blue light, followed by another incubation with ALA for 2-24 h. Enhancement of protoporphyrin accumula tion in irradiated HMEC-1 cells was observed 2-24 h after irradiation, which was associated with a decrease in ferrochelatase protein and ac tivity. Porphyrin accumulation from ALA after irradiation was signific antly decreased when catalase (750-3000 U/mL, 29.3-44.3% suppression) or superoxide dismutase (270 U/mL, 36.4% suppression) was present duri ng irradiation. These data demonstrate that HMEC-1 cells were capable of porphyrin biosynthesis, and that exposure of protoporphyrin-contain ing HMEC-1 cells to UVA and blue light, which includes the Soret band spectrum, decreased the ferrochelatase activity and its protein. These changes were mediated, at least in part, by reactive oxygen species.