Depletion of histidine and tryptophan serum levels in porphyria cutanea tarda and congenital erythropoietic porphyria patients after irradiation with visible light.

We have studied the photosensitized oxidation, via singlet oxygen production, of histidine (His) and tryptophan (Trp) in the serum of porphyria patients and in the serum of healthy volunteers before or after addition of hematoporphyrin. It was observed that free plasma His and Trp are good probes of singlet oxygen production in the blood under visible light irradiation. However, Trp, which is mostly bound to serum albumin, is much less susceptible to photooxidation than His, which remains free in the plasma. These results must not be ignored in systemic effects of porphyrias and in photochemotherapy with hematoporphyrin derivatives.