Studies of Photoprotection Against Porphyrin Photosensitization Using Dithiothreitol and Glycerol

Abstract

Although protection against ionizing radiation by compounds containing sulfhydryl (SN) groups, such as cysteine, has been reported, these agents have been unsuccessful to date in protecting mammals against non-ionizing radiation (>200nm). This study describes successful photoprotection by dithiothreitol (DTT) and glycerol against porphyrin photosensitization having an action spectrum of 400nm. Test models used were red blood cells (RBC) obtained from patients with erythropoietic protoporphyria (EPP) and mice photo-sensitized by hematoporphyrin (HP). A mortality rate approaching a lethal dose in 50% of the animals in 1 day (LD50/24 hrs) was established in 100 white mice that had received an intraperitoneal injection of 100mg HP/kg body wt., and were then irradiated with 5 × 106 ergs/cm2 from a fluorescent light source emitting 320-450nm radiation. Another 100 mice were treated in an identical manner except that they received, in addition to HP, 80mg DTT/kg body weight in a 5.5% glycerol solution. This group showed 75% reduction in mortality (p < 0.03). No lethal effects were observed in animals treated with DTT and glycerol or HP in the above concentrations without 400nm irradiation. RBC obtained from patients with EPP and exposed to 107 ergs/mm2 of 400nm radiation showed 100% hemolysis after 180min. These cells, when irradiated under identical conditions except for the addition of DTT, manifested only 19% hemolysis during this period of time. Measurements of SH groups of RBC from patients with EPP showed a progressive decrease during photohemolysis. Comparison of the rate of photohemolysis of normal and glucose-6-phosphate dehydrogenase (G6PD) deficient RBC irradiated in the presence of protoporphyrin IX revealed that G6PD deficient RBC, which have an impaired ability to produce reduced glutathione (GSH), were more susceptible to porphyrin-induced photohemolysis. These studies demonstrate that DTT and glycerol offer photoprotection in an in vivo mammalian system against porphyrin photosensitization. It is suggested that the mechanism of the photoprotective action against 320-450nm radiation has many features similar to that of radioprotection by thiols and glycerol against ionizing radiation