ABSTRACT
Supplementation of human mononuclear cells with 3 and 6 mM of lipoic acid produces an inhibition of the antioxidant adaptive response triggered by treatment with UV-B light (0.30 W/m2 for 15 min). Supplementation with 1.5 mM of lipoic acid gives no conclusive results. The adaptive response is characterized by an increase in the activities of superoxide dismutase, catalase, glutathione peroxidase and DT-diaphorase. Catalase (5.5 +/- 0.6 pmol/mg prot) increases its activity by up to 22 +/- 3 pmol/mg prot, after irradiation with UV-B. Supplementation with 3 and 6 mM of lipoic acid completely inhibits the adaptive response. The activities of the membrane-bound mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase do not increase after UV-B exposure. Moreover, their activities are found to decrease and the addition of lipoic acid does not prevent this effect. The inhibition of the antioxidant response by lipoic acid in human cells appears as indirect evidence of the existence of oxidative stress in the development of this response. As lipoic acid behaves as an effective antioxidant, it seems that its action decreases the intracellular oxidative signals necessary to develop the adaptive response in human mononuclear cells.
Subject(s)
Catalase/blood , Glutathione Peroxidase/blood , Leukocytes, Mononuclear/drug effects , Leukocytes, Mononuclear/radiation effects , Superoxide Dismutase/blood , Thioctic Acid/pharmacology , Ultraviolet Rays , Adult , Catalase/radiation effects , Cell Survival/drug effects , Cell Survival/radiation effects , Dihydrolipoamide Dehydrogenase/blood , Dihydrolipoamide Dehydrogenase/radiation effects , Female , Glutathione Peroxidase/radiation effects , Humans , In Vitro Techniques , Leukocytes, Mononuclear/enzymology , Male , Superoxide Dismutase/radiation effectsABSTRACT
One-electron redox radicals generated by radiation--chemical methods have been reacted with the oxidized (E) form of pig heart lipoamide dehydrogenase. The reducing radicals eaq- and CO2-. and O2-. do not measurably inactivate the enzyme, whereas the oxidizing species .OH and Br2-. do. The CO2-. anion forms the semiquinone radical .EH on the millisecond time scale, whereas at longer times only EH2 is observed. Evidence suggests that Br2-. oxidizes adjacent sulfhydryl groups to form a disulfide in a manner similar to the reaction of Cu2+ ions. With .OH, destruction of the flavin adenine dinucleotide (FAD) moiety is responsible for at least 50% of the enzyme inactivation. This destruction appears to be a result of secondary reactions which transfer damage from remote initial sites of attack to the flavin. Pathways for migration of eaq- damage also appear to exist.
