Dihydrolipoic acid--a universal antioxidant both in the membrane and in the aqueous phase. Reduction of peroxyl, ascorbyl and chromanoxyl radicals.
Kagan VE, Shvedova A, Serbinova E, Khan S, Swanson C, Powell R, Packer L.
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
Biochem Pharmacol. 1992 Oct 20;44(8):1637-49.

Thioctic (lipoic) acid is used as a therapeutic agent in a variety of diseases in which enhanced free radical peroxidation of membrane phospholipids has been shown to be a characteristic feature. It was suggested that the antioxidant properties of thioctic acid and its reduced form, dihydrolipoic acid, are at least in part responsible for the therapeutic potential. The reported results on the antioxidant efficiency of thioctic and dihydrolipoic acids obtained in oxidation models with complex multicomponent initiation systems are controversial. In the present work we used relatively simple oxidation systems to study the antioxidant effects of dihydrolipoic and thioctic acids based on their interactions with: (1) peroxyl radicals which are essential for the initiation of lipid peroxidation, (2) chromanoxyl radicals of vitamin E, and (3) ascorbyl radicals of vitamin C, the two major lipid- and water-soluble antioxidants, respectively. We demonstrated that: (1) dihydrolipoic acid (but not thioctic acid) was an efficient direct scavenger of peroxyl radicals generated in the aqueous phase by the water-soluble azoinitiator 2,2'-azobis(2-amidinopropane)-dihydrochloride, and in liposomes or in microsomal membranes by the lipid-soluble azoinitiator 2,2'-azobis(2,4-dimethylvaleronitrile); (2) both dihydrolipoic acid and thioctic acid did not interact directly with chromanoxyl radicals of vitamin E (or its synthetic homologues) generated in liposomes or in the membranes by three different ways: UV-irradiation, peroxyl radicals of 2,2'-azobis(2,4-dimethylvaleronitrile), or peroxyl radicals of linolenic acid formed by the lipoxygenase-catalyzed oxidation; and (3) dihydrolipoic acid (but not thioctic acid) reduced ascorbyl radicals (and dehydroascorbate) generated in the course of ascorbate oxidation by chromanoxyl radicals. This interaction resulted in ascorbate-mediated dihydrolipoic acid-dependent reduction of the vitamin E chromanoxyl radicals, i.e. vitamin E recycling. We conclude that dihydrolipoic acid may act as a strong direct chain-breaking antioxidant and may enhance the antioxidant potency of other antioxidants (ascorbate and vitamin E) in both the aqueous and the hydrophobic membraneous phases.

Radioprotection of hematopoietic tissues in mice by lipoic acid.
Ramakrishnan N, Wolfe WW, Catravas GN.
Office of the Chair of Science, Armed Forces Radiobiology Research Institute, Bethesda, Maryland 20889-5145. Radiat Res. 1992 Jun;130(3):360-5.

Lipoic acid is a lipophilic antioxidant that participates in many enzymatic reactions and is used clinically to treat mushroom poisoning and metal toxicity. In this report the protective effect of lipoic acid (oxidized form) against radiation injury to hematopoietic tissues in mice was assessed by the endogenous and exogenous spleen colony assays and survival (LD50/30) assay. Intraperitoneal administration of lipoic acid at a nonlethal concentration of 200 mg/kg body wt, 30 min before irradiation increased the LD50/30 from 8.67 to 10.93 Gy in male CD2F1 mice. Following a 9-Gy irradiation, the yield of endogenous spleen colony-forming units in mice treated with saline and lipoic acid was 0.75 +/- 0.5 and 8.9 +/- 1.6, respectively. Using the exogenous spleen colony assay, lipoic acid treatment increased the D0 from 0.81 +/- 0.01 to 1.09 +/- 0.01 Gy, yielding a dose modification factor of 1.34 +/- 0.01. Dihydrolipoic acid (reduced form) has no radioprotective effect in CD2F1 mice.

Antioxidant adaptive response of human mononuclear cells to UV-B: effect of lipoic acid.
Alvarez S, Boveris A.
Laboratory of Free Radical Biology, School of Pharmacy and Biochemistry, University of Buenos Aires, Argentina.
J Photochem Photobiol B. 2000 Apr-May;55(2-3):113-9.

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. Scientific abstracts from Pubmed