Alpha lipoic acid (ALA) protects proteins against the hydroxyl free radical-induced alterations: Rationale for its geriatric topical application

The well known OH^. free radical scavenging properties of α-lipoic acid (ALA) cannot be easily utilized for biological experiments, because the compound is practically insoluble in water. We elaborated a simple method of preparing its Na-salt (Na-ALA) which proved to be water soluble. It has been demonstrated by ESR spin trapping experiments with DMPO, using the Fenton reaction as the source of OH^. free radicals that Na-ALA maintains its OH^. free radical scavenging ability: it reacts nearly an order of magnitude faster with these radicals than the spin trap itself. It was tested in two different systems to determine whether Na-ALA was able to protect bovine serum albumin (BSA) against the OH^. free radical-induced polymerization and protein oxidation. (i) OH^. free radicals were generated by Fenton reaction in the presence of BSA. This protein is polymerized by these radicals shown by the loss of its water solubility; Na-ALA exerted a considerable protective effect against this type of protein damage. (ii) BSA oxidation was induced by Co-gamma irradiation of 80 krad, resulting in a strong increase in the protein carbonyl content. Na-ALA inhibited this carbonyl formation very efficiently. The data suggest that the interaction of the OH radical with Na-ALA takes place on the disulfide group, yielding thiosulfinate or thiosulfonate. The results indicate that the geriatric topical application of Na-ALA may have an established rationale. Copyright (C) 1999 Elsevier Science Ireland Ltd.