THIOCTIC ACID, (-)- is less biologically potent enantiomer of thioctic acid than (+)-enantiomer. Dihydrolipoamide dehydrogenase from human renal carcinoma does not accept the S-enantiomer as a substrate in comparison to R-enantiomer but S-enantiomer is an inhibitor of the overall reaction of the bovine pyruvate dehydrogenase complex. Chronic parenteral treatment with the antioxidant alpha-lipoic acid enhances insulin-stimulated glucose transport and non-oxidative and oxidative glucose metabolism in insulin-resistant rat skeletal muscle, with the R-(+) enantiomer being much more effective than the S-(-) enantiomer. The racemate of lipoic acid at high dosage reduced the life span significantly of immunosuppressed mice but the S(-)-enantiomer of lipoic acid increased the 50% survival rate. The thioctic acid, (-)- was shown to be toxic when administered to thiamine-deficient rats.