Alternative Medicine Review 2006 (Sep); 11 (3): 232–237 ~ FULL TEXT
Alpha-lipoic acid (ALA – also known as thioctic acid) was discovered in 1951 as a molecule that assists in acyl-group transfer and as a coenzyme in the Krebs cycle. In the 1980s, the scientific community realized alpha-lipoic acid is
a powerful antioxidant. Several qualities distinguish alpha-lipoic acid from other antioxidants: ALA can be synthesized
by animals and humans;  it neutralizes free radicals in both the fatty and watery regions of cells, in contrast
to vitamin C (water soluble) and vitamin E (fat soluble); and, ALA functions as an antioxidant in both its reduced
and oxidized forms. 
Alpha-lipoic acid is a potent antioxidant in both fat- and water-soluble mediums. Furthermore, its antioxidant
activity extends to both its oxidized and reduced forms. DHLA is capable of directly regenerating ascorbic
acid from dehydroascorbic acid and indirectly regenerating vitamin E.  Researchers have also found ALA increases
intracellular glutathione  and coenzyme Q10  levels.
Alpha-lipoic acid appears capable of chelating certain metals. It forms stable complexes with copper, manganese,
and zinc.  In animal studies, it has been found to protect against arsenic poisoning,  and, in both animal and in vitro studies, ALA reduced cadmium-induced hepatotoxicity.  In vitro, ALA chelated mercury from renal slices. 
Mechanisms that may account for lipoic acid’s benefit in preventing diabetic complications include prevention of protein glycosylation  and inhibition of the enzyme aldose reductase, the latter of which subsequently inhibits conversion of glucose and galactose to sorbitol. [15, 16] Accumulation of sorbitol has been implicated in the pathogenesis of various diabetic complications, including “sugar cataracts” where sorbitol accumulates in the lens.