Abstract
alpha-Lipoic acid (LA) is the one of the strongest antioxidants to be utilized in supplement, skin ointment and so on. The distorted five membered dithiolane ring of LA, which is necessary structure to work as a cofactor of enzyme, is considerably vulnerable to UV irradiation. LA is easily decomposed by photoirradiation resulting in the loss of its characteristic absorption band at 333 nm. The photodegradation of LA means loss of its physiological activity, so that protection of LA from UV light is eagerly desired. Thiol compounds can be regarded as a potential candidate. In order to pursue the possibility of the thiol compounds in prevention of LA degradation, we examined the photoirradiation of LA in the presence and absence of homocysteine.