Stoichiometric and kinetic studies on Ginkgo biloba extract and related antioxidants

Owing to increasing evidence showing the importance of lipid peroxidation in oxidative stress in vivo, the role and evaluation of antioxidants have received much attention. Ginkgo biloba extract (GBE), well-known as an efficient drug against diseases induced by free radicals, has been suggested to exert its effect by antioxidant action. A method was established to determine the activity of GBE as a hydrogen donor by stoichiometric and kinetic studies, and GBE was compared with several other antioxidants such as alpha-tocopherol, propyl gallate, and two kinds of flavonoids which are found in GBE, quercetin, and kaempferol. It was found that there were 6.62 x 10(19) active hydrogens in 1 g of GBE. Stoichiometric studies showed that one molecule of alpha-tocopherol reacted with one molecule of galvinoxyl radical. For quercetin, kaempferol and propyl gallate, the experimental stoichiometric numbers were 4.0, 1.9, and 3.1, respectively. The rates of reaction of antioxidants with galvinoxyl in ethanol were determined spectrophotometrically, using a stopped-flow technique. The second-order rate constant, k2, obtained at 25 degrees C was 0.13 (g/L)(-1)s(-1) for GBE and 5.9 x 10(3), 2.1 x 10(3), 1.2 x 10(4), and 2.4 x 10(3) M(-1)s(-1) for quercetin, kaempferol, propyl gallate, and alpha-tocopherol, respectively. The second-order rate constant, k2', on the molar basis of active hydroxyl groups in the tested substances obtained at 25 degrees C decreased in the order of propyl gallate > alpha-tocopherol > quercetin > GBE approximately kaempferol. This is the first study on GBE as an antioxidant which reports both stoichiometric and kinetic results.