STOICHIOMETRIC AND KINETIC-STUDIES ON GINKGO-BILOBA EXTRACT AND RELATED ANTIOXIDANTS

Owing to increasing evidence showing the importance oi lipid peroxidat ion in oxidative stress in vivo, the role and evaluation of antioxidan ts have received much attention. Ginkgo biloba extract (CBE), well-kno wn as an efficient drug against diseases induced by free radicals, has been suggested to exert its effect by antioxidant action. A method wa s established to determine the activity of GEE as a hydrogen donor by stoichiometric and kinetic studies, and GEE was compared with several other antioxidants such as alpha-tocopherol, propyl gallate, and two k inds of flavonoids which are found in CBE, 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-tocopher ol reacted with one molecule of galvinoxyl radical. For quercetin, kae mpferol and propyl gallate, the experimental stoichiometric numbers we re 4.0, 1.9, and 3.1, respectively. The rates of reaction oi antioxida nts with galvinoxyl in ethanol were determined spectrophotometrically, using a stopped-flow technique. The second-order rate constant, k(2), obtained at 25 degrees C was 0.13 (g/L)(-1)s(-1) for GEE 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 querceti n, kaempferol, propyl gallate, and alpha-tocopherol, respectively. The second-order rate constant, k(2)', On the molar basis of active hydro xyl groups in the tested substances obtained at 25 degrees C decreased in the order of propyl gallate > alpha-tocopherol > quercetin > CBE a pproximate to kaempferol. This is the first study on GEE as an antioxi dant which reports both stoichiometric and kinetic results.