Kubo Isao, Nihei Ken-ichi, Shimizu Kuniyoshi
Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720-3112, USA.
Bioorg Med Chem. 2004 Oct 15;12(20):5343-7. doi: 10.1016/j.bmc.2004.07.050.
Quercetin was oxidized as a substrate catalyzed by mushroom tyrosinase to the corresponding o-quinone and subsequent isomerization to p-quinone methide type intermediate; followed by the addition of water on C-2 yielding a relatively stable intermediate, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. In the presence of a catalytic amount of l-DOPA as a cofactor, the rate of this oxidation was enhanced. Fisetin, which lacks the C-5 hydroxyl group, was also oxidized but the rate of oxidation was faster than that of quercetin, indicating that the C-5 hydroxyl group is not essential but is associated with the activity.
槲皮素作为底物被蘑菇酪氨酸酶催化氧化为相应的邻醌，随后异构化为对醌甲基化物型中间体；接着在C-2位加水生成相对稳定的中间体2-(3,4-二羟基苯甲酰基)-2,4,6-三羟基-3(2H)-苯并呋喃酮。在催化量的L-多巴作为辅因子存在的情况下，这种氧化速率会提高。缺乏C-5羟基的漆黄素也会被氧化，但氧化速率比槲皮素快，这表明C-5羟基不是必需的，但与活性有关。
