抄録
Epidemiologic studies have shown that the coffee drinking habit can reduce the risk of gout and hyperuricemia. In our recent chemical studies, only the roasted coffee beans demonstrated xanthine oxidase (XO) inhibition, while green and dry beans did not. XO is the rate-determining enzyme in production of uric acid during human purine catabolism. Initially, several chlorogenic acid lactones were identified in hot-water extracts of roasted coffee beans after purification by SEC chromatography. Two lactones had moderate XO inhibitory activity and all the lactones were observed only in roasted coffee based on HPLC analysis of various kinds of coffee beans. We next found that the ethyl-acetate-soluble part of the hot-water extract of roasted coffee beans showed more potent XO inhibitory activity. This fraction is not considered to contain amounts of chlorogenic acid lactones. XO inhibitory assay-guided fractionation produced a predominantly active fraction in the faster eluting part, compared to that of chlorogenic acid lactone via medium-pressure chromatography using an octadecylsilyl column. Further purifications of the fraction produced pyrogallol as a very potent XO inhibitor, which had an IC50 of 0.73 μmol L-1, much stronger than that of other related polyphenolic compounds. The mechanism of XO inhibitory activity of pyrogallol was investigated. Pyrogallol was unstable and readily converted to purpurogallin in a pH 7.4 solution. The XO inhibitory activity of the purpurogallin (IC50 in pH 7.4 solution: 0.2 µmol L-1) was higher than that of pyrogallol (IC50 in pH 7.4 solution: 1.6 µmol L-1). Molecular docking simulations showed that purpurogallin had stronger affinity for the active site of XO than the corresponding pyrogallol. These results revealed that the potent XO inhibitory activity seemingly observed in pyrogallol is derived from its chemical conversion into purpurogallin. These identified XO inhibitors present in roasted coffee may work in the human body after coffee drinking to control overproduction of uric acid by XO.
