Abstract
Two flavonols, quercetin (1) and quercitrin (2), were isolated from the leaves of Zanthoxylum piperitum. Their structures were established by UV, one- and two-dimensional NMR, and mass spectroscopic methods. Quercetin showed significant inhibition against mushroom tyrosinase with an IC50 value of 3.8 microg/ml, and appeared to inhibit the polyphenol oxidase activity of tyrosinase in a competitive manner (Ki = 10 +/- 0.20 microM) when L-tyrosine was used as a substrate, although it did not inhibit the melanin production of Streptomyces bikiniensis.
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