Antioxidant Flavonols and Phenolic Compounds from Atraphaxis frutescens and Their Inhibitory Activities against Insect Phenoloxidase and Mushroom Tyrosinase

Chemical investigation of the aerial parts of Atraphaxis frutescens resulted in the isolation of five 7-methoxyflavonols with pyrogallol B-ring moieties (1–5), a fisetinidol glucoside (13), and a benzyl glycoside (18), together with 26 known compounds including flavonoids, phenylpropanoid amides, anthraquinone glycosides, lignans, and a benzyl derivative. The principal chemical structural feature of the isolated compounds was either a pyrogallol or catechol B-ring moiety, and they showed potent 1,1-diphenyl-2-picrylhydrazyl radical scavenging activities. To assess the effects of these antioxidants on biological enzymes, their inhibitory effects against an insect phenoloxidase and a mushroom tyrosinase were evaluated. This study indicated that insect phenoloxidase was inhibited by phenylpropanoid amides and that mushroom tyrosinase was inhibited by the characteristic 7-methoxyflavonol 3-O-rhamnopyranosides.