Inhibition of ?-Galactosidase of Apple by Flavonoids and Other Polyphenols

The inhibition of a purified β-galactosidase from “Spartan” apples by flavonoids and related compounds has been studied with a fluorimetric assay using the substrate 4-methylumbelliferyl β-D-galactoside. Certain flavonoids, flavonoid glycosides, phenolic acids, and polyphenols (quercetin, fustin, rutin, p-coumaric acid, and catechol) gave classical noncompetitive inhibition at moderate substrate concentrations. Others (kaempferol, chlorogenic acid) gave nonlinear Dixon plots. The best inhibitors were flavones. In general, the degree of inhibition increased with the degree of hydroxylation and glycosylation of the phenol. Correlation of the structure of these substances with the observed inhibition suggests that the trans-3-(4-hydroxyphenyl)-propenoyl group, common to 4′-hydroxyflavones and p-hydroxyphenolic acids, is important for effective inhibition.