Inhibition of Xanthine Oxidase by Phenolic Conjugates of Methylated Quinic Acid

Abstract

The caffeoyl conjugates of prenylhydroquinone glucoside and of quinic acid, either in the carboxyl-free or carboxymethyl forms, isolated from Phagnalon rupestre (Asteraceae), showed inhibitory activity on lipid peroxidation induced by Fe²⁺/ascorbate and by CCl₄/NADPH in rat liver microsomes, with IC₅₀ values ranging from 3 to 11 μM. After having demonstrated their effect on the xanthine oxidase-regulated superoxide production, the active compounds were tested for the direct inhibition of this enzyme. Methylated dicaffeoylquinic conjugates competitively inhibited the enzyme and the highest potency was obtained for the 4,5-diester, with an IC₅₀ value of 3.6 μM, nearly ten times lower than that of the 3,5-analogue. In conclusion, the presence of the caffeoyl moiety is essential for both the antiperoxidative and radical scavenging activities, and the methylation of the quinic carboxyl group enhances the potency on xanthine oxidase inhibitory activity.

Abbreviations

BHT:di-tert-butyl-hydroxytoluene

DCPIP:2,6-dichlorophenolindophenol

DMSO:dimethylsulfoxide

DPPH:2,2-diphenyl-1-picrylhydrazyl radical

IC₅₀:50 % inhibitory concentration

I_I:enzyme inhibition constant

LB:Lineweaver-Burk

PYR:pyrogallol

Q:quercetin-3-O-glucoside

ROS:reactive oxygen species

TBA:thiobarbituric acid

TBARS:thiobarbituric acid-reactive substances

Key words

Caffeoylquinic acids - hydroquinone - antioxidant - xanthine oxidase inhibitors - lipid peroxidation

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