Onion extract (Allium cepa L.) up-regulates paraoxonase 1 activity with concomitant protection against LDL oxidation in male wistar strain rats subjected to mercuric chloride induced oxidative stress

Abstract

Onion (Allium cepa L.) is one of the rich sources of flavonoids, consisting mainly of the major flavonols quercetin-3,4'-O-diglucoside (QDG) and quercetin-4'-O-monoglucoside (QMG) with high therapeutic properties. Paraoxonase1 (PON1) protects the oxidative modification of low density lipoprotein (LDL) and is a major anti-atherosclerotic protein component of high-density lipoprotein (HDL). We explored the roles of onion extracts and flavonoids (quercetin & catechin) in the regulation of PON1 expression by measuring its arylesterase activity and correlating with plasma MDA and oxidized LDL levels, as a marker of oxidative stress in wistar strain rats subjected to mercuric chloride (HgCl2) induced oxidative insult. Rats were divided into eight groups. Control, Experimental (HgCl2 treated), Exp + onion extract/catechin/quercetin, Positive control (Normal + onion/catechin/quercetin). Treatment continued for 4 weeks. PON1 activity decreased in rats treated with HgCl2 (HgCl2 treated 23.83 U/ml plasma; control value: 33.20 U/ml plasma) with an increase in susceptibility of LDL for oxidation (values derived after 3000 s; Pearson's r = 0.9970, p< 0.001; control value: Pearson's r = 0.9980, p< 0.001) and plasma MDA level (0.408 nmol/ml plasma; control value: 0.202 nmol/ml plasma). Onion extracts successfully and significantly attenuated the adverse effects of HgCl2 by up regulating PON1 activity (28.10 U/ml plasma) and its protective capacity against LDL oxidation (Pearson's r = 0.9978, p< 0.001) and lipid peroxidation (0.313 nmol/ml plasma). Similar effects were shown by quercetin and catechin with improved oxidative status either by enhancing PON1activities (31.36 & 24.76 U/ml plasma) and antioxidant defenses (0.24 & 0.28 nmol/ml plasma MDA level) or reducing susceptibility to LDL oxidation (Pearson's r = 0.9973 & 0.9984, p < 0.001), when given to oxidatively stressed rats.