Phenylpropanoids from Ballota nigra L. inhibit in vitro LDL peroxidation

From the European plant Ballota nigra L. various polyphenols including phenylpropanoid derivatives were isolated. There is increasing evidence that oxidized low-density lipoproteins (Ox-LDL) might be involved in the pathogenesis of atherosclerosis and it has been reported that polyphenols inhibit LDL peroxidation and atherogenesis. The goal of this study was to test whether the major polyphenolic compounds extracted from Ballota nigra, four phenylpropanoid glycosides, verbascoside, forsythoside B, arenarioside, and ballotetroside and one non-glycosidic phenylpropanoid, caffeoyl-L-malic acid, inhibit Cu(2+)-induced LDL peroxidation. The effectiveness of these compounds was compared to the activity of quercetin, a well-known polyphenol inhibitor of Cu(2+)-induced LDL oxidation. Antioxidant efficacious doses (ED 50) of arenarioside and ballotetroside were 1.8 microM and 7.5 microM respectively, while in the same conditions, the ED 50 of forsythoside B and verbascoside were similar (1 microM) and those of quercetin and of caffeoyl-L-malic acid were 2.3 microM and 9.5 microM respectively. Spectrophotometric studies show that quercetin is a Cu(2+) chelator while phenylpropanoid glycosides and caffeoyl-L-malic acid are not Cu(2+) chelators. Therefore, phenylpropanoid glycosides are strong inhibitors of Cu(2+)-induced LDL oxidation, independent of any capacity to act as Cu(2+) chelators.