Abstract

The effects of the methanol extracts of 20 selected medicinal plants on free radical scavenging capacity, human low-density lipoprotein (LDL) peroxidation and platelet aggregation were investigated. The antioxidant activity was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity and thiobarbituric acid reactive substances (TBARS) assay with LDL as the oxidation substrate. The antiplatelet activity in human whole blood was investigated using an electrical impedance method. The total phenolic contents (TPC) of the extracts were determined by the Folin-Ciocalteau method. Among the extracts, Phyllanthus amarus and Labisia pumila var. alata possessed potent radical scavenging activity with IC₅₀ values of 3.4 and 5.7 µg/ml, respectively. The extracts ofZingiber officinale, Curcuma xanthorrhiza and Curcuma domestica showed strong inhibition of LDL peroxidation with IC₅₀ values ranging from 0.1 to 0.9 µg/ml. The extract of Z. officinalewas the most effective sample against platelet aggregation caused by arachidonic acid (AA) and adenosine diphosphate (ADP) with IC₅₀ values of 10.9 and 7.7 µg/ml, respectively. The Pearson correlation analysis indicated that the TPC of the extracts showed significant positive correlations with DPPH scavenging activity (r = 0.846) and LDL antioxidant activity (r= 0.639) and moderate positive correlations with antiplatelet activities (AA-induced, r = 0.51; ADP-induced, r = 0.40; collagen-induced, r = 0.44). The antioxidant and antiplatelet activities of the plant extracts could partly be due to their TPC.

Key words: Medicinal plants, LDL antioxidant activity, antiplatelet activity, DPPH scavenging capacity, total phenolic contents.

Abbreviation

LDL, Low-density lipoprotein; TBARS, thiobarbituric acid reactive substances; TPC, total phenolic contents; AA, arachidonic acid; ADP, adenosine diphosphate; PAF, platelet activating factor; DMSO, dimethyl sulfoxide; PBS, phosphate buffer saline; ASA, acetyl salicylic acid; UKM, universiti Kebangsaan Malaysia; FC, folin-ciocalteau; GAE/g, gallic acid equivalents per gram; VLDL, very low-density lipoprotein;SDS, sodium dodecyl sulphate; TBA, thiobarbituric acid; MDA, malondialdehyde; HDL, highdensity lipoprotein; UV, ultraviolet; SEM, standard error of the mean; SPSS, statistically package for social  sciences;  ORAC,  oxygen radical absorbance capacity; FRAP, ferric reducing/antioxidant power; ABTS, 2,2'-azino-bis 3-ethylbenzothiazoline-6-sulphonic acid.