Antioxidant and Vascular Relaxation Activities of Tropical Plant Extracts

In this study, the methanolic extracts of twelve plants which are the leaves of noni (Morinda citrifolia L.), maman (Gynandropsis gynandra L.), cashew (Anacardium occidentale L.), sweet potato (Ipomoea batatas L.), papaya (Carica papaya L.), mint (Mentha arvensis L.), semambu (Azadirachta indica), betel (Piper betle L.); lemongrass stalk (Cymbopogon citrates Stapf.), bird chili fruit (Capsicum frutescens L.), roselle calyx (Hibiscus sabdariffa), and oil palm frond (Elaeis guineensis Jacq.) were analyzed for in vitro antioxidant and vascular activities. Their total phenolic content was also estimated. Plant extract showing high antioxidant and vascular relaxing activities were further tested for their in vivo antioxidant and cholesterol lowering effects in hypercholesterolemic New Zealand White rabbits.In vivo antioxidant test (ferric reducing/antioxidant potential [FRAP] assay) showed the highest activity in cashew leaf extract (4.3 mmol FRAP/g d.w.) and the lowest in lemongrass extract (0.2 mmol FRAP/g d.w). The antioxidant activity of the cashew leaf extract was equivalent to those of green tea extract while lemongrass extract's antioxidant activity was equivalent to those of purified apigenin. The total phenols ranged from 55-256 mg gallic acid equivalent / g dry weight (d.w.) extracted samples with the highest concentration in oil palm frond extract. In this study, the antioxidant activity of the extracts did not correlate with the total phenolic contents, mainly due to the heterogeneity of the samples tested. For vascular activity, sweet potato, betel, cashew, maman, mint, oil palm frond, semambu, bird chili, and papaya extracts showed more than 50% relaxing effects in Wistar Kyoto rat's isolated aortic ring preparation. Most of the relaxing effects were endothelium-dependent with nitric oxide as the main mediator. Prostacyclin and endothelium-derived hyperpolarizing factors (EDHF) effects were also observed in small amounts. For the smaller vessels, mesenteric arteries, the extent of relaxation is reduced, where only oil palm frond, betel and lemongrass extracts showed more than 50% relaxing effect. It is observed that in the smaller vessel, the EDHFs effects are more prominent, especially in the betel and lemongrass extracts. Oil palm frond was selected for the in vivo study for its high phenolic content and high vascular relaxing activities in both blood vessel preparations. This study showed that oil palm frond supplementation in the diets of hypercholesterolemic rabbits led to a delayed increment in serum total cholesterol levels. There was no significant antioxidant and toxicity effect on liver, kidney and muscle observed. This study shows that most of the selected plants possess a high antioxidant and vascular relaxing activities. Antioxidants in food are important in minimizing food deterioration i.e. rancidity which affect the taste and textures of food containing lipids. In human, dietary antioxidants help to reduce oxidative stress, which is associated with the etiology of several chronic diseases such as hypertension, cancer, aging diseases, atherosclerosis and coronary artery diseases. Similarly, consumption of plants possessing vascular relaxing activities may restore and/or improve vascular functions and hence protect against vascular diseases such as hypertension.

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