Study On Chemical Constituents And Antioxidant Activity Of Portulaca Oleracea L.

Portulaca oleracea(Portulacaceae family) is a well-known edible plant,which is widespread in temperate and tropical regions of the world.Many types of chemical compounds were present in this plant,including unsaturated fatty acids,terpenoids,alkaloids,coumarins,flavonoids,phenolic acids and volatile oil.It is also used as folk medicine in many countries with various pharmacological effects such as antibacterial,analgesic, antiinflammatory,muscle relaxation,anthelminthic,wound healing and neuroprotective activities.It is noteworthy that many researches have shown that P.oleracea has strong antioxidant activity.However,the antioxidant ingredients in P.oleracea is not fully understood yet.In this study,the crude drug of P.oleracea was extracted with 70%ethanol.The condensed extract was subjected on AB-8 resin,and the eluates were partitioned with ethyl acetate and n-butanol,sequentially.Eight compounds were isolated from above extracts using various chromatographic methods,such as silica gel column chromatography,polyamide column chromatography,ODS-A column chromatography and polydextran gel SephadexLH-20 column chromatography.Based on various spectroscopic data(¹H-NMR, ¹³C-NMR,H-H COSY,HMQC and HMBC),seven compounds were identified as caffeic acid(1),oleracein E (2),oleracein A(3),oleracein B(4),hesperidin(5),adenosine(6) and adenine(7).Compound 1,5,and 7 were isolated from P.oleracea L.for the first time.Antioxidant activities of different fractions and phenolic compounds from Portulaca oleracea were determined,based on scavenging activity against DPPH radical and inhibitory effect on hydrogen peroxide-induced lipid peroxidation in rat brain homogenates.In both assays,n-butanol fraction and ethyl acetate fraction were shown to be the more potent antioxidants,and were in a dose-dependent manner, followed by the whole water fraction.The remaining water fraction showed the weakest DPPH free-radical scavenging activity and stimulated lipid peroxidation significantly at the median concentration(92.81μg/mL and 185.63μg/mL).As for individual compound,caffeic acid showed the strongest antioxidant activity. Phenolic alkaloids also served as a new class of antioxidant agent in this plant.The DPPH radical-scavenging activities of three phenolic alkaloids,i.e.oleracein A(OA),oleracein B(OB) and oleracein E(OE),were higher than the positive control,ascorbic acid and a-tocopherol,being in the following order:OB>OA>OE. Among three alkaloids,OE was most potent in preventing hydrogen peroxide-induced formation of MDA in a dose-dependent manner,with IC₅₀ value of 73.13μM,close to that of caffeic acid(72.09μM).However,OA and OB showed no activity at low concentration and significant inhibitory activity occurred only at high concentrations above 200μM.The innovative point of this study was that we first determined the antioxidant activities of different fractions and phenolic compounds from Portulaca oleracea depending on the inhibitory effect on hydrogen peroxide-induced lipid peroxidation in rat brain homogenates.This study further elucidated the antioxidant ingredients of P.oleracea and their action mechanism.It would be very important for the future usage of P. oleracea as functional food or potential drugs for the prevention and treatment of some chronic diseases.