Fifteen indigenous medicinal plant species were collected. The study samples with their English, local, and scientific names are provided. All the analyzed samples were collected freshly and then processed at the Food Analysis Laboratory of Institute of Nutrition and Food Science, University of Dhaka for further analysis. Sample preparation The identified edible portion of collected samples were separated immediately. Tender stem, shoots and leaves were taken for A. viridis, A. philoxeroides, A. sessilis, A. spinosus and C. benghalensis and tender leaves for S. tora, M. koenigii, S. calva and M. oleifera. Tender stem and leaves for C. asiatica while the whole plant except root were taken for O. corniculata and O. corymbosa. Only stem for P. retrofractum, only leaves for Z. rhetsa and tender stem, leaves and fruits for P. angulata were taken as edible portions. The edible portion of samples were then washed with running tap water followed by distilled water. They were drained completely and air dried over a paper towel. Each sample was then freeze-dried at - 180°C, weighed, ground using homogenizer. The ground freeze-dried samples were stored in an air-tight packet at - 20°C in the refrigerator until analysis.
Extraction and determination of total phenolic content Total phenol content (TPC) of indigenous plant samples was determined according to Georgé et al. (2005) using solid phase extraction (SPE) method. Samples were weighed and ground into a fine powder with 70% acetone, stirred for 30 min with a magnetic stirrer and supernatant (i.e., raw extract (RE)) was collected after centrifugation at 5000×g for 7 min. Raw extracts (3 mL) were passed into the SPE–HLB (hydrophiliclipophilic balance) to recover the washing extract (WE). The different extracts (RE and WE), following addition to each of them of 3.5 mL of water diluted Folin – Ciocalteu reagent, were incubated for 2 min at room temperature. Then, each of the mixtures following addition 2.5 mL of sodium carbonate (75 g/L) was incubated again at 50ºC. After 15 minutes, absorbance was measured at 760 nm. The calibration curve of absorbance vs. concentration of standard (i.e., gallic acid) was used to quantify TPC. Results were expressed as mg gallic acid equivalents per 100 g fresh weight (FW) of plant sample (mg GAE/ 100 g FW).
Extraction and quantification of in vitro AC For measuring in vitro AC using the Blois (1958) method, methanol and water extracts were used. Each fraction (50 µL) was added to 150 µL of 62.5 µM DPPH (2,2-diphenyl-1- picrylhydrazyl) radical solution. After 30 minutes, absorbance was measured at 492 nm using a microplate reader (Multiskan 141 EX, Thermo Electron, USA). For positive control and blank control, ascorbate (vitamin C) and distilled water were used. For preparation of standard, 60% methanol was used to dissolve DPPH to have a final concentration of 0, 15.6, 31.25, 62.5, 125, 250, 500 and 1000 µM. The free radical scavenging capacity of the sample extracts were estimated against the calibration standard curve.
Extraction and quantification of individual phenolic compounds The polyphenolic compound of the indigenous plants was estimated following the method described by Sakakibara et al., 2003. Before estimation of polyphenolic profile, an aliquot of methanolic extract was taken. After that the samples were evaporated to make dry using a rotary evaporator. Dry samples were then reconstituted using 1.5 and 3 mL of dimethyl sulfoxide (DMSO). This reconstituted sample extract was then centrifuged at 4200 rpm for 10 min and filtered through a Whatman 0.2 μm syringe filter (PTFE). In the HPLC analysis for quantification of polyphenolic profile, standards naming gallic acid, gallocatechin, protocatechuic acid, 3,4-dihydroxy benzoic acid, epigallocatechin, chlorogenic acid, catechin, syringic acid, vanillic acid, 3-OH benzoic acid, epicatechin, caffeic acid, sinapic acid, (-) catechin gallate, procyanidin B2, epigallocatechin gallate, ferulic acid, isoferulic acid, 4-coumaric acid, hesperdin, gallocatechin gallate, epicatechin galate, 2-coumaric acid, rosmaric acid, isovitexin, luteolin-7-O-glucoside, hespertin, quercetin-3-beta-galactoside, hyperoside, ellagic acid, rutin, physcion, daidzein, galangin, salicylic acid, rhoifolin, naringenin, flavonone (internal standard), quercetin, luteolin, chrysophanol, chalcone, rhein, isorhamnetin, myricetin, emodin, kaempferol, cinnamic acid and curcumin were used. Dilution of the stock samples was done with DMSO to keep the concentration of the standards 1 mg/mL then the calibration curve was made. The calibration curves showed peak area vs. concentration of standard in chromatograms. The HPLC system used was a Dionex HPLC series Ultimate 3000 (Germany) equipped with Dionex model with Chromeleon software, autosampler U-3000, and 3000 RS diode array detection system to monitor at all wavelengths from 200 to 600 nm. For the column, Dionex PA2 RSLC 120 A, Column C18 Acclaim RSLC, (100 mm x 2.1dm) i.e., 2.1μ Thermo Scientific Ltd was used at 35ºC. Solution A was composed of 50 mM sodium phosphate (pH 3.3) and 10% methanol while Solution B composed of 70% methanol. Analysis was done using flow rate of 0.47 mL/min. Initially flow rate was 100% of Solution A; for the next 0.03 min, 70% A; for another 2.65 min, 65% A; for another 7.9 min, 60% A; for another 11.5 min 50% A and finally 0% A for 13.1 min , again 17 min 100 % A and 20.25 min 100% A. The injection volume of the extract was kept 5μL.