The methanolic extracts of 18 tropical and subtropical plants were evaluated using various in vitro assays: DPPH radical scavenging activity, H 2 O 2 radical scavenging activity, β-carotene bleaching assay, and reducing power assay. Total phenol content measurements of several tropical and subtropical plants were determined by Folin-Ciocalteau reagent equivalent to gallic acid and quercetin. The α-glucosidase inhibitory assay was conducted against α-glucosidase from Saccharomyces cerevisiae.

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... inhibitory mechanisms of the active compounds were analyzed using Lineweaver-Burk plots. The plant extracts were obtained by the maceration method using methanol as the solvent. A significant and linear correlation coefficient between the antioxidant activity and the total phenolic content was found in the methanol extracts. The result revealed that the subtropical plants Quercus gilva Blume and Quercus phillyraeoides A. Grey had the highest antioxidant activities (IC 50 ) with 38.53±3.10 and 25.00±1.43 μg/mL, respectively, while Mallotus japonicus and Quercus phillyraeoides A. Grey had the highest α-glucosidase inhibitory activities (IC 50 ) with 8.46±0.30 and 9.88±0.36 μg/mL, respectively. Thus, these plants could potentially be rich sources of natural antioxidants and anti-diabetes medicine.