Antioxidant and α-glucosidase inhibitor activities of Grammatophyllum speciosum Blume

Grammatophyllum speciosum Blume is the largest epiphytic orchid. In Thailand, it is used to prepare a herbal elixir against inflammation. This study determined antioxidant activity and α-glucosidase inhibitor activity in ethanol extracts prepared from leaf, rhizome+root and pseudobulbs of 3 different age plants: 6-year-old plants, 1-year-old plants and in vitro plantlets. For 1-year-old plants and in vitro plantlets, only 2 parts, leaf and rhizome+root, were used. The experimental design was a completely randomized design. The experiments were done in duplicate and measurements were applied in triplicate (n=3). After extraction of fresh and dried samples, antioxidant activity and α-glucosidase inhibitor activity were determined. Extractions from fresh samples of 6-year-old leaf and rhizome+root parts yielded the highest DPPH free radical scavenging activity (77.68 and 76.95%, respectively), but were not different from Trolox, 100 μg mL^‑1, the positive control (83.85%). The IC₅₀ value of the DPPH extract from 6-year-old leaves and rhizomes+roots and Trolox were 32.07, 17.87 and 7.43 µg mL^‑1, respectively. For α-glucosidase inhibitor activity, extractions from 6-year-old rhizome+root (both fresh and dry samples) showed strong activity (84.88 and 85.47%, respectively) and were significantly lower than acarbose, 1 mg mL^‑1, (positive control, 94.50%). The other plant samples had weak activity (8-13%). The IC₅₀ value of α-glucosidase inhibitor activity in extracts from 6-year-old rhizome+root (fresh sample) and acarbose were 443.52 and 390.42 µg mL^‑1, respectively. Results suggested that 6-year-old rhizome+root of G. speciosum could be a potential source of natural antidiabetic medicine. The 6-year-old leaf and 6-year-old rhizome+root could be sources of antioxidant.