Quality Assessment and In Vitro Anti-diabetic Activity of Thunbergia laurifolia Stems and Leaves

The study aimed to evaluate pharmacognostic parameters for standardization of raw materials, Thunbergia laurifolia Lindl. (family: Thunbergiaceae) stems and leaves, as well as their active phytochemical (rosmarinic acid) contents. The antidiabetic potential was evaluated by yeast α-glucosidase inhibitory activity using p-nitrophenyl-α-D-glucopyranoside as substrate. Dried stems and leaves of Thunbergia laurifolia were collected from 15 different locations in Thailand. The microscopic anatomical and histological characteristics of stem and leaf were illustrated. The physico-chemical contents, including loss on drying, acid-insoluble ash, total ash, ethanol-soluble extractives, water-soluble extractives, and moisture of dried stems and leaves, were established. TLC-densitometry of rosmarinic acid in dried T. laurifolia stems and leaves were developed and revealed contents of 0.120 ± 0.079 and 0.291 ± 0.150 g per 100 g, respectively. Similarly, TLC-image analysis by ImageJ showed contents of 0.127 ± 0.094 and 0.303 ± 0.162 g per 100 g respectively (p > 0.05). Both quantitative TLC demonstrated their validity due to specificity, accuracy, repeatability, intermediate precision, limit of detection, limit of quantitation, and robustness. The antidiabetic potential of rosmarinic acid, T. laurifolia leaf and stem ethanolic extracts, and acarbose (positive control) exhibited IC50 of 0.31, 0.80, 5.89, and 1.48 mg/ml, respectively.