Phenolic and flavonoid compounds in aqueous extracts of thunbergia laurifolia leaves and their effect on the toxicity of the carbamate insecticide methomyl to murine macrophage cells

Background: Thunbergia laurifolia is a Thai herb and has been used in Thai folklore medicine for centuries. Generally, Thais consume T. laurifolia as a herbal tea because of its beneficial properties as an antidote for chemical toxins, drug-, arsenic-, strychnine-, alcohol- and food-poisoning. However, its effectively against some insecticide compounds, e.g. methomyl, has not yet been determined. Objective: To examine the protective effect of aqueous extract from leaves of T. laurifolia on methomyl (MT) poisoning of murine macrophage cells (anti-MT effect) and to identify phenolic and flavonoid compounds in the extract. Methods: T. laurifolia was extracted with water and stored in freeze-dried form. The extract was investigated for its antioxidant activity and some phenolic and flavonoid compounds were identified using liquid chromatography-mass spectrometry (LC-MS). To study anti-MT effects in RAW264.7 murine macrophage cells, these were treated with leaf extract either before (pretreatment), concomitantly (combined) or after (post-treatment) exposure to MT and cell viability determined in an MTT test (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide). Results: The extract exhibited strong antioxidant properties based on total extractable phenolic content (TPC), total extractable flavonoid content (TFC), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and Ferric ion reducing antioxidant power (FRAP) activity. The LC-MS analyses of phenolic compounds indicated the presence of caffeic acid, rosmarinic acid, catechin, rutin, isoquercetin, quercetin and apigenin as bioactive compounds. Viability of RAW 264.7 murine macrophage cells treated with MT was increased significantly by post-treatment with leaf extract but not by combined or pre-treatments. Conclusion: The aqueous extract of T. laurifolia leaves contained abundant antioxidant activity. Flavonoids present were catechin, rutin, isoquercetin, quercetin and apigenin. The aqueous leaf extract was able to help cells recover from the effects of exposure to MT.