Effect of drying methods and solvent extraction on the phenolic compounds of Gynura pseudochina (L.) DC. leaf extracts and their anti-psoriatic property

Gynura pseudochina (L.) DC. is a local herb that has been used as an anti-inflammatory agent. To study the utilization of plant phenolics for psoriatic treatment, the effects of the drying process, solvent extraction process and age of the leaves on the total phenolic content (TPC), total flavonoid content (TFC), crude extract and anti-oxidant activity were evaluated. The phenolic composition in the plant extracts was investigated. The most efficient process was applied to obtain a suitable extract for in vitro studies. HaCaT cells subjected to non-TNF-α and TNF-α stimulation were utilized to evaluate the cytotoxicity and anti-RelA, anti-RelB and anti-IL-8 properties of the extracts. The results showed that freeze drying and microwave drying preserved high levels of TPC, TFC and anti-oxidant activity. The HPLC results indicated that 50% (v/v) methanol demonstrated a good efficiency for recovering phenolic compounds, specifically chlorogenic acid (CGA), caffeic acid (CA), p-coumaric acid (PCA) and rutin (RUT). CA content was increased with microwave drying. Microwave drying and mixed-aged leaves were utilized for further studies. Ethanol was used instead of methanol because the former is a safer solvent for health product application. The LC-MS/MS results revealed that 25% (v/v) and 99.9% (v/v) ethanol extracts of mixed-ages leaves dried using a microwave (MLM) contained phenolic acids, flavonoids, a xanthone derivative, a phenylpropanoid, a phenolic glycoside compound and a glycerol-phospholipid. The MLM extract additionally contained pyrrolizidine alkaloids (PAs) at a low concentration. Co-extraction with 25% and 50% (v/v) ethanol of the MLM extract (EMLM) was performed for recovering each phenolic compound. EMLM extracts and their marker compounds (CGA, CA, RUT and PCA) were not toxic to HaCaT cells subjected to non-TNF-α and TNF-α stimulation, except for CA. The EMLM extracts and certain concentrations of the marker compounds could inhibit the RelB canonical pathway and IL-8 production. All results support the possible application of G. pseudochina leaf extracts for psoriasis alleviation.