Quantitative UPLC-MS/MS analysis of obliquumol from Ptaeroxylon obliquum (Thunb.) Radlk. extracts and biological activities of its semi-synthesised derivative ptaeroxylinol

Abstract

Quantification of compounds in plant extracts is rarely conducted to determine variation in concentrations of bioactive constituents. The aim of the study was to develop a method using ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) to identify and quantify obliquumol (12-O-acetylptaeroxylinol) in Ptaeroxylon obliquum leaves collected from different localities in South Africa. Additionally, biological activity of a semi-synthesized derivative, ptaeroxylinol was investigated. Column chromatography was used to isolate obliquumol from P. obliquum leaves, and thereafter it was saponified to ptaeroxylinol. Ultra-performance liquid chromatography coupled to quadrupole time of flight mass spectrometry (UPLC-qTof-MS) was carried out on the different P. obliquum extracts to quantify obliquumol. A serial microdilution method was used to determine the minimum inhibitory concentration (MIC) against non-pathogenic mycobacteria and fungi. The cytotoxicity was determined using Vero monkey kidney and human liver (C3A) cells. A method was developed to isolate large quantities of obliquumol (0.14%) from dried P. obliquum leaves. The different P. obliquum acetone extracts had variable obliquumol concentrations between 0.1–38.5 µg/mg. Ptaeroxylinol had an MIC as low as 8 µg/mL and 16 µg/mL against Candida albicans ATCC 10,231 and Cryptococcus neoformans, respectively. With an IC50 of 85.7 μg/mL for Vero cells and 126.51 μg/mL for C3A cells, respectively, ptaeroxylinol had low cytotoxicity to the cells tested. A UPLC-MS/MS method was developed to quantify obliquumol content in the P. obliquum acetone extracts. Ptaeroxylinol had good activity against C. albicans (MIC = 8 µg/mL) and it appears that the cleavage of the acetoxy to alcohol group played a role in the antimicrobial activity.