Identification of adenosine A1 receptor ligands from Morus alba L. stem bark by NMR metabolomics approach

Comprehensive extraction coupled to NMR metabolomics was applied to the identification of active compounds from Morus alba stem bark binding to the adenosine A1 receptor. Orthogonal projection to the least square (OPLS) analysis was used to estimate which compounds significantly correlate with the activity. Based on the loading bi-plot and Y-related coefficient plot, unsaturated fatty acid signals strongly correlate to the activity. The characteristic NMR signals of alkaloids and prenylated aromatic compounds found to be abundant in this plant do not positively correlate with the activity. Characteristic signals from two compounds isolated from this plant (betulinic acid and morusin) also have negative Y-related coefficients and indeed they are inactive when tested. Aromatic compounds without prenyl or methoxy units are proposed to be the active ones. To further identify the possible active compounds, the comprehensive extraction needs to be optimized, particularly on the selection of a solvents combination which gives a high resolution of the metabolites over the fractions.