1H NMR-Based Metabolomics Approach Revealing Metabolite Variation of Black Turmeric (Curcuma caesia) Extracts and Correlation with Its Antioxidant and ?-Glucosidase Inhibitory Activities

Research background. Curcuma species (Zingiberaceae) are well known me-dicinal herbs in India and Southeast Asia. Despite various findings reporting their beneficial biological activities, very little information has been recorded on the Curcuma caesia. Thus, this study aims to determine the phenolic content, antiox-idant and alpha-glucosidase inhibitory activity of both rhizome and leaves of C. caesia. Experimental approach. Rhizome and leaves of C. caesia were dried with oven (OD) and freeze (FD)-drying methods, and extracted with different 0(etha-nol,water)=100:0, 80:20, 50:50 and 0:100. The bioactivities of C. caesia extracts were evaluated using in vitro tests; total phenolic content (TPC), antioxidant (DPPH and FRAP) and alpha-glucosidase inhibitory activity. Proton nuclear magnetic reso-nance (1H NMR)-based metabolomics approach was employed to differentiate the most active extracts based on their metabolite profiles and correlation with bioactivities. Results and conclusions. The FD rhizome extracted with 0(ethanol,water)=100:0 was observed to have potent TPC expressed as gallic acid equivalents, FRAP ex-pressed as Trolox equivalents and alpha-glucosidase inhibitory activity with values of (45.4 +/- 2.1) mg/g extract, (147.7 +/- 8.3) mg/g extract and (265.5 +/- 38.6) mu g/mL (IC50), respectively. Meanwhile, for DPPH scavenging activity, the 0(ethanol,water)=80:20 and 100:0 extracts of FD rhizome showed the highest activity with no significant difference between them. Hence, the FD rhizome extracts were selected for fur-ther metabolomics analysis. Principal component analysis (PCA) showed clear discrimination among the different extracts. Partial least square (PLS) analysis showed positive correlations of the metabolites, including xanthorrhizol deriva-tive, 1-hydroxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-(6E)-6-heptene-3,4-dione, valine, luteolin, zedoardiol, beta-turmerone, selina-4(15),7(11)-dien-8-one, zedoalac-tone B and germacrone, with the antioxidant and alpha-glucosidase inhibition activ-ities, whereas curdione and 1-(4-hydroxy-3,5-dimethoxyphenyl)-7-(4-hydroxy-3-methoxyphenyl)-(lE,6E)-1,6-heptadiene3,4-dione were correlated with alpha-glucosidase inhibitory activity. Novelty and scientific contribution. C. caesia rhizome and leaf extracts con-tained phenolic compounds and had varies antioxidant and alpha-glucosidase inhib-itory capacities. These findings strongly suggest that the rhizomes of C. caesia are an invaluable natural source of active ingredients for applications in pharmaceu-tical and food industries.