Lignans from the Tuber-barks of Colocasia antiquorum var. esculenta and Their Antimelanogenic Activity

Colocasia antiquorum var. esculenta, a variant of C. antiquorum, commonly known as “Imperial Taro”, is an edible vegetable in many tropical and subtropical regions of the world. This study with the aim of evaluating the potential of C. antiquorum var. esculenta as a functional food with a depigmenting effect resulted in the identification of a new sesquilignan, named colocasinol A (1), and a new acyclic phenylpropane lignanamide, named cis-grossamide K (2), together with 10 known compounds (3−12). The identification and structural elucidation of these compounds were based on 1D and 2D nuclear magnetic resonance (NMR) spectroscopic data analysis as well as high-resolution fast atom bombardment mass spectrometry (FABMS) and electron impact mass spectrometry (EIMS). Quantitation of the melanin contents and cell viability in murine melanocyte melan-a cells was used to assess the antimelanogenic activities of the isolated compounds. Among them, cis-grossamide K (2), isoamericanol A (3), americanol A (4), 2-hydroxy-3,2′-dimethoxy-4′-(2,3-epoxy-1-hydroxypropyl)-5-(3-hydroxy-1-propenyl)biphenyl (5), and (−)-pinoresinol (6) showed inhibitory effects on melanin production. Compounds 2, 5, and 6 exerted a particularly strong antimelanogenic activity on the cells without high cell toxicity (IC₅₀ = 54.24, 53.49, and 56.26 μM, and LD₅₀ = 163.60, 110.23, and >500 μM, respectively).