Chemical Constituents of Gold-red Apple and Their α-Glucosidase Inhibitory Activities

Ten compounds were isolated and purified from the peels of gold-red apple (Malus domestica) for the 1st time. The identified compounds are 3 beta, 20 beta-dihydroxyursan-28-oic acid (1), 2 alpha-hydroxyoleanolic acid (2), euscaphic acid (3), 3-O-p-coumaroyl tormentic acid (4), ursolic acid (5), 2 alpha-hydroxyursolic acid (6), oleanolic acid (7), betulinic acid (8), linolic acid (9), and a-linolenic acid (10). Their structures were determined by interpreting their nuclear magnetic resonance and mass spectrometry (MS) spectra, and by comparison with literature data. Compound 1 is new, and compound 2 is herein reported for the 1st time for the genus Malus. alpha-Glucosidase inhibition assay revealed 6 of the triterpenoid isolates as remarkable alpha-glucosidase inhibitors, with betulinic acid showing the strongest inhibition (IC50 = 15.19 mu M). Ultra-performance liquid chromatography-electrospray ionization MS analysis of the fruit peels, pomace, flesh, and juice revealed that the peels and pomace contained high levels of triterpenes, suggesting that wastes from the fruit juice industry could serve as rich sources of bioactive triterpenes.