Bioassay-guided isolation of novel and selective urease inhibitors from Diospyros lotus

Two new dimeric naphthoquinones, 5′,8′-dihydroxy-6,6′-dimethyl-7,3′-binaphthyl-1,4,1′,4′-tetraone (1; Di-naphthodiospyrol D) and 5′,8′-dihydroxy-5,8-dimethoxy-6,6′-dimethyl-7,3′-binaphthyl-1,4,1′,4′-tetraone (2; Di-naphthodiospyrol E), along with known naphthoquinones diospyrin (3) and 8-hydroxy diospyrin (4) were isolated from the chloroform fraction of extract of Diospyros lotus roots. Their structures were elucidated by advanced spectroscopic analyses, including HSQC, HMBC, NOESY, and J-resolved NMR experiments. The fractions and compounds 1−4 were evaluated for urease activity and phosphodiesterase-I, carbonic anhydrase-II and α-chymotrypsin enzyme inhibitory activities. Compounds 1 and 2 and their corresponding fractions showed significant and selective inhibitory effects on urease activities. The IC50 values of 1 and 2 were 260.4 ± 6.37 and 381.4 ± 4.80 µmol·L−1, respectively, using thiourea (IC50 = 21 ± 0.11 µmol·L−1) as the standard inhibitor. This was the first report demonstrating that the naphthoquinones class showed urease inhibition.