Isolation of Onosmins A and B, Lipoxygenase Inhibitors from Onosma hispida

Onosmins A (1) and B (2), lipoxygenase inhibitors, have been isolated from Onosma hispida. Their structures were established as 2-[(4-methylbenzyl)amino]benzoic acid (1) and methyl 2-[(4-methylbenzyl)amino]benzoate (2) through spectroscopic studies, including 2D-NMR. The known compounds apigenin (3), 6,4′-dimethoxy-3,5,7-trihydroxyflavone (4), 6,7-dimethoxy-3,5,4′-trihydroxyflavone (5) and apigenin 7-O-β-D-glucoside (6) are also reported for the first time from this species. Compounds (1) and (2) inhibited lipoxygenase (LOX, EC 1.13.11.12) enzyme in a concentration-dependent fashion with IC50 values of 24.0 and 36.2 μM, respectively. Lineweaver–Burk as well as Dixon plots and their secondary replots indicated that the nature of inhibition was purely a non-competitive type, with Ki values 22.0 μM and 31.1, respectively.