Inhibition of zeta-crystallin by Coumarins: a structure-activity study.

A structure-activity study was carried out to determine the important groups of coumarin derivatives in inhibiting the oxidoreductase activity of the camel lens zeta-crystallin. Coumarin, 4-hydroxycoumarin, 7-hydroxy-4-methylcoumarin, dicoumarol, and warfarin were screened for their inhibitory effect on zeta-crystallin activity. The sequence of potency for the inhibitors was dicoumarol > 4-hydroxycoumarin > warfarin > > coumarin. 7-Hydroxy-4-methylcoumarin was ineffective as an inhibitor. Only dicoumarol, 4-hydroxycoumarin, and warfarin were found to inhibit the oxidoreductase activity in micromolar ranges. All tested inhibitors seem to act in reversible and time-independent manner. Concentration causing 50% inhibition of the enzyme activity (IC50 value) was 34 microM for dicoumarol, 76 microM for 4-hydroxycoumarin, and approximately 515 microM for warfarin, while 1 mM coumarin showed less than 10% inhibition. Kinetic analysis revealed inhibition of camel lens zeta-crystallin by coumarin derivatives to occur in a competitive manner with respect to dichlorophenolindophenol (DCIP) as an electron acceptor and uncompetitive manner with respect to NADPH as an electron donor. The Ki values were found to be 16 microM for dicoumarol, 40 microM for 4-hydroxycoumarin, and 220 microM for warfarin. The structure-activity relationship of coumarin derivatives indicates that the phenolic hydroxyl group at the C-4 position in the coumarin skeleton is important for the maximal inhibition.