Inhibition of camel lens zeta-crystallin/NADPH:quinone oxidoreductase activity by Cibacron blue

Camel lens zeta-crystallin/NADPH:quinone oxidoreductase activity was inhibited by Cibacron blue 3GA (CB) with 9,10-phenanthrenequinone (PQ) as an electron acceptor and NADPH as an electron donor in a time-independent and concentration dependent manner. The IC50 value of CB was 50 nM. The Lineweaver-Burk plots and the secondary plots indicated that the inhibition was linear mixed type (partial competitive and pure noncompetitive) with respect to NADPH and noncompetitive with respect to PQ. The estimated inhibition constant (K-i) values were 26.0 nM for NADPH and 55.0 nM for PQ respectively, suggesting that CB has high affinity towards the NADPH binding site. The secondary plots of inhibition with respect to NADPH, also indicate a dissociation constant (K-l) value of 68.0 nM for the zeta-crystallin-NADPH-CB complex. This K-l bring greater than the K-i value suggests that noncompetitive inhibition is predominant over competitive inhibition at the NADPH binding site.