Camel lens zeta-crystallin/NADPH:quinone oxidoreductase activity was inhibited by chloranilic acid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) with NADPH as an electron donor and 9,10-phenanthrenequinone (PQ) as an electron acceptor in a time-independent but concentration dependent manner. The IC50 of chloranilic acid was 1 microM. The inhibition was noncompetitive with respect to both NADPH and PQ as deduced by Lineweaver-Burk plots. The estimated inhibition constant (Ki) was 0.8 microM for both NADPH and PQ. Examination of other benzoquinones suggested that the presence of -OH and -Cl on benzoquinone was essential for the inhibition.