Interaction of camel lens zeta-crystallin with aspirin was investigated by activity and fluorescence measurements. Aspirin minimally inhibited the oxidoreductase activity of the enzyme and weakly quenched its fluorescence. However, significant fluorescence quenching of zeta-crystallin coincided with the appearance of a fluorescence signal characteristic of salicylic acid thereby raising the possibility that salicylic acid might have been the moiety responsible for inhibition and fluorescence quenching. Direct fluorescence measurements showed that zeta-crystallin had a much higher affinity for salicylic acid than aspirin (K(i) of about 24 microM for salicylic acid versus 630 microM for aspirin). Salicylic acid was also far more effective in inhibiting zeta-crystallin than aspirin (K(i) values were 23 microM versus 820 microM, respectively). Inhibition kinetics suggested that salicylic acid interacted with zeta-crystallin via a binding site that was distinct from that of NADPH. Salicylic acid also interacted with and quenched the fluorescence of camel lens alpha-crystallin suggesting a general mode of interaction with lens proteins. Within the normal therapeutic concentrations of salicylic acid or aspirin, only crystallin-salicylic acid interactions might be significant. These results showed that camel lens zeta- and alpha-crystallin exhibited remarkable selectivity for salicylic acid over aspirin, and thus, could be considered as salicylate-binding proteins.