ζ-Crystallin displays strong selectivity for salicylic acid over aspirin

Abstract

Interaction of camel lens ζ-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 ζ-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 ζ-crystallin had a much higher affinity for salicylic acid than aspirin ( K i of about 24 μM for salicylic acid versus 630 μM for aspirin). Salicylic acid was also far more effective in inhibiting ζ-crystallin than aspirin ( K i values were 23 μM versus 820 μM , respectively). Inhibition kinetics suggested that salicylic acid interacted with ζ-crystallin via a binding site that was distinct from that of NADPH. Salicylic acid also interacted with and quenched the fluorescence of camel lens α-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 ζ- and α-crystallin exhibited remarkable selectivity for salicylic acid over aspirin, and thus, could be considered as salicylate-binding proteins.