Inhibition of camel lens ζ-crystallin by aspirin and aspirin-like analgesics

Camel lens zeta-crystallin was reversibly inhibited to various degrees by aspirin (acetyl salicylic acid) and the aspirin-like analgesics: paracetamol (acetaminophen) and ibuprofen (2-(4-isobutyl phenyl)-propionic acid). Among these. aspirin was the most potent inhibitor. causing nearly complete inhibition in a dose-dependent, but time-independent manner. Analysis of inhibition kinetics revealed that aspirin was uncompetitive inhibitor (K-i 0.64 mM) with respect to NADPH and non-competitive inhibitor (K-i 1.6 mM) with respect to the substrate, 9.10-phenan-threnequinone (PQ). Multiple-inhibition analysis showed that aspirin and pyridoxal 5' phosphate (PAL-P), a lysine specific reagent. simultaneously bound to a critical lysine residue located towards the NADPH binding region. Consistent with this. NADPH was able to substantially protect zeta-crystallin against aspirin. whereas PQ did not provide any protection. The results suggested that an essential lysine residue was the locus of aspirin binding. The inhibition of zeta-crystallin by aspirin and aspirin-like analgesics was reversible thus eliminating acetylation as a mechanism for inhibition. Reversible binding of aspirin to this lysine may cause steric hindrance resulting in uncompetitive inhibition with respect to NADPH. (C) 2001 Published by Elsevier Science Ltd. All rights reserved.