The extracellular control of intracellular aspirin hydrolysis

We have previously presented evidence that an intra-erythrocyte arylesterase is primarily responsible for the deacetylation of aspirin to salicylate. We now report studies on extracellular control factors. The fractional acetylsalicylic acid (ASA) hydrolysis at the completion of 20-minute incubations in suspensions of washed red blood cells (RBC) with human serum albumin (HSA) decreased sharply, from 0.6 at 0 gm/dl HSA to 0.2 at 5.0 gm/dl HSA. In RBC suspensions with oleic, linoleic, and lauric acid, ASA hydrolysis increased as fatty acid:HSA ratios increased. The addition of salicylate (SA) also resulted in an increase in the fraction hydrolyzed. In a separate, more direct, ultracentrifugation assay, unbound ASA levels were similarly affected by HSA and the other ligands. When SA and oleic acid were incubated with ASA in RBC, additive competition for binding sites was apparent at relatively low levels of the fatty acid. In the presence of increasing concentrations of sodium SA, 5-amino SA, and 4-amino SA, ASA hydrolysis in RBC suspensions was increased by an average of 137%, 75%, and 54%, respectively. The most reasonable explanation for these findings is that the intact ASA molecule binds to albumin and that the binding of competing ligands can modulate the levels of free ASA in the circulation. This, in turn, affects the amount of drug available as substrate for penetration into the RBC and reaction with the intracellular hydrolase which catalyzes ASA deacetylation. Such factors controlling drug availability, if validated in vivo, could be important with regard to the antiinflammatory and anti-platelet properties of aspirin.