Aspirin binding and the effect of albumin on spontaneous and enzyme-catalysed hydrolysis.

A method of measuring the binding of aspirin to albumin without the interference of hydrolysis was developed. At concentrations of 10 mg litre-1, aspirin is about 85% bound to bovine serum albumin (4 g %), whereas its hydrolysis product, salicylic acid, is 95% bound. Salicylic acid was shown to displace aspirin from albumin binding sites. Both salicylic acid and aspirin bind more strongly to bovine serum albumin than to human serum albumin at protein concentrations of 4 g %. Protein binding protected aspirin against spontaneous hydrolysis although protein-bound aspirin still hydrolysed at a finite rate. In contrast, albumin enhanced the enzyme-catalysed hydrolysis of aspirin. By using a simple model, the rate constants for the individual processes contributing to the overall hydrolysis rate constant in the presence of albumin and esterase are calculated.