Salicylate protein binding in serum from young and elderly subjects as measured by diafiltration

The protein binding of salicylate was measured by continuous ultrafiltration (diafiltration) at 22 degrees C in serum obtained from 5 healthy young (mean age: 27 years) and 5 healthy elderly (mean age: 73 years) male volunteers. Unbound salicylate increased disproportionately with increasing total salicylate concentration, up to 7000 mumol, in all sera. The fraction bound of salicylate was significantly lower in sera from elderly but this was not due to decreased albumin or total protein concentrations. The binding of salicylate to serum proteins was characterized by two classes of binding sites. The high affinity site had an association constant of either 9490 l/mol (young) or 7560 l/mol (elderly) and the number of binding sites was either 4.7 (young) or 3.7 (elderly). The total binding capacity of the low affinity site, 112 l/mol, in sera from elderly was significantly less than the binding capacity, 631 l/mol, in sera from young. Differences in binding capacity of the low affinity site partially accounted for a two to three-fold increase in the salicylate free fraction in elderly sera. These data suggest that age-related differences in serum protein binding may influence salicylate pharmacokinetics.