The protein binding of acetylsalicylic acid and salicylic acid.

Quantitative experiments to study the binding of acetylsalicylic acid (ASA) and salicylic acid (SA) by human serum albumin (HSA) were carried out using ‘4C-labeled ASA or SA and gel filtration to separate the free from the bound forms. Two binding procedures were employed: the ASA or SA was incubated with the HSA, or the mixture was placed in an equilibrium dialysis cell. By withdrawing samples at intervals, the extent of binding or the attainment of equilibrium could be assessed. Evidence that filtration by the gel caused unbinding of the bound SA was obtained, with resultant lower percentage binding of SA by HSA than that obtained by equilibrium dialysis without gel filtration. In either case, binding equilibrium was reached in 4-8 hr. The binding of ASA by HSA was markedly different from that of SA. The experiments both with or without gel filtration demonstrated a progressive increase in binding of ASA in the 20to 53-hr periods studied. In addition, ASA apparently displaces SA from its binding sites on albumin, an observation that may have therapeutic implications.