Clinical Chemistry, Vol 15, 1027-1038, Copyright © 1969 by the American Association for Clinical Chemistry

The Protein Binding of Acetylsalicylic Acid and Salicylic Acid

¹ Department of Biochemistry, College of Medicine, University of Iowa, Iowa City, Iowa 52240.

Quantitative experiments to study the binding of acetylsalicylic acid (ASA) and salicylic acid (SA) by human serum albumin (HSA) were carried out using ¹⁴C-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 20- to 53-hr periods studied. In addition, ASA apparently displaces SA from its binding sites on albumin, an observation that may have therapeutic implications.