Interaction of Protizinic Acid with Human Serum Albumin: Site-to-Site Displacement of Protizinic Acid by Ibuprofen

Author(s): Koji Nishi, Keishi Yamasaki and Masaki Otagiri

The binding of protizinic acid [(10-methyl-7-methoxy-2- phenoxythiazinyl)-2-propionic acid, PA], a non-steroidal anti-inflammatory drug, to human serum albumin (HSA) was studied by dialysis and spectroscopic methods. A scatchard analysis of equilibrium dialysis (ED) data and the results of site marker displacement experiments suggest that PA predominantly binds to site II or the benzodiazepine site on HSA, although the drug also has a low affinity for site I. Circular dichroism (CD) spectra of PA bound to HSA in low drug to HSA and high drug to HSA ratios were completely different, indicating that the extrinsic mechanism responsible for the high and low affinity of PA to HSA are probably different. In the presence of ibuprofen, a site II-specific drug, the CD spectra of the PA-HSA complex (0.5:1) changed to that for a high ratio (5: 1) of the drug to HSA in the absence of ibuprofen. This characteristic change in the CD spectra of the PA-HSA complex indicates that PA is displaced from a high to low affinity binding site after it is displaced by ibuprofen. ED findings also indicate site-to-site displacement of PA, i.e., when PA is displaced from the high affinity site, it re-binds to its low affinity site

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