In order to analyze the mutual displacement effects on the protein binding of/3-lactam antibiotics, binding experiments with the human serum albumin (HSA) were performed for cephalothin (CET) and phenoxymethylpenicillin (PCV) by using the centrifugal ultrafiltration method. The numbers of primary and secondary bindin g sites, �, and n2, and the affinity constants for the primary and secondary binding sites, Kx and K2 were determined for CET to be 1.00±0.06 (mean±S.D.) and 4.54±0.12 and 2.59 x 103 ±0.10x 103 (m-1) and 2.59 x 102±0.16 x 102 (m_i), respectively, and for PCV to be 0.94±0.10 and 5.41 ±0.40 and 3.52x 103 ±0.25 x 103 (m-1) and 4. 07 x 102 ±0.54x 102 (m_i), respectively. Using the predicted optimum unbound concentration of PCV, i.e., 4.6 x 10�4 M, the displacement effect of PCV to the binding of CET at the primary site has been demonstrated, while no significant effect was observed at the secondary binding site. Moreover, a competivive displacement effe ct of CET was also demonstrated for the binding of PCV to HSA at the primary binding site, suggesting that CET and PCV bound to HSA at the same primary binding site.
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