Structure-binding relationship and binding sites of cephalosporins in human serum albumin

The binding of some cephalosporins to human serum albumin (HSA) was studied by an ultrafiltration technique. Changes in C-3 side chain resulted in marked changes in the binding to HSA, but changes in C-7 side chain did not. Cephalosporins were classified into three groups by C-3 side chain: (i) Cationic side chain with low affinity for HSA; (ii) anionic side chain with high affinity for HSA; (iii) non ionized side chain, in which binding to HSA was dependent on lipophilicity. These findings suggest that electrostatic and hydrophobic forces play a role in the binding affinity of cephalosporins for HSA. The binding of cephalosporins with high HSA affinity was displaced significantly by warfarin but not by phenylbutazone, L-tryptophan, or diazepam. The interaction of the cephalosporins with high affinity for HSA with chemically modified HSA was investigated to clarify the amino acid residues of HSA involved in the cephalosporin binding sites. The binding of the cephalosporins decreased remarkably with the modification of the tyrosine residues. These results suggest that the binding site of cephalosporins is located in the vicinity of warfarin binding site rather than benzodiazepine binding site and that tyrosine residues are involved in the cephalosporin binding site.