Binding of sisomicin and streptomycin to human albumin

The aminoglycosides of the kanamycin and neomycin series are displaced from their protein binding by very low Ca++ ion concentrations, whereas streptomycin is displaced only by very high Ca++ ion concentrations. For streptomycin, therefore, the binding mechanism must differ from that for the other aminoglycosides. The displacing reaction cannot be described by the law of mass action, combining the binding equations. Several possible reasons, such as complex formation and changes in the conformation of albumin, are discussed. Complex formation between sisomicin and the lanthanide ion Nd+++, which has the same ionic radius as the Ca++ ion, could be excluded by recording the difference absorption spectrum. The displacement of the aminoglycosides from their protein binding by Ca++ ions (as well as the pH dependence of the protein binding) suggests that ionic bonds are involved in the interactions between aminoglycosides and serum proteins.