Binding of aminoglycoside antibiotics to human serum proteins. III. Effect of experimental conditions.

The binding of several aminoglycoside antibiotics to human serum proteins was studied under varying experimental conditions. The protein binding was determined by means of equilibrium dialysis and, with sisomicin and gentamicin, also by the ultracentrifuge method in the presence and in the absence of Ca++ and Mg++ ions. The technical experimental procedure (dialysis, ultrafiltration, ultracentrifugation) has no effect on protein binding contrary to the physico-chemical conditions (varying concentrations of Ca++ and Mg++ ions). Under experimental conditions closely conforming to physiological conditions, the aminoglycosides of the kanamycin and neomycin series are not bound by the serum proteins, independent of the assay method used, whereas streptomycin is bound under these conditions. In the absence of divalent cations all the aminoglycosides studied were bound by the serum proteins to varying degrees; the fewer the OH groups contained in the aminoglycoside molecule the greater the rate of protein binding. At equal protein concentration, the albumin fraction of the serum has as great a binding capacity for sisomicin as gamma globulin. Alpha-1 and beta-1 globulin, however, are unable to bind sisomicin.