Ionic binding of aminoglycosides to human serum albumin in the absence of divalent cations. IV. Effect of structure, ph and concentration.

The binding of sisomicin and streptomycin to human serum albumin was studied in the absence of divalent cations by means of the dialysis method. Hydrophobic bonds between albumin and sisomicin or streptomycin can be excluded by nuclear magnetic resonance measurements. The presence of hydrogen bonds is made unlikely according to the result that the binding of the aminoglycosides decreases with increasing number of OH groups in the aminoglycoside molecule. The pH dependence of protein binding suggests that ionic bonds are involved in the binding of aminoglycosides. On the basis of the concentration dependence of the albumin binding of sisomicin and streptomycin we determined the binding affinities delta F degrees, the binding constants K1, and the maximum number n of aminoglycoside molecules that can be bound by a molecule of albumin in the absence of Ca++ ions. The results were as follows: Sisomicin: delta F degrees = -4189 cal/mole, K1 = 900 1/mole, n = 12; Streptomycin: delta F degrees = 3512 cal/mole, K1 = 300 1/mole, n = 17.