Erythromycin binding to human serum.

Erythromycin binding to human serum was measured under conditions of binding equilibrium. The binding is sensitive to pH changes, decreasing at acid pH. Over a great range of serum dilution, the bound fraction is semilogarithmically related to serum concentration. Binding is shown to be completely reversible. With increasing erythromycin concentration a specific part of binding is saturable and specifically displaceable by erythromycin is specifically bound to a single class of noninteracting binding sites with an apparent dissociation constant Kd = 5.9 microM (38 degrees C). The kinetic and thermodynamic parameters at 25 degrees are: Kd = 8.4 microM, delta H degrees = +4.4 X 10(3) cal per mole, delta G degrees = 6.9 X 10(3) cal per mole, delta S degrees = +38 e.u.