Printed in U.S.A. DETERMINATION OF BINDING CONSTANTS OF SERUM ALBUMIN FOR PENICILLIN1

(benzylpenicillin, oxacillin, methicillin, nafcillin) and cephaloridine were measured in the presence of human and bovine serum albumin by means of a photometric assay of growth inhibition of Staphylococcus aureus. The decrease in antibiotic activity in the presence of albumin was assumed to reflect the antibiotic binding to the protein which other workers have demonstrated by direct physicochemical means and by the diminution of the concen-tration of free, active antibiotic. The percent of penicillin bound or inactivated was calcu-lated from the decreased antimicrobial activity in the presence of albumin. In the concen-tration range near therapeutic dosages, this percent antibiotic bound was independent of the antibiotic concentration and resembled a logarithmic function of the albumin concen-tration. Binding constants for bovine serum albumin were calculated from a simple equation for chemical equilibrium giving these values for the primary binding site: nafcillin, 3.1