Ceftriaxone binding to human serum albumin: competition with bilirubin

Ceftriaxone, a cephalosporin, is bound reversibly to defatted human serum albumin from adults, with a first stoichiometric binding constant of 60,000 M-1, as found by equilibrium dialysis at pH 7.4, 37 degrees. A second molecule is weakly bound, with a binding constant of 500 M-1. Possible cobinding with bilirubin was studied by the peroxidase method and by equilibrium dialysis with and without added bilirubin. Results indicated competitive binding; formation of an albumin complex containing both bilirubin and ceftriaxone could not be demonstrated. Light absorption spectra of bilirubin-albumin showed little change on addition of ceftriaxone, in agreement with the competitive biding mechanism. Binding to serum albumin from newborn infants is weaker than to the protein from adults, with the first binding constant being about 36,000 M-1. Cobinding of ceftriaxone and bilirubin to albumin from newborn infants is likewise competitive. It is concluded that ceftriaxone is a strong bilirubin displacer with a potential of inducing bilirubin encephalopathy in predisposed newborns.