1037 FACTORS DETERMINING THE CELLULAR UPTAKE OF BILIRUBIN

The toxicity of bilirubin is a function of the amount of bilirubin bound to cellular elements and the sensitivity of the cell to a given bilirubin load. We have examined the stoichiometry of cell binding of bilirubin by incubating red blood cells with various concentrations of bilirubin and serum in vitro, measuring the serum unbound bilirubin concentration (UBC) with the peroxidase assay, and extracting the RBC bilirubin with a human serum albumin solution.Results: (1) The partitioning of the total bilirubin pool between serum and cells reached equilibrium within 10 minutes.(2) At equilibrium, the cellular content of bilirubin was a function of the UBC and not the total bilirubin concentration or bilirubin/albumin molar ratio.(3) Albumin binding of bilirubin was not affected by pH.(4) Cellular uptake of bilirubin was greatly influenced by pH. At physiological pH, bilirubin exists predominantly as an anion with only a small fraction present as the bilirubin acid. The increased bilirubin uptake at pH 7.0 vs. pH 7.4 corresponded to the calculated increase in the less soluble protonated bilirubin.Conclusions: Cellular uptake of bilirubin is a function of the concentration of free bilirubin acid. It is probable that the risk for kernicterus (with respect to blood chemistries) can be estimated by measuring the serum UBC and blood pH.