INFLUENCE OF HEME ON THE KINETICS OF BILIRUBIN BINDING

Abstract

Non-bilirubin products of hemolysis have been suggested to reduce the bilirubin binding capacity of serum albumin. If these products interfere with the albumin binding of bilirubin a child with hemolytic disease would be at a higher risk for bilirubin encephalopathy at a given level of serum bilirubin than a child without hemolytic disease. Experiments were conducted to teat the hypothesis that heme interferes with the albumin binding of bilirubin. Aliquots of pooled cord sera with increasing concentrations of heme and unconjugated bilirubin were compared to control aliquots without heme. Unbound, unconjugated bilirubin was measured by enzymatic oxidation. Heme concentrations of greater than 7.5μM resulted in significantly higher levels of unbound bilirubin (paired t, p<0.001) over a wide range of total bilirubin concentrations. The dissociation constant of the primary binding site for control and heme containing aliquots (Heme=15μM) ware 3.9 × 10−8 and 8.3 × 10−8 reapectivly. The second class of binding sites appeared to be similarly affected. Aa one of the non-bilirubin products of hemolysis, heme appears to interfere with both the primary and secondary binding sites for unconjugated bilirubin.