Effect of pH and temperature on the binding of bilirubin to human erythrocyte membranes

Effect of pH and temperature on the binding of bilirubin to human erythrocyte membranes was studied by incubating the membranes at different pH and temperatures and determining the bound bilirubin. At all pH values, the amount of membrane-bound bilirubin increased with the increase in bilirubin-to-albumin molar ratios (B/As), being highest at lower pH values in all cases. Further, linear increase in bound bilirubin with the increase in bilirubin concentration in the incubate was observed at a constant B/A and at all pH values. However, the slope value increased with the decrease in pH suggesting more bilirubin binding to membranes at lower pH values. Increase in bilirubin binding at lower pH can be explained on the basis of increased free bilirubin concentration as well as more conversion of bilirubin dianion to monoanion. Temperature dependence of bilirubin binding to membranes was observed within the temperature range of 7 degrees-60 degrees C, showing minimum binding at 27 degrees C and 37 degrees C which increased on either side. Increase in bilirubin binding at temperatures lower than 20 degrees C and higher than 40 degrees C can be ascribed to the change in membrane topography as well as bilirubin-albumin interaction.