Binding of bilirubin to mammalian erythrocytes

Binding of bilirubin to erythrocytes of various mammalian species in the presence of their respective plasma albumins was studied at pH 8.0, ionic strength 0.41 and at 37 degrees C. In human, buffalo, goat, and sheep, the amount of erythrocyte-bound bilirubin increased with the increase in both the bilirubin/albumin molar ratio (B/A) and the total bilirubin concentration. In all species, the binding patterns were qualitatively similar. However, at any given B/A, goat erythrocytes bound the highest amount of bilirubin, followed by buffalo and human erythrocytes; sheep erythrocytes bound the lowest amount of bilirubin. Increase in erythrocyte-bound bilirubin per unit increase in bilirubin concentration at a constant B/A as obtained from the values of the slope of the plot between erythrocyte-bound bilirubin and total bilirubin in the incubate, was found to be highest for goat erythrocytes, followed by buffalo, human, and sheep erythrocytes. At given bilirubin concentration, percentage fractional binding of bilirubin between any two B/As was found to be highest between 1.5 and 2.0, suggesting that a greater amount of bilirubin was transferred from plasma to cells between B/As 1.5 and 2.0. Percentage fractional binding of bilirubin was highest in goat erythrocytes, followed by buffalo, human, and sheep erythrocytes. These differences in the amount of erythrocyte-bound bilirubin were not due to the effect of various plasma albumins. These results suggest that the difference in bilirubin binding by different mammalian erythrocytes can be attributed to the difference in either the affinity of the erythrocyte receptors or their number or both.