The effect of pH on bilirubin binding by human erythrocytes.

The effect of pH on bilirubin binding by human erythrocytes was studied by incubating erythrocytes with solutions of bilirubin and albumin at pH levels 7.4-6.8. A reduction in pH from 7.4 greatly increased the cellular binding of bilirubin at all molar bilirubin/albumin ratios, even below the 1/1 limit. Spectrophotometric studies indicated that mainly the spectrum of the bilirubin molecule bound at the second binding site on the albumin molecule is affected by low pH. When cells were incubated with bilirubin without albumin, increased cellular binding of bilirubin at low pH was also found. However, the bilirubin concentrations greatly exceeded the solubility of bilirubin. The results and their possible clinical implications are discussed.