THE KINETICS OF INTERACTIONS OF BILIRUBIN WITH LIPID BILAYERS AND WITH SERUM-ALBUMIN

Rate constants for the hydration of bilirubin bound to unilamellar bilayers of dioleoylphosphatidylcholine and albumin were measured by stopped-flow methods. Rate constants for association of bilirubin with these vesicles and albumin were calculated from measured rate constants for dissociation and the equilibrium binding constants of bilirubin and lipids or albumin. Rate constants for hydration (dissociation) for bilirubin bound to dioleoylphosphatidylcholine and albumin were 71 s-1 and 1.8 s-1 respectively. Rate constants for association were 4.0 10(7) s-1 and 1.1 10(9) M-1 s-1, respectively. Both rates for interactions of bilirubin with bilayers were essentially independent of temperature in the range 0-40-degrees-C, indicating that barriers to entry and exit of bilirubin from bilayers were entropic. Rates of transbilayer movement of bilirubin in dioleoylphosphatidylcholine were too fast to resolve by measuring rates of hydration of bilirubin. Rate constants for hydration of bilirubin bound to bilayers with less avidity for bilirubin as compared with dioleoylphosphatidylcholine also were too fast to measure with stopped-flow methods. In addition to providing details of the energetic basis for interactions between bilirubin and membranes, the data allow for calculating the maximal rates at which bilirubin could transfer spontaneously from sites on albumin in blood to the interior of cells. The data show, in this regard, that this rate is 10-50 fold faster than measured rates of uptake of bilirubin by intact liver.