Interaction of bilirubin with small unilamellar vesicles of dipalmitoylphosphatidylcholine.

Interaction of bilirubin with phospholipid bilayers was studied at physiological pH above and below the gel-liquid crystalline phase transition of small unilamellar vesicles of dipalmitoylphosphatidylcholine. Chromatographic, calorimetric and 1H-NMR evidences strongly suggest that dianion form of bilirubin binds to the polar heads of the phosphatidylcholines protruding from the outer leaflet of the vesicles, whilst acid bilirubin, which is insoluble in water, is hydrophobically inserted into the lipophilic region of the bilayers. The surface-bound bilirubin is promptly removed from vesicles, whilst the acid form hydrophobically inserted into the vesicles is firmly bound to the membrane in the gel state. This pool of bilirubin could perturb the chemico-physical properties of the membrane (i.e., fluidity, phase transition, etc. ...) thus contributing to perturbation of the biological properties of living cells.