1708 INTERACTIONS OF UNCONJUGATED BILIRUBIN WITH PHOSPHOLIPID COMPONENTS OF LUNG SURFACTANT

Abstract

This work examines the surface tension-area (O-A) behavior of mixed films of unconjugated bilirubin with dipalmitoyl phosphatidylcholine (DPPC) and 9:1 DPPC:di-oleoyl phosphatidylcholine (DOPC) at 22°C on 0.15M NaCl and buffered phosphate subphases (pH=5.5, 7.4 and 8.0). Also pure films of unconjugated bilirubin spread from chloroform solution are studied under similar temperature and pH conditions. In pure films bilirubin is found to be a surprisingly surface active molecule able to reach minimum surface tension values of ∼10 dynes/cm at pH5.5. In mixed films bilirubin exhibits interfacial interactions with DPPC and DPPC:DOPC in both high and low surface tension regimes. Most importantly bilirubin acts to increase respreading in films of DPPC and 9:1 DPPC:DOPC compressed past collapse,without any meaningful increase in the minimum surface tension achieved. Interactions in spread films of bilirubinphospholipid are of greatest magnitude at low pH values (5.5), where bilirubin is constrained to remain at the interface by virtue of negligible subphase solubility. The interactions decrease as pH increases to physiologic values and beyond (7.4 and 8.0), consistent with concomitant increasesin bilirubin solubility. Many infants with neonatal RDS also suffer hyperbilirubinenia with possible deposition of bilirubin in the alveoli (yellow hyaline membranes). Our results suggest that this bilirubin does not impair the activity of pulmonary surfactant in vivo, but further experiments with more complex films and subphases at body temperature would be required to make this definitive (Supp. by HL-25170)