Surfactant apolipoprotein from adult and fetal lung is related to cytoplasmic actin

The concentration of the major apolipoprotein of pulmonary surfactant (M.Wt. = 36,000 - 45,000) has been reported to increase in amniotic fluid after 29 weeks gestation. We wished to evaluate the role of this protein in the development of surfactant secretion by fetal lung. Apolipoprotein was identified in bronchioalveolar lavage of adult lung and then purified from adult human lung homogenate. The purified protein was a polymer of sub-unit M.Wt 42,000 and was virtually the only protein from adult lung cytosol that bound to an emulsion of dipalmitoylphosphatidylcholine. Surprisingly, the concentration of this protein in immature fetal lung (14 - 17 weeks gestation) was similar to that of adult lung. It was detected not only in the surfactant fraction of term amniotic fluid but also in the particulate fraction of immature amniotic fluid. A comparison of their sub-unit M.Wts., isoelectric points and limited proteolysis patterns suggested that surfactant apolipoprotein is related to cytoplasmic actin. The physiological role of surfactant apolipoprotein is probably involved in the intra-cellular migration and exocytosis of lamellar bodies. Its presence early in gestation could then be due to actin not specifically related to surfactant.