Two hydrophobic low-molecular-mass protein fractions of pulmonary surfactant. Characterization and biophysical activity.

Hydrophobic low-molecular-mass proteins were isolated from minced pig lungs and separated into two fractions. Electrophoresis of protein fraction 1 showed two major bands. Calculations of molecular masses from the electrophoretic mobilities are unreliable because of the extreme hydrophobicity of the peptides. However, the two bands were at positions corresponding to apparent molecular masses of about 3 kDa and 14 kDa, while sequence degradation disclosed only one major structure. Electrophoretic separation of protein fraction 2 revealed one band, at an apparent molecular mass of about 6 kDa. Microheterogeneities at the N terminus of both fractions were observed. However, the two fractions had different N-terminal structures and amino acid compositions. Consequently they are concluded to represent different polypeptides without common segments. Bronchoalveolar lavage from humans also contains surfactant polypeptides and at least the fraction 2 peptide is highly similar in human and porcine surfactants. Artificial surfactant preparations, obtained by recombination of protein fraction 1 or 2 with a mixture of synthetic phospholipids, were evaluated with the pulsating bubble method and in experiments on artificially ventilated premature newborn rabbits. The addition of protein fraction 1 to the phospholipid mixture improved surface adsorption from more than 300 s to about 2 s and reduced minimum surface tension from more than 20 mN/m to nearly 0 as measured with a pulsating bubble. When this surfactant preparation was instilled into the airways of newborn rabbits, the tidal volumes at insufflation pressure 25 cm H2O was increased about twentyfold compared to the volumes obtained in non-treated controls. Preparations based on protein fraction 1 had better in vitro and in vivo properties than those based on protein fraction 2. Both these protein-based preparations were decidedly more effective than phospholipids alone.