Experimental studies on the pulmonary surfactant. Reconstitution of surface-active material.

The conditions for the reconstitution of surfactant lipoprotein were investigated using pig pulmonary surfactant. Lipids of surface-active material (SAM) were extracted with ethanol-ether and the residue, containing mainly protein, was extracted first with dilute sodium borate buffer (pH 10.0) (fraction A) followed by extraction with deoxycholate solution (fraction B). The former (A) contained mainly protein having a nominal molecular weight of 34,000 daltons, albumin and globulin, and the latter (B) 34,000 and 15,000 daltons. Reconstitution of lipids and proteins using these two fractions revealed a greater affinity of fraction B to lipids than that of fraction A, when lipids and proteins were dialyzed against pH 7.4 Tris-HCl. Observation by freeze-fracture methods revealed that protein particles were incorporated into liposomes. Protein-lipid ratio of this complex (LB) could be regulated by altering the amount of lipids and protein during the dialysis. Fraction A was incorporated into LB (LBA) by dialyzing LB with fraction A at an acidic pH. With this procedure, protein having a nominal molecular weight of 34,000 daltons was selectively incorporated. In negatively stained preparations, LBA had a granular appearance in the area limited by lipid membrane. We suggest that in the lipid-protein complex of SAM, two different proteins are present; a hydrophobic one in lipids and more hydrophilic ones in the water phase that have a close association with the former.