Dimeric N-terminal segment of human surfactant protein B (dSP-B1-25) has enhanced surface properties compared to monomeric SP-B1-25

Surfactant protein B (SP-B) is a 17-kDa dimeric protein produced by alveola r type II cells. Its main function is to lower the surface tension by inser ting lipids into the air/liquid interface of the lung. SP-B's function can be mimicked by a 25-amino acid peptide, SP-B1-25, which is based on the N-t erminal sequence of SP-B. We synthesized a dimeric version of this peptide, dSP-B1-25, and the two peptides were tested for their surface activity. Bo th SP-B1-25, and dSP-B1-25, showed good lipid mixing and adsorption activit ies. The dimeric peptide showed activity comparable to that of native SP-B in the pressure-driven captive bubble surfactometer. Spread surface films l ed to stable near-zero minimum surface tensions during cycling while protei n free, and films containing SP-B1-25, lost material from the interface dur ing compression. We propose that dimerization of the peptide is required to create a lipid reservoir attached to the monolayer from which new material can enter the surface film upon expansion of the air/liquid interface. The dimeric state of SP-B can fulfill the same function in vivo.