Characterization of rabbit surfactant-associated proteins.

In the present study, the apolipoproteins associated with a purified surfactant fraction isolated from lung lavage of adult rabbits were characterized. Surfactant purity was assessed by the glycerophospholipid composition and by electron microscopic examination. The purified surfactant was delipidated and the apolipoproteins were analyzed by two-dimensional polyacrylamide gel electrophoresis. By use of this technique at least eight proteins or families of proteins were found to be associated with surfactant. Four of these apolipoproteins were families of proteins of 55-70, 29-36, 26-28 and 22-23 kilodaltons (kDa). All of these apolipoprotein families had acidic isoelectric points (pI less than or equal to 5.6), and were specifically bound to a Con A-Sepharose matrix, indicative that these apolipoprotein families are modified by oligosaccharide side-chains. The finding that neuraminidase treatment degraded the 29-36 kDa family is suggestive that this apolipoprotein family contains sialic acid residues. Three major proteins of 66, 43-45 and 35 kDa and a minor protein of 86 kDa were also observed. These proteins had isoelectric points in the more neutral range (pI 6.0-6.5). The 66 kDa protein (pI 6.4) had the same apparent molecular weight and isoelectric point as the major protein of delipidated rabbit serum and as purified rabbit albumin, which suggests that this protein is albumin. These findings are indicative that the apolipoproteins of surfactant are more numerous and complex than previously reported.