Revised Manuscript Received May 22, 1990 abstract: Although a monolayer of dipalmitoylphosphatidylcholine, the major component of pulmonary surfactant, is thought to be responsible for the reduction of the surface tension at the air-liquid interface of the alveolus, the participation of unsaturated and anionic phospholipids and the three surfactant-associated proteins is suggested in the generation and maintenance of this surface-active monolayer. We have examined the effects of surfactant-associated protein A (SP-A) purified from bovine lavage material on the surface activity of lipid extract surfactant (LES), an organic extract of pulmonary surfactant containing all of the phospholipids and SP-B and SP-C, but lacking SP-A. Measurements of the surface tension during dynamic compression were made on a pulsating bubble surfactometer. Additionof SP-A to LES reduces the number of pulsations required to attain surface tensions near zero at minimum bubbleradius. This increase in surface activity is dependent upon the presence of Ca2+ in the assay mixture. Maximal enhancement is observed at or below 1% of the lipid concentration (w/w). The addition of two blood proteins, fibrinogen and albumin, at physiological concentrations to LES causes severe inhibition of surface activity. Addition of SP-A in the presence of Ca2+ completely counteracts the inhibition by fibrinogen. The amount of SP-A required for full reversal of this inhibition was less than 0.5% of the lipid concentration. Complete reversal of inhibition by albumin was also observed, even though there was a~

5000-fold molar excess of inhibitor. Addition of lysophosphatidylcholine also inhibits LES; however, SP-A has no effect on this inhibition. e type II epithelial cell produces and secretes pulmonary surfactant which serves to reduce the surface tension across the air-liquid interface of the alveolus, andhence facilitates breathing. This substance, composed of approximately 90% lipids and 10% protein, reduces surface tension during breathing by generating a monolayer at the interface enriched in the saturated phospholipiddipalmitoylphosphatidylcholine (DPPC)'(Goerke, 1974; King, 1984; Notter et al., 1984;