Abstract. This article starts with a brief account of the history of research on pulmonary surfactant. We will then discuss the morphological aspects and composition of the pulmonary surfactant system. We describe the hydrophilic surfactant proteins A and D and the hydrophobic surfactant proteins B and C, with focus on the crucial roles of these proteins in the dynamics, metabolism, and functions of pulmonary surfactant. Next we discuss the major disorders of the surfactant system. The final part of the review will be focused on the potentials and complications of surfactant therapy in the treatment of some of these disorders. It is our belief that increased knowledge of the surfactant system and its functions will lead to a more optimal composition of the exogenous surfactants and, perhaps, widen their applicability to treatment of surfactant disorders other than neonatal respiratory distress syndrome.Key words: Surfactant protein-Pulmonary surfactant-Respiratory distress syndrome. HistoryResearch on surfactant goes back to 1929 when von Neergaard published the first paper about the difference in pressure needed to inflate lungs with air or with liquid [333]. He found that the pressure necessary for filling the lungs with air was higher than when the lungs were filled with liquid. To explain this result he stated that the alveoli were stabilized by lowering the naturally high surface tension of the air/water interface. In 1946 Thannhauser and co-workers reported that lung tissue has a remarkably high content of the lipid dipalmityl lecithin (current name, dipalmitoylphosphatidylcholine)Offprint requests to: Henk P. Haagsman.

Pulmonary surfactant protein C (SP-C) is a small, extremely hydrophobic peptide with a highly conservative primary structure. The protein is characterized by two adjacent palmitoylated cysteine residues, two positively charged residues (one arginine residue and one lysine residue) in the N-terminal region, and a long hydrophobic stretch. SP-C enhances the adsorption of phospholipids into an air-water interface. To determine the importance of the positively charged residues, we carried out experiments with natural porcine SP-C and modified porcine SP-C (SP-Cm) in which the positive charges had been blocked by phenylglyoxal. Circular dichroism experiments showed that SP-Cm had an increased content of alpha-helix. Natural SP-C, but not SP-Cm, catalyzed insertion of phospholipids into a monolayer at the airwater interface. This reduced insertion was due to a strong reduction of binding of phospholipid vesicles to the monolayer. The insertion catalyzed by the natural porcine SP-C was decreased by an increased pH of the subphase. In contrast to natural SP-C, SP-Cm induced lipid mixing between phospholipid vesicles. The extent of lipid mixing was a function of the SP-C content. We conclude that the positively charged residues of SP-C are important for the binding of phospholipid vesicles to the monolayer, a process that precedes the insertion of phospholipids into the monolayer.

Separation of surfactant subtypes is now commonly used as a parameter in assessing the amount of active compared with inactive material in various models of lung injury. The protein content, morphology and surface activity were determined of the heavy and light subtype isolated by differential centrifugation. Here we report the presence of surfactant proteins B and C in the heavy subtype but not in the light subtype. Adsorption studies revealed that separation of fast adsorbing bronchoalveolar lavage resulted in slowly adsorbing heavy and light subtypes. Surfactant, reconstituted from heavy and light fractions, did not show a high adsorption rate. It is concluded that the isolation procedures might result in a loss of fast adsorbing surfactant structures. Surface area cycling was used as a model in vitro for the extracellular surfactant metabolism. The heavy subtype is converted into the light subtype during conversion. Conversion performed with resuspended heavy subtype revealed the generation of a disparate subtype. Furthermore it was found that the conversion was dependent on preparation and handling of the samples before cycling. Finally, adsorption studies at low surfactant concentrations revealed a delayed adsorption of lipid-extracted surfactants compared with natural surfactants. These observations emphasize the importance of the (surfactant-associated protein A-dependent) structural organization of surfactant lipids in the adsorption process.

No abstract

Pulmonary surfactant is a mixture of (phospho)lipids and surfactant specific proteins, lining the alveolar space. During each respiration cycle phospholipids are transferred between the phospholipid monolayer at the air/water interface and a variety of underlying membranes. Surfactant proteins may play a role in facilitating the insertion and removal of phospholipids by affecting the lipid organization of the bilayer and monolayer. The experiments described in this article were carried out in order to investigate the influence of surfactant protein B (SP-B) on the distribution of phospholipids in membranes and on the mixing of lipids between membranes. To determine the distribution of the non-labeled phospholipids in small unilamellar vesicles (SUV), the relative clustering of pyrene-labeled phospholipids was used, by measuring the ratio of excimer-to-monomer (E/M) pyrene fluorescence. In the absence of SP-B it was found that the clustering of the pyrenePC molecules was dependent on the proportion of saturated acyl chains and not on the proportion of negative charges. Addition of the positively charged SP-B to a mixture of DPPC and PG, led to an increase of approximately 20% in E/M ratio, indicating a clustering of the negatively charged PG molecules. This effect was intensified by addition of calcium ions. If pyrenePC-containing SUV were mixed with excess non-labeled SUV in the presence of SP-B and calcium ions, the E/M ratio decreased, corresponding with a flow of the pyrenePC molecules into the acceptor membranes. It is concluded that presence of domains of phospholipids can be detected with the use of pyrene-labeled phospholipids. Furthermore, SP-B showed a concentrating effect on the distribution of the negatively charged phospholipids, a process that could be important in regulating the phospholipid composition of the monolayer.