Phospholipid binding and biophysical activity of pulmonary surfactant-associated protein (SAP)-35 and its non-collagenous COOH-terminal domains

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Surfactant dysfunction in SP-A-/- and iNOS-/- mice with mycoplasma infection

Surfactant dysfunction was studied in C57BL/6 (B6), B6.SP-A(-/-), and B6.iNOS(-/-) mice with pulmonary mycoplasma infection (10(7) colony-forming units). Cell-free bronchoalveolar lavage (BAL) from uninfected B6.SP-A(-/-) versus B6 mice had a reduced content of very large aggregates (VLA) and an increase in intermediate large aggregates (ILA), with no difference in total large aggregates (LA = VLA + ILA). However, LA from uninfected B6.SP-A(-/-) versus B6 mice contained less protein and were more sensitive to inhibition by serum albumin and lysophosphatidylcholine in pulsating bubble studies in vitro. Infection with Mycoplasma pulmonis caused significant lung injury and surfactant abnormalities in B6.SP-A(-/-), B6.iNOS(-/-), and B6 mice at 24, 48, 72 h after infection compared with uninfected mice of the same strain. Analyses of time-pooled data indicated that mycoplasma-infected B6.SP-A(-/-) and B6.iNOS(-/-) mice had significantly lower levels of LA and higher protein/phospholipid ratios in BAL compared with infected B6 mice. Infected B6.iNOS(-/-) versus B6 mice also had increased minimum surface tensions on the pulsating bubble and decreased levels of surfactant protein (SP)-B in BAL. These results indicate that pulmonary mycoplasma infection in vivo causes lung injury and surfactant abnormalities that are dependent in part on iNOS and SP-A. In addition, SP-A deficiency modifies surfactant aggregate content and lowers the inhibition resistance of LA surfactant in vitro compared with congenic normal mice.

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.

Surfactant proteins A and D bind CD14 by different mechanisms

Surfactant proteins A (SP-A) and D (SP-D) are lung collectins that are constituents of the innate immune system of the lung. Recent evidence (Sano, H., Sohma, H., Muta, T., Nomura, S., Voelker, D. R., and Kuroki, Y. (1999) J. Immunol. 163, 387-395) demonstrates that SP-A modulates lipopolysaccharide (LPS)-induced cellular responses by direct interaction with CD14. In this report we examined the structural elements of the lung collectins involved in CD14 recognition and the consequences for CD14/LPS interaction. Rat SP-A and SP-D bound CD14 in a concentration-dependent manner. Mannose and EDTA inhibited SP-D binding to CD14 but did not decrease SP-A binding. The SP-A binding to CD14 was completely blocked by a monoclonal antibody that binds to the SP-A neck domain but only partially blocked by an antibody that binds to the SP-A lectin domain. SP-A but not SP-D bound to deglycosylated CD14. SP-D decreased CD14 binding to both smooth and rough LPS, whereas SP-A enhanced CD14 binding to rough LPS and inhibited binding to smooth LPS. SP-A also altered the migration profile of LPS on a sucrose density gradient in the presence of CD14. From these results, we conclude that 1) lung collectins bind CD14, 2) the SP-A neck domain and SP-D lectin domain participate in CD14 binding, 3) SP-A recognizes a peptide component and SP-D recognizes a carbohydrate moiety of CD14, and 4) lung collectins alter LPS/CD14 interactions.

Lipoglycans are putative ligands for the human pulmonary surfactant protein A attachment to mycobacteria. Critical role of the lipids for lectin-carbohydrate recognition

The human pulmonary surfactant protein A (hSP-A) has been implicated in the early capture and phagocytosis of the pathogenic Mycobacterium tuberculosis by alveolar macrophages. In this report, we examined the interaction of alveolar proteinosis patient hSP-A with Mycobacterium bovis BCG, the vaccinating strain, as a model of pathogenic mycobacteria, and Mycobacterium smegmatis, a nonpathogenic strain. We found that hSP-A binds to the surface of M. bovis BCG, but also to a slightly lesser extent, to M. smegmatis, indicating that hSP-A does not discriminate between virulent and nonpathogenic strains. Among the various glycoconjugates isolated from the mycobacterial envelope, we found that the best ligands are the two major lipoglycans: the mannosylated lipoarabinomannan (ManLAM) and the lipomannan. In contrast, the mannose-capped arabinomannan, structurally close to the ManLAM, as well as the LAMs from the non pathogenic M. smegmatis are poorly recognized by hSP-A. These results clearly show that the presence of both the terminal mannose residues and the phophatidyl-myo-inositol anchor are necessary to achieve the highest binding affinity. Selective removal of either the terminal mannose or the acyl residues esterifying the glycerol moiety of the ManLAM abrogates the interaction with hSP-A, further supporting the notion that the hSP-A recognition of the carbohydrate epitopes of the lipoglycans is dependent of the presence of the fatty acids.

Deletion mapping of N-terminal domains of surfactant protein A. The N-terminal segment is required for phospholipid aggregation and specific inhibition of surfactant secretion

The objective of the current study was to examine the functional importance of the N-terminal domains of surfactant protein A (SP-A) including the N-terminal segment from Asn1 to Ala7 (denoted domain 1), the N-terminal portion of the collagen domain from Gly8 to Gly44 (domain 2), and the C-terminal portion of the collagen-like domain from Gly45 to Pro80 (domain 3). Wild type recombinant SP-A (SP-Ahyp; where hyp indicates hydroxyproline-deficient) and truncated mutant (TM) SP-As containing deletions of domain(s) 1 (TM1), 2 (TM2), 1 and 2 (TM1-2), and 1, 2, and 3 (TM1-2-3) were synthesized in insect cells and purified by mannose-Sepharose affinity chromatography. N-terminal disulfide-dependent dimerization was preserved at near wild type levels in the TM1-2 (at Cys-1) and TM2 proteins (at Cys-1 and Cys6), and to a lesser extent in TM1 (at Cys-1), but not in TM1-2-3. Cross-linking analyses demonstrated that the neck + CRD was sufficient for assembly of monomers into noncovalent trimers and that the N-terminal segment was required for the association of trimers to form higher oligomers. All TM proteins except TM1-2-3 bound to phospholipid, but only the N-terminal segment containing TM proteins aggregated phospholipid vesicles. The TM1, TM1-2, and TM2 but not the TM1-2-3 inhibited the secretion of surfactant from type II cells as effectively as SP-Ahyp, but the inhibitory activity of each mutant was blocked by excess alpha-methylmannoside and therefore nonspecific. TM1 and TM1-2-3 did not enhance the uptake of phospholipids by isolated type II cells, but the TM1-2 and TM2 had activities that were 72 and 83% of SP-Ahyp, respectively. We conclude the following for SP-A: 1) trimerization does not require the collagen-like region or interchain disulfide linkage; 2) the N-terminal portion of the collagen-like domain is required for specific inhibition of surfactant secretion but not for binding to liposomes or for enhanced uptake of phospholipids into type II cells; 3) N-terminal interchain disulfide linkage can functionally replace the N-terminal segment for lipid binding, receptor binding, and enhancement of lipid uptake; 4) the N-terminal segment is required for the association of trimeric subunits into higher oligomers, for phospholipid aggregation, and for specific inhibition of surfactant secretion and cannot be functionally replaced by disulfide linkage alone for these activities.

Cation-mediated conformational variants of surfactant protein A

Surfactant protein A (SP-A) is the major protein of pulmonary surfactant. This protein is implicated in regulating surfactant secretion, alveolar processing, recycling, and in non-serum-induced immune response. An increasing body of work indicates the importance of cations, particularly calcium, on SP-A function. However, little information exists on the effects of cations on SP-A quaternary structure. Here, the quaternary organisation of bovine surfactant protein A in the presence of cations has been quantitatively and systematically studied by transmission electron microscopy. The conformation of SP-A is altered by the presence of cations, especially calcium, then sodium, and to a small extent, magnesium. There is a transition concentration, unique for each cation, at which a conformational switch occurs. These transition concentrations are: 5 mM for CaCl2, 100 mM for NaCl and 1 mM for MgCl2. Below these concentrations, SP-A exists primarily in an opened form with a large head diameter of 20 nm; above it, SP-A is mostly in a closed form due to a compaction of the headgroups resulting in a head diameter of 11 nm. There is a corresponding increase in particle length from 17 nm for opened SP-A to 20 nm for closed SP-A. The fact that the transition concentrations are within physiological range suggests that cation-mediated conformational changes of SP-A could be operative in vivo.

Structure, processing and properties of surfactant protein A

Surfactant protein A (SP-A) is a highly ordered, oligomeric glycoprotein that is secreted into the airspaces of the lung by the pulmonary epithelium. The in vitro activities of protein suggest diverse roles in pulmonary host defense and surfactant homeostasis, structure and surface activity. Functional mapping of SP-A using directed mutagenesis has identified domains which interact with surfactant phospholipids, alveolar type II cells and microbes. Recently developed genetically manipulated animal models are beginning to clarify the critical physiological roles for SP-A in the normal lung, and in the pathophysiology of pulmonary disease.

Structural changes of surfactant protein A induced by cations reorient the protein on lipid bilayers

Surfactant protein A (SP-A) is an octadecameric hydrophilic glycoprotein and is the major protein component of pulmonary surfactant. This protein complex plays several roles in the body, such as regulation of surfactant secretion, recycling and adsorption of surfactant lipids, and non-serum-induced immune response. Many of SP-A's activities are dependent upon the presence of cations, especially calcium. Here, we have studied in vitro the effect of cations on the interaction of purified bovine SP-A with phospholipid vesicles made of dipalmitoylphosphatidylcholine and unsaturated phosphatidylcholine. We have found that SP-A octadecamers exist in an "opened-bouquet" conformation in the absence of cations and interact with lipid membranes via one or two globular headgroups. Calcium-induced structural changes in SP-A lead to the formation of a clearly identifiable stem in a "closed-bouquet" conformation. This change, in turn, seemingly results in all of SP-A's globular headgroups interacting with the lipid membrane surface and with the stem pointing away from the membrane surface. These results represent direct evidence that the headgroups of SP-A (comprising carbohydrate recognition domains), and not the stem (comprising the amino-terminus and collagen-like region), interact with lipid bilayers. Our data support models of tubular myelin in which the headgroups, not the tails, interact with the lipid walls of the lattice.

Surfactant protein-A: new insights into an old protein--Part I

Pulmonary surfactant is a lipoprotein substance that lines the lungs and helps reduce surface tension. Surfactant associated protein-A (SP-A) is the most abundant non-serum protein in pulmonary surfactant. This complex glycoprotein aids in the synthesis, secretion and recycling of surfactant phospholipids, and facilitates the reduction of surface tension by surfactant phospholipids. Recent evidence has highlighted the role of SP-A in the innate immune system present in the lung. SP-A may play a major role in defense against pathogens by interacting with both infectious agents and the immune system. Factors that affect fetal lung maturation, e.g., gestational age and hormones, regulate SP-A gene expression. Mediators of immune function also regulate SP-A levels. A number of lung disorders, including infectious diseases and respiratory distress syndrome are associated with abnormal alveolar SP-A levels. SP-A can no longer be called a lung-specific protein, since it has recently been detected in other tissues. In most species, SP-A is encoded by a single gene, however in humans it is encoded by two, very similar genes. Models for the structure of the human SP-A protein molecule have been proposed, suggesting that the mature alveolar SP-A molecule is composed of both gene products. The study of SP-A may provide information helpful in understanding disease processes and formulating new treatment modalities.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

Effect of acidic pH on the structure and lipid binding properties of porcine surfactant protein A. Potential role of acidification along its exocytic pathway

Pulmonary surfactant protein A (SP-A) is synthesized by type II cells and stored intracellularly in secretory granules (lamellar bodies) together with surfactant lipids and hydrophobic surfactant proteins B and C (SP-B and SP-C). We asked whether the progressive decrease in pH along the exocytic pathway could influence the secondary structure and lipid binding and aggregation properties of porcine SP-A. Conformational analysis from CD spectra of SP-A at various pH values indicated that the percentage of alpha-helix progressively decreased and that of beta-sheet increased as the pH was reduced. The protein underwent a marked self-aggregation at mildly acidic pH in the presence of Ca2+, conditions thought to resemble those existing in the trans-Golgi network. Protein aggregation was greater as the pH was reduced. We also found that both neutral and acidic vesicles either with or without SP-B or SP-C bound to SP-A at acidic pH as demonstrated by co-migration during centrifugation. However, the binding of acidic but not neutral vesicles to SP-A led to 1) a striking change in the CD spectra of the protein, which was interpreted as a decrease of the level of SP-A self-aggregation, and 2) a protection of the protein from endoproteinase Glu-C degradation at pH 4.5. SP-A massively aggregated acidic vesicles but poorly aggregated neutral vesicles at acidic pH. Aggregation of dipalmitoylphosphatidylcholine (DPPC) vesicles either with or without SP-B and/or SP-C strongly depended on pH, being progressively decreased as the pH was reduced and markedly increased when pH was shifted back to 7.0. At the pH of lamellar bodies, SP-A-induced aggregation of DPPC vesicles containing SP-B or a mixture of SP-B and SP-C was very low, although SP-A bound to these vesicles. These results indicate that 1) DPPC binding and DPPC aggregation are different phenomena that probably have different SP-A structural requirements and 2) aggregation of membranes induced by SP-A at acidic pH is critically dependent on the presence of acidic phospholipids, which affect protein structure, probably preventing the formation of large aggregates of protein.

Mutational analysis of Arg197 of rat surfactant protein A. His197 creates specific lipid uptake defects

In previous studies, tandem mutagenesis of Glu195 and Arg197 of surfactant protein A (SP-A) has implicated both residues as critical participants in the interaction of the molecule with alveolar type II cells and phospholipids. We substituted Ala, Lys, His, Asp, and Asn mutations for Arg to evaluate the role of a basic amino acid at position 197 in SP-A action. Unexpectedly, Ala197 retained complete activity in the SP-A functions of carbohydrate binding, type II cell binding, inhibition of surfactant secretion, lipid binding, lipid aggregation, and lipid uptake by type II cells. The results unambiguously demonstrate that Arg197 is not mechanistically essential for SP-A function. The Lys197 mutation displayed all functions of the wild type protein but exhibited a 2-fold increase in lipid uptake activity. The His197 mutation displayed all SP-A functions studied except for lipid uptake. The results obtained with the His197 mutation clearly demonstrate that lipid aggregation alone by SP-A is insufficient to promote lipid uptake by type II cells. These findings indicate that specific interactions between type II cells and SP-A are involved in the phospholipid uptake processes.

The Cys6 intermolecular disulfide bond and the collagen-like region of rat SP-A play critical roles in interactions with alveolar type II cells and surfactant lipids

Rat pulmonary surfactant protein A is an oligomer of 18 polypeptide chains which are associated by triple helix formation in the collagen-like domain and interchain disulfide bridges at the NH2 terminus. The roles of the intermolecular bond at Cys6 and the collagen-like domain (Gly8-Pro80) in the interactions of SP-A with phospholipids and alveolar type II cells were investigated using mutant forms of the protein. Wild type SP-A (SP-Ahyp), SP-A with the substitution Cys6 --> Ser to prevent disulfide formation (SP-Ahyp, C6S), and SP-A with the collagen-domain deleted (SP-ADeltaG8-P80) were synthesized in insect cells using recombinant baculoviruses. The SP-As were glycosylated and secreted from the invertebrate cells and the binding affinities of the wild type and mutant proteins for the mannose-Sepharose matrix used for purification were nearly identical. The SP-Ahyp and SP-ADeltaG8-P80 were at least nonameric in solution based on gel exclusion chromatography, and demonstrated extensive sulfhydryl-dependent oligomerization under nonreducing conditions. The SP-Ahyp,C6S was also oligomeric in solution and formed disulfide-dependent dimers, indicating the presence of at least one additional interchain disulfide bond. The SPADeltaG8-P80 but not the SP-Ahyp,C6S aggregated lipid vesicles at 20 degrees C and augmented the surface tension lowering effect of extracts of natural surfactant. The SP-ADeltaG8-P80 competed poorly with native SP-A for receptor occupancy on isolated alveolar type II cells and was a potent but nonspecific (concanavalin A-like) inhibitor of surfactant secretion. In contrast, the SP-Ahyp,C6S partially competed for receptor occupancy and weakly inhibited surfactant secretion in a specific manner. Neither the SP-ADeltaG8-P80 nor the SP-Ahyp,C6S supported the association of phospholipid liposomes with type II cells. We conclude that: 1) the Cys6 interchain disulfide bond of SP-A is required for aggregation of liposomes and for potent inhibition of surfactant secretion. 2) The collagen-like region is required for competition with 125I-SP-A for receptor occupancy and specific inhibition of surfactant secretion in the presence of competing sugars. 3) Both the NH2-terminal disulfide and the collagen-like region are required to enhance the association of phospholipid vesicles with type II cells.

Reversible calcium-dependent interaction of liposomes with pulmonary surfactant protein A. Analysis by resonant mirror technique and near-infrared light scattering

Surfactant protein A (SP-A) is crucial for lung function, including tubular myelin formation and lipid uptake by type II pneumocytes. Known properties of SP-A in vitro are its Ca2+-dependent interaction with phospholipids and its role in the aggregation of liposomes. To dissect and to analyze these processes, we have immobilized SP-A and measured binding of liposomes by the resonant mirror technique. Liposome aggregation was followed separately by kinetic light scattering in suspensions. It was found that SP-A-mediated binding and aggregation of liposomes have a common K0.5 of 20 microM for free Ca2+, independent of the species (sheep, rat, or cow) and of the phospholipid composition, and that both reactions exhibit the same high cooperativity (Hill coefficients of 6-9) for Ca2+ ions. However, binding of liposomes to SP-A is >10-fold faster than aggregation. Both processes are completely reversed by low Ca2+ concentrations, but liposomes dissociate from SP-A in <0.3 s, whereas disaggregation of the liposomes takes approximately 30 s. At equilibrium, the level of aggregation depends on the concentration of free SP-A. We interpret these results to be a rapid and reversible sequence of three reactions: (i) a cooperative Ca2+-dependent conformational change in SP-A, (ii) binding of Ca2+-bound SP-A to liposomes, and (iii) aggregation of the Ca2+/SP-A-bound liposomes.

Secretagogues increase the expression of surfactant protein A receptors on lung type II cells

Since secretagogues have been shown to increase the internalization of surfactant phospholipid and protein by lung cells, we postulated that their action occurred through a mechanism involving increased surfactant protein A (SP-A) receptor density. Therefore, we evaluated the influence of secretagogues on the binding of iodinated SP-A to alveolar type II cells. Type II cells were isolated from rat lung and maintained in primary culture for 18 h on Transwell membranes. Upon exposure to 8-bromo-cyclic AMP (cAMP, 0.1 mM), phorbol 12-myristate 13-acetate (PMA, 10 nM), terbutaline (0.1 mM), or ATP (1 mM), the binding of SP-A increased 1.5-2-fold. This stimulation was cell substrate-dependent since type II cells plated on plastic dishes did not show this effect. A time course of the stimulation of SP-A binding due to secretagogues showed that both cAMP and PMA increased SP-A binding by 2-fold after 20 min. With cAMP, binding remained elevated for 2 h, while binding in the presence of PMA had returned to control values. The effects of submaximal concentrations of cAMP and PMA on binding were additive. Inhibition of cellular protein synthesis with cycloheximide did not alter the increase of SP-A binding stimulated by the secretagogues. Type II cells pretreated with PMA responded to subsequent treatment with cAMP by increasing SP-A binding, while these cells were refractory to subsequent treatment with PMA. Both constitutive and regulated binding of SP-A to type II cells were sensitive to trypsin. The binding of SP-A to type II cells showed saturation at a concentration of 1 microg/ml SP-A under control and secretagogue-stimulated conditions, with both total and calcium-dependent binding showing a 2-fold increase upon secretagogue exposure. The data are consistent with the hypothesis that secretagogues stimulate surfactant uptake, at least in part, through recruitment of SP-A receptors to the type II cell surface, resulting in an increase in the number of SP-A binding sites.

Acylation of pulmonary surfactant protein-C is required for its optimal surface active interactions with phospholipids

This study investigates the importance of thioester-linked acyl groups in lung surfactant protein C (SP-C) in facilitating interactions with phospholipids that yield functionally important surface active behaviors. Native SP-C, palmitoylated at cysteine residues at positions 5 and 6, was isolated from bovine lung surfactant by liquid chromatography. Deacylated SP-C (dSP-C), unchanged in composition and sequence from SP-C but having a decreased alpha-helical content in films with dipalmitoyl phosphatidylcholine (DPPC) of 52 versus 70%, was obtained by treatment with 0.1 M sodium carbonate buffer at pH 10. Surface activity was studied for SP-C and dSP-C combined with column-purified phospholipids (PPL) from calf lung surfactant or with synthetic phospholipids (DPPC or a synthetic phospholipid mixture (SPL) containing 50:35:15, DPPC:egg phosphatidylcholine:egg phosphatidylglycerol). Interfacial measurements included surface pressure time adsorption isotherms for dispersed surfactants with diffusion minimized, dynamic surface pressure area isotherms and respreading for films in the Wilhelmy balance, and overall surface tension lowering at physiologic cycling rate in oscillating bubble experiments. Dispersions of PPL:SP-C and SPL:SP-C rapidly adsorbed to high equilibrium surface pressures of 47-48 mN/m, significantly better than corresponding dispersions containing dSP-C. The adsorption of PPL:dSP-C was essentially unchanged from that of PPL alone, and the adsorption of SPL:dSP-C was improved only slightly over SPL alone. In Wilhelmy balance studies, dynamic respreading was significantly improved over phospholipids alone in films of SP-C plus PPL, SPL, or DPPC. Respreading was improved less markedly by dSP-C in corresponding films with SPL or DPPC and not at all in films with PPL. Maximum surface pressures were also higher in cycled films of SP-C versus dSP-C combined with PPL or SPL. In bubble experiments (37 degrees C, 20 cycles/min), dispersions of PPL:SP-C and SPL:SP-C reached low minimum surface tensions of <1 and 5 mN/m, respectively, whereas PPL:dSP-C and SPL:dSP-C only reached minima of approximately 20 mN/m as did PPL and SPL alone. Acylation in SP-C is crucial for its interactions with phospholipids over the full spectrum of adsorption and dynamic surface behaviors important for lung surfactant.

A synthetic segment of surfactant protein A: structure, in vitro surface activity, and in vivo efficacy

Surfactant protein A (SP-A) is a 248-residue, water-soluble, lipid-associating protein found in lung surfactant. Analysis of the amino acid sequence using the Eisenberg hydrophobic moment algorithm predicts that the SP-A segment spanning residues 114-144 has high hydrophobic moments, typical of lipid-associating amphipathic domains. The secondary structure, in vitro surface activity and in vivo lung activity of this SP-A sequence were studied with a 31-residue synthetic peptide analog (A114-144). Analysis of the secondary structure using circular dichroism and Fourier transform infrared spectroscopy indicated association with lipid dispersions and a dominant helical content. Surface activity measurements of A114-144 with surfactant lipid dispersions and the hydrophobic surfactant proteins B and C (SP-B/C) showed that A114-144 enhances surface activity under conditions of dynamic compression and respreading on a Langmuir/Wilhelmy surface balance. Synthetic surfactant dispersions containing A114-144 improved lung compliance in spontaneously breathing, 28-d premature rabbits to a greater degree than surfactant dispersions with synthetic SP-B/C and synthetic surfactant lipids alone. These observations indicate that inclusion of A114-144 may improve synthetic preparations currently used for surfactant replacement therapy.

The role of the amino-terminal domain and the collagenous region in the structure and the function of rat surfactant protein D

Surfactant protein D (SP-D) is a member of the C-type lectin superfamily with four distinct structural domains: an amino terminus involved in forming intermolecular disulfides, a collagen-like domain, a neck region, and a carbohydrate recognition domain. A collagen domain deletion mutant (CDM) of SP-D was created by site-directed mutagenesis. A second variant lacking both the amino-terminal region and the collagen-like domain was generated by collagenase treatment and purification of the collagenase-resistant fragment (CRF). The CDM expressed in CHO-K1 cells formed the covalent trimers, but not the noncovalent dodecamers, typical of native SP-D. The CRF derived from recombinant SP-D formed only monomers. The CDM bound mannose-Sepharose and phosphatidylinositol (PI) as well as SP-D, but the binding to mannosyl bovine serum albumin and glucosylceramide was diminished by approximately 60%. The CRF displayed weak binding to mannose-Sepharose and PI and essentially no binding to mannosyl bovine serum albumin and glucosylceramide. Both SP-D and CDM altered the self-aggregation of PI-containing liposomes. SP-D reduced the density and the light scattering properties of PI aggregates. These results demonstrate that the collagen-like domain is required for dodecamer but not covalent trimer formation of SP-D and plays an important, but not essential, role in the interaction of SP-D with PI and GlcCer. Removal of the amino-terminal domain of SP-D along with the collagen-like domain diminishes PI binding and effectively eliminates GlcCer binding.

Epitope mapping for monoclonal antibody against human surfactant protein A (SP-A) that alters receptor binding of SP-A and the SP-A-dependent regulation of phospholipid secretion by alveolar type II cells

Surfactant protein A (SP-A) is a lung-specific glycoprotein in pulmonary surfactant and has a collagen like sequence on its N-terminal. SP-A has been shown to function as an inhibitor of phospholipid secretion by primary culture of alveolar type II cells via cell surface receptor(s) for SP-A. In a previous report, we showed that the C-terminal non-collagen like domain of human SP-A possessed the biological activities, and that a monoclonal antibody against human SP-A, PE10, abolished the biological activity of SP-A (Murata et al. (1993) Biochem. J. 291, 71-76). In the present study, we investigated an epitope of SP-A for PE10. Western blot analysis with fragmented peptides of human SP-A generated by both lysyl endopeptidase and BrCN showed that PE10 reacted with the peptide corresponding with Glu202 to the C-terminal but that it lacked the ability to bind to the peptide corresponding with Tyr208 to the C-terminal. The antibodies against a synthetic peptide (P1) corresponding with Glu202 to Asn217 of human SP-A inhibited the binding of PE10 to SP-A, suggesting that a similar site was recognized by both PE10 and anti-P1 antibodies. Anti-P1 antibodies as well as PE10 suppressed the biological activity of SP-A. A direct interaction between P1 and rat lung membranes, or between P1 and alveolar type II cell membranes was shown from the measurement of the fluorescence emission spectra of dansyl-labeled P1. These results suggest that an area contiguous to or near the region from Glu202 to Met207 of SP-A is important for expressing the biological activities.

Exclusion of SP-C, but not SP-B, by gel phase palmitoyl lipids

The interactions of the hydrophobic pulmonary surfactant proteins, SP-C and SP-B, with lipid bilayers were assessed by fluorescence energy transfer. SP-C and SP-B were labeled with the fluorescent probe, succinimidyl nitrobenzoxadiazolyl amino hexanoate (NBD). Fluorescence energy transfer from NBD-SP-C and NBD-SP-B to four distinct indocarbocyanine probes (CnDiI) was utilized to determine the association of the surfactant proteins with various lipid acyl chains. In lipid mixtures including DPPC and DPPG, SP-C was associated with shorter chain and unsaturated lipids below the bulk lipid phase transition. Longer chain saturated CnDiI were excluded from SP-C aggregates. In contrast, SP-B demonstrated little acyl chain preference. The association of SP-C with shorter chain and unsaturated lipids below the bulk phase transition is interpreted to arise from a mismatch in the length of the hydrophobic region of the SP-C alpha-helix relative to the length of the hydrophobic region of dipalmitoyl lipids in the gel phase.

Host defence capacities of pulmonary surfactant: evidence for 'non-surfactant' functions of the surfactant system

The most well characterized function of pulmonary surfactant is its ability to reduce surface tension at the alveolar air-liquid interface, thereby preventing lung collapse. However, several lines of evidence suggest that surfactant may also have 'non-surfactant' functions: specific components of surfactant (proteins and phospholipids) may interact with different alveolar cells, inhaled particles and micro-organisms modulating pulmonary host defence systems. SP-A, the most abundant surfactant protein, binds to alveolar macrophages via a specific surface receptor with high affinity [128]. Such binding effects the release of reactive oxygen species from resident alveolar macrophages if SP-A is properly presented to the target cell. SP-A also stimulates chemotaxis of alveolar macrophages [142], and serves as an opsonin in the phagocytosis of herpes simplex virus [161] Candida tropicalis [138] and various bacteria [137]. In addition, SP-A enhances the uptake of particles by monocytes and culture-derived macrophages [140] and improves bacterial killing. SP-D, another hydrophobic surfactant-associated protein, might interact with alveolar macrophages as well, stimulating the release of oxygen radicals [148], while for the hydrophilic surfactant proteins SP-B and SP-C no macrophage interactions have been described so far. SP-A and SP-D are members of the so-called 'collectins', pattern recognition molecules involved in first line defence. While some surfactant proteins appear to stimulate certain macrophage defence functions, surfactant phospholipids seem to inhibit those of lymphocytes. Suppressed lymphocyte functions include lymphoproliferation in response to mitogens and alloantigens, B cell immunoglobulin production and natural killer cell cytotoxicity. Concerning surfactant's phospholipid composition phosphatidylglycerol is more suppressive than phosphatidylcholine on a molar basis [38]. Bovine surfactant has an immunosuppressive effect on the development of hypersensitivity pneumonitis in a guinea pig model [150]. Despite these interesting observations, several important questions concerning the interactions of surfactant components with pulmonary host defence systems remain unanswered. Sufficient host defence in the lungs works through various humoral-cellular systems in conjunction with the specific anatomy of the airways and the gas exchange surface--how does the surfactant system fit into this network? Surfactant and alveolar cells are both altered during lung injury--is there a relationship between alveolar cells from RDS patients and the endogenous surfactant isolated from such patients? How does exogenous surfactant as used for substitution therapy modulate the defence system of the host? Some of those artificial surfactants have been shown to inhibit the endotoxin-alveolar macrophages, PMNs and monocytes including IL-1, IL-6 and TNF [139,152].(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of pulmonary surfactant protein A (SP-A) and calcium on the adsorption of cholesterol and film stability

The effect of exogenous cholesterol on the stability of surface films at 37 degrees C from various surfactants was studied with the pulsating bubble surfactometer. Addition of cholesterol (5%, w/w) to bovine lipid extract surfactant (bLES) or mixtures of dipalmitoylphosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol /SP-B (7:3:1%) dispersed in 1.5 mM CaCl2/0.9% NaCl resulted in unstable surface films. Although 10% cholesterol only partially impaired the surface activity of bLES, it virtually abolished that of the reconstituted surfactant. The inhibitory effects of cholesterol were significantly repressed by SP-A (10%, w/w of lipid) and 3 mM CaCl2 or 5 mM CaCl2 without SP-A. Adsorption of cholesterol from various surfactants into the air/water interface was examined by measuring the surface radioactivity of [14C]cholesterol. Cholesterol alone dispersed in 1.5 mM CaCl2/0.9% NaCl could not adsorb to the interface, but it adsorbed readily when mixed with bLES. Cholesterol adsorption was markedly suppressed by SP-A in 3 mM CaCl2/0.9% NaCl or 5 mM CaCl2/0.9% NaCl without SP-A. Electron microscopy revealed striking ultrastructural differences between bLES/5% cholesterol/10% SP-A in 3 mM CaCl2/0.9% NaCl and bLES/5% cholesterol in 3 or 5 mM CaCl2/0.9% NaCl. The former exhibited large multilayer and small unilamellar vesicles, while the latter displayed condensed patches of aggregates. Adsorption studies showed aggregated patches adsorbed more rapidly than vesicles but attained lower equilibrium surface pressures. These results indicate SP-A and calcium limit the adsorption of surfactant cholesterol to the air-water interface.

Lipid effects on aggregation of pulmonary surfactant protein SP-C studied by fluorescence energy transfer

The self-association of pulmonary surfactant protein SP-C in lipid vesicles was studied using fluorescence energy transfer. Bovine SP-C was labeled with two fluorescent probes, succinimidyl 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate and eosin isothiocyanate, on the amino terminus of the protein, producing NBD-SP-C and EITC-SP-C, respectively. The N-terminus of SP-C was relatively immobile between 20 and 37 degrees C, as demonstrated by high fluorescence anisotropy of NBD-SP-C and EITC-SP-C. The mobility increased at the transition of the lipid to the fluid phase. Using fluorescence energy transfer, with NBD-SP-C as the donor and EITC-SP-C as the acceptor, a high degree of SP-C/SP-C association was found below 25 degrees C, decreasing to very little self-association above 42 degrees C in 7:1 1,2-dipalmitoylphosphatidylcholine-1,2-dipalmitoylphosphatidylglycerol (DPPC-DPPG) vesicles. The fraction of SP-C aggregated below 37 degrees C in 7:1 DPPC-DPPG was estimated from the observed energy transfer to be more than 70% of total SP-C. In various lipid mixtures, self-association of SP-C was dependent on the presence of at least some gel-phase lipids. In a lipid mixture resembling pulmonary surfactant, gradually increasing self-association was observed below 38 degrees C. The relation of the present data to the state of aggregation of SP-C in pulmonary surfactant is discussed.

Amphipathic alpha-helical peptides based on surfactant apoprotein SP-A

Three peptides based on the putative amphipathic helical region of the major pulmonary surfactant apoprotein (SP-A) were synthesized by solid-phase techniques, mixed with DPPC and tested for efficacy as lung surfactants in an in vitro adult rat lavaged lung model. The peptides correspond to residues 81-102 (SP-A81-102) and 78-101 (SP-A78-101) of the native human sequence and an analog with increased hydrophobicity, Leu84,90SP-A78-101. Neither native sequence was effective in simple mixtures with DPPC. However, substitution of leucine residues for Asp84 and Thr90 of SP-A81-102 yielded a peptide which was active in mixtures with DPPC, restoring quasi-static lung compliance to 90% of the unlavaged value. In the absence of peptide, DPPC had no effect on the P-V curve of the lavaged lung. The activity of the Leu84,90 analog correlated with an increased amphipathic alpha-helical potential and an improvement in several predictive parameters for lipid-binding. The similarities between this active peptide and other active amphipathic alpha-helical peptides lend support to the hypothesis that amphipathic alpha-helical potential and the size of the hydrophobic face are critical for functional synthetic surfactant peptides in simple mixtures with dipalmitoylphosphatidylcholine.

Binding of pulmonary surfactant protein A to galactosylceramide and asialo-GM2

The binding of pulmonary surfactant protein A (SP-A) to glycolipids was examined in the present study. The direct binding of SP-A on a thin-layer chromatogram was visualized using 125I-SP-A as a probe. 125I-SP-A bound to galactosylceramide and asialo-GM2, but failed to exhibit significant binding to GM1, GM2, asialo-GM1, sulfatide, and Forssman antigen. The study of 125I-SP-A binding to glycolipids coated onto microtiter wells also revealed that SP-A bound to galactosylceramide and asialo-GM2. SP-A bound to galactosylceramides with non-hydroxy or hydroxy fatty acids, but showed no binding to either glucosylceramide or galactosylsphingosine. Excess native SP-A competed with 125I-SP-A for the binding to asialo-GM2 and galactosylceramide. Specific antibody to rat SP-A inhibited 125I-SP-A binding to glycolipids. In spite of chelation of Ca2+ with EDTA or EGTA, SP-A retained a significant binding to glycolipids. Inclusion of excess monosaccharides in the binding buffer reduced the glycolipid binding of SP-A, but failed to achieve complete abolishment. The oligosaccharide isolated from asialo-GM2 is also effective at reducing 125I-SP-A binding to the solid-phase asialo-GM2. From these data, we conclude that SP-A binds to galactosylceramide and asialo-GM2, and that both saccharide and ceramide moieties in the glycolipid molecule are important for the binding of SP-A to glycolipids.

Structure and functions of a dimeric form of surfactant protein SP-C: a Fourier transform infrared and surfactometry study

Surfactant proteins SP-B (M(r) = 8700, reduced) and SP-C (M(r) = 3000-6000, major form, non-reduced) interact with surfactant phospholipids to enhance their surface active properties. In the present study, we describe the structural and functional characteristics of a novel dimeric form of bovine SP-C (M(r) = 9000, non-reduced), which is identified as [SP-C]2. Dimeric SP-C exhibits surface tension-lowering properties differing from those of monomeric SP-C and enhances the surface properties of bovine SP-B/phospholipid mixtures. Chemical analysis indicated that [SP-C]2 was not acylated at the cysteinyl residues. Fourier transform-infrared spectroscopy (FT-IR) was utilized to determine the secondary structures of [SP-C]2 in DPPC films. Relative percentages of alpha-helical, beta-sheet, beta-turn and random coil structures were calculated by peak fit analysis of the amide I band of the FT-IR spectra indicating that, in contrast to the helical structure of monomeric SP-C, [SP-C]2 exhibits almost exclusively beta-sheet structure. In addition, only 10% of the amide (backbone) hydrogens exchanged with deuterium of D2O, indicating that the remaining 90% of amide hydrogens were not accessible to D2O due to strong hydrogen bonding or their location in a hydrophobic environment. Dimerization of SP-C effects a major change in secondary structure, a factor which may play a role in the interaction of SP-C with phospholipids in pulmonary surfactant.

Calcium dependent conformational changes of surfactant protein A (SP-A) and its collagenase resistant fragment with or without dithiothreitol

Calcium-dependent conformational changes of surfactant protein A (SP-A) and the collagenase resistant fragment (CRF) of SP-A were studied by measuring fluorescence spectra. The emission peaks of both SP-A and CRF in the absence of Ca2+ appeared at 343 nm when they were excited at 280 nm. In the presence of Ca2+, the peaks appeared at 340 nm and were accompanied by an increase in the fluorescence intensity. The magnitude of the fluorescence intensity change induced by Ca2+ was amplified by the addition of dithiothreitol (DTT) in both SP-A and CRF. The Ca2+ binding of CRF was measured by a flow dialysis method with 45CaCl2 in the Ca2+ concentration range where the Ca(2+)-induced fluorescence changes occurred. The maximum binding number of Ca2+ to CRF was about 2 mol per mol of CRF, and the value was independent of the presence of DTT.

Effects of lung surfactant proteolipid SP-C on the organization of model membrane lipids: a fluorescence study

Lipid-protein interactions of pulmonary surfactant-associated protein SP-C in model DPPC/DPPG and DPPC/DPPG/eggPC vesicles were studied using steady-state and time-resolved fluorescence measurements of two fluorescent phospholipid probes, NBD-PC and NBD-PG. These fluorescent probes were utilized to determine SP-C-induced lipid perturbations near the bilayer surface, and to investigate possible lipid headgroup-specific interactions of SP-C. The presence of SP-C in DPPC/DPPG membrane vesicles resulted in (1) a dramatic increase in steady-state anisotropy of NBD-PC and NBD-PG at gel phase temperatures, (2) a broadening of the gel-fluid phase transition, (3) a decrease in self-quenching of NBD-PC and NBD-PG probes, and (4) a slight increase in steady-state anisotropy of NBD-PG at fluid phase temperatures. Time-resolved measurements, as well as steady-state intensity measurements indicate that incorporation of SP-C into DPPC/DPPG or DPPC/DPPG/eggPC vesicles results in a increase in the fraction of the long-lifetime species of NBD-PC. The results presented here indicate that SP-C orders the membrane bilayer surface, disrupts acyl chain packing, and may increase the lateral pressure within the bilayer.

Interaction of C1q receptor with lung surfactant protein A

Earlier we reported the purification of C1q receptor (C1qR) from U937 cells and human tonsil lymphocytes (Malhotra, R. and Sim, R. B., Biochem. J. 1989. 218: 625) and showed that C1qR interacts with the ligands C1q, mannose-binding protein, conglutinin and lung surfactant protein A (SP-A) (Malhotra, R., Thiel, S., Reid, K. B. M. and Sim, R. B., J. Exp. Med. 1990. 172: 955). C1qR was characterized as an acidic glycoprotein, which, when solubilized, exists as a dimer of Mr 115,000 under non-denaturing conditions. In this article we provide evidence for binding of radioiodinated SP-A to U937 cells and show that binding of radioiodinated SP-A to U937 cells is specific, saturable, salt dependent and is inhibited by purified C1qR and by C1q. The interaction of SP-A with U937 cells was found to up-regulate the surface expression of C1qR. Incubation of SP-A with U937 cells at 37 degrees C for 80 min was found to increase the receptor number per cell. Increase in receptor number was inhibited in the presence of sodium azide and monensin. Incubation of cells with calcium ionophore A23187 induced increased surface expression in the absence of SP-A. The results indicate that interaction of SP-A with U937 cells triggers the expression of an intracellular pool of C1qR.

Surfactant protein C precursor is palmitoylated and associates with subcellular membranes

Surfactant protein C (SP-C) is a 3.7 kDa, hydrophobic protein that enhances the adsorption of phospholipids in pulmonary surfactant. SP-C is generated by proteolytic processing of a 21 kDa precursor. Murine fetal lung explant cultures and a Chinese hamster ovary cell line expressing recombinant human SP-C gene (CHO/SPC) were used to determine the subcellular location and post-translational modification(s) of proSP-C. After in vitro translation, proSP-C of Mr = 21,000 was generated. ProSP-C was associated with canine pancreatic microsomes during co-translation and was partially protected from digestion with proteinase K, supporting the concept that proSP-C enters but does not completely traverse the membrane of the endoplasmic reticulum (ER). Association of proSP-C isoforms of 21 and 26 kDa with intracellular membranes was demonstrated by subcellular fractionation of CHO/SPC cells. Pulse/chase experiments demonstrated that the 21 kDa SP-C proprotein was synthesized first and after 15 min was modified to produce a 26 kDa isoform in CHO/SPC cells or a 24 kDa isoform in murine fetal lung. Both the 21 and 26 kDa proSP-C isoforms were detected after labelling CHO/SPC cells with [3H]palmitic acid. The formation of the 26 kDa proSP-C isoform in CHO/SPC cells and the 24 kDa proSP-C isoform in murine fetal lung was blocked by cerulenin, an inhibitor of fatty acid synthesis. In conclusion, proSP-C is associated with subcellular membranes. ProSP-C is palmitoylated and undergoes additional post-translational modification that is blocked by an inhibitor of fatty acid synthesis.

Characterization of pulmonary surfactant protein D: its copurification with lipids

Surfactant protein D (SP-D) is a collagenous surfactant associated protein synthesized by alveolar type II cells. SP-D was purified from the supernatant of rat bronchoalveolar lavage fluids obtained by centrifugation at 33,000 x gav for 16 h. The contents of SP-D and SP-A in fractions obtained by the centrifugation of rat bronchoalveolar lavage were determined by enzyme-linked immunoassay. The total content of SP-D was approximately 12% of that of SP-A in these lavage fluids. 99.1% of SP-A was present in the 33,000g pellet, whereas 71.1% of SP-D was in the 33,000g supernatant. Analysis by high performance liquid chromatography reveals that lipids are copurified with isolated SP-D. Phosphatidylcholine accounted for 84.8% of the phospholipids copurified with SP-D. Unlike SP-A, SP-D in the purified and delipidated form failed to compete with 125I-labeled SP-A for phosphatidylcholine binding, and to aggregate phospholipid liposomes. The present study demonstrates that lipids are copurified with SP-D, that SP-D and SP-A distribute differently in rat bronchoalveolar lavage fluids, and that SP-D in the purified and delipidated form does not exhibit interaction with lipids in the same fashion as SP-A.

Human surfactant protein D: SP-D contains a C-type lectin carbohydrate recognition domain

Lung surfactant protein D (SP-D) shows calcium-dependent binding to specific saccharides, and is similar in domain structure to certain members of the calcium-dependent (C-type) lectin family. Using a degenerate oligomeric probe corresponding to a conserved peptide sequence derived from the amino-terminus of the putative carbohydrate binding domain of rat and bovine SP-D, we screened a human lung cDNA library and isolated a 1.4-kb cDNA for the human protein. The relationship of the cDNA to SP-D was established by several techniques including amino-terminal microsequencing of SP-D-derived peptides, and immunoprecipitation of translation products of transcribed mRNA with monospecific antibodies to SP-D. In addition, antibodies to a synthetic peptide derived from a predicted unique epitope within the carbohydrate recognition domain of SP-D specifically reacted with SP-D. DNA sequencing demonstrated a noncollagenous carboxy-terminal domain that is highly homologous with the carboxy-terminal globular domain of previously described C-type lectins. This domain contains all of the so-called "invariant residues," including four conserved cysteine residues, and shows high homology with the mannose-binding subfamily of C-type lectins. Sequencing also demonstrated an amino-terminal collagenous domain that contains an uninterrupted sequence of 59 Gly-X-Y triplets and that also contains the only identified consensus for asparagine-linked oligosaccharides. The studies demonstrate that SP-D is a member of the C-type lectin family, and confirm predicted structural similarities to conglutinin, SP-D, and the serum mannose binding proteins.

Purification of surfactant protein A from dog lung by reconstitution with surfactant lipids

We have developed a simple method for purification of surfactant major apoprotein (SP-A, MW 34-38 kD) from dog lungs with high yield and purity. Lipids and proteins of partially purified surfactant were dissociated by sodium deoxycholate (DOC, 100 mM, 37 degrees C, 30 min), diluted 1:10 with borate buffer containing 3 mM CaCl2, and dialysate in the same buffer to reconstitute the lipids and proteins (4 degrees C, 48 h). The reconstituent and the partially purified surfactant were purified by ultracentrifugation on a discontinuous sucrose density gradient. Protein was isolated from the reconstituent and from the purified surfactant by delipidation, and the yields and purities were assessed by one-dimensional SDS-PAGE and 2-dimensional electrophoresis (isoelectric focusing, SDS-PAGE). We found that the surface pressure-time adsorption isotherm, minimum surface tension, and the yield and composition of lipids of the reconstituent were identical with those from the purified surfactant. Only about 0.25% of the DOC used for dissociation remained with the reconstituent and it did not affect surface properties of the reconstituent. The yield of SP-A in the reconstituent was almost the same as that in the purified surfactant, but the former contained no plasma protein whereas the latter contained significant amounts. The amino acid composition and the partial N-terminal amino acid sequence of SP-A were the same as those from the purified surfactant. Reconstituent prepared from surfactant lipids and SP-A adsorbed more rapidly and reached a higher final surface pressure than did the surfactant lipids alone. These results demonstrate that large quantities of SP-A can be purified by reconstitution with surfactant lipids, and that the purified protein is biophysically active.