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BACKGROUND: Lung surfactant protein A (SP-A) is a complex molecule composed of up to 18 polypeptide chains. In vivo, SP-A probably binds to a wide range of inhaled materials via the interaction of surface carbohydrates with the lectin domains of SP-A and mediates their interaction with cells as part of a natural defense system. Multiplicity of lectin domains gives high-affinity binding to carbohydrate-bearing surfaces. MATERIALS AND METHODS: Gel filtration analyses were performed on bronchoalveolar lavage (BAL) fluid samples from three patient groups: pulmonary alveolar proteinosis (n = 12), birch pollen allergy (n = 11), and healthy volunteers (n = 4). Sucrose density gradient centrifugation was employed to determine molecular weights of SP-A oligomers. SP-A was solubilized from the lipid phase to compare oligomeric state with that of water soluble SP-A. RESULTS: SP-A exists as fully assembled complexes with 18 polypeptide chains, but it is also consistently found in smaller oligomeric forms. This is true for both the water- and lipid-soluble fractions of SP-A. CONCLUSION: The three patient groups analyzed show a shift towards lower oligomeric forms of SP-A in the following sequence: healthy-pulmonary alveolar proteinosis-pollen allergy. Depolymerization would be expected to lead to loss of binding affinity for carbohydrate-rich surfaces, with loss or alteration of biological function. While there are many complex factors involved in the establishment of an allergy, it is possible that reduced participation of SP-A in clearing a potential allergen from the lungs could be an early step in the chain of events.