Calcium Dependent Association of Surfactant Protein A with Pulmonary Surfactant: Application to Simple Surfactant Protein A Purification

Surfactant protein A (SP-A) is an abundant lipoprotein component of pulmonary surfactant that plays multiple roles in surfactant homeostasis within the lung. A simple and rapid purification procedure for SP-A is described. Purified surfactant is washed by centrifugation with Ca2+containing buffer to remove residual soluble proteins. Following the Ca2+buffer wash, the surfactant pellet is washed in buffer containing EGTA and Mg2+which releases the bound SP-A in almost pure form. Subsequent chromatography of the SP-A on Sephacryl S-500 yields homogeneous preparations of the protein. The SP-A purified using this procedure requires no exposure to either detergents or organic solvents to remove lipid. SP-A prepared by this new method inhibits lipid secretion from alveolar type II cells as effectively as SP-A prepared by other methods. In addition, the SP-A depleted surfactant produced in the first step of this procedure is capable of binding exogenous SP-A in a time dependent, saturable and Ca2+dependent manner.