Characterization of radioiodinated lung surfactant protein A (SP-A) and the effects of oxidation on SP-A quaternary structure and activity

Lung surfactant protein A (SP-A) is the most abundant surfactant-associated protein present in the lung and respiratory tract. SP-A binds to several pathogens via its C-type lectin domains, and may act as an opsonin, mediating adhesion to cells via the collectin receptor. Binding studies using SP-A are made difficult by its apparent instability following radioiodination. This study Investigated the effect of oxidation (via radioiodination and exposure to H2O2) on the structural and functional characteristics of SP-A. Radioiodinated SP-A, stored at 4 degrees C, retained carbohydrate binding activity after labeling. After 10 days storage, the radioiodinated SP-A was indistinguishable on SDS-PAGE from freshly radioiodinated SP-A, but sedimentation coefficient and Stokes radius values changed dramatically, indicating SP-A depolymerization. Such a quaternary structural breakdown, with a concomitant reduction in carbohydrate binding activity, is likely to be due to oxidative cleavage of disulfide bonds. Comparable results were observed upon radioiodination of the structurally similar molecule C1q. Consequently, the effect of prolonged incubation with H2O2 upon SP-A was investigated, with similar results. Thus, exposure to oxidizing agents leads to breakdown of the hexameric quaternary structure of SP-A, often to native polypeptides, with an attendant loss of binding activity. Such an effect may have consequences for the physiological role of SP-A in the lung.