INHIBITION OF LUNG CALCIUM-INDEPENDENT PHOSPHOLIPASE A(2) BY SURFACTANT PROTEIN-A

The effect of lung surfactant protein A (SP-A) on lung phospholipase A(2) (PLA(2)) activity was investigated. SP-A was purified from bovine surfactant obtained by lung lavage. PLA(2) was assayed using radiolabeled 1,2-dipalmitoyl phosphatidylcholine (DPPC) in surfactant-like unilamellar liposomes with Ca2+-free acidic (pH 4) or 10 mM Ca2+, alkaline (pH 8.5) buffer. SP-A significantly inhibited Ca2+-independent acidic PLA(2) of rat lung homogenate or isolated lamellar bodies but had no effect on the Ca2+-dependent alkaline enzyme. Lamellar body PLA(2) was inhibited by 50% with 0.25 mu g SP-A/mu g lamellar body protein. Similar inhibition by SP-A was observed when 1-palmitoyl,2-oleoyl PC (POPC) was the substrate. Binding assay showed binding of I-125-labeled SP-A to DPPC but not to POPC, indicating that removal of substrate was not the mechanism for inhibition of the enzyme by SP-A. Chemical reduction or alkylation of SP-A abolished its inhibitory effect on PLA(2) activity. Inactivation of endogenous SP-A in isolated lamellar bodies or surfactant increased Ca2+-independent PLA(2) activity in these fractions. The presence of SP-A in liposomes stimulated the uptake of DPPC by isolated granular pneumocytes in primary culture but significantly inhibited its degradation. These results indicate that the Ca2+-independent acidic PLA(2) has a role in the metabolism of internalized surfactant phospholipid and that SP-A can modulate the activity of this enzyme.