Inhibition of lung calcium-independent phospholipase A2 by surfactant protein A.
The effect of lung surfactant protein A (SP-A) on lung phospholipase A2 (PLA2) activity was investigated. SP-A was purified from bovine surfactant obtained by lung lavage. PLA2 was assayed using radiolabeled 1,2-dipalmitoyl phosphatidylcholine (DPPC) in surfactant-like unilamellar liposomes with Ca(2+)-free acidic (pH 4) or 10 mM Ca2+, alkaline (pH 8.5) buffer. SP-A significantly inhibited Ca(2+)-independent acidic PLA2 of rat lung homogenate or isolated lamellar bodies but had no effect on the Ca(2+)-dependent alkaline enzyme. Lamellar body PLA2 was inhibited by 50% with 0.25 micrograms SP-A/microgram 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 125I-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 PLA2 activity. Inactivation of endogenous SP-A in isolated lamellar bodies or surfactant increased Ca(2+)-independent PLA2 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 Ca(2+)-independent acidic PLA2 has a role in the metabolism of internalized surfactant phospholipid and that SP-A can modulate the activity of this enzyme.
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Institute for Environmental Medicine, University of Pennsylvania, School of Medicine, Philadelphia 19104-6068.
Dodia
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Chander
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D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D002478:Cells, Cultured; D005994:Glycerophosphates / Q000494:pharmacology; D007460:Iodoacetamide / Q000494:pharmacology; D008168:Lung / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008623:Mercaptoethanol / Q000494:pharmacology; D010741:Phospholipases A / Q000037:antagonists & inhibitors / Q000378:metabolism*; D054467:Phospholipases A2; D011510:Proteolipids / Q000494:pharmacology*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D005994:Glycerophosphates; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; C078587:1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol; D015060:1,2-Dipalmitoylphosphatidylcholine; D008623:Mercaptoethanol; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium; D007460:Iodoacetamide