Phospholipases A 1 and A 2 from rabbit lung lysosomes have maximal activity at pH 4.0 (Fig. 1). Lamellar bodies from rabbit lung contain phospholipases A which resemble the lysosomal enzymes in being active at pH 4 and showing sensitivity to inhibition by calcium ions (80% inhibition at 10 mM Ca 2+). Lamellar body phospholipases hydrolyse dipalmitoyl phosphatidylcholine (containing 10% phosphatidylglycerol and thus approximating the phospholipids composition of mature lamellar bodies) at only 1% of their rate of hydrolysis of unsaturated molecules (Tables 1 and 2). This substrate preference corresponds to that shown by phospholipases from lysosomes.

Speculation: The substrate preferences of the lamellar body phospholipases may explain the ability of these enzymes to coexist with surfactant phospholipid in the mature organelle. In the developing lamellar body, phospholipase A 2 may have a role to perform in the removal of unsaturated phosphatidylocholine molecules and their remodeling to the dipalmitoyl species.