SERUM AMYLOID-A PROTEIN ENHANCES THE ACTIVITY OF SECRETORY NONPANCREATIC PHOSPHOLIPASE-A(2)

The acute-phase proteins serum amyloid A protein (SAA) and secretory phospholipase A(2) (sPLA(2)) are simultaneously expressed during inflammatory conditions. SAA associates with high-density lipoprotein (HDL) altering its physicochemical composition. We found that purified acute-phase SAA, but not the constitutive form, markedly enhances the lipolytic activity of sPLA(2) in a dose-related manner with phosphatidylcholine/lysophosphatidylcholine or phosphatidylethanolamine/lysophosphatidylethanolamine liposomal substrates. Normal HDL was found to reduce activity of sPLA(2) in a dose-dependent manner, but when acute-phase HDL containing 27% SAA was tested, it enhanced sPLA(2) activity. Immunopurified monospecific antibodies against SAA completely abolished the enhancing activity of SAA and acute-phase HDL. Given the central role of HDL in lipoprotein metabolism, the interaction between HDL, SAA and sPLA(2) may account for changes detected in lipoprotein metabolism during the acute phase.