Binding of human serum amyloid A (hSAA) and its high-density Iipoprotein3 complex (hSAA-HDL3) to human neutrophils. Possible implication to the function of a protein of an unknown physiological role

Serum amyloid A (SAA) is an acute-phase serum protein which exists in the body in a complex with high-density lipoprotein (HDL₃). It is involved in chronic inflammation and neoplastic diseases in an as yet unknown manner. Toward an understanding of the possible physiological role of SAA we initiated a study of its association with blood proinflammatory cells with which it may interact functionally in vivo. In the following we describe the binding characteristics of recombinant human SAA to human neutrophils (polymorphonuclear leukocytes; PMNLs) and their plasma membranes. Scatchard analysis of rSAA binding and displacement curves revealed K_d in the nanomolar range. The C-terminal domain of the protein, i.e. amino acid residues 77-104, which might reside in serum following SAA degradation and amyloid A formation, was found to inhibit efficiently the binding of the whole protein to neutrophils. The interaction of SAA, and of its related peptides while complexed in HDL₃, with human PMNs was also studied. The results suggest that SAA may be involved, in an as yet unknown manner, in the neutrophil-associated inflammatory mechanism.