BINDING OF HUMAN SERUM AMYLOID-P COMPONENT (HSAP) TO HUMAN NEUTROPHILS

Human serum amyloid P component (hSAP) and human C-reactive protein (h CRP) are normal serum constituents related to the pentraxin family of plasma proteins. hSAP has morphological and immunochemical identity an d extensive sequence similarity to the amyloid P (AP) component found in normal tissues and particularly in amyloid deposits. hCRP and its p roteolytic products have been previously shown to bind and to interact with various types of human leukocytes. Binding-displacement experime nts with I-125-labeled hSAP and hCRP show that both proteins have spec ific high affinity binding sites on normal human polymorphonuclear leu kocytes (PMN) and each can compete efficiently with the binding of the other. Scatchard analysis of hSAP-displacement curves reveals a heter ogeneous population of hSAP-binding sites existing on the PMN cells, a mong them about 300000 low-affinity binding sites with K-d less than o r equal to 5X10(-6)M and about 30000 high-affinity binding sites with K-d less than or equal to 5x10(-8)M. hAP was found to be degraded by e nzymes from human neutrophils to yield a mixture of low-molecular-mass peptides, similarly to the case of CRP reported previously. The bindi ng of hSAP can be efficiently inhibited by this peptide mixture. The r esults suggest that both hCRP and hSAP, together with related peptides , may participate in vivo in an unknown mechanism of regulation of hum an neutrophils.