Amyloid deposits almost invariably contain a pentagonal-shaped protein (a so-called pentraxin), termed amyloid P component (AP), in close apposition to the amyloid fibrils. AP is also detected alongside normal elastin fibres in skin and basement membrane. In the present studies, purified human AP was shown to inhibit the activity of porcine pancreatic elastase. The inhibition of elastolytic activity was not abolished by heating AP to 70 degrees C. Furthermore, two other human serum proteins used as controls did not inhibit elastase activity: albumin, which has a similarly acidic pI, and C-reactive protein, which is a pentraxin, sharing 50% sequence homology with AP. Enzyme kinetic studies showed that elastase treated with AP had a slower elastolytic rate than untreated elastase. The inhibitory effect of AP was reversed by high substrate (fivefold) concentration. These observations suggest that AP may function in vivo to protect elastin and amyloid fibrils from proteolytic cleavage. Indeed, this may in part account for the relative resistance of amyloid deposits to resorption and proteolysis.

Amyloid P component (AP/SAP), a glycoprotein, precipitated with purified snail galactans from Helix pomatia and Ariania arbustorum in a dose‐dependent manner. Radiolabelled AP hinds to human peripheral blood mononuclear cells (PBMC), erythrocytes, and cells derived from human non‐T, non‐B acute lymphocytic leukaemia. The AP cell binding is specific in that it is dose‐dependent and can be blocked both by excess cold AP and by Helix promatia galactan. although it cannot be blocked by an equal amount of the monosaccharide galactose. In vitro studies of human PBMC immune responses demonstrated that AP inhibits PBMC proliferation responses to mycobacterial purified protein derivative and to phytohaemagglutinin and the humoral, antibody response to pokeweed mitogen. The AP‐induced suppression of non‐specific antibody production by human PBMC was dependent on the time at which AP was added to the culture. AP was suppressive if added in the first 48 h of the 7‐day culture, and the suppression could not be reversed by washing the cells after the exposure to AP. The mechanism of AP‐induced immunosuppression is still unclear, but human SAP circulates as a pair of pentameric rings, having ten identical subunits that bind to galactose polymers, and our present data suggest that AP affects the immune response through its properties as a lectin.

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