The structure and binding characteristics of serum amyloid protein (9.5S alpha 1-glycoprotein).

No difference have been observed in the properties of amyloid P-component (AP) and its serum counterpart (SAP) which might account for the deposition of the former in amyloidosis. Purified by nonselective techniques, preparations of AP and SAP were shown to have similar molecular weights and peptide composition, identical morphology in the electron microscope (EM) and to be antigenically indistinguishable. Both proteins were soluble in EDTA but readily precipitated in the presence of calcium ions, forming characteristic two-dimensional arrays in the EM. In serum however, SAP was not aggregated and sedimented at 9.5S in Ca2+ as did the purified protein in EDTA. Precipitation of purified SAP by calcium could be prevented by pretreatment with acid hydrolysates of agarose or SP Sephadex, matrices for which SAP has a calcium-dependent affinity. It is proposed that SAP circulates in combination with a low molecular weight natural ligand which prevents its aggregation. While the identity of natural ligand for SAP is as yet unknown, it is likely to resemble the glycosidic subunits in agarose.