Identification of multiple forms of the P component of amyloid isolated from human serum

We examined isolated serum amyloid P component (SAP), the circulating counterpart of the amyloid P component, and established a previously unreported heterogeneity for SAP by immunological and biochemical methods. Highly purified SAP had a gel filtration Mr of 255,000, a Stokes radius of 57 A, a calculated frictional coefficient of 1.4, and 10 subunits of Mr of approximately 25,000. We present evidence suggestive of varying affinities of SAP for agarose, to which SAP is known to adsorb in the presence of calcium, by fused rocket immunoelectrophoresis. We resolved SAP subunits by isoelectric focusing into multiple species with isoelectric points of 6.08 (two forms), 6.02, and 5.95; three of these forms were delineated by high resolution two-dimensional SDS gel electrophoresis to have an Mr = 25,500, while a fourth (pI = 6.08) had an Mr = 24,500. The observed isoelectric charge heterogeneity could not be eliminated by neuraminidase treatment event though the electrophoretic migration of native desialized SAP was impeded (alpha 1 to beta) when examined by crossed immunoelectrophoresis, being consistent with the removal of anionic charges. Further, an additional neuraminidase-generated component was detected at the beta-position which could be removed by concanavalin A affinity. These data suggest SAP may exist in microheterologous or allotypic forms, the significance of which is under investigation.