Purification and characterization of a senile amyloid-related antigenic substance (apoSASSAM) from mouse serum. apoSASSAM is an apoA-II apolipoprotein of mouse high density lipoproteins

Two putative serum precursors which cross-react with antiserum against murine senile amyloid protein (ASSAM) were isolated from the high density lipoprotein (HDL) of normal mouse serum. Apolipoproteins designated "apoSASSAM-1" and "apoSASSAM-2" have the same molecular weight as tissue amyloid fibril protein. ApoSASSAM-1 and apoSASSAM-2 migrate to an intermediate position between apoA-I and apoC on alkaline-urea polyacrylamide gel electrophoresis and are present mainly in HDL apoproteins and to a slight extent in very low density lipoprotein apoproteins when compared to apoC. ApoSASSAM-1 and apoSASSAM-2 are polymorphic; there are two apparent isoproteins of apoSASSAM-1 with isoelectric points of 4.72 and 4.79 and two major isoproteins of apo-SASSAM-2. Subunit bands of ASSAM separated by alkaline-urea polyacrylamide gel electrophoresis and that migrated to the same positions as apoSASSAM-1 and apoSASSAM-2 were labeled by anti-apoSASSAM-1 antiserum. The amino acid compositions of apoSASSAM-1 and apoSASSAM-2 were much the same and closely resembled those of ASSAM and mouse apoA-II. Sequence analysis of apoSASSAM and ASSAM revealed a blocked amino terminus. ApoSASSAM is considered to be a mouse apoA-II and probably transforms to amyloid fibril "ASSAM" in tissues through a process yet to be clarified.