Two apoproteins immunologically related to the 9000-dalton abnormal tissue constituent known as amyloid protein AA were isolated from the lipoprotein density interval 1.125-1.21 g/cm3 (HDL3) of a pool of human serums by delipidation, gel filtration, and ion-exchange chromatography. Lesser amounts of the same apoproteins were isolated from the density interval 1.063-1.125 g/cm3 (HDL2). These apoproteins, designated apoSAA1 and apoSAA2, have molecular weights near 11,500, almost identical amino acid compositions, and slightly different isoelectric points. Their amino acid sequences are identical as far as determined (30 residues), except that apoSAA2 lacks the NH2-terminal arginine found in apoSAA1. The sequence is homologous with that of amyloid protein AA, which thus has residing in the plasma high density lipoproteins a potential precursor whose biological significance and function remain to be determined.
