Human serums contain a protein antigenically related to protein AA, the principal protein of a major class of amyloid substance. The serum antigen, SAA, occurs mainly in a hig molecular weight form, 1 to 2 X 105. This work shows that the bulk of the SAA sediments at density 1.12 g/cm3 and floats at density 1.21 g/cm3, as does the high density lipoprotein HDL3. SAA is associated with the apolipoproteins ApoA-I and ApoA-II. The total cholesterol:total protein ratio of the fraction with density 1.12-1.21 g/cm3 is 0.2:1, consistent with that of HDL3. Acid treatment dissociates a significant portion of the SAA constituent of the HDL3 fraction into low molecular weight species of the order of 13,000. The quantity of SAA may vary from 0.1% up to 1% or more of the total protein of HDL3.Amyloidosis is the term given to a group of diseases characterized by extracellular deposits in various tissues of a material, amyloid substance, now known to be comprised largely of protein. Chemically distinct varieties of amyloid deposits are known (1). Recently most emphasis has been placed upon those cases in which the amyloid substance has a significant complement of immunoglobulin light chain fragments (2). However, the first protein extracted from amyloid substance and recognized as a major constituent of one group of amyloid substance was the polypeptide now called amyloid protein AA (3). This protein was found in the amyloid deposits associated with various chronic inflammatory diseases, such as rheumatoid arthritis, ankylosing spondylitis, chronic osteomeylitis, and tuberculosis in humans. All amyloid deposits of animals so far examined, including monkey, mink, mouse, guinea pig, and duck, yield upon extraction a polypeptide homologous by amino acid sequence with the human protein AA (4-7). Human and monkey protein AA are most nearly alike and consist of 76 amino acid residues, of molecular weights about 8500 and 8620, respectively (8,9). The amino acid sequence of protein AA is not homologous with any known immunoglobulin or with any protein so far identified.Antibodies raised in rabbits against protein AA derived from human amyloid tissue deposits react with a serum constituent called SAA (10-12). The immunoreactive protein SAA has been variously reported to have a molecular weight ranging from 80,000 to 200,000, as estimated by gel chromatography or electrophoresis (10-13). Treatment of serum containing macromolecular SAA with acid causes partial degradation of SAA to a peptide with an estimated molecular weight of about 11 to 15 X 103 (13-15); this protein has an NH2-terminal amino acid sequence identical with that of tissue protein AA (14, 15). It is thought that the low molecular weight peptide is derived from the "native" macromolecular SAA and that the low molecular weight SAA component is the precursor of the protein AA of tissue amyloid deposits.The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordan...

The serum amyloid A (SAA) proteins make up a multigene family of apolipoproteins associated with high density lipoproteins. They are of ancient origin; the filnding of a highly homologous protein in mammals and ducks indicates that SAAs have been in existence for at least 300 million years. The interspecies similarity among the SAAs makes the mouse, in which they have been most thoroughly studied, a reasonable model to use for defining the function(s) of this family of proteins in humans. Originally it was observed that the SAA proteins were made in the liver and represented a set ofproteins belonging to acute-phase reactants. SAA3 is a unique member of the SAA multigene family in mice in that its mRNA is also expressed in extrahepatic tissues by a variety of cell types, mainly macrophages and adipocytes. To date, nothing has been reported regarding the fate or function of the SAA3 translation product. To identify the SAA3 protein, we developed SAA3-specific antibodies by immunizing rabbits against a portion of SAA3 protein synthesized in a bacterial fusion protein expression system. Electroimmunoblot analysis of serum and lipoprotein fractions of it showed SAA3 to be associated with high density lipoproteins of mice treated with lipopolysaccharide. Furthermore, a continuous mouse macrophage cell line (J-774.1), when exposed to lipopolysaccharide, expressed SAA3 mRNA in a dose-dependent manner and secreted SAA3 protein. The expression and secretion of SAA3 by macrophages stimulated with lipopolysaccharide suggest a role for this SAA in local responses to injury and inflammation.

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Amyloid protein AA is the presumptive fragment of an acute phase serum apolipoprotein, apoSAA. Two major murine apoSAA isotypes (apoSAA1 and apoSAA2) have been identified. The NH2-terminal amino acid sequences of purified murine apoSAA1 and apoSAA2 have been examined and compared with that of murine amyloid protein AA. Our results indicate that apoSAA1 and apoSAA2 are separate gene products and that amyloid protein AA has NH2-terminal amino acid sequence identity with only one of these isotypes, namely apoSAA2.

Mouse plasma contains a protein antigenically related to mouse protein AA, the principal protein derived from tissue deposits of amyloid substance in mice. In this work the plasma antigen, SAA, was found mainly as a high molecular weight form (2 x 10(5)) residing for the most part in the density interval 1.063-1.21 g/cm3 (high density lipoprotein, HDL); the largest amount of SAA, absolute and relative to total protein, was found in the density interval 1.125-12.1 g/cm3 (HDL3). When apoproteins of the mouse HDL obtained by delipidation of the lipoprotein particles were chromatographed in acid/urea, the antigenic activity appeared in the 10,000- to 15,000-dalton portion of the apoprotein complex. In these characteristics mouse SAA closely resembles human SAA and the behavior of the protein related to amyloid protein AA indicates that is one of the apoproteins of the HDL complex in both species. Therefore we suggest that it be named apoSAA.