Heterogeneity of human serum amyloid A proteins.

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Abstract
Serum amyloid A proteins (SAA), presumed precursors of the tissue amyloid A proteins (AA) characteristic of secondary amyloidosis, were isolated from the plasma high-density lipoproteins (HDL) of normals after etiocholanolone-induced inflammation and from patients with Wegener''s granulomatosis, systemic lupus erythematosis, juvenile rheumatoid arthritis, Waldenstrom''s macroglobulinemia and Goodpasture''s syndrome. At least 6 polymorphic forms of SAA were identified among the low MW proteins of HDL and these comprised up to 27% of the total HDL protein. Gel and ion-exchange chromatography permitted isolation of the SAA polymorphs in homogeneous form. Their amino acid compositions were very similar, they were indistinguishable in cationic and sodium dodecyl sulfate-polyacrylamide gel electrophoresis [PAGE] systems and each had the terminal sequence COOH-Tyr-Lys-Phe-. Charge heterogeneity in anionic-urea PAGE was unaffected by neuraminidase treatment and none of the SAA protein bands stained with the periodate-Schiff reagent. The 2 major SAA polymorphs, designated SAA4 and SAA5 according to their order of elution from DEAE-cellulose, had different NH2-terminal sequences. Manual Edman degradation demonstrated NH2-Arg-Ser-Phe-Phe- for SAA4 and NH2-Ser-Phe-Phe- for SAA5. This NH2-terminal heterogeneity corresponds to that most frequently reported for AA and suggests that microheterogeneity in SAA may underlie that already documented in AA. Sufficient quantities of the other SAA polymorphs were not available for similar analyses but the amino acid compositions do not indicate that NH2-terminal heterogeneity accounts for all of the observed polymorphism. Artifactual polymorphism appears unlikely and the heterogeneity of SAA may reflect origin from more than one cell type with or without posttranslational modification. From quantitative COOH-terminal analyses, it was calculated that SAA is of 11,000-11,900 MW. Primary structure studies show AA to be a single chain protein of 76 residues and SAA appears to contain a peptide of 33 amino acids that is missing from AA.