A Novel Polymorphism of Human Serum Amyloid A Protein, SAA1γ, Is Characterized by Alanines at Both Residues 52 and 57

We have identified a hitherto unknown polymorphism of serum amyloid A protein (SAA) in the pooled serum of nonamyloidotic inflammatory patients. The molecular weight of the novel SAA determined by ion-spray mass spectrometry appeared 28 amu lower in mass than the known SAA1 subsets, indicating that the substitution of a valine residue by an alanine was present. Further analyses by the combination of liquid chromatography/fast atom bombardment mass spectrometry, collision-activated dissociation mass spectrometry, and amino acid analysis revealed that this novel SAA, designated SAA1γ, has alanines at both residues 52 and 57 while the two known SAA1 subsets have a valine at either residue 52 or 57. Furthermore, des-1Arg and des-1Arg-2Ser forms of SAA1γ as well as those of the known SAA1 subsets were identified. This indicated that the common modification must have occurred at the N-termini of all SAA1 subsets. The existence of this SAA1γ had been suggested from one of the two novel AA proteins that we recently found (Baba, S. et al., 1992, Biochim. Biophys. Acta, 1180, 195-200).