Serum amyloid A (SAA) protein was isolated from acute phase sheep sera by ultracentrifugation, gel filtration and ion-exchange chromatography. The purified protein was characterized by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing, amino acid composition and Edman degradation. Protein SAA sheep consists of 112 amino acid residues and has a blocked N-terminus. The amino acid sequence showed a high degree of homology with SAA proteins from other species, especially at positions 32 to 54, indicating that this particular part of the protein is important for its function. When compared to human protein SAA, nine inserted amino acids could be demonstrated, located in regions 69 to 77. Similar observations have been seen in cow, horse, dog, cat, and mink protein SAA. Heterogeneities were found in positions 28, 55, 63, 64, 66, 75, 77, 78, 80 and 89. Positions 63, 64, 66, 75, 77, 78 and 80 revealed the existence of a minor gene product of protein SAA sheep. The minor variant of protein SAA sheep is identical in these positions with the corresponding positions in protein SAA cow. By comparing the amino acid sequences of the different SAA proteins, two separate branches in the evolutionary pattern of protein SAA appear. One of the branches includes the species with the insertion which represents also one of the more heterogeneous part of the protein.

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Serum amyloid A (SAA) protein, a sensitive acute phase protein and the precursor of protein AA in secondary amyloid, was purified from pooled acute phase rabbit serum using two different methods: isolation of protein SAA directly by octyl-Sepharose chromatography of total serum, and dissociation and isolation of apoSAA from acute phase high density lipoprotein (HDL). The protein SAA fraction obtained was further purified using gel filtration and ion exchange chromatography. Rabbit protein SAA has 104 amino acid residues, like human SAA, and has a partially blocked N terminus. The highly conserved region from position 33 to position 63 found in SAA from all species studied was confirmed also in rabbit SAA. No microheterogeneities were observed. The amino acid sequence showed extensive N-terminal homology with the rabbit amyloid A protein, except for the microheterogeneity in position 12 in protein AA. It also showed identical amino acid sequence with that deduced from the rabbit cDNA clone pSAA 55. Complete homologies were found with clone SAA 2, except for positions 22 and 78, clone SA8-1, except for positions 22 and 79 and clone SA7-3, except for position 22. This pSAA 55/SA7-3/SA8-1/SAA2-like protein was the only SAA isotype found both in total serum and in the HDL fraction. Isotypes corresponding to other SAA-like genes could not be found in this pool of acute phase rabbit sera.

The revised amino acid sequence of mink protein SAA was shown to be composed of 111 amino acid residues. The protein has an insertion of eight amino acid residues compared with that reported earlier. Microheterogeneities were observed in positions 6, 10, 24, 27, 67 and 71. The amino acid sequence is in accordance with the SAA mink cDNA sequences, except for the phenylalanine in position 6. The data indicate a third SAA gene in mink.

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