Pig plasma alpha-protease inhibitors PI2, PI3 and PI4 are members of the antichymotrypsin family.

Three related alpha-protease inhibitors, PI2 I, PI3 C and PI4 C2, of blood serum of the pig (Sus scrofa) were isolated. PI2 I inhibited both trypsin and chymotrypsin; PI3 C and PI4 C2 strongly inhibited chymotrypsin, but did not significantly inhibit trypsin. By using SDS-PAGE, the three proteins were found to be composed of single polypeptide chains, and molecular weights were 63,000 for PI2 I, 58,000 for PI3 C and 64,000 for PI4 C2. All three proteins were shown to be glycoproteins. In PI3 C, eight sialic acid residues were found, and in PI4 C2 (similarly as in PI2 F) 10-11 residues were found. Amino acid composition as well as N-terminal sequences of the three proteins were very similar, indicating close homology. Comparison of these partial amino acid sequences with the cDNA-deduced amino acid sequence of pig alpha-antichymotrypsin (AACT; Buchman, 1989, GenBank, Accession No. M29508) revealed great similarities, the sequence of PI2 I being virtually identical with the pig AACT. On the basis of all available results, PI2 is proposed to be pig AACT, an orthologue of human AACT.

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