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 b lood serum of the pig (Sus scrofa) were isolated. PI2 I inhibited both trypsin and chymotrypsin; PI3 C and PI4 C2 strongly inhibited chymotr ypsin, but did not significantly inhibit trypsin. By using SDS-PAGE, t he three proteins were found to be composed of single polypeptide chai ns, 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 (simila rly 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 sequ ences with the cDNA-deduced amino acid sequence of pig alpha-antichymo trypsin (AACT; Buchman, 1989, GenBank, Accession No. M29508) revealed great similarities, the sequence of PI2 I being virtually identical wi th the pig AACT. On the basis of all available results, PI2 is propose d to be pig AACT, an orthologue of human AACT.