Pig plasma protease inhibitor gene complex: isolation and partial characterization of three inhibitors.

1. Pig plasma alpha-protease inhibitors (protease inhibitor-1, PI1; protease inhibitor-2, PI2; postalbumin-1A, PO1A; postalbumin-1B, PO1B), all encoded by one gene complex (gene cluster), were isolated by rivanol-ammonium sulphate fractionation and double-one dimensional IPG-PAGE. The proteins were recovered from the polyacrylamide gel by a combination of electrophoresis and isoelectric focusing. 2. Molecular wt estimated by SDS-PAGE under reducing conditions was 63,000 each for PI1 and PI2 and 60,000 each for PO1A and PO1B. The two main components of a genetic variant of PI2 differed in mol. wt by approx. 1000. 3. PO1A, PO1B and PI2 were shown to be glycoproteins. The major component of both PO1A and PO1B contained about 15% carbohydrate and the two components of PI2 had about 24 per cent and 21 per cent carbohydrate, respectively. 4. Neuraminidase treatment showed that the main component of PO1A had 8 sialic acid residues and fast and slow components of PI2 had respectively 11 and 10 residues. 5. Amino acid compositions of PO1A, PO1B and PI2 were very similar to one another, indicating that the genes for these proteins have evolved by regional duplications of a common ancestral gene. 6. The results (mol. wt, amino acid and carbohydrate compositions) confirm that pig PI2 is homologous to human plasma alpha 1-antichymotrypsin.