Purification and partial characterization of an ostrich α₁-Antichymotrypsin-like serum inhibitor

α₁-Antichymotrypsin, a member of the serpins, is the predominant plasma inhibitor of neutrophil cathepsin G. The aim of this study was to purify ostrich α₁-antichymotrypsin and to compare its biochemical properties with those of other species. Ostrich α₁-antichymotrypsin was purified from serum by ammonium sulphate fractionation, QAE-Sephadex C-50 and phenyl-Toyopearl chromatography. N-terminal sequence, amino acid composition, molecular mass, isoelectric point and reaction with cathepsin G, elastase and chymotrypsin, were determined. SDS-PAGE revealed a M(r) of 55 000 for ostrich α₁-antichymotrypsin and pI values of 6.8 and 4.1-4.3 were obtained. The amino acid composition revealed 444 residues and the N-terminal sequence of the first 20 residues revealed a homology of 30% when compared with several other α₁-antichymotrypsin sequences. Total inhibition of cathepsin G by ostrich α₁-antichymotrypsin was found at a 4:1 molar ratio of inhibitor to enzyme which was similar to that found for commercial α₁-antichymotrypsin. Immunological studies highlighted the lack of cross-reactivity between ostrich and human α₁-antichymotrypsin. The study indicated that ostrich α₁-antichymotrypsin-like molecule exhibited similar properties to human α₁-antichymotrypsin although there were notable differences.