Molecular cloning reveals isoforms of bovine alpha 1-antichymotrypsin

Comparison of bovine alpha 1-antichymotrypsin (ACT) protease inhibitor with that in human was achieved by cloning a nearly full-length bovine ACT cDNA of 1.5 kb, obtained by screening a bovine liver cDNA library with the human liver ACT cDNA. The deduced primary sequence indicated that the 1456-bp bovine ACT cDNA encodes a protein of 416 amino acids that contains the predicted full-length ACT with a 26-residue NH2-terminal signal sequence. Overall, the primary sequence of bovine ACT possesses a high degree of homology (55%) with human ACT; both bovine and human ACTs share common sequences in the reactive-site domains. Importantly, the reactive site of bovine ACT possesses serine as the predicted P1 position (residue at the NH2-terminal side of the cleaved peptide bond) of the reactive site, whereas human ACT contains leucine in the P1 position. Interestingly, further evidence for heterogeneity in P1 residues was provided by a second partial 0.9-kb bovine liver ACT cDNA clone (pHHK11) that contains isoleucine as P1 residue and shares only partial homology (68%) with the deduced primary sequence of the full-length bovine liver ACT cDNA clone (pHHK12). These findings suggest that isoforms of ACT in bovine liver vary in reactive-site P1 residues; the P1 position of the reactive site is often involved in protease inhibitor specificity. Consistent with the hypothesis of ACT isoforms was the demonstration of multiple copies of the bovine ACT gene by genomic blots.