MOLECULAR-CLONING REVEALS ISOFORMS OF BOVINE ALPHA(1)-ANTICHYMOTRYPSIN

Comparison of bovine alpha(1)-antichymotrypsin (ACT) protease inhibito r with that in human was achieved by cloning a nearly full-length bovi ne ACT cDNA of 1.5 kb, obtained by screening a bovine liver cDNA libra ry with the human liver ACT cDNA. The deduced primary sequence indicat ed that the 1456-bp bovine ACT cDNA encodes a protein of 416 amino aci ds that contains the predicted full-length ACT with a 26 residue NH2-t erminal signal sequence. Overall, the primary sequence of bovine ACT p ossesses a high degree of homology (55%) with human ACT; both bovine a nd human ACTs share common sequences in the reactive-site domains. Imp ortantly, the reactive site of bovine ACT possesses serine as the pred icted P-1 position (residue at the NH2-terminal side of the cleaved pe ptide bond) of the reactive site, whereas human ACT contains leucine i n the P-1 position. Interestingly, further evidence for heterogeneity in P-1 residues was provided by a second partial 0.9-kb bovine liver A CT cDNA clone (pHHK11) that contains isoleucine as P-1 residue and sha res only partial homology (68%) with the deduced primary sequence of t he full-length bovine liver ACT cDNA clone (pHHK12). These findings su ggest that isoforms of ACT in bovine liver vary in reactive-site P-1 r esidues; the P-1 position of the reactive site is often involved in pr otease inhibitor specificity. Consistent with the hypothesis of ACT is oforms was the demonstration of multiple copies of the bovine ACT gene by genomic blots.