Characterization of Sheep Alpha-1-Proteinase Inhibitor: Important Differences from the Human Protein

The plasma proteinase inhibitor corresponding to alpha-1-proteinase inhibitor (α1PI) in humans was isolated from sheep plasma. Ovine α1PI is of higher molecular weight (62,000 daltons) than is human α1PI, is resistant to chemical oxidation by N-chlorosuccinimide, and has poor elastase-inactivating power compared with the corresponding inhibitor in humans. However, ovine α1PI is a potent trypsin inhibitor. Despite the differences indicated above, a partial homology (22 to 35%) exists between human and sheep α1PI, at least as analyzed through the first 20 residues of the sheep inhibitor. The weak elastase-inhibitory capacity of sheep α1PI is paralleled by the low content of elastase in the sheep neutrophil granule. These important differences between sheep and human neutrophils and plasma proteinase inhibitors should be borne in mind in designing experiments related to proteolytically mediated lung injury in the former species.