Sheep are often used to study tissue damage following shock after traumatic injury and in the course of other diseases. The processes involved are thought to be caused at least in part by elastase released from polymorphonuclear leukocytes (PMNs). Since little is known about elastase and its role as a mediator of tissue damage in sheep, we studied the biochemical properties and release characteristics to sheep leukocyte elastase (SLE) in comparison of those of human leukocyte elastase (HLE). Both enzymes showed similar molecular masses, amino-acid compositions, N-terminal amino-acid sequences, and abilities to digest elastin substrates. Differences, however, were found in kinetic parameters measured with the elastase-specific substrate N-methoxysuccinyl-(L-alanyl)2-L-prolyl-L- valine-4-nitroanilide (MeoSuc-AAPV-pNa). The Michaelis constant (Km) of ovine elastase was nearly 10 times higher (1.82 mM) than the Km of HLE (0.21 mM). Values of SLE calculated for kcat were 70% and for kcat/Km 8% of corresponding values determined for HLE. In addition, significant differences between sheep and human PMNs were found in in vitro stimulation experiments. In contrast to human PMNs, sheep neutrophils released no active elastase, and only 50 to 70% of the H2O2 produced by human PMNs. This failure to release active elastase could not be explained by a lower elastase content of sheep PMNs, as there were no significant differences found between the elastase contents of sheep and human PMNs. We conclude that elastase liberated by stimulated sheep PMNs is inactivated by a concomitantly released proteinase inhibitor also located within the sheep PMNs.(ABSTRACT TRUNCATED AT 250 WORDS)