Rat is a likely test animal for determining the efficacy of proteinase inhibitor drugs directed toward human leukocyte elastase and cathepsin G. We therefore sought to assess and compare relevant properties of both human and rat leukocyte elastase and cathepsin G. Some differences between the pairs of proteinases from the two species were found, however both pairs of enzymes displayed comparable specificity toward various natural (plant and animal) proteinase inhibitors and also toward specific peptide substrates and a serine proteinase-specific reagent. Such overlapping specificity implies similarity of reactive center topography and sequence homology around the extended substrate/inhibitor binding regions of these proteinases. This apparent homology leads us to conclude that a pharmacologically effective inhibitor of leukocyte proteinases in the rat would probably also be effective in man.Human leukocyte elastase is arguably the most destructive enzyme present in the body. Its ability to degrade virtually all connective tissue components [l -31 has implicated its involvement in a wide variety of pathological conditions [I, 4 -141. Although the body normally possesses abundant levels of endogenous proteinase inhibitors, two of which (a-1 -proteinase inhibitor and a-2-macroglobulin) react very rapidly with human leukocyte elastase, elastase-mediated destruction of tissue nevertheless may occur when the enzyme/inhibitor balance is upset in favor of elastase. This suggests a potential therapeutic value for human leukocyte elastase-specific proteinase inhibitor drugs.Human cathepsin G has been implicated in a wide, and seemingly unrelated variety of disease processes by virtue of its ability to cleave many physiologically important proteins. It has been suggested that cathepsin G may play a role in the development of hypertension, based on its ability to convert both angiotensinogen [15] and angiotensin I [16] directly to angiotensin 11. Human cathepsin G has also been reported to cleave myosin and thus may be involved in muscle catabolism [17]. Fibronectin is cleaved into a reproducible set of fragments by the enzyme [IS] thus implicating cathepsin G in the regulation of fibronectin-mediated processes, such as cell adhesion [19], cell motility [20] and cell migration [21]. Although the involvement of cathepsin G in disease is less well documented than, e. g. human leukocyte elastase in emphysema [S -101, if the enzyme is inappropriately regulated Abhreviutions. %,PI, human alpha-I -proteinase inhibitor; Ac-AlaAla-(aza)Nle-ONp), acetyl-L-alanine-L-alanine-u-azanor-leucine-pnitrophenyl ester; Ac-Phe-ONp, N-acetyl-L-phenylalanine-p-nitrophenyl ester; Bz-Tyr-pNA, benzoyl-L-tyrosine-p-nitroanilide; BzTyr-OEt, N-benzoyl-L-tyrosine ethyl ester; HSA, human serum albumin; MeO-Suc-Ala-Ala-Pro-Phe-pNA, methoxy succinyl-L-ala-Enzymes. Leukocyte cathepsinG, both rat and human (EC 3.4.21.20); leukocyte elastase, both rat and human (EC 3.4.21.11). extracellularly the organism will likely suffer deleterious effects.Since anim...