Chymostatin is a bioactive peptide of microbial origin that acts as a protease inhibitor with selectivity for chymotryptase-like serine proteases.[1] It potently inhibits chymotrypsin and chymase (Ki = 9.36 and 13.1 nM, respectively) while less effectively blocking the activity of cathepsins, papain, and leukocyte elastase.[2],[1],[3],[4],[5] It is without effect on trypsin, thrombin, plasmin, pepsin, and kallikrein.[1]

Reference:
[1]. Umezawa, H., Aoyagi, T., Morishima, H., et al. Chymostatin, a new chymotrypsin inhibitor produced by actinomycetes. J.Antibiot.(Tokyo) 23(8), 425-427 (1970).
[2]. Akahoshi, F., Ashimori, A., Sakashita, H., et al. Synthesis, structure-activity relationships, and pharmacokinetic profiles of nonpeptidic difluoromethylene ketones as novel inhibitors of human chymase. Journal of Medicinal Chemistry 44(8), 1297-1304 (2001).
[3]. Feinstein, G., Malemud, C.J., and Janoff, A. The inhibition of human leucocyte elastase and chymotrypsin-like protease by elastatinal and chymostatin. Biochim.Biophys.Acta. 429(3), 925-932 (1976).
[4]. Stein, R.L., and Strimpler, A.M. Slow-binding inhibition of chymotrypsin and cathepsin G by the peptide aldehyde chymostatin. Biochemistry 26(9), 2611-2615 (1987).
[5]. Yamamoto, K., Takeda, M., and Kato, Y. Characteristics of activation of cathepsin B by sodium salicylate and comparison of catalytic site properties of cathepsins B and H. Japanese Journal of Pharmacology 39(2), 207-215 (1985).