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The physical, chemical, and immunologic properties of a protease from rat skeletal muscle, proposed to function in the degradation of certain intracellular enzymes, are identical to those of a chymotrypsin-like serine protease isolated from peritoneal mast cells. The results of polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and 8 M urea indicate that the two rat proteases have identical mobilities corresponding to a molecular weight of 26,000. The relative amino acid compositions of the proteases are nearly identical. Immunodiffusion tests for crossreaction between the muscle protease and antisera directed toward mast cell protease indicate that the former is immunologically identical to mast cell protease. The first 35 amino-terminal residues of the two enzymes are identical and indicate homology of these proteins to other mammalian serine proteases. The sequence analysis of the protease from muscle was extended -for an additional 16 positions, and comparison of this amino-terminal sequence with that of a similar enzyme from small intestine showed approximately 75% sequence identity. In contrast, only 40% of the residues in this region of bovine chyrnotrypsin A were found at corresponding loci in rat muscle protease. It is concluded that the protease from muscle or mast cells is closely related to the enzyme from small intestine which recently was localized in the "atypical" mast cells of gut mucosa Katunuma and coworkers (1, 2) have isolated several chymotrypsin-like serine proteases from various rat tissues and proposed that these enzymes initiate the degradation of several intracellular pyridoxal phosphate-dependent enzymes such as ornithine aminotransferase. The apo forms of these enzymes were inactivated by the proteases, but not the holo enzymes nor the apo forms of enzymes requiring other cofactors.Recently, an improved method was developed to purify a serine protease from rat skeletal muscle (unpublished data). During that study it was observed that the relative amino acid composition of this enzyme and that of the chymotrypsin-like protease (chymase) of peritoneal mast cells (3) were similar. The present study shows that the amino-terminal sequences and the immunologic properties of these two proteases are identical. EXPERIMENTAL Materials. For the purpose of developing specific antisera, a small amount of the chymotrypsin-like protease of rat peritoneal mast cells was purified as described by Lagunoff and Pritzl (4). Larger quantities of enzymes were prepared by a method which includes affinity adsorption chromatography on ovoinhibitor immobilized on Sepharose 4B (unpublished data). The protease from skeletal muscle was purified by the method of Sanada et al. (5) or by the affinity adsorption method described above. Rabbit antisera directed toward mast cell protease or the "atypical" mast cell protease of small intestine were prepared and their specificities were determined as described (3).Methods. Proteins were reduced and pyridylethylated (6) before sequence a...

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