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The generation of a different type of β-kallikrein, designated Cβ-kallikrein, from α-kallikrein by chymotryptic action was ascertained by the following observations : 1) When α-kallikrein was incubated with chymotrypsin, an increase of esterolytic activity of kallikrein was observed. 2) In sodium dodecyl sulfate polyacrylamide gel electrophoresis, Cβ-kallikrein was found to be different from the β-kallikrein obtained from α-kallikrein by tryptic digestion, and was designated Tβ-kallikrein. 3) N-Terminal amino acid sequence analyses of internal light and heavy chains of Cβ-kallikrein indicated that N-termini of the light and the heavy chains were isoleucine and lysine, respectively, and that the heavy chain had most of the "kallikrein autolysis loop" sequence in its N-terminal end. In the case of Tβ-kallikrein, N-termini of the light and the heavy chains were isoleucine and alanine, respectively, and the light chain retained the "kallikrein autolysis loop" region in its C-terminal end. These observations demonstrated that Cβ-kallikrein was different from the β-kallikrein prepared from autolyzed pancreas, Aβ-kallikrein, which had lost the "kallikrein autolysis loop" sequence. Structural differences of the above four kallikreins (α-, Tβ-, Cβ- and Aβ-) result in somewhat different enzyme properties. The kinetic constants for the hydrolysis of synthetic substrates (Nα-benzoyl-L-arginine ethyl ester and Nα-tosyl-L-arginine methyl ester) of these kallikreins differed from each other, and inhibitory profiles against α1-antitrypsin were also different. These observations suggest that the "kallikrein autolysis loop" region may play an important role in the regulation of kallikrein activity in vivo.