Enterokinase （enteropeptidase, EC 3.4.4.8） occupies a key position in the utilization of dietary proteins. The enzyme initiates intraluminar digestion of proteins by the proteolytic conversion of trypsinogen to trypsin, which in turn activates the other pancreatic zymogens （Kunitz, 1939a,b; Hadorn et al., 1969）. The proteolytic attack of enterokinase is directed exclusively toward the Lys6-Ile7 peptide bond of trypsinogen leaving all other lysine and arginine bonds in the molecule unaffected （Maroux et al., 1971）. The resultant cleavage produces the simultaneous release of active trypsin and of the?
amino-terminal hexapeptide Val-（Asp）d-Lys （Rovery et al., 1953; Davie and Neurath, 1955）. This unique specificity exhibited by enterokinase is of interest as it relates to both the molecular basis of substrate recognition and the control of the digestive process.?