The active site of porcine enteropeptidase. Selective inactivation of the peptidase activity

Bovine proenteropeptidase is activated by trypsin, and the specificity of enteropeptidase depends on the heavy chain

Enteropeptidase, also known as enterokinase, initiates the activation of pancreatic hydrolases by cleaving and activating trypsinogen. Enteropeptidase is synthesized as a single-chain protein, whereas purified enteropeptidase contains a approximately 47-kDa serine protease domain (light chain) and a disulfide-linked approximately 120-kDa heavy chain. The heavy chain contains an amino-terminal membrane-spanning segment and several repeated structural motifs of unknown function. To study the role of heavy chain motifs in substrate recognition, secreted variants of recombinant bovine proenteropeptidase were constructed by replacing the transmembrane domain with a signal peptide. Secreted variants containing both the heavy chain (minus the transmembrane domain) and the catalytic light chain (pro-HL-BEK (where BEK is bovine enteropeptidase)) or only the catalytic domain (pro-L-BEK) were expressed in baby hamster kidney cells and purified. Single-chain pro-HL-BEK and pro-L-BEK were zymogens with extremely low catalytic activity, and both were activated readily by trypsin cleavage. Trypsinogen was activated efficiently by purified enteropeptidase from bovine intestine (Km = 5.6 microM and kcat = 4.0 s-1) and by HL-BEK (Km = 5.6 microM and kcat = 2.2 s-1), but not by L-BEK (Km = 133 microM and kcat = 0.1 s-1); HL-BEK cleaved trypsinogen at pH 5.6 with 520-fold greater catalytic efficiency than did L-BEK. Qualitatively similar results were obtained at pH 8.4. In contrast to this striking difference in trypsinogen recognition, the small synthetic substrate Gly-Asp-Asp-Asp-Asp-Lys-beta-naphthylamide was cleaved with similar kinetic parameters by both HL-BEK (Km = 0.27 mM and kcat = 0.07 s-1) and L-BEK (Km = 0.60 mM and kcat = 0.06 s-1). The presence of the heavy chain also influenced the rate of reaction with protease inhibitors. Bovine pancreatic trypsin inhibitor preferred HL-BEK (initial Ki = 99 nM and final Ki* = 1.8 nM) over L-BEK (Ki = 698 nM and Ki* = 6.2 nM). Soybean trypsin inhibitor exhibited a reciprocal pattern, inhibiting L-BEK (Ki* = 1.6 nM), but not HL-BEK. These kinetic data indicate that the enteropeptidase heavy chain has little influence on the recognition of small peptides, but strongly influences macromolecular substrate recognition and inhibitor specificity.

Determination of enteropeptidase activity in human duodenal aspirates

A sensitive procedure is described for the determination in duodenal aspirates of enteropeptidase activity based on the activation of trypsinogen and the estimation of trypsin formed with benzoyl-arginine-p-nitroanilide. Using the recovery approach where a known amount of purified human enteropeptidase is diluted in duodenal fluid and the recoverable activity determined, this method was shown to give a sensitive and reliable estimate of the enteropeptidase activity in duodenal fluid although it was shown that the enzyme was subject to a 10% activation by components in the duodenal fluid. The reported 5-fold stimulation of enteropeptidase activity by bile salts could not be demonstrated.

On the catalytic and binding sites of porcine enteropeptidase

The active site of porcine enteropeptidase (EC 3.4.21.9) was investigated in order to characterize better both catalytic and binding sites. The participation of a serine and a histidine residue in the catalytic process was fully confirmed and the two residues were located on the light chain of the enzyme. The binding site was found to be composed of at least 2 subsites S1 and S2. The subsite S1 (similar to the trypsin-binding site) is responsible for the interactions with the small substrates of trypsin and the lysine side chain of trypsinogen, while subsite S2 (probably a cluster of lysines) is responsible for the interactions with the polyanionic sequence found in all trypsinogens. Binding of substrate by subsite S2 led to an increased efficiency of the catalytic site which can be correlated to the known high specificity of enteropeptidase.

The effect of calcium ions on the hydrolysis of benzoylarginine ethyl ester by porcine enteropeptidase

Calcium ions are shown to have a marked pH-dependent effect on the kinetics of benzoyllarginine ethyl ester hydrolysis by porcine enteropeptidase (EC 3.4.21.9). Below pH 6.0, calcium ions stimulate benzoylarginine ethyl ester hydrolysis but inhibit this activity above pH 6.0. This effect is mainly on the Km for benzoylarginine ethyl ester. At pH 5.3, 2mM calcium ions reduce the Km for benzoylarginine ethyl ester from 0.31 mM to 0.26 mM while at pH 6.5 the Km increases four-fold from 0.035 mM to 0.12 mM in the presence of calcium ions. Enteropeptidase activity is not inhibited by ethylenediaminetetra-acetate indicating that calcium ions are a non-essential cofactor for benzoylarginine ethyl ester hydrolysis.

Interaction of C1s and C4. A binding phenomenon

Addition of enzymatically active 125-I-labeled C1s (the esterase which is part of the activated complex protein of serum designated as the first component of complement or C1) to purified C4 (the naturally occurring fourth component of human serum complement) results in binding of a portion of the C1s to C4 as indicated by sucrose density gradient ultracentrifugation. Demonstration of binding requires hemolytically active C4, but not enzymatically active C1s. The latter was demonstrated by using DFP inactivated C1s as well as fragments of C1s produced by prior protease treatment of the C1s. While treatment of C1s with proteases (human leukocyte lysosomal enzymes, trypsin or plasmin) resulted in progressive inactivation of the enzymatic activity, the decline in esteratic activity occurred at a much slower rate than the decline in functional activity (inactivation of C4 in free solution). The data lead to the probable conclusion that C1s contains an enzymatic (or esteratic) site in addition to a binding site. The latter might be important for positioning a large molecule, such as C4, in order to effect proteolytic cleavage at the proper bond and hence prepare C4 to participate in the complement sequence.