Studies on curare-like action of the tripeptide carbobenzoxy-Gly-Gly-Arg-beta-naphthylamide in mouse diaphragm.

The effects of several protease substrates or protease inhibitors on neuromuscular transmission in the isolated mouse phrenic nerve-diaphragm were studied. N-Carbobenzoxy-Gly-Gly-Arg-beta-naphthylamide (Z-GGR-N) but none of the other agents inhibited the nerve-evoked muscle contractions. By means of electrophysiological studies, Z-GGR-N was found to inhibit the amplitudes of both end-plate potentials (epps) (IC50 approximately 50 microM) and miniature end-plate potentials (mepps) but to increase the frequencies of mepps. This tripeptide could protect the nicotinic acetylcholine receptor from the irreversible inhibitory action of alpha-bungarotoxin on the mouse diaphragm. Similar to D-tubocurarine, Z-GGR-N induced tetanic fading both of nerve-evoked muscle contractions and of the amplitude of epps. Furthermore, Z-GGR-N exhibited a greater depression of the amplitudes of train-epps than those of mepps, similar to that of hexamethonium and D-tubocurarine, indicating an effect on presynaptic autoreceptors. Suramin, which could competitively reverse the inhibitory effects of non-depolarizing relaxants, acted in this study as an antagonist of all the effects of Z-GGR-N, especially those at the presynaptic site. All of these findings suggest that Z-GGR-N is a novel tripeptide possessing curare-like actions at both presynaptic and postsynaptic sites and that these actions are independent of its protease substrate property.

The combined action of two enzymes in human serum can mimic the activity of cathepsin B.

Synthetic substrates are often used to measure the activity of proteolytic enzymes. We have investigated the activities which cleave synthetic substrates such as alpha-N-benzyloxycarbonyl-Arg-Arg-beta-naphthylamide, for which the lysosomal proteinase cathepsin B has a high affinity, in sera from normal individuals, pregnant women and patients with breast cancer. As reported by other workers, activities against these substrates were elevated during pregnancy. Naphthylamine release, however, was shown to be the result of the combined action of two enzymes. The substrate is first cleaved by an endopeptidase to yield alpha-N-benzyloxycarbonyl-Arg and the aminopeptidase substrate Arg-beta-naphthylamide, which is then cleaved by serum aminopeptidases, particularly oxytocinase. A similar mechanism of cleavage was also found in the sera of breast cancer patients, where the endopeptidase catalyzing the first reaction was characterized as plasma kallikrein and the second reaction was carried out by serum leucine aminopeptidase. In no serum sample was there evidence for true cathepsin B activity.

Fluorometric microassay of trypsin and enteropeptidase in children--comparison with a titrimetic assay.

Two trypsin assay methods for the estimation of this enzyme in duodenal fluid from children have been compared. Assay results for a fluorometric method based on the use of N-carbobenzoxy-diglycyl-L-arginyl-2-naphthylamide hydrochloride (GANA) as the trypsin substrate were found to correlate well (r = 0.91, P less than 0.001) with those obtained with a much less sensitive titrimetric assay which used benzoylarginine ethylester hydrochloride (BAEE) as substrate. The higher sensitivity of the fluorometric assay has allowed accurate determination of trypsin activity in 10 microliter aliquots of duodenal fluid. This low volume requirement makes the assay suitable for studies on infants of all ages and conserves duodenal fluid for use in other investigations often warranted during the assessment of childhood malabsorption. The fluorometric assay has also been used to monitor the separation of enteropeptidase from trypsin(ogen) by chromatography on Sephacryl S-200 in samples of duodenal fluid from two children. Different proteolytic pathway deficiencies were confirmed in these children.

Human cathepsin B. Application of the substrate N-benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide to a study of the inhibition by leupeptin.

1. The kinetic parameters Kcat. and Km were determined for the hydrolysis of some arginine naphthylamides by human cathepsin B. 2. A new and efficient synthesis of Z-Arg-Arg-NNap (benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide) was developed. 3. Z-Arg-Arg-NNap was a specific and sensitive substrate for cathepsin B, and was used for kinetic studies. 4. Values of kcat. were maximal in the pH range 5.4--6.2, and depended on a single ionizing group of pKa 4.4. 5. Leupeptin was a purely competitive inhibitor of human cathepsin B. 6. The effect of pH on the apparent inhibitor constant, Ki (app.), was determined. Ki (app.) was pH-independent in the range pH 4.3--6.0, with the mean value 7 x 10(-9) M.

Design of kallidin-releasing tissue kallikrein inhibitors based on the specificities of the enzyme's binding subsites.

The tissue kallikrein inhibitors reported in the present work were derived by selectively replacing residues in Nalpha-substituted arginine- or phenylalanine-pNA (where pNA is p-nitroanilide), and in peptide substrates for these enzymes. Phenylacetyl-Arg-pNA was found to be an efficient inhibitor of human tissue kallikrein (Ki 0.4 microM) and was neither a substrate nor an inhibitor of plasma kallikrein. The peptide inhibitors having phenylalanine as the P1 residue behaved as specific inhibitors for kallidin-releasing tissue kallikreins, while plasma kallikrein showed high affinity for inhibitors containing (p-nitro)phenylalanine at the same position. The Ki value of the most potent inhibitor developed, Abz-Phe-Arg-Arg-Pro-Arg-EDDnp [where Abz is o-aminobenzoyl and EDDnp is N-(2,4-dinitrophenyl)-ethylenediamine], was 0.08 microM for human tissue kallikrein. Progress curve analyses of the inhibition of human tissue kallikrein by benzoyl-Arg-pNA and phenylacetyl-Phe-Ser-Arg-EDDnp indicated a single-step mechanism for reversible formation of the enzyme-inhibitor complex.

A general framework of cysteine-proteinase mechanism deduced from studies on enzymes with structurally different analogous catalytic-site residues Asp-158 and -161 (papain and actinidin), Gly-196 (cathepsin B) and Asn-165 (cathepsin H). Kinetic studies up to pH 8 of the hydrolysis of N-alpha-benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide catalysed by cathepsin B and of L-arginine 2-naphthylamide catalysed by cathepsin H.

The pH-dependences of kcat, Km and kcat./Km for the hydrolysis at 25 degrees C at I 0.1 of L-arginine 2-naphthylamide catalysed by cathepsin H from bovine spleen were determined in the pH range approx. 4-8. The pH-dependences of these kinetic parameters were determined also for the hydrolysis at 25 degrees C at I 0.1 of N-alpha-benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide catalysed by cathepsin B (EC 3.4.22.1) from bovine spleen in the pH range 7-8, which extends the studies in acidic media reported by Willenbrock & Brocklehurst [(1984) Biochem. J. 222, 805-814]. These results are discussed and related to those from the reactivity-probe kinetics reported in the preceding paper [Willenbrock & Brocklehurst (1985) Biochem. J. 227, 511-519] and to known structural features present in rat liver cathepsins B and H and in papain (EC 3.4.22.2) and actinidin (EC 3.4.22.14). Consideration of the kinetic data leads to the suggestion that in the cysteine proteinases rearrangement of intimate S-/ImH+ ion-pairs in catalytic sites is brought about by a combination of field effects in the immediate vicinity of the ion-pair and consequences of protonic dissociation of a group with pKa 5-6 remote from the catalytic site. The contributions of the two types of effect seem to differ from enzyme to enzyme. Of the four cysteine proteinases considered, only cathepsin B exerts an absolute requirement for the proton-deficient form of a group with pKa 5-6 for catalytic activity. Protonic dissociation with pKa 5-6 enhances catalytic activity in cathepsin H and in actinidin and appears to have little or no effect in papain. Only cathepsin B lacks a polar or negatively charged side chain in the residue analogous to Asp-158 in papain, and this is suggested to account for its total dependence on a protonic dissociation remote from the catalytic site.

Natural structural variation in enzymes as a tool in the study of mechanism exemplified by a comparison of the catalytic-site structure and characteristics of cathepsin B and papain. pH-dependent kinetics of the reactions of cathepsin B from bovine spleen and from rat liver with a thiol-specific two-protonic-state probe (2,2'-dipyridyl disulphide) and with a specific synthetic substrate (N-alpha-benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide).

Cathepsin B (EC 3.4.22.1) from bovine spleen and the analogous enzyme from rat liver were investigated at 25 degrees C at I0.1 in acidic media by kinetic study of (a) the reactions of their catalytic-site thiol groups towards the two-protonic-state reactivity probe 2,2'-dipyridyl disulphide and (b) their catalysis of the hydrolysis of N-alpha-benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide. Reactivity-probe kinetics showed that nucleophilic character is generated in the sulphur atom of cathepsin B by protonic dissociation with pKa 3.4, presumably to form an S-/ImH+ ion-pair. Substrate-catalysis kinetics showed that ion-pair formation is not sufficient to generate catalytic competence in cathepsin B, because catalytic activity is not generated as the pH is raised across pKa 3.4 but rather as it is raised across pKa 5-6 (5.1 for kcat; 5.6 for kcat./Km for the bovine spleen enzyme and 5.8 for kcat./Km for the rat liver enzyme). The implications of these results and of known structural differences between the catalytic sites of the rat liver enzyme and papain (EC 3.4.22.2) for the mechanism of cysteine-proteinase-catalysed hydrolysis are discussed.