Peptidase specificity characterization of C- and N-terminal catalytic sites of angiotensin I-converting enzyme

Quenched fluorescence peptides were used to investigate the substrate specificity requirements for recombinant wild-type angiotensin I-converting enzyme (ACE) and two full-length mutants bearing a single functional active site (N- or C-domain). We assayed two series of bradykinin-related peptides flanked by o-aminobenzoic acid (Abz) and N-(2,4-dinitrophenyl)ethylenediamine (EDDnp), namely, Abz-GFSPFXQ-EDDnp and Abz-GFSPFRX-EDDnp (X = natural amino acids), in which the fluorescence appeared when Abz/EDDnp are separated by substrate hydrolysis. Abz-GFSPFFQ-EDDnp was preferentially hydrolyzed by the C-domain while Abz-GFSPFQQ-EDDnp exhibits higher N-domain specificity. Internally quenched fluorescent analogues of N-acetyl-SDKP-OH were also synthesized and assayed. Abz-SDK(Dnp)P-OH, in which Abz and Dnp (2,4-dinitrophenyl) are the fluorescent donor-acceptor pair, was cleaved at the D-K(Dnp) bond with high specificity by the ACE N-domain (k(cat)/K(m) = 1.1 microM(-)(1) s(-)(1)) being practically resistant to hydrolysis by the C-domain. The importance of hydroxyl-containing amino acids at the P(2) position for N-domain specificity was shown by performing the kinetics of hydrolysis of Abz-TDK(Dnp)P-OH and Abz-YDK(Dnp)P-OH. The peptides Abz-YRK(Dnp)P-OH and Abz-FRK(Dnp)P-OH which were hydrolyzed by wild-type ACE with K(m) values of 5.1 and 4.0 microM and k(cat) values of 246 and 210 s(-)(1), respectively, have been shown to be excellent substrates for ACE. The differentiation of the catalytic specificity of the C- and N-domains of ACE seems to depend on very subtle variations on substrate-specific amino acids. The presence of a free C-terminal carboxyl group or an aromatic moiety at the same substrate position determines specific interactions with the ACE active site which is regulated by chloride and seems to distinguish the activities of both domains.

Development of an operational substrate for ZapA, a metalloprotease secreted by the bacterium Proteus mirabilis

The protease ZapA, secreted by Proteus mirabilis, has been considered to be a virulence factor of this opportunistic bacterium. The control of its expression requires the use of an appropriate methodology, which until now has not been developed. The present study focused on the replacement of azocasein with fluorogenic substrates, and on the definition of enzyme specificity. Eight fluorogenic substrates were tested, and the peptide Abz-Ala-Phe-Arg-Ser-Ala-Ala-Gln-EDDnp was found to be the most convenient for use as an operational substrate for ZapA. A single peptide bond (Arg-Ser) was cleaved with a Km of 4.6 microM, a k cat of 1.73 s-1, and a catalytic efficiency of 376 (mM s)-1. Another good substrate for ZapA was peptide 6 (Abz-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg-Gln-EDDnp) which was cleaved at a single bond (Phe-Ser) with a Km of 13.6 microM, a k cat of 3.96 s-1 and a catalytic efficiency of 291 (mM s)-1. The properties of the amino acids flanking the scissile bonds were also evaluated, and no clear requirement for the amino acid residue at P1 was found, although the enzyme seems to have a preference for a hydrophobic residue at P2.

Alpha1-antichymotrypsin and kallistatin hydrolysis by human cathepsin D

In the present paper, we demonstrate that alpha1-antichymotrypsin, a serpin with high inhibitory specificity toward cathepsin G, and kallistatin, a human serpin with high specificity toward tissue kallikrein, are digested by cathepsin D. Alpha1-Antichymotrypsin was hydrolyzed essentially in the reactive center loop at L-S, A-L, or L-V bonds; kallistatin was split into small fragments, but we detected the cleavage at F-F and F-S bonds in its reactive center loop in the first 15 min of digestion. In contrast to alpha1-antichymotrypsin, kallistatin is irreversibly inactivated at pH 4.0. Synthetic internally quenched fluorescent peptides containing sequences similar to the reactive center loops of these serpins were hydrolyzed by cathepsin D. The peptides derived from kallistatin were hydrolyzed more efficiently, and particularly relevant was the high susceptibility of the substrates Abz-AIKFFSAQTNRHILRFNRQ-EDDnp (Km = 0.08 microM, kcat = 2.4 s(-1)) and Abz-AIKFFSAQTNRQ-EDDnp (Km = 0.8 microM, kcat = 17.8 s(-1)), which were hydrolyzed at the F-F bond. Therefore, besides the description of a new class of very efficient internally quenched substrates for cathepsin D, we give evidence for the downregulation role of this proteinase on alpha1-antichymotrypsin and kallistatin. The acidification of extracellular milieu by tumor cells can result in activation of cathepsin D; as a consequence, kinins can be released, improving blood supply and leaving more cathepsin G available for the degradation of extracellular matrix.

Studies on the subsite specificity of rat nardilysin (N-arginine dibasic convertase)

The subsite specificity of rat nardilysin was investigated using fluorogenic substrates of the type 2-aminobenzoyl-GGX(1)X(2)RKX(3)GQ-ethylenediamine-2,4- dinitrophenyl, where P(2), P(2)', and P(3) residues were varied. (The nomenclature of Schechter and Berger (Schechter, I., and Berger, A. (1967) Biochem. Biophys. Res. Commun. 27, 157-162) is used where cleavage of a peptide occurs between the P(1) and P(1)' residues, and adjacent residues are designated P(2), P(3), P(2)', P(3)', etc.) There was little effect on K(m) among different residues at any of these positions. In contrast, residues at each position affected k(cat), with P(2) residues having the greatest effect. The S(3), S(2), and S(2)' subsites differed in their amino acid preference. Tryptophan and serine, which produced poor substrates at the P(2) position, were among the best P(2)' residues. The specificity at P(3) was generally opposite that of P(2). Residues at P(2), and to a lesser extent at P(3), influenced the cleavage site. At the P(2) position, His, Phe, Tyr, Asn, or Trp produced cleavage at the amino side of the first basic residue. In contrast, a P(2) Ile or Val produced cleavage between the dibasic pair. Other residues produced intermediate effects. The pH dependence for substrate binding showed that the enzyme prefers to bind a protonated histidine. A comparison of the effect of arginine or lysine at the P(1)' or P(1) position showed that there is a tendency to cleave on the amino side of arginine and that this cleavage produces the highest k(cat) values.

Simple modifications of the serpin reactive site loop convert SCCA2 into a cysteine proteinase inhibitor: a critical role for the P3' proline in facilitating RSL cleavage

The human squamous cell carcinoma antigens (SCCA) 1 and 2 are members of the serpin family that are 92% identical in their amino acid sequence. Despite this similarity, they inhibit distinct classes of proteinases. SCCA1 neutralizes the papain-like cysteine proteinases, cathepsins (cat) S, L, and K; and SCCA2 inhibits the chymotrypsin-like serine proteinases, catG and human mast cell chymase. SCCA2 also can inhibit catS, as well as other papain-like cysteine proteinases, albeit at a rate 50-fold less than that of SCCA1. Analysis of the mechanism of inhibition by SCCA1 revealed that the reactive site loop (RSL) is important for cysteine proteinase inhibition. The inhibition of catS by a mutant SCCA2 containing the RSL of SCCA1 is comparable to that of wild-type SCCA1. This finding suggested that there were no motifs outside and only eight residues within the RSL that were directing catS-specific inhibition. The purpose of this study was to determine which of these residues might account for the marked difference in the ability of SCCA1 and SCCA2 to inhibit papain-like cysteine proteinases. SCCA2 molecules containing different RSL mutations showed that no single amino acid substitution could convert SCCA2 into a more potent cysteine proteinase inhibitor. Rather, different combinations of mutations led to incremental increases in catS inhibitory activity with residues in four positions (P1, P3', P4', and P11') accounting for 80% of the difference in activity between SCCA1 and SCCA2. Interestingly, the RSL cleavage site differed between wild-type SCCA2 and this mutant. Moreover, these data established the importance of a Pro residue in the P3' position for efficient inhibition of catS by both wild-type SCCA1 and mutated SCCA2. Molecular modeling studies suggested that this residue might facilitate positioning of the RSL within the active site of the cysteine proteinase.

Hydrolysis by plasma kallikrein of fluorogenic peptides derived from prorenin processing site

Human plasma kallikrein (HPK) activates plasma prorenin to renin, and the physiological significance of this activation is still unknown. In this paper we investigated the efficiency and the cleavage pattern of the hydrolysis by HPK of the internally quenched fluorescent peptides (qf-peptides) derived from the amino acid sequence of human prorenin cleavage site. The peptide Abz-F-S-Q-P-M-K-R-L-T-L-G-N-T-T-Q-EDDnp (Abz=ortho-aminobenzoic acid, and EDDnp=N-[2,4-dinitrophenyl]-ethylene diamine), that corresponds to the amino acid sequence P(7) to P(7)' of human prorenin cleavage site, is hydrolyzed at the correct processing site (R-L bond) with k(cat)/K(m)=85 mM(-1) s(-1). Alanine was scanned in all positions from P(5) to P(5)' in order to investigate the substrate specificity requirements of HPK. The qf-peptides derived from the equivalent segment of rat prorenin, that has Lys-Lys as basic amino acid pair, and the peptide Abz-NVTSPVQ-EDDnp that contains the proposed cleavage site of rat prorenin have very low susceptibility to hydrolysis by rat plasma kallikrein. These data are according to the previously reported absence of rat plasma prorenin activation by rat plasma kallikrein (RPK), and with the view that prorenin activation in rat requires alternative enzymes and/or mechanism. All the obtained peptides described in this paper were also assayed with bovine trypsin that was taken as a reference protease because it is commonly used to activate prorenin.

Altered expression of cruzipain and a cathepsin B-like target in a Trypanosoma cruzi cell line displaying resistance to synthetic inhibitors of cysteine-proteinases

The therapeutic potential of synthetic inhibitors to the major cysteine-proteinase from Trypanosoma cruzi (cruzain or cruzipain) was recently demonstrated in animal models of Chagas' disease. A possible limitation of this strategy would be the emergence of parasite populations developing resistance to cysteine-proteinase inhibitors. Here, we describe the properties of a phenotypically stable T. cruzi cell line (R-Dm28) that displays increased resistance to Z-(SBz)Cys-Phe-CHN2, an irreversible cysteine-proteinase inhibitor which preferentially inactivates cathepsin L-like enzymes. Isolated from axenic cultures of the parental cells (IC50 1.5 microM), R-Dm28 epimastigotes exhibited 13-fold (IC50) 20 microM) higher resistance to this inhibitor and did not display cross-resistance to unrelated trypanocidal drugs, such as benznidazol and nifurtimox. Western blotting (with mAb), affinity labeling (with biotin-LVG-CHN2) and FACS analysis of R-Dm28 log-phase epimastigotes revealed that the cruzipain target was expressed at lower levels, as compared with Dm28c. Interestingly, this deficit was paralleled by increased expression of an unrelated Mr 30 000 cysteine-proteinase whose activity was somewhat refractory to inhibition by Z-(SBz)Cys-Phe-CHN,. N-terminal sequencing of the affinity-purified biotin-LVG-proteinase complex allowed its identification as a cathepsin B-like enzyme. Increased antigenic deposits of this proteinase were found in the grossly enlarged and electron dense reservosomes from R-Dm28 epimastigotes. Our data suggest that R-Dm28 resistance to toxic effects induced by the synthetic inhibitor may result from decreased availability of the most sensitive cysteine-proteinase target, cruzipain. The deficit in metabolic functions otherwise mediated by this cathepsin L-like proteinase is likely compensated by increased expression/accumulation of a cathepsin B-like target.

Hydrolysis by cathepsin B of fluorescent peptides derived from human prorenin

Cathepsin B is a lysosomal thiolprotease that, because of its colocalization with renin and its ability to activate prorenin, has been proposed as a prorenin processing enzyme. To characterize the biochemical aspect of this potential cathepsin B activity in more detail, we synthesized and assayed with human cathepsin B the internally quenched fluorescent peptide Abz-FSQPMKRLTLGNTTQ-EDDnp (Abz, ortho-aminobenzoic acid fluorescent group and EDDnp, N-¿2, 4-dinitrophenyl-ethylenediamine quencher group) that contains 7 amino acids for each side of the R-L bond that is the processing site of human prorenin. Human cathepsin B hydrolyzed this peptide at the correct site (R-L bond), with k(cat)/K(m)=75 mmol/L(-1) s(-1). Analogues of this peptide obtained by Ala scanning at positions P(5) to P(5)' were also synthesized and assayed as substrates for human cathepsin B. The obtained specificity constant (k(cat)/K(m)) values have a significant parallel with the previous data of prorenin activation by AtT-20 cells and in vitro by cathepsin B. In addition, we demonstrated the presence of cathepsin B-like activity in rat mesangial cells and the ability of its whole soluble fraction lysates, as well as that of purified cloned rat cathepsin B, to hydrolyze Abz-IKKSSF-EDDnp at the K-S bond, which contains 6 amino acids of rat prorenin processing site. The specificity data of cathepsin B toward peptides derived from prorenin processing site support the view that human or rodent cathepsin B could be involved in the intracellular processing of prorenin that is locally synthesized or taken up from the extracellular compartment.

Expression and characterization of a recombinant cysteine proteinase of Leishmania mexicana

A major cysteine proteinase (CPB) of Leishmania mexicana, that is predominantly expressed in the form of the parasite that causes disease in mammals, has been overexpressed in Escherichia coli and purified from inclusion bodies to apparent homogeneity. The CPB enzyme, CPB2.8, was expressed as an inactive pro-form lacking the characteristic C-terminal extension (CPB2.8DeltaCTE). Pro-region processing was initiated during protein refolding and proceeded through several intermediate stages. Maximum enzyme activity accompanied removal of the entire pro-region. This was facilitated by acidification. Purified mature enzyme gave a single band on SDS/PAGE and gelatin SDS/PAGE gels, co-migrated with native enzyme in L. mexicana lysates, and had the same N-terminal sequence as the native enzyme. The procedure yielded >3.5 mg of active enzyme per litre of E. coli culture.

End-to-end distance distribution in bradykinin observed by Förster resonance energy transfer

Förster resonance energy transfer (FRET) was used to study the conformational dynamics of bradykinin related peptides. The fluorescent probe aminobenzoic acid (Abz) bound to the amino terminal of bradykinin maintained its fluorescence characteristics, like high quantum yield and excited state decay dominated by a lifetime of 8.3 ns. The binding of the acceptor group N-[2, 4-dinitrophenyl]-ethylenediamine (EDDnp) to the carboxy terminal of Abz labeled bradykinin resulted in a drastic decrease of the fluorescence intensity and in a fastening of the excited state decay. The change of the decay kinetics to an heterogeneous process, precludes the use of energy transfer models based on a single fixed distance between donor and acceptor. The computational package CONTIN was employed to the analysis of time-resolved fluorescence data, allowing the recovery of a distance distribution between donor and acceptor corresponding to the end-to-end distance of the labeled peptide. The distance distribution reflects the occurrence of distinct conformations for the peptide, that coexist in equilibrium during the fluorescence lifetime. We observed three distance populations for bradykinin in water, that merged to two populations when the solvent was trifluoroethanol (TFE). The results were consistent with those obtained from circular dichroism spectroscopy, that showed structural flexibility in water and the presence of more defined secondary structure in TFE. We also studied several peptides related to bradykinin, and the results emphasized the formation of turns involving the proline residues and the decrease of conformational flexibility induced by using TFE as the solvent.

Probing the specificity of cysteine proteinases at subsites remote from the active site: analysis of P4, P3, P2' and P3' variations in extended substrates

We have determined the kinetic parameters for the hydrolysis by papain, cathepsin B and cathepsin L of internally quenched fluorescent peptides derived from the lead peptides Abz-AAFRSAQ-EDDnp [in which Abz and EDDnp stand for o-aminobenzoic acid and N-(2,4-dinitrophenyl)ethylenediamine respectively], to map the specificity of S(4) and S(3) subsites, and Abz-AFRSAAQ-EDDnp, to identify the specificity of S(2)' and S(3)'. Abz and EDDnp were the fluorescent quencher pair. These two series of peptides were cleaved at the Arg-Ser bond and systematic modifications at P(4), P(3), P(2)' and P(3)' were made. The S(4) to S(2)' subsites had a significant influence on the hydrolytic efficiencies of the three enzymes. Only papain activity was observed to be dependent on S(3)', indicating that its binding site is larger than those of cathepsins B and L. Hydrophobic amino acids were accepted at S(4), S(3), S(2)' and S(3)' of the three enzymes. The best substrates for cathepsins L and B had Trp and Asn at P(2)' respectively; variations at this position were less accepted by these enzymes. The best substrates for papain were peptides containing Trp, Tyr or Asn at P(3)'. Basic residues at P(3) and P(4) were well accepted by cathepsin L and papain. We also explored the susceptibility of substrates Abz-AFRSXAQ-EDDnp, modified at P(2)' (X), to human cathepsin B mutants from which one or two occluding loop contacts had been removed. The modifications at His(111) (H111A) and His(110) (H110A) of cathepsin B led to an increase in k(cat) values of one or two orders of magnitude. The hydrolytic efficiencies of these cathepsin B mutants became closer to those of papain or cathepsin L.

Crystal structure of cathepsin X: a flip-flop of the ring of His23 allows carboxy-monopeptidase and carboxy-dipeptidase activity of the protease

BACKGROUND

Cathepsin X is a widespread, abundantly expressed papain-like mammalian lysosomal cysteine protease. It exhibits carboxy-monopeptidase as well as carboxy-dipeptidase activity and shares a similar activity profile with cathepsin B. The latter has been implicated in normal physiological events as well as in various pathological states such as rheumatoid arthritis, Alzheimer's disease and cancer progression. Thus the question is raised as to which of the two enzyme activities has actually been monitored.

RESULTS

The crystal structure of human cathepsin X has been determined at 2.67 A resolution. The structure shares the common features of a papain-like enzyme fold, but with a unique active site. The most pronounced feature of the cathepsin X structure is the mini-loop that includes a short three-residue insertion protruding into the active site of the protease. The residue Tyr27 on one side of the loop forms the surface of the S1 substrate-binding site, and His23 on the other side modulates both carboxy-monopeptidase as well as carboxy-dipeptidase activity of the enzyme by binding the C-terminal carboxyl group of a substrate in two different sidechain conformations.

CONCLUSIONS

The structure of cathepsin X exhibits a binding surface that will assist in the design of specific inhibitors of cathepsin X as well as of cathepsin B and thereby help to clarify the physiological roles of both proteases.

Characterization of the cell adhesion site of Trypanosoma cruzi metacyclic stage surface glycoprotein gp82

The surface glycoprotein gp82, expressed in the insect-stage metacyclic trypomastigotes of Trypanosoma cruzi, has been implicated in mammalian cell invasion. Here we have characterized the cell adhesion site of gp82 by using recombinant proteins and synthetic peptides based on gp82. The recombinant protein Del-4/8, lacking 65 amino acids of gp82 central domain (at positions 257 to 321), was virtually devoid of cell-binding activity and lacked the ability to inhibit parasite invasion, in contrast to J18, the construct containing the full-length gp82 sequence (amino acids 1 to 516). Constructs with shorter deletions, i.e., Del-4 (deleted from 257 to 271) and Del-8 (deleted from 293 to 321), bound to target cells to a significantly lesser degree than did J18. The sites deleted in recombinant proteins Del-4 and Del-8 contained acidic amino acids critical for cell adhesion. Thus, the cell-binding capacity of protein Del-E/D, lacking the glutamic acid (259/260) and aspartic acid (303/304) pairs, was negligible, as was its capacity to inhibit parasite internalization. Of a set of synthetic peptides spanning the gp82 central domain, a 22-mer hybrid peptide, p4/8, formed by two noncontiguous sequences (at positions 257 to 273 and 302 to 306) and containing the four acidic residues, competed with the binding of J18 protein to target cells and significantly inhibited ( approximately 60%) the penetration of parasites. This peptide, generated by the juxtaposition of sequences that are separated by a hydrophobic stretch in the linear molecule, appears to be mimicking a conformation-dependent cell-binding site of gp82. Experiments of antibody competition with a set of 20-mer overlapping peptides mapped the epitope for 3F6, a monoclonal antibody directed to gp82 that inhibits parasite invasion, to the sequence represented by peptide p3 (244 to 263), which has a partial overlap with the cell adhesion site.

Structure-function analysis of the 7B2 CT peptide

Prohormone convertases play important roles in the proteolytic conversion of many protein precursors. The neuroendocrine protein 7B2 and its 31-residue carboxyl-terminal (CT) peptide potently and specifically inhibit prohormone convertase 2 (PC2). We have analyzed the residues contributing to inhibition using N-terminal truncation and alanine scanning. Removal of more than 3 residues from the amino-terminal end of CT1-18 resulted in a more than 190-fold drop in inhibitory activity, showing that most of the residues between 3 and 18 are required for inhibition. In agreement, an Ala scan indicated that only 4 residues could be replaced with Ala without losing mid-nanomolar inhibitory potency; in particular, Gln7, Gln9, and Asp12 could be Ala-substituted to yield peptides with a similar inhibitory potency to the starting peptide. The all-d-retro-inverso, all-l-inverso, and all-d analogues of CT peptide were completely inactive, indicating that amino acid side chains and the CT peptide main chain interact with PC2. CT peptide inhibition could not be competitively blocked by preincubation with truncated CT peptide forms, supporting an absolute requirement for the Lys-Lys pair in initial binding of the CT peptide to the active site.

The role of Glu(192) in the allosteric control of the S(2)' and S(3)' subsites of thrombin

Thrombin is an allosteric protease controlled through exosites flanking the catalytic groove. Binding of a peptide derived from hirudin (Hir(52-65)) and/or of heparin to these opposing exosites alters catalysis. We have investigated the contribution of subsites S(2)' and S(3)' to this allosteric transition by comparing the hydrolysis of two sets of fluorescence-quenched substrates having all natural amino acids at positions P(2)' and P(3)'. Regardless of the amino acids, Hir(52-65) decreased, and heparin increased the k(cat)/K(m) value of hydrolysis by thrombin. Several lines of evidence have suggested that Glu(192) participates in this modulation. We have examined the role of Glu(192) by comparing the catalytic activity of thrombin and its E192Q mutant. Mutation substantially diminishes the selectivity of thrombin. The substrate with the "best" P(2)' residue was cleaved with a k(cat)/K(m) value only 49 times higher than the one having the "least favorable" P(2)' residue (versus 636-fold with thrombin). Mutant E192Q also lost the strong preference of thrombin for positively charged P(3)' residues and its strong aversion for negatively charged P(3)' residues. Furthermore, both Hir(52-65) and heparin increased the k(cat)/K(m) value of substrate hydrolysis. We conclude that Glu(192) is critical for the P(2)' and P(3)' specificities of thrombin and for the allostery mediated through exosite 1.

Specificity of cathepsin B to fluorescent substrates containing benzyl side-chain-substituted amino acids at P1 subsite

We have determined the kinetic parameters for the hydrolysis by cathepsin B of peptidyl-coumarin amide and intramolecularly quenched fluorogenic peptides with the general structures epsilonNH2-Cap-Leu-X-MCA and Abz-Lys-Leu-X-Phe-Ser-Lys-Gln-EDDnp, respectively. Abz (orthoaminobenzoic acid) and EDDnp (2,4-dinitrophenyl-ethylenediamine) are the fluorescent donor-acceptor pair, and X was Cys(SBzl), Ser(OBzl), and Thr(OBzl) containing benzyl group (Bzl) at the functional side chain of Cys, Ser, and Thr. The peptidyl-coumarin-containing Cys(SBz1), Ser(OBzl), and Thr(OBzl) have higher affinity cathepsin B, supporting the interpretation of the crystal structure of rat cathepsin B complexed with the inhibitor Z-Arg-Ser(OBzl)-CH2Cl that the benzyl group attached to Ser hydroxyl side chain occupies the enzyme S'(1) subsite [Jia et al. (1995), J. Biol. Chem. 270, 5527]. A similar effect of benzyl group was also detected in the internally quenched peptides. Finally, the benzyl group in substrates containing Cys(SBzl) amino acid at P1 seems to compensate the absence of adequate S2-P2 interaction in the hydrolysis of the peptides having Pro or Ala at P2 position.

Characterization of thiol-, aspartyl-, and thiol-metallo-peptidase activities in Madin-Darby canine kidney cells

We combined fluorogenic substrates or internally quenched fluorescent peptides with specific inhibitors in the pH profile of proteolytic activity experiments in order to detect proteolytic activities in lysates of MDCK cells. Hydrolytic activities related to cathepsin B, L, and D were observed. Serine-proteinase was not detected; however, we clearly demonstrated the presence of a thiol-metallo-endo-oligopeptidase, also called thimet-oligopeptidase (TOP). This peptidase from MDCK cells has substrate and inhibitor specificities as well as an activation profile with mercaptoethanol that are indistinguishable from the recombinant rat testis TOP (EC 3. 4.24.15). In addition, polyclonal purified antibodies to this enzyme depleted the TOP activity of MDCK cells in whole homogenate. Although we present only preliminary data, TOP is secreted by MDCK cells. The presence of TOP in a phenotype polarized MDCK cells can have special significance in the cytoplasmic selection, transport, or clearance of short peptides due to restriction of the enzyme to sequences from 6 to 17 amino acids. Therefore, the MDCK cell could be a very useful cellular model with which to study some of the suggested TOP biological functions as processing of biological active peptides and antigen presentation.

Characterization of a kinin inactivating serine endopeptidase H2 (kininase) from human urine using fluorogenic substrates

We have previously described a kinin-inactivating endopeptidase (H2), which was purified 19-fold from human urine by DEAE-cellulose chromatography and gel filtration. The enzyme was inhibited 100% by PMSF, TPCK and pOHMB. In the present communication, we further characterized this enzyme using the fluorogenic substrates Abz-RPPGFSPFRQ-EDDnp (Abz-BKQ-EDDnp) and Abz-FRQ-EDDnp (Abz = ortho-aminobenzoic acid; EDDnp = N-[2,4-dinitrophenyl] ethylenediamine). Also a rapid, sensitive and specific assay for the H2 was developed. The enzyme hydrolyzed bradykinin (BK = RPPGFSPFR) at the F-S peptide bond, differing from the cleavage site F-R, in the fluorogenic substrates Abz-BKQ-EDDnp and Abz-FRQ-EDDnp. Other enzymes present in urine as the serine endopeptidase H1, prolyl endopeptidase and neutral endopeptidase-like were not able to hydrolyze the related substrate Abz-FRQ-EDDnp. The determined Km for Abz-BKQ-EDDnp and Abz-FRQ-EDDnp were 0.79 microM and 3.02 microM, respectively. Using the fluorogenic substrates, we observed that PMSF and p-hydroxymercuribenzoate irreversibly inhibited the enzyme H2. E-64 was a weak and reversible inhibitor, whereas EDTA and pepstatin were not inhibitory. The inhibition observed in the presence of pOHMB was partially reversed by 2 mM cysteine. These results suggest that the H2 enzyme belongs to the subfamily of SH-containing serine proteases. Based on the molecular weight of isolated H2 (60 kDa), we believe that this enzyme originated from the kidney and may cleave the kinins filtered through the glomerulus and also that produced in the kidney.

Comparison of human and porcine tissue kallikrein substrate specificities

Little is known about the species specificity of tissue kallikrein-kininogen interaction since the kinetic parameters for Lys-bradykinin release from kininogen by tissue kallikreins from different animal species have not been reported. We have now determined the kinetic parameters for hydrolysis by human and porcine tissue kallikrein, hK1 and pK1, respectively (Berg et al., 1992) of two series of intramolecularly quenched fluorogenic peptides having the sequences that flank the scissile Arg-Ser or Met-Lys bond in human and bovine kininogen. Results have shown that peptides having sequences from human kininogen are better substrates for hK1 and peptides derived from bovine kininogen are better substrates for pK1. Kinetic data for hydrolysis of the Arg-Ser bond showed that differences in the interaction of residue(s) in positions P2'-P10' contribute to the efficiency of the cleavage and may be responsible for differences in their susceptibilities to the two kallikreins. Significant variations in the kinetic data were observed for the hydrolysis of the Met-Lys bond in substrates with an N-terminal extension at sites P3-P9. The highest k(cat)/Km value in the hydrolysis of Abz-[Gln370-Gln381]-bkng-EDDnp by pk1 demonstrates an important interaction of subsites S5-S4 with Gln and Thr residues in the bovine kininogen segment. A Gln370-Gln391 bovine kininogen fragment used to study the cleavage of both Met-Lys and Arg-Ser bonds in the same molecule confirmed the importance of an extended interaction site for species specificity among tissue kallikreins.

Fluorescence-quenched solid phase combinatorial libraries in the characterization of cysteine protease substrate specificity

To map the substrate specificity of cysteine proteases, two combinatorial peptide libraries were synthesized and screened using the archetypal protease, papain. The use of PEGA resin as the solid support for library synthesis facilitated the application of an on-resin fluorescence-quenched assay. Results from the screening of library 2 indicated a preference for Pro or Val in the S3 subsite and hydrophobic residues in S2; the most prevalent residue not being Phe but Val. The S1 subsite exhibited a dual specificity for both small, nonpolar residues, Ala or Gly, as well as larger, Gln, and charged residues, Arg. Small residues predominated in the S1'-S4' subsites. Active peptides from the libraries and variations thereof were resynthesized and their kinetics of hydrolysis by papain assessed in solution phase assays. Generally, there was a good correlation between the extent of substrate cleavage on solid phase and the kcat/KM's obtained in solution phase assays. Several good substrates for papain were obtained, the best substrates being Y(NO2)PMPPLCTSMK(Abz) (kcat/KM = 2109 (mM s)-1), Y(NO2)PYAVQSPQK(Abz) (kcat/KM = 1524 (mM s)-1), and Y(NO2)PVLRQQRSK(Abz) (kcat/KM = 1450 (mM s)-1). These results were interpreted in structural terms by the use of molecular dynamics (MD). These MD calculations indicated two different modes for the binding of substrates in the narrow enzyme cleft.

Cysteine proteinase activity regulation. A possible role of heparin and heparin-like glycosaminoglycans

Papain is considered to be the archetype of cysteine proteinases. The interaction of heparin and other glycosaminoglycans with papain may be representative of many mammalian cysteine proteinase-glycosaminoglycan interactions that can regulate the function of this class of proteinases in vivo. The conformational changes in papain structure due to glycosaminoglycan interaction were studied by circular dichroism spectroscopy, and the changes in enzyme behavior were studied by kinetic analysis, monitored with fluorogenic substrate. The presence of heparin significantly increases the alpha-helix content of papain. Heparin binding to papain was demonstrated by affinity chromatography and shown to be mediated by electrostatic interactions. The incubation of papain with heparin promoted a powerful increase in the affinity of the enzyme for the substrate. In order to probe the glycosaminoglycan structure requirements for the papain interaction, the effects of two other glycosaminoglycans were tested. Like heparin, heparan sulfate, to a lesser degree, was able to decrease the papain substrate affinity, and it simultaneously induced alpha-helix structure in papain. On the other hand, dermatan sulfate was not able to decrease the substrate affinity and did not induce alpha-helix structure in papain. Heparin stabilizes the papain structure and thereby its activity at alkaline pH.

A 36-residue peptide contains all of the information required for 7B2-mediated activation of prohormone convertase 2

The prohormone convertases (PCs) are serine proteinases responsible for the processing of secretory protein precursors. PC2 is the only member of this family whose activation requires intracellular interaction with a helper protein, the neuroendocrine protein 7B2. In order to gain a better understanding of the mechanism of proPC2 activation, we have characterized the structural determinants of 7B2 required for proPC2 activation. We had already identified a proline-rich binding determinant in the 21-kDa domain, the portion of 7B2 responsible for proPC2 activation. We have now investigated the function of the weakly conserved amino-terminal portion of 21-kDa 7B2 by sequential deletions. Mutant proteins were analyzed in four assays: binding to proPC2, facilitation of proPC2 maturation, and activation of proPC2 in vivo and in vitro. We found that the amino-terminal half of 7B2 is not involved in proPC2 activation, and we identified an active 36-residue peptide that contains the previously characterized proline-rich sequence as well as an alpha-helix and the only disulfide bond of 7B2. Mutation of the alpha-helix and of the cysteines demonstrated that these determinants are absolutely required for PC2 activation. Thus, the 186-residue full-length 7B2 rat protein can be functionally reduced to an internal segment of only 36 residues.

Endocytic delivery of intramolecularly quenched substrates and inhibitors to the intracellular yeast Kex2 protease1

Kex2 in the yeast Saccharomyces cerevisiae is a transmembrane, Ca2+-dependent serine protease of the subtilisin-like pro-protein convertase (SPC) family with specificity for cleavage after paired basic amino acids. At steady state, Kex2 is predominantly localized in late Golgi compartments and initiates the proteolytic maturation of pro-protein precursors that transit the distal secretory pathway. However, Kex2 localization is not static, and its itinerary apparently involves transiting out of the late Golgi and cycling back from post-Golgi endosomal compartments during its lifetime. We tested whether the endocytic pathway could deliver small molecules to Kex2 from the extracellular medium. Here we report that intramolecularly quenched fluorogenic substrates taken up into intact yeast revealed fluorescence due to specific cleavage by Kex2 protease in endosomal compartments. Furthermore, the endocytic delivery of protease inhibitors interfered with Kex2 activity for precursor protein processing. These observations reveal that the endocytic pathway does intersect with the cycling itinerary of active Kex2 protease. This strategy of endocytic drug delivery has implications for modulating SPC protease activity needed for hormone, toxin and viral glycoprotein precursor processing in human cells.

New, sensitive fluorogenic substrates for human cathepsin G based on the sequence of serpin-reactive site loops

Cathepsin G has both trypsin- and chymotrypsin-like activity, but studies on its enzymatic properties have been limited by a lack of sensitive synthetic substrates. Cathepsin G activity is physiologically controlled by the fast acting serpin inhibitors alpha1-antichymotrypsin and alpha1-proteinase inhibitor, in which the reactive site loops are cleaved during interaction with their target enzymes. We therefore synthesized a series of intramolecularly quenched fluorogenic peptides based on the sequence of various serpin loops. Those peptides were assayed as substrates for cathepsin G and other chymotrypsin-like enzymes including chymotrypsin and chymase. Peptide substrates derived from the alpha1-antichymotrypsin loop were the most sensitive for cathepsin G with kcat/Km values of 5-20 mM-1 s-1. Substitutions were introduced at positions P1 and P2 in alpha1-antichymotrypsin-derived substrates to tentatively improve their sensitivity. Replacement of Leu-Leu in ortho-aminobenzoyl (Abz)-Thr-Leu-Leu-Ser-Ala-Leu-Gln-N-(2, 4-dinitrophenyl)ethylenediamine (EDDnp) by Pro-Phe in Abz-Thr-Pro-Phe-Ser-Ala-Leu-Gln-EDDnp produced the most sensitive substrate of cathepsin G ever reported. It was cleaved with a specificity constant kcat/Km of 150 mM-1 s-1. Analysis by molecular modeling of a peptide substrate bound into the cathepsin G active site revealed that, in addition to the protease S1 subsite, subsites S1' and S2' significantly contribute to the definition of the substrate specificity of cathepsin G.

Specificity of the dynorphin-processing endoprotease: comparison with prohormone convertases

The cleavage specificity of a monobasic processing dynorphin converting endoprotease is examined with a series of quench fluorescent peptide substrates and compared with the cleavage specificity of prohormone convertases. A dynorphin B-29-derived peptide, Abz-Arg-Arg-Gln-Phe-Lys-Val-Val-Thr-Arg-Ser-Glneddnp (where Abz is o-aminobenzoyl and eddnp is ethylenediamine 2,4-dinitrophenyl), that contains both dibasic and monobasic cleavage sites is efficiently cleaved by the dynorphin converting enzyme and not cleaved by two propeptide processing enzymes, furin and prohormone convertase 1. A shorter prorenin-related peptide, Dnp-Arg-Met-Ala-Arg-Leu-Thr-Leu-eddnp, that contains a monobasic cleavage site is cleaved by the dynorphin converting enzyme and prohormone convertase 1 and not by furin. Substitution of the P1' position by Ala moderately affects cleavage by the dynorphin-processing enzyme and prohormone convertase 1. It is interesting that this substitution results in efficient cleavage by furin. The site of cleavage, as determined by matrix-assisted laser desorption/ionization time of flight mass spectrometry, is N-terminal to the Arg at the P1 position for the dynorphin converting enzyme and C-terminal to the Arg at the P1 position for furin and prohormone convertase 1. Peptides with additional basic residues at the P2 and at P4 positions also serve as substrates for the dynorphin converting enzyme. This enzyme cleaves shorter peptide substrates with significantly lower efficiency as compared with the longer peptide substrates, suggesting that the dynorphin converting enzyme prefers longer peptides that contain monobasic processing sites as substrates. Taken together, these results suggest that the cleavage specificity of the dynorphin converting enzyme is distinct but related to the cleavage specificity of the prohormone convertases and that multiple enzymes could be involved in the processing of peptide hormones and neuropeptides at monobasic and dibasic sites.