Substrate specificity of prostate-specific antigen (PSA)

Direct cancer tissue proteomics: a method to identify candidate cancer biomarkers from formalin-fixed paraffin-embedded archival tissues

Successful treatment of multiple cancer types requires early detection and identification of reliable biomarkers present in specific cancer tissues. To test the feasibility of identifying proteins from archival cancer tissues, we have developed a methodology, termed direct tissue proteomics (DTP), which can be used to identify proteins directly from formalin-fixed paraffin-embedded prostate cancer tissue samples. Using minute prostate biopsy sections, we demonstrate the identification of 428 prostate-expressed proteins using the shotgun method. Because the DTP method is not quantitative, we employed the absolute quantification method and demonstrate picogram level quantification of prostate-specific antigen. In depth bioinformatics analysis of these expressed proteins affords the categorization of metabolic pathways that may be important for distinct stages of prostate carcinogenesis. Furthermore, we validate Wnt-3 as an upregulated protein in cancerous prostate cells by immunohistochemistry. We propose that this general strategy provides a roadmap for successful identification of critical molecular targets of multiple cancer types.

Development of Albumin-binding Doxorubicin Prodrugs that are Cleaved by Prostate-specific Antigen

Prostate-specific antigen (PSA) is a serine protease that is overexpressed in prostate carcinoma and represents a molecular target for selectively releasing an anticancer agent from a prodrug formulation. In this work, we developed albumin-binding prodrugs with the structures MT-Ser-Ser-Tyr-Tyr- Ser-Gly-DOXO, MT-Asn-Ser-Ser-Tyr-Phe-Gln-DOXO (MT = maleimidotriethyleneglycol acid; DOXO = Doxorubicin) or EMC-Arg-Arg-Ser-Ser-Tyr-Tyr-Ser-Gly-DOXO (EMC = epsilon-maleimidocaproic acid; X = amino acid). The maleimide Doxorubicin derivatives bound rapidly to the cysteine-34 position of endogenous and exogenous albumin and were efficiently cleaved by PSA at the P(1)-P'(1) scissile bond, releasing a respective Doxorubicin dipeptide (Ser-Gly-DOXO or Phe-Gln-DOXO). The derivative containing arginine residues (EMC-Arg-Arg-Ser-Ser-Tyr-Tyr-Ser-Gly-DOXO) exhibited excellent water solubility for intravenous administration. Subsequent biological evaluation was focused on a PSA-negative xenograft model (PC 3) and a PSA-positive xenograft model (CWR22) in order to assess the selectivity of our therapeutic approach. EMC-Arg-Arg-Ser-Ser-Tyr-Tyr-Ser-Gly-DOXO showed no in vivo activity in the PSA-negative PC 3 model, but good activity in the CWR22 PSA-positive model that was comparable to Doxorubicin.

Intermolecular complementation achieves high-specificity tumor targeting by anthrax toxin

Anthrax toxin protective antigen (PrAg) forms a heptamer in which the binding site for lethal factor (LF) spans two adjacent monomers. This suggested that high cell-type specificity in tumor targeting could be obtained using monomers that generate functional LF-binding sites only through intermolecular complementation. We created PrAg mutants with mutations affecting different LF-binding subsites and containing either urokinase plasminogen activator (uPA) or matrix metalloproteinase (MMP) cleavage sites. Individually, these PrAg mutants had low toxicity as a result of impaired LF binding, but when administered together to uPA- and MMP-expressing tumor cells, they assembled into functional LF-binding heteroheptamers. The mixture of two complementing PrAg variants had greatly reduced toxicity in mice and was highly effective in the treatment of aggressive transplanted tumors of diverse origin. These results show that anthrax toxin, and by implication other multimeric toxins, offer excellent opportunities to introduce multiple-specificity determinants and thereby achieve high therapeutic indices.

Construction of tumor-specific toxins using ubiquitin fusion technique

The use of cytotoxic agents to eliminate cancer cells is limited because of their nonselective toxicity and unwanted side effects. One of the strategies to overcome these limitations is to use latent prodrugs that become toxic in situ after being enzymatically activated in target cells. In this work we describe a method for producing tumor-specific toxins by using a ubiquitin fusion technique. The method is illustrated by the production of recombinant toxins by in-frame fusion of ubiquitin to saporin, a toxin from the plant Saponaria officinalis. Ubiquitin-fused toxins were rapidly degraded via the ubiquitin-proteasome system, significantly reducing their nonspecific toxicity. The insertion of the protease-cleavage sequence between ubiquitin and saporin led to the removal of ubiquitin by the protease and resulted in protease-dependent stabilization of the toxin. We engineered toxins that can be stabilized by specific proteases such as deubiquitinating enzymes and prostate-specific antigen (PSA). Both constructs were activated in vitro and in cultured cells by the appropriate enzyme. Processing by the protease resulted in a greater than 10-fold increase in the toxicity of these constructs. Importantly, the PSA-cleavable toxin was able to kill specifically the PSA-producing prostate cancer cells. The ubiquitin fusion technique is thus a versatile and reliable method for obtaining selective cytotoxic agents and can easily be adapted for different kinds of toxins and activating proteases.

Engineering of human coagulation factor x variants activated by prostate-specific antigen

In this study, we present a novel approach for the induction of tumor vessel thrombosis using genetically modified coagulation factor X. Human factor X was engineered in its activation peptide in a way that it can be specifically activated by prostate-specific antigen (PSA), a tumor-specific proteinase secreted into the bloodstream by prostate cancer cells. For this purpose we inserted different sequences of known PSA cleavage sites from the natural substrate of PSA, semenogelin I, into the activation peptide of factor X. One FX variant (FX-V4) was further optimized by site-directed mutagenesis of the P2 position and the P5 position (FX-V4-P2YP5R). After preincubation with PSA, FX-V4-P2YP5R was able to efficiently induce coagulation in vitro. These FX variants should be useful for site-specific induction of blood coagulation in the tumor vasculature.

The emerging roles of human tissue kallikreins in cancer

Human tissue kallikreins (hKs), which are encoded by the largest contiguous cluster of protease genes in the human genome, are secreted serine proteases with diverse expression patterns and physiological roles. Although primarily known for their clinical applicability as cancer biomarkers, recent evidence implicates hKs in many cancer-related processes, including cell-growth regulation, angiogenesis, invasion and metastasis. They have been shown to promote or inhibit neoplastic progression, acting individually and/or in cascades with other hKs and proteases, and might represent attractive targets for therapeutic intervention.

Engineering exosite peptides for complete inhibition of factor VIIa using a protease switch with substrate phage

Limitations of current anticoagulant therapies have led us to develop two distinct classes of exosite peptide inhibitors for the initiator of the clotting process, the tissue factor-factor VIIa (TF.FVIIa) complex (Roberge, M., Santell, L., Dennis, M. S., Eigenbrot, C., Dwyer, M. A., and Lazarus, R. A. (2001) Biochemistry 40, 9522-9531). Although both peptide classes are potent and selective inhibitors of TF.FVIIa, neither showed 100% inhibition at saturating concentrations. Crystal structures of these peptides in complex with the FVII/FVIIa protease domain revealed their distinct binding sites and close proximity to the active site. The favorable orientation of the 15-mer A-site peptide A-183 (EEWEVLCWTWETCER) suggested that a C-terminal extension into the FVIIa active site could yield a chimeric inhibitor that was not only potent and selective but complete as well. A novel two-step "protease switch" approach using substrate phage display was developed by first binding all phage containing A-183 and C-terminal extension libraries to immobilized and inactive FVIIa. Upon altering pH and adding TF to switch on FVIIa enzymatic activity, only those phage released by proteolytic cleavage within the extension were propagated. This process selected for both preferred sequence and length in the extension, leading to a 27-mer peptide A-183X (EEWEVLCWTWETCERGEGVEEELWEWR) with a C-terminal 12-mer extension containing an Arg in the P1 position. A-183X was a more potent and complete inhibitor of FX activation, having a maximal extent of inhibition of approximately 99% with an IC50 of 230 pm versus A-183 which maximally inhibited to 74% with an IC50 of 1.5 nm. A-183X also had a maximal prolongation of the prothrombin time of 7.6- versus 1.9-fold for A-183, making it a more effective anticoagulant.

Crystal structure of a prostate kallikrein isolated from stallion seminal plasma: a homologue of human PSA

Prostate-specific kallikrein, a member of the gene family of serine proteases, was initially discovered in semen and is the most useful serum marker for prostate cancer diagnosis and prognosis. We report the crystal structure at 1.42A resolution of horse prostate kallikrein (HPK). This is the first structure of a serine protease purified from seminal plasma. HPK shares extensive sequence homology with human prostate-specific antigen (PSA), including a predicted chymotrypsin-like specificity, as suggested by the presence of a serine residue at position S1 of the specificity pocket. In contrast to other kallikreins, HPK shows a structurally distinct specificity pocket. Its entrance is blocked by the kallikrein loop, suggesting a possible protective or substrate-selective role for this loop. The HPK structure seems to be in an inactivated state and further processing might be required to allow the binding of substrate molecules. Crystal soaking experiments revealed a binding site for Zn(2+) and Hg(2+), two known PSA inhibitors.

Phage display substrate: a blind method for determining protease specificity

Phage display substrate enables rapid determination of protease specificity by exposing vast numbers of recombinant peptides to a given protease. Peptides released through specific cleavage are amplified in an expression system. Phage display substrate has been widely exploited and developed further. The number of proteases (from various sources) characterized by this approach testifies to its power. To conserve their advantage over chemical methods, however, phage libraries must be constructed accordingly. The current phenomenal progress in genomics steadily increases the number of protease to be studied. Phage display substrate should prove a powerful method to exploit this wealth of new knowledge.

Substrate specificity of human kallikrein 2 (hK2) as determined by phage display technology

Human glandular kallikrein 2 (hK2) is a trypsin-like serine protease expressed predominantly in the prostate epithelium. Recently, hK2 has proven to be a useful marker that can be used in combination with prostate specific antigen for screening and diagnosis of prostate cancer. The cleavage by hK2 of certain substrates in the proteolytic cascade suggest that the kallikrein may be involved in prostate cancer development; however, there has been very little other progress toward its biochemical characterization or elucidation of its true physiological role. In the present work, we adapt phage substrate technology to study the substrate specificity of hK2. A phage-displayed random pentapeptide library with exhaustive diversity was generated and then screened with purified hK2. Phages displaying peptides susceptible to hK2 cleavage were amplified in eight rounds of selection and genes encoding substrates were transferred from the phage to a fluorescent system using cyan fluorescent protein (derived from green fluorescent protein) that enables rapid determination of specificity constants. This study shows that hK2 has a strict preference for Arg in the P1 position, which is further enhanced by a Ser in P'1 position. The scissile bonds identified by phage display substrate selection correspond to those of the natural biological substrates of hK2, which include protein C inhibitor, semenogelins, and fibronectin. Moreover, three new putative hK2 protein substrates, shown elsewhere to be involved in the biology of the cancer, have been identified thus reinforcing the importance of hK2 in prostate cancer development.

Design of new and sensitive fluorogenic substrates for human kallikrein hK3 (prostate-specific antigen) derived from semenogelin sequences

Human kallikrein hK3 (prostate-specific antigen) is a chymotrypsin-like serine protease which is widely used in the diagnosis of prostate cancer. Assays of the enzymatic activity of hK3 in extracellular fluids have been limited by a lack of sensitive synthetic substrates. This report describes the design of a series of internally quenched fluorescent peptides containing an amino acid sequence based on preferential hK3 cleavage sites in semenogelins. Those were identified by 2-D gel electrophoresis analysis and N-terminal sequencing of semenogelin fragments generated by ex vivo proteolysis in freshly ejaculated semen. These peptides were cleaved by hK3 at the C-terminal of certain tyrosyl or glutaminyl residues with k(cat)/K(m) values of 15000-60000 M(-1) s(-1). The substrate Abz-SSIYSQTEEQ-EDDnp was cleaved at the Tyr-Ser bond with a specificity constant k(cat)/K(m) of 60000 M(-1) s(-1), making it the best substrate for hK3 described to date.

Discovery of substrate for type I signal peptidase SpsB from Staphylococcus aureus

Based on the kinetic model of substrate phage proteolysis, we have formulated a strategy for best manipulating the conditions in screening phage display libraries for protease substrates (Sharkov, N. A., Davis, R. M., Reidhaar-Olson, J. F., Navre, M., and Cai, D. (2001) J. Biol. Chem. 276, 10788-10793). This strategy is exploited in the present study with signal peptidase SpsB from Staphylococcus aureus. We demonstrate that highly active substrate phage clones can be isolated from a phage display library by systematically tuning the selection stringency in screening. Several of the selected clones exhibit superior reactivity over a control, the best clone, SIIIRIII-8, showing >100-fold improvement. Because no conserved sequence features were readily revealed that could allow delineation of the active and unreactive clones, the sequences identified in five of the active clones were tested as synthetic dodecamers, Ac-AGX(8)GA-NH(2). Using electrospray ionization mass spectrometry, we show that four of these peptides can be cleaved by SpsB and that Ala is the P1 residue exclusively and Ala or Leu the P3 residue, in keeping with the (-3, -1) rule for substrate recognition by signal peptidase. Our successful screening with SpsB demonstrated the general applicability of the screening strategy and allowed us to isolate the first peptide substrates for the enzyme.

A unique substrate recognition profile for matrix metalloproteinase-2

The catalytic domains of the matrix metalloproteinases (MMPs) are structurally homologous, raising questions as to the degree of distinction, or overlap, in substrate recognition. The primary objective of the present study was to define the substrate recognition profile of MMP-2, a protease that was historically referred to as gelatinase A. By cleaving a phage peptide library with recombinant MMP-2, four distinct sets of substrates were identified. The first set is structurally related to substrates previously reported for other MMPs. These substrates contain the PXX/X(Hy) consensus motif (where X(Hy) is a hydrophobic residue) and are not generally selective for MMP-2 over the other MMPs tested. Two other groups of substrates were selected from the phage library with similar frequency. Substrates in group II contain the L/IXX/X(Hy) consensus motif. Substrates in group III contain a consensus motif with a sequence of X(Hy)SX/L, and the fourth set of substrates contain the HXX/X(Hy) sequence. Substrates in Group II, III, and IV were found to be 8- to almost 200-fold more selective for MMP-2 over MMP-9. To gain an understanding of the structural basis for substrate selectivity, individual residues within substrates were mutated, revealing that the P(2) residue is a key element in conferring selectivity. These findings indicate that MMP-2 and MMP-9 exhibit different substrate recognition profiles and point to the P(2) subsite as a primary determinant in substrate distinction.

Amidolytic activity of prostatic acid phosphatase on human semenogelins and semenogelin-derived synthetic substrates

In addition to kallikrein hK3, a serine protease generally reported as PSA (prostate-specific antigen), at least two other enzymes in human seminal plasma also cleave synthetic peptidyl substrates derived from the sequence of human semenogelins. We have identified one of these as prostatic acid phosphatase (PAP), a major component of prostatic fluid whose physiological function is unclear. The other is a high Mr basic protein present at low concentrations in seminal plasma and that remains to be characterized. PAP was purified to homogeneity from freshly ejaculated seminal plasma. Its N-terminal sequence and its phosphatase properties (hydrolysis of para-nitrophenylphosphate at low pH) were determined, and its inhibition by sodium fluoride measured. Both purified and commercial PAP also had amidolytic activity on peptide substrates derived from the semenogelin sequence at neutral and slightly basic pH. The k(cat)/K(m) values were in the 10(2)-10(3) m(-1) x s(-1) range using fluorogenic semenogelin-derived substrates whose peptidyl moiety included cleavage sites that had been identified ex vivo. PAP cleavage sites differed from those of hK3 and were mainly at P1 = Gln residues or between residues bearing hydroxyl groups. PAP amidolytic activity was poorly inhibited by all currently used wide spectrum proteinase inhibitors. Only 3-4 dichloroisocoumarin and benzamidine inhibited purified PAP. Purified human semenogelin was cleaved by purified and commercial PAP at neutral pH; the two main cleavage sites were at Tyr292 and Ser170 (semenogelin I sequence), only the former has been identified ex vivo by analysis of seminal plasma.

A substrate phage enzyme-linked immunosorbent assay to profile panels of proteases

It is estimated that proteases comprise nearly 2% of the human genome. Given that the primary structure of all known proteases will soon be available, an important challenge is to define the structure-activity relationships that govern substrate hydrolysis. Ideally this would be accomplished on a genome-wide scale. To this end, we have developed a one-pot phage selection system that yields the substrate recognition profile of multiple proteases from a single round of selection. The system meets five key criteria: (i) multiple proteases can be analyzed simultaneously, (ii) prior knowledge of substrate preference is not required, (iii) information regarding substrate preferences on both side of the scissile bond is obtained, (iv) the system yields selective substrates that distinguish closely related proteases, and (v) semiquantitative information on substrate hydrolysis is obtained, allowing for the assignment of initial rank-order preferences. As an illustration, a phage selection with a mixture of thrombin and factor Xa (serine proteases) along with matrix-metalloproteinase-9 and atrolysin C (metalloproteinases) was performed. Peptide substrates were identified that (i) have high k(cat)/K(m) ratios, (ii) are selective for individual proteases, and (iii) match the sequences of known physiological substrates.

Substrate hydrolysis by matrix metalloproteinase-9

The catalytic clefts of all matrix metalloproteinases (MMPs) have a similar architecture, raising questions about the redundancy in substrate recognition across the protein family. In the present study, an unbiased phage display strategy was applied to define the substrate recognition profile of MMP-9. Three groups of substrates were identified, each occupying a distinct set of subsites within the catalytic pocket. The most prevalent motif contains the sequence Pro-X-X-Hy-(Ser/Thr) at P(3) through P(2'). This sequence is similar to the MMP cleavage sites within the collagens and is homologous to substrates the have been selected for other MMPs. Despite this similarity, most of the substrates identified here are selective for MMP-9 over MMP-7 and MMP-13. This observation indicates that substrate selectivity is conferred by key subsite interactions at positions other than P(3) and P(1'). This study shows that MMP-9 has a unique preference for Arg at both P(2) and P(1), and a preference for Ser/Thr at P(2'). Substrates containing the consensus MMP-9 recognition motif were used to query the protein data bases. A surprisingly limited list of putative physiologic substrates was identified. The functional implications of these proteins lead to testable hypotheses regarding physiologic substrates for MMP-9.

Reaction kinetics of protease with substrate phage. Kinetic model developed using stromelysin

Peptide libraries generated using phage display have been widely applied to proteolytic enzymes for substrate selection and optimization, but the reaction kinetics between the enzyme and substrate phage are not well understood. Using a quantitative ELISA assay to monitor the disappearance of substrate, we have been able to follow the course of reaction between stromelysin, a metalloprotease, and its substrate phage. We found that under the proteolytic conditions where the enzyme was present in nanomolar concentration or higher, in excess over the substrate, the proteolysis of substrate phage was a single exponential event and the observed rate linear with respect to enzyme concentration. The enzyme concentration dependence could be described by pseudo first-order kinetic equations. Our data suggest that substrate binding is slow relative to the subsequent hydrolysis step, implying that the phage display selection process enriches clones that have high binding affinity to the protease, and the selection may not discriminate those of different chemical reactivity toward the enzyme. Considering that multiple substrate molecules may be present on a single phage particle, we regard the substrate phage reaction kinetic model as empirical. The validity of the model was ascertained when we successfully applied it to determine the binding affinity of a competitive inhibitor of stromelysin.

Screening combinatorial libraries for optimal enzyme substrates by mass spectrometry

A method has been developed for the rapid identification of optimal enzyme substrates from combinatorial libraries. This methodology was validated by screening a 361-member N-terminally formylated tripeptide library, f-XXR (X = 19 different amino acids), for optimal substrates of Escherichia coli peptide deformylase (PDF). The library was synthesized on a solid phase via the split-pool synthesis method. The N-terminal formyl group was added by treating the resin with a 1:1 (mol/mol) mixture of HCO(2)H and DCO(2)D in the presence of dicyclohexylcarbodiimide. In a mass spectrum, each member of the library produced a doublet peak (separated by 1.0063 Da). Limited treatment of this library with E. coli PDF resulted in the deformylation of those peptides that are the most efficient substrates of the enzyme. The deformylated products, due to loss of the mass-degenerate formyl group, each generated a singlet peak in the mass spectrum. Thus, the PDF product peaks were readily identified and sequenced via tandem mass spectrometry. The results showed that PDF strongly prefers a norleucine and, to a lesser extent, a phenylalanine as the N-terminal residue, whereas it has little selectivity at the penultimate position. This result is in excellent agreement with the literature data and therefore demonstrates the methodology as an effective approach to the identification of optimal enzyme substrates. This method should be generally applicable to other enzymes as well as synthetic catalysts.

Identification of novel prostate-specific antigen-binding peptides modulating its enzyme activity

Prostate-specific antigen (PSA) is a serine protease with highly prostate-specific expression. Measurement of PSA in serum is widely used for diagnosis and monitoring of prostate cancer. PSA dissolves the seminal gel forming after ejaculation. It has been suggested to mediate invasion and metastasis of prostate cancer but also to exert antiangiogenic activity. We have identified peptides specific for PSA by screening cyclic phage display peptide libraries. PSA-binding peptides were isolated from four different libraries and produced as a fusion protein with glutathione S-transferase (GST). The phage and fusion proteins were shown to bind to PSA specifically as indicated by lack of binding to other serine proteinases. A peptide with four cysteines showed the highest affinity for PSA. Zn2+, an inhibitor of PSA activity, increased the affinity of the peptides to PSA. The binding specificity was characterized by cross-inhibition using monoclonal anti-PSA antibodies of known epitope specificities. The peptides bound to the same region as mAbs specific for free PSA indicating that they bind close to the active site of the enzyme. The peptides enhanced the enzyme activity of PSA against a chromogenic substrate. These results show that peptides binding to PSA and modulating its enzyme activity can be developed by phage display technique. The peptides have the potential to be used for identification of PSA variants and for imaging and targeting of prostatic tumors.

Enzymatic action of prostate-specific antigen (PSA or hK3): substrate specificity and regulation by Zn(2+), a tight-binding inhibitor

BACKGROUND

In semen, prostate-specific antigen (PSA or hK3) digests the gel proteins semenogelin I and II, resulting in liquefaction and the release of motile spermatozoa. We characterized the substrate specificity and zinc-mediated inhibition of PSA.

METHODS

The proteolysis of human semenogelin I (SgI) and II (SgII) by PSA was characterized by purification of generated SgI and SgII fragments, N-terminal sequencing, and mass spectrometry. Zn(2+)-inhibition of PSA was studied using a chromogenic substrate.

RESULTS

Eighteen cleavage sites in SgI and 16 in SgII were identified. Cleavages were identified mainly as the C-terminal of certain tyrosine and glutamine residues, but also the C-terminal of histidine, aspartic acid, leucine, serine, and asparagine residues. No cleavages were identified at any arginine, lysine, phenylalanine, tryptophan, or methionine residues, indicating that the substrate specificity of PSA is distinct from that of trypsin, chymotrypsin, tissue kallkrein (hK1), and kallikrein 2 (hK2). Zn(2+) ions have a dramatic effect on PSA activity; the data indicate that Zn(2+) is a tight-binding inhibitor of PSA activity.

CONCLUSIONS

The data will enable the optimized design of PSA activity assays, which may prove instrumental to uncovering the role of PSA in cancer and reproduction. The inhibition data indicate that Zn(2+) could regulate PSA activity, which may prove important in the development of efficient inhibitors of PSA activity.

Negative selectivity and the evolution of protease cascades: the specificity of plasmin for peptide and protein substrates

BACKGROUND

Understanding the networks of selective proteolysis that regulate complex biological systems requires an appreciation of the molecular mechanisms used to maintain substrate specificity. Human plasmin, a serine protease that promotes the dissolution of blood clots and is essential in maintaining normal hemostasis, is usually described as having broad substrate specificity. Recent evidence that plasmin also plays a key role in a variety of other important biological and pathological processes, however, has suggested that this description might need to be re-evaluated.

RESULTS

We used substrate phage display to elucidate optimal subsite occupancy for substrates of plasmin. We identified a peptide substrate that is cleaved 710,000-fold more efficiently by plasmin than a peptide containing the activation sequence of plasminogen. Plasmin achieves this unexpected, large differential activity even though both target sequences possess an arginine residue in the P1 position. We also demonstrate that proteolysis by plasmin can be targeted to an engineered protein substrate and that introduction of substrate sequences identified by phage display into plasminogen increases plasmin-mediated cleavage of the mutant 2000-fold.

CONCLUSIONS

The specificity of plasmin is more tightly controlled than previously recognized; interactions with substrates at all subsites between S4 and S2' contribute to catalysis. Furthermore, in contrast to most enzymes that exhibit positive selectivity for substrate, the evolution of substrate specificity by plasmin has apparently been dominated by a strong negative selection against development of autoactivation activity. This 'negative selectivity' avoids short-circuiting regulation of the fibrinolytic system and other important biological processes, and might be an important general mechanism for controlling protease cascades.

Identification of substrate sequences for membrane type-1 matrix metalloproteinase using bacteriophage peptide display library

Membrane type-1 matrix metalloproteinase (MT1-MMP) has been reported to mediate the activation of progelatinase A (proMMP-2) which is associated with tumor invasion and metastasis, and also known to have an ability to digest extracellular matrix components. To clarify substrate specificity of MT1-MMP, we have searched for amino acid sequences cleaved by this protease using the hexamer substrate phage library consisting of a large number of randomized amino acids sequences. The consensus substrate sequences for MT1-MMP were deduced from the selected clones and appeared to be P-X-G/P-L at the P3-P1' sites. Peptide cleavage assay revealed that MT1-MMP preferentially digested a synthetic substrate containing Pro of the P1 position compared to that being substituted with Gly. Our results may have an important implication to identifying new target proteins for MT1-MMP and leading to the design of its selective inhibitors suitable for cancer chemotherapy.

Design of synthetic hexapeptide substrates for prostate-specific antigen using single-position minilibraries

Prostate-specific antigen (PSA), a serine endoprotease with chymotrypsin-like substrate specificity, is a marker used widely for detection of prostate cancer and other prostate diseases, catalyzing hydrolysis of the gel-forming proteins semenogelins I and II, which are synthesized and secreted by the seminal vesicle. In this study we report the use of two single-position minilibraries and RP-HPLC selection to optimize a hexapeptide substrate for PSA, spanning substrate positions P3 to P3'. PSA has been shown previously to prefer tyrosine in position P1 [Denmeade et al. (1997) Cancer Research, 57, 4924-4930]. Here we demonstrate preference for serine in position P1' and strong preference for phenylalanine in position P2. Based on these results we have designed and demonstrated the utility of the optimized fluorogenic PSA substrate 7-methoxy-coumarin-4-acetylGlnPheTyrSerSerAsnLys(epsilon-2,4-dinit rophenyl)amide, 1, which permits continuous monitoring of PSA endopeptidase activity at high sensitivity.

Enzymatic action of human glandular kallikrein 2 (hK2). Substrate specificity and regulation by Zn2+ and extracellular protease inhibitors

Human glandular kallikrein 2 (hK2) is a serine protease expressed by the prostate gland with 80% identity in primary structure to prostate-specific antigen (PSA). Recently, hK2 was shown to activate the zymogen form of PSA (proPSA) in vitro and is likely to be the physiological activator of PSA in the prostate. hK2 is also able to activate urokinase and effectively cleave fibronectin. We studied the substrate specificity of hK2 and regulation of its activity by zinc and extracellular protease inhibitors present in the prostate and seminal plasma. The enzymatic activity and substrate specificity was studied by determining hK2 cleavage sites in the major gel proteins in semen, semenogelin I and II, and by measuring hydrolysis of various tripeptide aminomethylcoumarin substrates. HK2 cleaves substrates C-terminal of single or double arginines. Basic amino acids were also occasionally found at several other positions N-terminal of the cleavage site. Therefore, the substrate specificity of hK2 fits in well with that of a processor of protein precursors. Possible regulation mechanisms were studied by testing the ability of Zn2+ and different protease inhibitors to inhibit hK2 by kinetic measurements. Inhibitory constants were determined for the most effective inhibitors PCI and Zn2+. The high affinity of PCI for hK2 (kass = 2.0 x 10(5) M-1 x s-1) and the high concentrations of PCI (4 microM) and hK2 (0.2 microM) in seminal plasma make hK2 a very likely physiological target protease for PCI. hK2 is inhibited by Zn2+ at micromolar concentrations well below the 9 mM zinc concentration found in the prostate. The enzymatic activity of hK2 is likely to be reversibly regulated by Zn2+ in prostatic fluid. This regulation may be impaired in CAP and advanced metastatic cancer resulting in lack of control of the hK2 activity and a need for other means of control.