Proteolytic dissection of rat brain hexokinase: determination of the cleavage pattern during limited digestion with trypsin

Limited treatment of rat brain hexokinase (ATP: D-hexose-6-phosphotransferase; EC 2.7.1.1) with trypsin causes cleavage of the Mr 98K enzyme into three major fragments having molecular weights of 10K, 40K, and 50K, with intermediates of Mr 60K and 90K being detected. This information, in conjunction with N- and C-terminal analysis of the intact enzyme and tryptic cleavage products, has established the tryptic cleavage pattern as where T1 and T2 indicate tryptic cleavage sites; cleavage at only T1 or T2 gives rise to the 90K or 60K intermediate, respectively. Confirmation of this cleavage pattern has been provided by two-dimensional peptide mapping using Staphylococcus aureus V8 protease, and epitope mapping with two monoclonal antibodies directed against rat brain hexokinase. The epitopes recognized by one of the monoclonal antibodies is located within the 40K C-terminal fragment while the epitope for the other monoclonal antibody lies within the 50K fragment. A two-dimensional peptide mapping-immunoblotting technique has permitted a more defined localization of these epitopes to specific regions within these major tryptic cleavage fragments. Complete tryptic cleavage of the enzyme occurs with only modest (approximately 20%) loss of catalytic activity, and the cleaved enzyme retains many of the properties of intact hexokinase. Specifically, there was no effect of cleavage on the Km for Glc or the Ki for Glc-6-P, though a slight decrease in Km for ATP was consistently noted to result from cleavage. Furthermore, like the intact enzyme, cleaved hexokinase retained the ability to bind to outer mitochondrial membranes in a Glc-6-P-sensitive manner. Under nondenaturing conditions, the cleaved fragments remain associated by noncovalent forces. Thus, the cleaved enzyme sedimented at a rate comparable to intact enzyme during centrifugation on sucrose density gradients, and migrated only slightly faster when electrophoresed on gradient acrylamide gels under nondenaturing conditions.     

Type X collagen contains two cleavage sites for a vertebrate collagenase

Type X collagen was cleaved at two sites by a purified human skin collagenase. Two experimental approaches were used to identify the location of the cleavage sites. First, native type X collagen was digested with the enzyme, and the rotary-shadowed products were visualized in the electron microscope. The major collagenase fragment of type X contained the epitope recognized by a monoclonal antibody (X-AC9). The antibody was used as a point of reference to locate the position of the cleavage fragment within the native molecule. Second, the digestion of radiolabeled type X collagen substrates was analyzed by gel electrophoresis. The complete cleavage of type X generated three products with 32-, 18-, and 9-kDa chains. The 32-kDa peptides were present in a triple-helical conformation and demonstrated a midpoint denaturation temperature of 43 degrees C in CD experiments. The 18-kDa peptide contained the tyrosine-rich globular domain of the molecule. The 9-kDa peptide was derived from the triple-helical end of the native molecule. Type X collagen was cleaved more rapidly by the vertebrate collagenase than was type II collagen in in vitro solution studies.     

The effects of substrates, products and other ligands on the susceptibility of inositol monophosphatase to proteolysis by endoprotease lys-C.

Inositol monophosphatase is cleaved by endoprotease lys-C at a single site (Lys36-Ser37). The rate of proteolysis is greatly reduced in the presence of substrate (D,L-Ins(1)P) and Mg2+, and less so in the presence of Pi and Mg2+, consistent with protection of the susceptible bond in the E-P or E-Pi states of the enzyme. Potentiation by Li+ of the protection afforded by a substrate analogue, 1S-phosphoryloxy-2R,4S-dihydroxycyclohexane, and Mg2+ supports the idea that Li+ binds to the E-P state.     

A novel protocol for the production of recombinant LL-37 expressed as a thioredoxin fusion protein

LL-37 is the only cathelicidin-derived antimicrobial peptide found in humans and it has a multifunctional role in host defense. The peptide has been shown to possess immunomodulatory functions in addition to antimicrobial activity. To provide sufficient material for biological and structural characterization of this important peptide, various systems were developed to produce recombinant LL-37 in Escherichia coli. In one previous approach, LL-37 coding sequence was cloned into vector pET-32a, allowing the peptide to be expressed as a thioredoxin fusion. The fusion protein contains two thrombin cleavage sites: a vector-encoded one that is 30-residue upstream of the insert and an engineered one that is immediately adjacent to LL-37. Cleavage at these two sites shall generate three fragments, one of which is the target peptide. However, when the fusion protein was treated with thrombin, cleavage only occurred at the remote upstream site. A plausible explanation is that the thrombin site adjacent to LL-37 is less accessible due to the peptide's aggregation tendency and cleavage at the remote site generates a fragment, which forms a large aggregate that buries the intended site. In this study, I deleted the vector-encoded thrombin site and S tag in pET-32a, and then inserted the coding sequence for LL-37 plus a thrombin site into the modified vector. Although removing the S tag did not change the oligomeric state of the fusion protein, deletion of the vector-encoded thrombin site allowed the fusion to be cleaved at the engineered site to release LL-37. The released peptide was separated from the carrier and cleavage enzyme by size-exclusion chromatography. This new approach enables a quick production of high quality active LL-37 with a decent amount.     

Evidence for two extracellular domains in the human interleukin-2 receptor: localization of IL-2 binding.

The human interleukin-2 (IL-2) receptor was quantitatively cleaved into two large disulfide-bonded fragments by either trypsin or endoproteinase lys-C (endo lys-C). The smaller fragment contains both N-linked oligosaccharides found in the intact receptor and is derived from the amino terminus of the molecule. The larger proteolytic fragment was metabolically labeled with 32PO4 and represents the carboxy terminus. The predicted cleavage sites of both enzymes lie in the region of the molecule encoded by exon 3. This pattern of limited proteolysis provides biochemical evidence that the extracellular region of the receptor is organized into two domains. This supports a structural model of the receptor in which the regions of internal homology encoded by exons 2 and 4 form independent disulfide-bonded domains connected by a hydrophilic segment. To determine the role of these domains in IL-2 binding, [125I]IL-2 was chemically cross-linked to the proteolytically cleaved receptor on the cell surface. The 125I-labeled complex obtained displayed N-linked oligosaccharides and had an Mr consistent with one molecule of IL-2 cross-linked to the smaller proteolytic fragment of the receptor. Thus, the amino-terminal domain of the IL-2 receptor appears to form an integral part of the IL-2 binding site.     

Identification and Mechanistic Analysis of a Novel Tick-Derived Inhibitor of Thrombin

A group of peptides from the salivary gland of the tick Hyalomma marginatum rufipes, a vector of Crimean Congo hemorrhagic fever show weak similarity to the madanins, a group of thrombin-inhibitory peptides from a second tick species, Haemaphysalis longicornis. We have evaluated the anti-serine protease activity of one of these H. marginatum peptides that has been given the name hyalomin-1. Hyalomin-1 was found to be a selective inhibitor of thrombin, blocking coagulation of plasma and inhibiting S2238 hydrolysis in a competitive manner with an inhibition constant (Ki) of 12 nM at an ionic strength of 150 mM. It also blocks the thrombin-mediated activation of coagulation factor XI, thrombin-mediated platelet aggregation, and the activation of coagulation factor V by thrombin. Hyalomin-1 is cleaved at a canonical thrombin cleavage site but the cleaved products do not inhibit coagulation. However, the C-terminal cleavage product showed non-competitive inhibition of S2238 hydrolysis. A peptide combining the N-terminal parts of the molecule with the cleavage region did not interact strongly with thrombin, but a 24-residue fragment containing the cleavage region and the C-terminal fragment inhibited the enzyme in a competitive manner and also inhibited coagulation of plasma. These results suggest that the peptide acts by binding to the active site as well as exosite I or the autolysis loop of thrombin. Injection of 2.5 mg/kg of hyalomin-1 increased arterial occlusion time in a mouse model of thrombosis, suggesting this peptide could be a candidate for clinical use as an antithrombotic.     

Isolation and characterization of a peptide containing the site of cleavage of the chick skin collagen alpha 1[I] chain by animal collagenases.

A 36-residue peptide containing the bond cleaved by animal collagenases was isolated from a digest of chick skin collagen α1-CB7 by Staphylococcus V8 protease. This cleavage site peptide, in contrast to the 36-residue α1-CB2, showed no tendency to renature to the triple helical form, as monitored by molecular sieve chromatography and the determination of circular dichroism spectra. These results provide a direct demonstration that the conformation of the α1[I] chain immediately around the collagenase cleavage site in the native molecule must be of a lower degree of helicity than other portions of the chain. This is considered to be an important factor in the collagenase specificity, in providing access to the sensitive bonds, but enzyme binding sites, probably located in the adjacent region(s) of maximum helicity, are also considered necessary to produce the maximum reaction rate.     

Evidence for domain organization within the 61-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase from bovine brain.

The complete amino acid sequence of the 61-kDa calmodulin-dependent, cyclic nucleotide phosphodiesterase (CaM-PDE) from bovine brain has been determined. The native protein is a homodimer of N alpha-acetylated, 529-residue polypeptide chains, each of which has a calculated molecular weight of 60,755. The structural organization of this CaM-PDE has been investigated with use of limited proteolysis and synthetic peptide analogues. A site capable of interacting with CaM has been identified, and the position of the catalytic domain has been mapped. A fully active, CaM-independent fragment (Mr = 36,000), produced by limited tryptic cleavage in the absence of CaM, represents a functional catalytic domain. N-Terminal sequence and size indicate that this 36-kDa fragment is comprised of residues 136 to approximately 450 of the CaM-PDE. This catalytic domain encompasses a approximately 250 residue sequence that is conserved among PDE isozymes of diverse size, phylogeny, and function. CaM-PDE and its PDE homologues comprise a unique family of proteins, each having a catalytic domain that evolved from a common progenitor. A search of the sequence for potential CaM-binding sites revealed only one 15-residue segment with both a net positive charge and the ability to form an amphiphilic alpha-helix. Peptide analogues that include this amphiphilic segment were synthesized. Each was found to inhibit the CaM-dependent activation of the enzyme and to bind directly to CaM with high affinity in a calcium-dependent manner. This site is among the sequences cleaved from a 45-kDa chymotryptic fragment that has the complete catalytic domain but no longer binds CaM. These results indicate that residues located between position 23 and 41 of the native enzyme contribute significantly to the binding of CaM although the involvement of residues from additional sites is not excluded.     

Expression, purification, and isotope labeling of a gp120 V3 peptide and production of a Fab from a HIV-1 neutralizing antibody for NMR studies

Most human immunodeficiency virus type 1 (HIV-1) neutralizing antibodies in infected individuals and in immunized animals are directed against the third variable loop (V3) of the envelope glycoprotein (gp120) of the virus. This loop plays a crucial role in phenotypic determination, cytopathicity (syncytium induction), and coreceptor usage of HIV-1. The human monoclonal antibody 447-52D was found to neutralize a broad spectrum of HIV-1 strains. In order to solve the solution structure of the V3MN peptide bound to the 447-52D Fab fragment by NMR, large quantities of labeled peptide and a protocol for the purification of the Fab fragment were needed. An expression plasmid coding for the 23-residue V3 peptide of the HIV-1MN strain (V3MN peptide, YNKRKRIHIGPGRAFYTTKNIIG) linked to a derivative of the RNA-binding domain of hnRNCP1 was constructed. The fusion protein attached to the V3 peptide prevents its degradation. Using this system, U-15N, U-13C,15N, and U-13C,15N, 50% 2H labeled fusion protein molecules were expressed in Escherichia coli grown on rich Celtone medium with yields of about 240 mg/liter. The V3MN peptide was released by CNBr cleavage and purified by RP-HPLC, giving final yields of 6-13 mg/liter. This expression system is generally applicable for biosynthesis of V3-related peptides and was also used to prepare the V3JR-FL. The 447-52D Fab fragment was obtained by a short enzymatic papain cleavage of the whole antibody. Preliminary NMR spectra demonstrate that full structural analysis of the V3MN complexed to the 447-52D Fab is feasible. This system enables studies of the same epitope bound to different HIV-1 neutralizing antibodies.     

Cleavage of nonmuscle myosin heavy chain-A during apoptosis in human Jurkat T cells

We have previously reported that calpastatin, an endogenous inhibitory protein of calpain, is cleaved by a caspase-3-like protease during apoptosis in human Jurkat T cells [Kato, M. et al. (2000) J. Biochem. 127, 297-305]. In this study, we found that nonmuscle myosin heavy chain-A (NMHC-A) is cleaved during apoptosis in Jurkat cells by using a cleavage-site-directed antibody for calpastatin. The cleavage-site-directed antibody was raised against the amino-terminal fragment of calpastatin, and this antibody detected the in vitro cleaved calpastatin fragment. Although cleaved calpastatin was not detected, a 95-kDa polypeptide (p95) was detected in apoptotic cells by this antibody. This p95 was identified as the carboxyl-terminal fragment of NMHC-A based on the results of peptide mass spectrometry fingerprinting and amino-terminal sequencing. Furthermore, two cleavage sites on NMHC-A, Asp-1153 and Asp-1948, were determined, and three cleaved fragments of NMHC-A, one cleaved at Asp-1153 and the other two cleaved at Asp-1948, were detected by cleavage-site-directed antibodies against each cleavage site. The results of confocal immunofluorescence microscopic analysis show that the cleavage at Asp-1948 occurs faster than that at Asp-1153 during apoptosis. In addition, the Asp-1153 cleaved fragment was distributed diffusely in the cytoplasm of apoptotic cells, whereas the Asp-1948 cleaved fragments were detected as condensed dots. In conclusion, our findings can be summarized as follows: (i) NMHC-A is cleaved at two sites during apoptosis, (ii) the timing of cleavage is different between these two cleavage sites, and (iii) the distribution of cleaved fragments is different in apoptotic cells.     

Human furin is a calcium-dependent serine endoprotease that recognizes the sequence Arg-X-X-Arg and efficiently cleaves anthrax toxin protective antigen.

Previous work demonstrated that human furin is a predominantly Golgi membrane-localized endoprotease that can efficiently process precursor proteins at paired basic residues (-Lys-Arg- or -Arg-Arg-) in transfected cells. Anion-exchange chromatography of culture supernatant from cells expressing a soluble truncated form of human furin resulted in a greatly enriched preparation of the endoprotease (approximately 70% pure as determined by protein staining). Enzymatic studies show that furin is a calcium-dependent (K0.5 = 200 microM) serine endoprotease which has greater than 50% of maximal activity between pH 6.0 and 8.5. The inhibitor sensitivity of furin suggests that it is similar to, yet distinct from, other calcium-dependent proteases. Evidence that furin may require a P4 Arg in fluorogenic peptide substrates suggested that this enzyme might cleave the protective antigen (PA) component of anthrax toxin at the sequence -Arg-Lys-Lys-Arg-. Indeed, PA was cleaved by purified furin at the proposed consensus site (-Arg-X-Lys/Arg-Arg decreases-) at a rate (8 mumol/min/mg total protein) 400-fold higher than that observed with synthetic peptides. In addition, the processing of mutant PA molecules with altered cleavage sites suggests that furin-catalyzed endoproteolysis minimally requires an -Arg-X-X-Arg- recognition sequence for efficient cleavage. Together, these results support the hypothesis that furin processes protein precursors containing this cleavage site motif in the exocytic pathway and in addition, raises the possibility that the enzyme also cleaves extracellular substrates, including PA.     

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.     

The epsilon subunit and inhibitory monoclonal antibodies interact with the carboxyl-terminal region of the beta subunit of Escherichia coli F1-ATPase

The epitopes of two classes of monoclonal antibody and the binding site for the epsilon subunit have been mapped to the carboxyl-terminal region of the beta subunit of Escherichia coli F1-ATPase using partial CNBr cleavage, weak acid hydrolysis, and Western blots. One class of antibody, B-I, inhibits ATPase activity; the other class, B-II, recognizes an epitope not exposed on the surface of intact F1. Data from two-dimensional gels and blots of beta cleaved with CNBr/weak acid showed that the B-I epitope lies between Asp-381 and the carboxyl-terminal Leu-459, and the B-II epitope lies between Asp-345 and Met-380. Weak acid hydrolysis of the beta-epsilon product obtained by cross-linking F1 with a water-soluble carbodiimide yielded a fragment containing epsilon and a 13-kDa carboxyl-terminal fragment of beta indicating that epsilon interacts with this portion of beta as well. Fab fragments from the B-I antibody beta-6 could be cross-linked to the epsilon subunit in native F1 by various cross-linking agents demonstrating that the antibody and the epsilon subunit occupy adjacent, nonoverlapping sites on the beta subunit. Implications of these results for the roles of the epsilon subunit and of the carboxyl-terminal region of the beta subunit in F1 are discussed.     

Delivering multiple anticancer peptides as a single prodrug using lysyl-lysine as a facile linker.

A large 40-residue precursor peptide (propeptide 5) was synthesized by linking together four designed anticancer peptide analogs to the neuropeptides: vasoactive intestinal peptide, somatostatin, bombesin and substance P, using enzyme cleavable lysyl-lysine linkers. On incubation with the enzyme trypsin, propeptide 5 was cleaved in a sequence-specific manner at the lysyl-lysine residues in the linker to release the individual peptide fragments which were identified by LC-MS. Another precursor peptide (propeptide 5a), consisting of two of the peptide analogs linked through lysyl-lysine linker, was also preferentially cleaved at the Lys-Lys site on incubation with the enzyme trypsin. Propeptide 5 showed potent anticancer activity, both in vitro and in vivo, which was greater than that of the individual component peptides. The enhanced activity suggests that the propeptide is possibly cleaved in the biological system at the lysyl-lysine site to yield the individual peptide analogs, which together show a synergistic effect. On the basis of these experimental findings, it can be concluded that pairs of basic amino acids such as Lys-Lys can be used as facile linkers for delivering multiple biologically active peptides.     

Complex recognition site for the group I intron-encoded endonuclease I-SceII.

We have characterized features of the site recognized by a double-stranded DNA endonuclease, I-SceII, encoded by intron 4 alpha of the yeast mitochondrial COX1 gene. We determined the effects of 36 point mutations on the cleavage efficiency of natural and synthetic substrates containing the Saccharomyces capensis I-SceII site. Most mutations of the 18-bp I-SceII recognition site are tolerated by the enzyme, and those mutant sites are cleaved between 42 and 100% as well as the wild-type substrate is. Nine mutants blocked cleavage to less than or equal to 33% of the wild-type, whereas only three point mutations, G-4----C, G-12----T, and G-15----C, block cleavage completely. Competition experiments indicate that these three substrates are not cleaved, at least in part because of a marked reduction in the affinity of the enzyme for those mutant DNAs. About 90% of the DNAs derived from randomization of the nucleotide sequence of the 4-bp staggered I-SceII cleavage site are not cleaved by the enzyme. I-SceII cleaves cloned DNA derived from human chromosome 3 about once every 110 kbp. The I-SceII recognition sites in four randomly chosen human DNA clones have 56 to 78% identity with the 18-bp site in yeast mitochondrial DNA; they are cleaved at least 50% as well as the wild-type mitochondrial substrate despite the presence of some substitutions that individually compromise cleavage of the mitochondrial substrate. Analysis of these data suggests that the effect of a given base substitution in I-SceII cleavage may depend on the sequence at other positions.     

Epithelial Na+ channels are fully activated by furin- and prostasin-dependent release of an inhibitory peptide from the gamma-subunit

Epithelial sodium channels (ENaC) are expressed in the apical membrane of high resistance Na(+) transporting epithelia and have a key role in regulating extracellular fluid volume and the volume of airway surface liquids. Maturation and activation of ENaC subunits involves furin-dependent cleavage of the ectodomain at two sites in the alpha subunit and at a single site within the gamma subunit. We now report that the serine protease prostasin further activates ENaC by inducing cleavage of the gamma subunit at a site distal to the furin cleavage site. Dual cleavage of the gamma subunit is predicted to release a 43-amino acid peptide. Channels with a gamma subunit lacking this 43-residue tract have increased activity due to a high open probability. A synthetic peptide corresponding to the fragment cleaved from the gamma subunit is a reversible inhibitor of endogenous ENaCs in mouse cortical-collecting duct cells and in primary cultures of human airway epithelial cells. Our results suggest that multiple proteases cleave ENaC gamma subunits to fully activate the channel.     

Rearranging the domains of pepsinogen.

Most eukaryotic aspartic protease zymogens are synthesized as a single polypeptide chain that contains two distinct homologous lobes and a pro peptide, which is removed upon activation. In pepsinogen, the pro peptide precedes the N-terminal lobe (designated pep) and the C-terminal lobe (designated sin). Based on the three-dimensional structure of pepsinogen, we have designed a pepsinogen polypeptide with the internal rearrangement of domains from pro-pep-sin (native pepsinogen) to sin-pro-pep. The domain-rearranged zymogen also contains a 10-residue linker designed to connect sin and pro domains. Recombinant sin-pro-pep was synthesized in Escherichia coli, refolded from 8 M urea, and purified. Upon acidification, sin-pro-pep autoactivates to a two-chain enzyme. However, the emergence of activity is much slower than the conversion of the single-chain zymogen to a two-chain intermediate. In the activation of native pepsinogen and sin-pro-pep, the pro region is cleaved at two sites between residues 16P and 17P and 44P and 1 successively, and complete activation of sin-pro-pep requires an additional cleavage at a third site between residues 1P and 2P. In pepsinogen activation, the cleavage of the first site is rate limiting because the second site is cleaved more rapidly to generate activity. In the activation of sin-pro-pep, however, the second site is cleaved slower than the first, and cleavage of the third site is the rate limiting step.(ABSTRACT TRUNCATED AT 250 WORDS)     

A monoclonal antibody that triggers deacylation of an intermediate thrombin-antithrombin III complex

Upon incubation of antithrombin III with thrombin in the presence of a monoclonal antibody recognizing an epitope exposed on the heavy chain part of thrombin-cleaved two-chain antithrombin III, antithrombin III was preferentially cleaved by the enzyme as a substrate, rather than covalently complexed with the enzyme to form an equimolar, stable acyl complex. Once the stable acyl complex was formed between the enzyme and antithrombin III, however, no further liberation of two-chain antithrombin III was observed. Kinetic studies showed that heparin does not affect this reaction, although generation of thrombin-cleaved two-chain antithrombin III is apparently accelerated in accordance with the rate constant for heparin-enhanced thrombin-antithrombin III complex formation. Here we propose the term "switching antibody" for an antibody that triggers deacylation of an intermediate enzyme-inhibitor complex by switching the enzyme-inhibitor reaction from the major pathway of stable acyl complex formation to an alternative pathway of cleavage of the inhibitor as a substrate.     

A functional dual-coated (FDC) microtiter plate method to replace the botulinum toxin LD50 test

Conventional capture ("Sandwich") ELISAs equally detect denatured inactive and native active botulinum type A toxin. Light chain endoprotease activity assays also fail to distinguish between various inactive molecules including partially denatured and fragmented material still retaining this protease activity. By co-coating microtiter plates with SNAP25 substrate and a monoclonal antibody specific for a conformational epitope of the toxin's Hc domain, it was possible to develop a highly sensitive (130 aM LoD), precise (1.4% GCV) new assay specific for the biologically active toxin molecule. Capture was performed in phosphate buffer with a fixed optimal concentration of chaotropic agent (e.g., 1.2 M urea) to differentially isolate functional toxin molecules. Addition of enzymatically favorable buffer containing zinc and DTT reduced the interchain disulfide bond releasing and activating the captured L-chain with subsequent specific cleavage of the SNAP25(1-206) substrate. A neoepitope antibody specific for the newly exposed Q(197) epitope was used to quantify the cleaved SNAP25(1-197). The assay's requirement for the intact toxin molecule was demonstrated with pre-reduced toxin (heavy and light chains), recombinant LHn fragments, and stressed samples containing partially or fully denatured material. This is the first known immunobiochemical assay that correlates with in vivo potency and provides a realistic alternative.     

Isolation and functional properties of an arginine-selective endoprotease from rat intestinal mucosa. A putative prosomatostatin convertase.

The endoproteolytic activity previously detected in rat intestinal mucosal extracts (Beinfeld M., Bourdais, J., Kuks, P., Morel, A., and Cohen, P. (1989) J. Biol. Chem. 264, 4460-4465), was purified to homogeneity as a 65-kDa molecular species. This putative proprotein-processing enzyme cleaves the peptide bond on the carboxyl side of a single arginine residue in hepta-[Leu62-Gln-Arg-Ser-Ala-Asn-Ser68] or trideca-[Asp56-Glu-Met-Arg-Leu-Glu-Leu-Gln-Arg-Ser-Ala-Asn-+ ++Ser68] peptides, reproducing the prosomatostatin sequence around Arg64, the locus for endoproteolytic release of either somatostatin-28 or its NH2-terminal fragment, somatostatin-28-(1-12), from their common precursor. This enzyme exhibits a strict selectivity for arginyl residues, as demonstrated with related substrates, and did not cleave at lysyl residues. Moreover, only arginyl residues belonging to peptides of the prosomatostatin family were cleaved, since no hydrolysis of peptides from other prohormones was detected. In addition, the arginine residue situated at position -5 on the NH2-terminal side of Arg64 not only did not function as a cleavage locus, but had no effect on the overall cleavage kinetics of the prosomatostatin-(56-68) peptide substrate. This enzyme also cleaved, but with much less efficiency, the peptide bond on the carboxyl side of an arginine in peptides containing either an Arg-Lys or a Lys-Arg doublet corresponding to prohormone cleavage sites. This enzyme was insensitive to divalent cation chelators, was completely inhibited by aprotinin and leupeptin, and was somewhat inhibited by other serine-protease inhibitors. It is concluded that this endoprotease is a serine protease and could be involved in prohormone or proprotein post-translational processing at single arginine cleavage sites.