Human plasma alpha 1- and alpha 2-thiol proteinase inhibitors strongly inhibit Ca-activated neutral protease from muscle

Cysteine proteases as therapeutic targets: does selectivity matter? A systematic review of calpain and cathepsin inhibitors

Cysteine proteases continue to provide validated targets for treatment of human diseases. In neurodegenerative disorders, multiple cysteine proteases provide targets for enzyme inhibitors, notably caspases, calpains, and cathepsins. The reactive, active-site cysteine provides specificity for many inhibitor designs over other families of proteases, such as aspartate and serine; however, a) inhibitor strategies often use covalent enzyme modification, and b) obtaining selectivity within families of cysteine proteases and their isozymes is problematic. This review provides a general update on strategies for cysteine protease inhibitor design and a focus on cathepsin B and calpain 1 as drug targets for neurodegenerative disorders; the latter focus providing an interesting query for the contemporary assumptions that irreversible, covalent protein modification and low selectivity are anathema to therapeutic safety and efficacy.

Evidence for tissue-associated alpha(2) macroglobulin in mouse skeletal muscle

Alpha(2)-Macroglobulin (alpha(2)M), a major serum protease inhibitor, was localized in mouse skeletal muscle by immunoperoxidase histochemistry. In all muscles examined (mm. soleus, plantaris, and extensor digitorum longus) specific immunoreactivity occurred diffusely in extracellular structures (periendomysium, blood vessel wall) as well as inside about a half of the muscle fibers. This localization pattern did not change substantially by extensively perfusing deeply anesthetized mice with phosphate buffered saline (PBS) to remove serum alpha(2)M. In release experiments on fresh (nonfixed) cryostat sections, specific immunoreactivity persisted after an extensive prewash with PBS (up to 5-6 h), but a new specific staining appeared inside those fibers that were originally negative. Western blotting experiments were negative on the soluble fraction of muscle homogenate, thus confirming that the perfusion procedure was effective in removing serum alpha(2)M. By contrast, three specific bands (185, 165, and 35 kDa) appeared in detergent-solubilized extracts (0.3% Triton X-100), indicating the occurrence of tissue-associated alpha(2)M. Confocal immunofluorescence microscopy revealed that the intracellular specific staining was associated to a longitudinal network, probably corresponding to the sarcoplasmic reticulum. A multifunctional role of alpha(2)M in skeletal muscle was hypothesized.

Hybrids of chicken cystatin with human kininogen domain 2 sequences exhibit novel inhibition of calpain, improved inhibition of actinidin and impaired inhibition of papain, cathepsin L and cathepsin B

Chicken cystatin and human kininogen domain 2 are members of the cystatin superfamily of protein-type cysteine proteinase inhibitors. They show structural and functional similarities, but only human kininogen domain 2 inhibits calpain. Using recombinant chicken cystatin as a scaffold for hybrid cassette analysis, the known reactive-site regions (N-terminus, first hairpin loop and second hairpin loop) were substituted by the corresponding sequences of human kininogen domain 2 in a single and combined manner. Seven hybrids were expressed, purified to homogeneity, characterized protein-chemically, and their inhibition of papain, actinidin, human cathepsin B, human cathepsin L and calpain (80-kDa subunit of rabbit skeletal muscle calpain II and porcine erthrocyte calpain 1) was determined. Strong but temporary inhibition of calpain by chicken cystatin hybrids carrying the N-terminus alone (variant sc1-KD2) or the N-terminus together with the first hairpin loop (variant sc1/2-KD2) was observed; hybrids of the second hairpin loop (sc3-KD2, sc1/3-KD2, sc2/3-KD2, sc1/2/3-KD2) were less strong calpain inhibitors. These data indicate that the inhibiton of calpain by human kininogen domain 2 requires the correct conformation and combination of several contact sites, and suggest that the N-terminus and the first hairpin loop play a major role in this ensemble. Remarkably, hybrid sc2-KD2 exhibited 5 or 150 times stronger inhibition of actinidin compared to native chicken cystatin or to proteolytically isolated human kininogen domain 2, respectively. This indicates an important role of the first hairpin loop of cystatins in the interaction with actinidin. Along with the impaired inhibition of cathepsin L, papain, actinidin, cathepsin B and calpain by the hybrids sc1/3-KD2, sc2/3-KD2 and sc1/2/3-KD2, these results support our hypothesis that all three predicted contact regions of kininogen domain 2 contribute to binding in the active-site clefts of papain-like enzymes in a finely balanced manner.

Cysteine-proteinase-inhibiting function of T kininogen and of its proteolytic fragments

Previous attempts to liberate T kinin from T kininogen [Moreau et al. (1986) Eur. J. Biochem. 159, 341-346; Gutman et al. (1988) Eur. J. Biochem. 171, 577-582] have shown that complete fragmentation of the precursor molecule into inhibitory peptides was achieved before any vasoactive peptide was released, suggesting a possible physiological significance for this phenomenon. In this study, cysteine-proteinase-inhibiting properties of rat T kininogen and of its proteolytic fragments issuing from trypsin and submaxillary gland endopeptidase k hydrolysis, have been investigated using rat lysosomal cathepsins B, H and L, papain and bovine calpains I and II. All three lysosomal cathepsins were inhibited by T kininogen but tighter interactions were observed with cathepsin L and papain. Though higher Ki values were obtained for cathepsins B and H, rate constants for association were found to have high and almost similar values (in the 10(6) M-1 s-1 range) whatever the enzyme used. Proteolytic fragments also inhibited cathepsin L and papain very strongly and even better than the entire molecule for some of them, but no significant inhibition of cathepsins B and H was observed. Bovine calpains were not inhibited by T kininogen nor by its proteolytic fragments. From the results of this kinetic analysis, which indicates that both the association and the dissociation of lysosomal cysteine proteinases with T kininogen may occur rapidly, an hypothesis has been put forward on the possible in vivo functioning of T kininogen as a proteinase inhibitor.

Tissue distribution of an endogenous inhibitor of calcium-activated neutral protease and age-related changes in its activity in rats

1. The distribution of an endogenous inhibitor of calcium-activated neutral protease (CANP) and age-related changes in its activity were studied in male and female rats of different ages by a fluorometric assay on tissue extracts after heat treatment. 2. Ubiquitous distribution of CANP inhibitor in brain, cardiac muscle, lung, spleen, liver, skeletal muscle, kidney and testis and its abundance in spleen, liver and kidney was demonstrated. 3. Comparison in terms of units/ml of crude extracts showed that the level of CANP inhibitor exceeded that of CANP in most tissues and that the relative content of CANP inhibitor to mCANP and microCANP differed greatly among tissues. 4. Sex and species differences in CANP inhibitor activity in each tissue were of little significance. 5. Changes in CANP inhibitor during aging from 6 to 12 months was not obvious but senescent rats showed a tendency toward increased inhibitor activity. This increase was especially evident in the testis.

Inhibition of chicken calpain II by proteins of the cystatin superfamily and alpha 2-macroglobulin

Inhibition of chicken calpain II by proteins of the cystatin superfamily and alpha 2-macroglobulin was investigated. Human liver cystatins A and B, human cystatin C, chicken cystatin and rat T-kininogen were found not to be inhibitory. Inhibition was, however, observed for bovine and rat kininogens, with Ki (inhibition constant) values of 0.8 nM and 30 nM respectively. alpha 2-Macroglobulin inhibits calpain with an initial rate constant of the order of 3 X 10(4) M-1.S-1. Calpain complexed with alpha 2-macroglobulin showed only limited reactivity towards azocasein, but reacted readily with the peptide substrate Suc-Leu-Tyr-4-methyl-7-coumarylamide and with L-3-carboxy-trans-2,3-epoxypropionyl-leucylamido-(4-guanidin o)butane (E-64). The calpain in the complexes was at least partially protected from loss of activity due to autolysis. The calpain-alpha 2-macroglobulin complexes contained both the calpain subunits.

Porcine high molecular weight kininogen: its purification and properties as a thiol proteinase inhibitor as compared to human high molecular weight kininogen

1. High mol. wt kininogen (HMW kininogen) was purified to a homogeneous state from porcine plasma. 2. The protein exhibited a strong inhibitory activity for thiol proteinases such as ficin, papain and calpain I, whereas it did not inhibit serine proteinases, trypsin and chymotrypsin. 3. The mol. wt, isoelectric point, amino acid and carbohydrate compositions, stabilities to temperature and pH, kinetic constants, and immunological properties of the porcine HMW kininogen were determined and compared with those of human HMW kininogen.

Human low-Mr kininogen contains three copies of a cystatin sequence that are divergent in structure and in inhibitory activity for cysteine proteinases

We point out that human low-Mr kininogen contains three cystatin-like sequences, rather than two, as had previously been thought. The protein was purified by affinity chromatography on carboxymethyl-papain-Sepharose, and subjected to limited proteolysis by trypsin and chymotrypsin. Fragments were isolated, and three corresponding to the individual cystatin-like domains were identified. By comparison with the known amino acid sequence of the protein they were numbered 1 to 3 from the N-terminus. Domain 1 was not found to have any inhibitory activity for cysteine proteinases, which is consistent with the absence of residues that are highly conserved in inhibitors of the cystatin superfamily, and have previously been suggested to be essential for activity. Domain 2 was a good inhibitor of chicken calpain, and also papain and cathepsin L. Domain 3 showed negligible inhibition of calpain, but inhibited papain and cathepsin L strongly. The probable arrangement of disulphide bonds in the heavy chain of low-Mr kininogen is deduced from the homology with the cystatins and other evidence contained in the present paper.

A new function of kininogens as thiol-proteinase inhibitors: inhibition of papain and cathepsins B, H and L by bovine, rat and human plasma kininogens

The amidolytic activities of papain and rat liver cathepsins B, H and L were strongly inhibited by high (HMM) and low (LMM) molecular mass kininogens from bovine, human and rat plasmas, and their Ki values were estimated to be in the order of 10(-10) - 10(-11)M for papain and 10(-8) - 10(-9)M for cathepsins. The derivatives of bovine kininogens, HMM kinin-free protein, HMM kinin- and fragment 1 X 2-free protein, and LMM kinin-free protein also showed strong inhibitory activity toward these thiol-proteinases. These results suggest that a reactive site which interacts with thiol-proteinases is contained in the heavy chain portion in kininogens.

Isolation of a human cDNA for alpha 2-thiol proteinase inhibitor and its identity with low molecular weight kininogen

A lambda gt11 cDNA library containing DNA inserts prepared from human liver mRNA has been screened with an antibody to human alpha 2-thiol proteinase inhibitor that was isolated from fresh plasma. Eighteen positive clones were isolated from one million phage, and each was plaque purified. The cDNA insert of one of these phage was sequenced and shown to code for alpha 2-thiol proteinase inhibitor as identified by a partial amino acid sequence of the light chain of alpha 2-thiol proteinase inhibitor. This cDNA insert contained 1529 base pairs coding for the complete alpha 2-thiol proteinase inhibitor. It included 45 base pairs of 5' noncoding sequence, 1281 base pairs that code for pre alpha 2-thiol proteinase inhibitor, a stop codon, 160 base pairs of 3' noncoding sequence, and 40 base pairs of poly(A) tail. The noncoding sequence on the 3' end contained a potential recognition site (AATAAA) for processing and polyadenylation of precursor messenger RNA. The amino acid sequence of alpha 2-thiol proteinase inhibitor deduced from the cDNA showed a striking similarity (overall homology at 74%) to that of bovine low molecular weight (LMW) kininogen, including two internally repeated sequences and a nonapeptide sequence of bradykinin. These data clearly indicated that alpha 2-thiol proteinase inhibitor and LMW kininogen are identical. This was further supported by immunological cross-reactivity between alpha 2-thiol proteinase inhibitor and LMW kininogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Human plasma alpha-cysteine proteinase inhibitor. Purification by affinity chromatography, characterization and isolation of an active fragment

Human plasma alpha-cysteine proteinase inhibitor (alpha CPI) was purified by a two-stage method: affinity chromatography on S-carboxymethyl-papain-Sepharose, and high-resolution anion-exchange chromatography. The protein was obtained as a form of Mr about 64 000 and material of higher Mr (about 100 000). In sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with reduction, both forms showed a major component of Mr 64 000. An antiserum was raised against alpha CPI, and 'rocket' immunoassays showed the mean concentration in sera from 19 individuals to be 35.9 mg/dl. Both low-Mr and high-Mr forms of alpha CPI were confirmed to be sialoglycoproteins by the decrease in electrophoretic mobility after treatment with neuraminidase. alpha CPI was shown immunologically to be distinct from antithrombin III and alpha 1-antichymotrypsin, two serine proteinase inhibitors from plasma with somewhat similar Mr values. alpha CPI was also distinct from cystatins A and B, the two intracellular low-Mr cysteine proteinase inhibitors from human liver. Complexes of alpha CPI with papain were detectable in immunoelectrophoresis, but dissociated to free enzyme and intact inhibitor in sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The stoichiometry of binding of papain was close to 1:1 for both low-Mr and high-Mr forms. alpha CPI was found to be a tight-binding inhibitor of papain and human cathepsins H and L (Ki 34 pM, 1.1 nM and 62 pM respectively). By contrast, inhibition of cathepsin B was much weaker, Ki being about 35 microM. Dipeptidyl peptidase I also was weakly inhibited. Digestion of alpha CPI with bromelain gave rise to an inhibitory fragment of Mr about 22 000, which was isolated.

Partial purification and some properties of a new papain inhibitor from psoriatic scales

A new papain inhibitor was purified from psoriatic epidermal scales using gel chromatography and anion exchange chromatography. The purified protein inhibited papain and ficin but not cathepsin B, cathepsin H, trypsin, or chymotrypsin. Isoelectric focusing revealed 3 major inhibitor variants with pI's of 7.3, 6.9, and 6.5. A Mr of 38,000 was obtained by a gel chromatographic method for the crude inhibitor. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis the Mr values of the isoelectric variants were: 43,000 for the variant pI 7.3, 43,000 and 35,000 for the variant pI 6.9, and 34,000-35,000 for the variant pI 6.5. An antiserum of the inhibitor was used to locate the inhibitor in the psoriatic and normal epidermis. In psoriatic epidermis, the inhibitor was found in the peripheral cytoplasm of spinous cells and in the scale. In normal epidermis, the staining was seen only in orifices of hair follicles. An inhibitor with similar size and antigenic properties to that isolated from the psoriatic scales was demonstrated in extracts made from the whole-thickness epidermis but not in extracts from the healthy epidermal scales, the dermis, the liver, the spleen, or the blood serum.

Cysteine proteinases and metastasis

Cysteine proteinases are a subclass of endopeptidases which require activation by thiol reagents. A tumor cysteine proteinase which appears to be related to lysosomal cathepsin B has been implicated in the ability of tumor cells to invade the extracellular matrix and to metastasize to secondary sites. Lysosomal cathepsin B can degrade such components of the extracellular matrix as collagen, fibronectin and proteoglycans. Activity of this cathepsin B-like cysteine proteinase (CB) has been correlated with tumor malignancy in a number of tumor lines yet not in all tumor lines studied. CB activity in tumors seems to be associated with the viable tumor cells, probably with the plasma membrane of these tumor cells. CB activity has been measured in the sera, urine, ascites fluid and pancreatic fluid of tumor-bearing patients. CB is released from tumor explants and tumor cells in vitro as well as from normal subcutaneous tissue exposed to tumor-conditioned medium. Cathepsin B from normal tissues is rapidly inactivated above pH 7.0. Therefore, CB in tumor cell membranes or released from tumor cells (or from host cells in response to tumor cells) may not possess proteolytic activity at neutral pH and thus may not facilitate tumor cell invasion. However, CB exhibits enhanced stability at neutral or slightly alkaline pH's. There is not yet definitive proof that CB plays a role in tumor invasion and metastasis. There is, however, an increasing body of correlative evidence relating CB activity and tumor malignancy. This correlative evidence plus preliminary evidence that tumor CB can degrade components of the extracellular matrix in vitro suggests that CB may be one proteinase active in a proteolytic cascade resulting in tumor invasion and metastasis.

Purification and characterization of Ca2+-activated neutral protease inhibitor from human platelets

An endogenous inhibitor of Ca2+-activated neutral protease (CANP) was purified to homogeneity from the soluble fraction of human platelets by the combination of heat treatment, ammonium sulfate fractionation, ion exchange chromatography and gel filtration. The purified inhibitor was found to be a tetramer composed of identical subunits and each subunit has a molecular weight of 63 K. The purified protein exerted specific inhibition against the low Ca2+-requiring form of CANP (mu-CANP) purified from human platelets in the presence of micromolar concentration of Ca2+. The kinetic study revealed that the inhibition is non-competitive with Ki value of 3.2 X 10(-8) M.