Interaction of the cysteine proteinase inhibitor chicken cystatin with papain

The two forms of chicken cystatin, with different isoelectric points, that have been described previously were indistinguishable in analyses of amino- and carboxy-terminal residues, amino acid composition, and peptide maps. The two forms thus are highly similar and most likely differ only in an amide group or in a small charged substituent. The binding of either cystatin form to highly purified, active papain was accompanied by the same pronounced changes in near-ultraviolet circular dichroism, ultraviolet absorption, and fluorescence emission. These changes were compatible with perturbations of the environment of aromatic residues in one or both proteins of the complex, arising from local interactions or from a conformational change. Modification of the single tryptophan residue of cystatin, at position 104, with N-bromosuccinimide resulted in considerably smaller spectroscopic changes on binding of the inhibitor to papain, indicating that the environment of this residue is affected by the binding. Analogous modification of Trp-69 and Trp-177 of papain markedly affected the fluorescence changes observed on binding of cystatin to the enzyme, similarly suggesting that these two residues of papain are involved in the interaction. The fluorescence increase of papain at alkaline pH, arising from Trp-177 and due to deprotonization of the adjacent His-159, was abolished on binding of cystatin to the enzyme, further supporting the proposal that this region of papain participates in the interaction with the inhibitor. A stoichiometry of binding of either cystatin form to papain of 1:1 and a lower limit for the binding constant of 10(9) M-1 were determined by titrations monitored by either the ultraviolet absorption or fluorescence changes induced by the interaction.     

Interaction between chicken cystatin and the cysteine proteinases actinidin, chymopapain A, and ficin

The cysteine proteinase inhibitor cystatin, from chicken egg white, bound with equimolar stoichiometry to the cysteine proteinases actinidin, chymopapain A, and ficin. The changes of near-ultraviolet absorption and fluorescence induced by the binding differed appreciably for the three enzymes, indicating that these spectral changes arise predominantly from aromatic residues in the proteinases. In contrast, the near-ultraviolet circular dichroism changes were similar for all three enzymes, supporting previous evidence that these changes originate mainly from the single tryptophan residue in cystatin, Trp-104. The pseudo-first-order rate constant for the binding increased linearly with the inhibitor concentration up to as high concentrations as could be measured for the three proteinases. This behavior is consistent with the complexes being formed by simple, bimolecular reactions, as was concluded previously for the reaction of cystatin with active and inactivated forms of papain. The second-order association rate constant varied only about 4-fold, from 2.2 X 10(6) to 9.6 X 10(6) M-1.s-1, for the three enzymes, the higher of these values being similar to that measured previously for the reaction with papain. These observations are consistent with the association rate being governed mainly by the frequency of collision between the binding areas of enzyme and inhibitor. All three cystatin-proteinase complexes dissociated to intact inhibitor, demonstrating reversibility. The dissociation rate constants varied about 20000-fold, from 4.6 X 10(-7) s-1 for ficin to 1.1 X 10(-2) s-1 for actinidin, reflecting substantial differences between the enzymes in the nature of the interactions with the inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of the probable inhibitory reactive sites of the cysteine proteinase inhibitors human cystatin C and chicken cystatin

When an excess of human cystatin C or chicken cystatin was mixed with papain, an enzyme-inhibitor complex was formed immediately. The residual free cystatin was then progressively converted to a form with different electrophoretic mobility and chromatographic properties. The modified cystatins were isolated and sequenced, showing that there had been cleavage of a single peptide bond in each molecule: Gly11-Gly12 in cystatin C, and Gly9-Ala10 in chicken cystatin. The residues Gly11 (cystatin C) and Gly9 (chicken cystatin) are among only three residues conserved in all known sequences of inhibitory cystatins. The modified cystatins were at least 1000-fold weaker inhibitors of papain than the native cystatins. An 18-residue synthetic peptide corresponding to residues 4-21 of cystatin C did not inhibit papain but was cleaved at the same Gly-Gly bond as cystatin C. When iodoacetate or L-3-carboxy-trans-2,3-epoxypropionyl-leucylamido-(4-guanidin o)butane was added to the mixtures of either cystatin with papain, modification of the excess cystatin was blocked. Papain-cystatin complexes were stable to prolonged incubation, even in the presence of excess papain. We conclude that the peptidyl bond of the conserved glycine residue in human cystatin C and chicken cystatin probably is part of a substrate-like inhibitory reactive site of these cysteine proteinase inhibitors of the cystatin superfamily and that this may be true also for other inhibitors of this superfamily. We also propose that human cystatin C and chicken cystatin, and probably other cystatins as well, inhibit cysteine proteinases by the simultaneous interactions with such proteinases of the inhibitory reactive sites and other, so far not identified, areas of the cystatins. The cleavage of the inhibitory reactive site glycyl bond in mixtures of papain with excess quantities of cystatins is apparently due to the activity of a small percentage of atypical cysteine proteinase molecules in the papain preparation that form only very loose complexes with cystatins under the conditions employed and degrade the free cystatin molecules.     

Inhibition of cysteine proteinases and dipeptidyl peptidase I by egg-white cystatin.

The interactions between egg-white cystatin and the cysteine proteinases papain, human cathepsin B and bovine dipeptidyl peptidase I were studied. Cystatin was shown to be a competitive reversible inhibitor of cathepsin B (Ki 1.7 nM, k-1 about 2.3 X 10(-3) s-1). The inhibition of dipeptidyl peptidase I was shown to be reversible (Ki(app.) 0.22 nM, k-1 about 2.2 X 10(-3) s-1). Cystatin bound papain too tightly for Ki to be determined, but an upper limit of 5 pM was estimated. The association was a second-order process, with k+1 1.0 X 10(7) M-1 X s-1. Papain was shown to form equimolar complexes with cystatin. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of complexes formed between papain or cathepsin B and an excess of cystatin showed no peptide bond cleavage after incubation for 72 h. The reaction of the active-site thiol group of papain with 5,5'-dithiobis-(2-nitrobenzoic acid) at pH 8 and 2,2'-dithiobispyridine at pH 4 was blocked by complex-formation. Dipeptidyl peptidase I and papain were found to compete for binding to cystatin, contrary to a previous report. The two major isoelectric forms of cystatin were found to have similar specific inhibitory activities for papain, and similar affinities for papain, cathepsin B and dipeptidyl peptidase I. This, together with specific oxidation of the N-terminal serine residue with periodate, showed the N-terminal amino group of cystatin 1 to be unimportant for inhibition. General citraconylation of amino groups resulted in a large decrease in the affinity of cystatin for dipeptidyl peptidase I. It is concluded that the interaction of cystatin with cysteine proteinases has many characteristics similar to those of an inhibitor such as aprotinin with serine proteinases.     

The affinity and kinetics of inhibition of cysteine proteinases by intact recombinant bovine cystatin C.

Recent studies have shown that the bovine cysteine proteinase inhibitor, cystatin C, is synthesized as a preprotein containing a 118-residue mature protein. However, the forms of the inhibitor isolated previously from bovine tissues had shorter N-terminal regions than expected from these results, and also lower affinity for proteinases than human cystatin C. In this work, we report the properties of recombinant, full-length bovine cystatin C having a complete N-terminal region. The general characteristics of this form of the inhibitor, as reflected by the isoelectric point, the far-ultraviolet circular dichroism spectrum, the thermal stability and the changes of tryptophan fluorescence on interaction with papain, resembled those of human cystatin C. The affinity and kinetics of inhibition of papain and cathepsins B, H and L by the bovine inhibitor were also comparable with those of the human inhibitor, although certain differences were apparent. Notably, the affinity of bovine cystatin C for cathepsin H was somewhat weaker than that of human cystatin C, and bovine cystatin C bound to cathepsin L with about a four-fold higher association rate constant than the human inhibitor. This rate constant is comparable with the highest values reported previously for cystatin-cysteine proteinase reactions. The full-length, recombinant bovine cystatin C bound appreciably more tightly to proteinases than the shorter form characterized previously. Digestion of the recombinant inhibitor with neutrophil elastase resulted in forms with truncated N-terminal regions and appreciably decreased affinity for papain, consistent with the forms of bovine cystatin C isolated previously having arisen by proteolytic cleavage of a mature, full-length inhibitor.     

Role of the single cysteine residue, Cys 3, of human and bovine cystatin B (stefin B) in the inhibition of cysteine proteinases

Cystatin B is unique among cysteine proteinase inhibitors of the cystatin superfamily in having a free Cys in the N-terminal segment of the proteinase binding region. The importance of this residue for inhibition of target proteinases was assessed by studies of the affinity and kinetics of interaction of human and bovine wild-type cystatin B and the Cys 3-to-Ser mutants of the inhibitors with papain and cathepsins L, H, and B. The wild-type forms from the two species had about the same affinity for each proteinase, binding tightly to papain and cathepsin L and more weakly to cathepsins H and B. In general, these affinities were appreciably higher than those reported earlier, perhaps because of irreversible oxidation of Cys 3 in previous work. The Cys-to-Ser mutation resulted in weaker binding of cystatin B to all four proteinases examined, the effect varying with both the proteinase and the species variant of the inhibitor. The affinities of the human inhibitor for papain and cathepsin H were decreased by threefold to fourfold and that for cathepsin B by approximately 20-fold, whereas the reductions in the affinities of the bovine inhibitor for papain and cathepsins H and B were approximately 14-fold, approximately 10-fold and approximately 300-fold, respectively. The decreases in affinity for cathepsin L could not be properly quantified but were greater than threefold. Increased dissociation rate constants were responsible for the weaker binding of both mutants to papain. By contrast, the reduced affinities for cathepsins H and B were due to decreased association rate constants. Cys 3 of both human and bovine cystatin B is thus of appreciable importance for inhibition of cysteine proteinases, in particular cathepsin B.     

Characterization and amino acid sequence of a new acidic cysteine proteinase inhibitor (cystatin SA) structurally closely related to cystatin S, from human whole saliva

A cysteine proteinase inhibitor (designated as cystatin SA) was isolated from human whole saliva by procedures including chromatography on DE 32 and DEAE-Sepharose CL-6B. The amino acid sequence determined by conventional methods showed sequence homology of 90 and 87% as compared with the sequences of cystatin S and cystatin SN, respectively, both of which are salivary inhibitors characterized previously. The new inhibitor consisted of 117 residues and had a pI value of 4.3. Cystatin SA inhibited ficin and papain more strongly than cystatin S or cystatin SN did. It also exhibited inhibitory activity toward dipeptidyl peptidase I but the activity was much weaker than those toward ficin and papain.     

Grafting of features of cystatins C or B into the N-terminal region or second binding loop of cystatin A (stefin A) substantially enhances inhibition of cysteine proteinases

Replacement of the three N-terminal residues preceding the conserved Gly of cystatin A by the corresponding 10-residue long segment of cystatin C increased the affinity of the inhibitor for the major lysosomal cysteine proteinase, cathepsin B, by approximately 15-fold. This tighter binding was predominantly due to a higher overall association rate constant. Characterization of the interaction with an inactive Cys29 to Ala variant of cathepsin B indicated that the higher rate constant was a result of an increased ability of the N-terminal region of the chimeric inhibitor to promote displacement of the cathepsin B occluding loop in the second binding step. The low dissociation rate constant for the binding of cystatin A to cathepsin B was retained by the chimeric inhibitor, which therefore had a higher affinity for this enzyme than any natural cystatin identified so far. In contrast, the N-terminal substitution negligibly affected the ability of cystatin A to inhibit papain. However, substitutions of Gly75 in the second binding loop of cystatin A by Trp or His, making the loop similar to those of cystatins C or B, respectively, increased the affinity for papain by approximately 10-fold. This enhanced affinity was due to both a higher association rate constant and a lower dissociation rate constant. Modeling of complexes between the two variants and papain indicated the possibility of favorable interactions being established between the substituting residues and the enzyme. The second-loop substitutions negligibly affected or moderately reduced the affinity for cathepsin B. Together, these results show that the inhibitory ability of cystatins can be substantially improved by protein engineering.     

Interaction of chicken cystatin with inactivated papains.

Papain which was inactivated by covalent attachment of small substituents to the active-site cysteine, up to the size of a carbamoylmethyl group, bound with high affinity to chicken cystatin (Kd less than approximately 15 pM), although less tightly than did active papain (Kd approximately 60 fM). However, as the size of the substituent was increased further, the affinity decreased appreciably, generally in proportion to the size of the inactivating group. For instance the dissociation constants for papain inactivated with N-ethylmaleimide and [N-(L-3-trans-carboxyoxiran-2-carbonyl)-L-leucyl]-amido-(4-guanido )butane were 0.17 and approximately 10 microM respectively. The spectroscopic changes accompanying the reaction of all but the most weakly binding (Kd greater than or equal to 2 microM) inactivated papains with cystatin were similar to those induced by the active enzyme. Interactions involving the reactive cysteine residue of papain are thus not crucial for high-affinity binding of the enzyme to cystatin, in accordance with a recently proposed model for the enzyme-inhibitor complex, based on computer docking experiments. In this model there is sufficient space around the reactive cysteine in the complex for a small inactivating group, explaining the tight binding of papains with such substituents. However, larger inactivating groups cannot be accommodated in this space and therefore must displace the inhibitor out of the tight fit with the enzyme, in agreement with the observed decrease in binding affinity with increasing size of bulkier substituents. The kinetics of binding of cystatin to inactivated papains were compatible with simple, reversible, bimolecular reactions, having association rate constants of (7-9) x 10(6) M-1 s-1 at pH 7.4, 25 degrees C, similar to what was shown previously for the binding of cystatin to active papain. The rate of association of the inhibitor with either active or inactivated papain thus appears to be primarily diffusion-controlled. The decreasing affinity of cystatin for papains inactivated with groups of increasing size was shown to be due to progressively higher dissociation rate constants, consistent with the greater impairment of fit between the binding regions of the two molecules.     

Induction of cystatin S in rat submandibular glands by papain.

1. Papain (a cysteine proteinase) were administered into the oral cavity of rats twice daily for 5 days. This treatment caused a dramatic increase in the level of cystatin S (a cysteine proteinase inhibitor belonging to family 2 of cystatin superfamily) in enlarged submandibular glands. 2. Immunochemical analysis using antibody against rat cystatin S and electrophoretic analysis confirmed that the protein induced by papain was identical to that induced by isoproterenol. 3. Induction of the cystatin S in the submandibular glands by oral administration of papain suggested a biological response which plays a role in preventing injury exogenous proteinase.     

Hydrophobic sequences can substitute for the wild-type N-terminal sequence of cystatin A (stefin A) in tight binding to cysteine proteinases selection of high-affinity N-terminal region variants by phage display.

A phage-display library of the cysteine-proteinase inhibitor, cystatin A, was constructed in which variants with the four N-terminal amino acids randomly mutated were expressed on the surface of filamenteous phage. Screening of this library for binding to papain gave predominantly variants with a glycine residue in position 4. This finding is in agreement with previous conclusions that glycine in this position is essential for tight binding of cystatin A to cysteine proteinases by allowing optimal interaction of the N-terminal region of the inhibitor with the enzyme. In contrast, the first three residues of the variants obtained by the screening were more variable. Two variants were identified with similar affinities for papain as the wild-type inhibitor, but with these residues, Val-Phe-Thr- or Ile-Leu-Leu, differing appreciably from those of the wild-type, Met-Ile-Pro. Other sequences of the N-terminal region, presumably mainly hydrophobic, can thus substitute for the wild-type sequence and contribute similar energy to the inhibitor-proteinase interaction. The two variants binding tightly to papain differed in their affinity for cathepsin B, demonstrating that cystatin variants with increased selectivity for a particular target cysteine proteinase can be obtained by phage-display technology.     

Importance of the second binding loop and the C-terminal end of cystatin B (stefin B) for inhibition of cysteine proteinases.

The importance of residues in the second hairpin loop and the C-terminal end of mammalian cystatin B for binding of proteinases was elucidated by mutagenesis of the bovine inhibitor. Bovine cystatin B was modeled onto the crystal structure of the human inhibitor in complex with papain with minimal structural changes. Substitution of the two deduced contact residues in the second hairpin loop, Leu-73 and His-75, with Gly resulted in appreciably reduced affinities for papain and cathepsins H and B. These losses indicated that the two residues together contribute 20-30% of the free energy of binding of cystatin B to these enzymes and that Leu-73 is responsible for most of this contribution. In contrast, the small decrease in the affinity for cathepsin L suggested that the second hairpin loop is less important for inhibition of this proteinase. Replacement of the contact residue in the C-terminal end, Tyr-97, with Ala resulted in losses in affinity for papain and cathepsins L and H that were consistent with Tyr-97 contributing 6-12% of the energy of binding of cystatin B to these enzymes. However, this substitution minimally affected the affinity for cathepsin B, indicating that the C-terminal end is of limited importance for binding of this proteinase. All affinity decreases were due predominantly to increased dissociation rate constants. These results show that both the second hairpin loop and the C-terminal end of cystatin B contribute to anchoring the inhibitor to target proteinases, each of the two regions interacting with a different domain of the enzyme. However, the relative contributions of these two interactions vary with the proteinase.     

Purification and characterization of a cystatin from the leaves of methyl jasmonate treated tomato plants

A multidomain cystatin was purified from the leaves of mature and seedling tomato plants (Lycopersicon esculentum, cv Bonnie Best) that had been sprayed with methyl jasmonate. For seedlings, cystatin purification was accomplished using EDTA washing, KCI extraction, 70 degrees C heat treatment, ammonium sulfate fractionation and gel filtration chromatography. For mature plants, DEAE chromatography was also needed to separate a protease, hydrolysis products of cystatin and serine proteinase inhibitors from the intact cystatin. Purified tomato cystatin has a molecular weight (Mr) of 88 kDa, eight papain binding domains, is a non-competitive inhibitor of papain with K1 of 1.4 nM and is not a glycoprotein. Tryptic peptides (Mr 26, 13 kDa) and most chymotryptic peptides (Mr 33, 13 kDa) of tomato cystatin retain inhibitor activity. Amino acid analysis revealed no Cys; Asx, Glx, Gly, Ser accounted for almost half the residues and there was some homology with potato multicystatin. Activity is stable at pH 4-11 at 4 degrees C, but unstable at neutral pH at > 60 degrees C (Ea = 92.5 kJ/mole). Extracts of mature plants treated with methyl jasmonate contain lower Mr cystatins that appear to result from the action of an endogenous 26 kDa protease on the 88 kDa inhibitor.     

Modelling family 2 cystatins and their interaction with papain

Cystatins are extensively studied cysteine protease inhibitors, found in wide range of organisms with highly conserved structural folds. S-type of cystatins is well known for their abundance in saliva, high selectivity and poorer activity towards host cysteine proteases in comparison to their immediate ancestor cystatin C. Despite more than 90% sequence similarity, the members of this group show highly dissimilar binding affinity towards papain. Cystatin M/E is a potent inhibitor of legumain and papain like cysteine proteases and recognized for its involvement in skin barrier formation and potential role as a tumor suppressor gene. However, the structures of these proteins and their complexes with papain or legumain are still unknown. In the present study, we have employed computational methods to get insight into the interactions between papain and cystatins. Three-dimensional structures of the cystatins are generated by homology modelling, refined with molecular dynamics simulation, validated through numerous web servers and finally complexed with papain using ZDOCK algorithm in Discovery Studio. A high degree of shape complementarity is observed within the complexes, stabilized by numerous hydrogen bonds (HB) and hydrophobic interactions. Using interaction energy, HB and solvent accessible surface area analyses, we have identified a series of key residues that may be involved in papain–cystatin interaction. Differential approaches of cystatins towards papain are also noticed which are possibly responsible for diverse inhibitory activity within the group. These findings will improve our understanding of fundamental inhibitory mechanisms of cystatin and provide clues for further research.     

Internalization of cystatin C in human cell lines

Altered protease activity is considered important for tumour invasion and metastasis, processes in which the cysteine proteases cathepsin B and L are involved. Their natural inhibitor cystatin C is a secreted protein, suggesting that it functions to control extracellular protease activity. Because cystatins added to cell cultures can inhibit polio, herpes simplex and coronavirus replication, which are intracellular processes, the internalization and intracellular regulation of cysteine proteases by cystatin C should be considered. The extension, mechanism and biological importance of this hypothetical process are unknown. We investigated whether internalization of cystatin C occurs in a set of human cell lines. Demonstrated by flow cytometry and confocal microscopy, A-431, MCF-7, MDA-MB-453, MDA-MB-468 and Capan-1 cells internalized fluorophore-conjugated cystatin C when exposed to physiological concentrations (1 microm). During cystatin C incubation, intracellular cystatin C increased after 5 min and accumulated for at least 6 h, reaching four to six times the baseline level. Western blotting showed that the internalized inhibitor was not degraded. It was functionally intact and extracts of cells exposed to cystatin C showed a higher capacity to inhibit papain and cathepsin B than control cells (decrease in enzyme activity of 34% and 37%, respectively). The uptake of labelled cystatin C was inhibited by unlabelled inhibitor, suggesting a specific pathway for the internalization. We conclude that the cysteine protease inhibitor cystatin C is internalized in significant quantities in various cancer cell lines. This is a potentially important physiological phenomenon not previously described for this group of inhibitors.     

中华鲟半胱氨酸蛋白酶抑制剂在毕赤酵母中的表达和活性分析

为研究鱼类半胱氨酸蛋白酶抑制剂（Cystatin）的功能并探索其在水产加工和病害防治中的应用潜力,将PCR改造后的编码成熟肽中华鲟（Acipenser sinensis）cystatin 基因亚克隆到毕赤酵母整合型表达载体pPICZαA,氯化锂法转化毕赤酵母菌株GS115,构建表达cystatin的酵母基因工程菌。经甲醇诱导、SDSPAGE检测培养基上清液,表明中华鲟cystatin在毕赤酵母中实现了高效表达,重组cystatin表达量约为215mg·L^-1。纯化后重组蛋白纯度达94.2%。生物活性检测结果表明,1μg重组中华鲟cystatin约能抑制15μg木瓜蛋白酶的水解活性。     

Proteolysis of the 85-kilodalton crystalline cysteine proteinase inhibitor from potato releases functional cystatin domains.

The protein crystals found in potato (Solanum tuberosum L.) tuber cells consist of a single 85-kD polypeptide. This polypeptide is an inhibitor of papain and other cysteine proteinases and is capable of binding several proteinase molecules simultaneously (P. Rodis, J.E. Hoff [1984] Plant Physiol 74: 907-911). We have characterized this unusual inhibitor in more detail. Titrations of papain activity with the potato papain inhibitor showed that there are eight papain binding sites per inhibitor molecule. The inhibition constant (Ki) value for papain inhibition was 0.1 nM. Treatment of the inhibitor with trypsin resulted in fragmentation of the 85-kD polypeptide into a 32-kD polypeptide and five 10-kD polypeptides. The 32-kD and 10-kD fragments all retained the ability to potently inhibit papain (Ki values against papain were 0.5 and 0.7 nM, respectively) and the molar stoichiometries of papain binding were 2 to 3:1 and 1:1, respectively. Other nonspecific proteinases such as chymotrypsin, subtilisin Carlsberg, thermolysin, and proteinase K also cleaved the 85-kD inhibitor polypeptide into functional 22-kD and several 10-kD fragments. The fragments obtained by digestion of the potato papain inhibitor with trypsin were purified by reverse-phase high-performance liquid chromatography, and the N-terminal amino acid sequence was obtained for each fragment. Comparison of these sequences showed that the fragments shared a high degree of homology but were not identical. The sequences were homologous to the N termini of members of the cystatin superfamily of cysteine proteinase inhibitors. Therefore, the inhibitor appears to comprise eight tandem cystatin domains linked by preteolytically sensitive junctions. We have called the inhibitor potato multicystatin (PMC). By immunoblot analysis and measurement of papain inhibitory activity, PMC was found at high levels in potato leaves (up to 0.6 microgram/g fresh weight tissue), where it accumulated under conditions that induce the accumulation of other proteinase inhibitors linked to plant defense. PMC may have a similar defensive role, for example in protecting the plant from phytophagous insects that utilize cysteine proteinases for dietary protein digestion.     

Contributions of individual residues in the N-terminal region of cystatin B (stefin B) to inhibition of cysteine proteinases

The importance of individual residues in the N-terminal region of cystatin B for proteinase inhibition was elucidated by measurements of the affinity and kinetics of binding of N-terminally truncated, recombinant variants of the bovine inhibitor to cysteine proteinases. Removal of Met-1 caused an 8- to 10-fold lower affinity for papain and cathepsin B, decreased the affinity also for cathepsin L but only minimally affected cathepsin H affinity. Additional truncation of Met-2 further weakened the binding to papain and cathepsin B by 40-70-fold, whereas the affinity for cathepsins L and H was essentially unaffected. Removal of Cys-3 had the most drastic effects on the interactions, resulting in a further affinity decrease of approximately 1500-fold for papain, approximately 700-fold for cathepsin L and approximately 15-fold for cathepsin H; the binding to cathepsin B could not be assessed. The binding kinetics could only be evaluated for papain and cathepsin H and showed that the reduced affinities for these enzymes were predominantly due to increased dissociation rate constants. These results demonstrate that the N-terminal region of cystatin B contributes appreciably to proteinase inhibition, in contrast to previous proposals. It is responsible for 12-40% of the total binding energy of the inhibitor to the proteinases investigated, being of least importance for cathepsin H binding. Cys-3 is the most important residue of the N-terminal region for inhibition of papain, cathepsin L and cathepsin H, the role of the other residues of this region varying with the target proteinase.     

Affinity of chicken cystatin for S-(carbamoylmethyl)-papain,measured by fluorescence at acidic pH

We have characterized the interaction between S-(carbamoylmethyl)-papain and chicken cystatin by means of fluorescence titration at acidic pH. The calculated binding constant was 32 x 10(6), which is less than the tight affinity at pH 7.4 (approximately 7 x 10(10)). Results have been discussed in terms of the burial of Trp-104 of cystatin and the possible role of the charge of the carbamoylmethyl moiety.     

Characterization of a new cysteine proteinase inhibitor of human saliva, cystatin SN, which is immunologically related to cystatin S

A new cysteine proteinase inhibitor, cystatin SN, was purified from human whole saliva by chromatography with DE32, Sephacryl S200, and CM-Sepharose CL6B. Cystatin SN is immunologically related to cystatin S and both inhibitors have a similar molecular mass of about 13 kDa. The new inhibitor, however, was clearly distinguished from cystatin S by its much higher pI value. These inhibitors showed similar inhibitory activity for ficin, but cystatin SN was a much better inhibitor for papain and dipeptidyl peptidase I. The amino acid sequence of cystatin SN deduced in the light of the known structure of cystatin S indicates that they have 10 different amino acid residues in the sequence comprising in total 113 residues.