Properties Of Cathepsin Research Articles

Hydroxyl Radical-Induced Oxidation on the Properties of Cathepsin H and Its Influence in Myofibrillar Proteins Degradation of Coregonus peled In Vitro.

The most frequently occurring protein modification in fish postmortem is oxidization, which further affects meat quality through multiple biochemical pathways. To investigate how hydroxyl radicals affect the structure of cathepsin H and its ability to break down myofibrillar proteins in Coregonus peled, cathepsin H was oxidized with 0, 0.1, 0.5, 1, 5, and 10 mM H2O2 and subsequently incubated with isolated myofibrillar proteins. The results showed that as the H2O2 concentration increased, the carbonyl and sulfhydryl contents of cathepsin H significantly increased and decreased, respectively. There were noticeable changes in the α-helix structures and a gradual reduction in UV absorbance and fluorescence intensity, indicating that oxidation can induce the cross-linking and aggregation of cathepsin H. These structural changes further reduced the activity of cathepsin H, reaching its lowest at 10 mM H2O2, which was 53.63% of the activity at 0 mM H2O2. Moreover, desmin and troponin-T all degraded at faster rates when cathepsin H and myofibrillar proteins were oxidized concurrently as opposed to when cathepsin H was oxidized alone. These findings provide vital insights into the interaction mechanism between oxidation, cathepsin H, as well as myofibrillar protein degradation, laying a groundwork for understanding the molecular mechanisms underlying changes in fish meat quality after slaughter and during processing.

Molecular characterization and determination of the biochemical properties of cathepsin L of Trichinella spiralis

Cathepsin L is an important cysteine protease, but its function in T. spiralis remains unclear. The aim of this research was to explore the biological characteristics of T. spiralis cathepsin L (TsCatL) and its role in T. spiralis-host interactions. Bioinformatic analysis revealed the presence of the cysteine protease active site residues Gln, Cys, His and Asn in mature TsCatL, as well as specific motifs of cathepsin L similar to ERFNIN and GYLND in the prepeptide of TsCatL. Molecular docking of mature TsCatL and E64 revealed hydrophobic effects and hydrogen bonding interactions. Two domains of TsCatL (TsCatL2) were cloned and expressed, and recombinant TsCatL2 (rTsCatL2) was autocatalytically cleaved under acidic conditions to form mature TsCatL. TsCatL was transcribed and expressed in larvae and adults and located in the stichosome, gut and embryo. Enzyme kinetic tests showed that rTsCatL2 degraded the substrate Z-Phe-Arg-AMC under acidic conditions, which was inhibited by E64 and PMSF and enhanced by EDTA, L-cysteine and DTT. The kinetic parameters of rTsCatL2 were a Km value of 48.82 μM and Vmax of 374.4 nM/min at pH 4.5, 37 °C and 5 mM DTT. In addition, it was shown that rTsCatL2 degraded haemoglobin, serum albumin, immunoglobulins (mouse IgG, human IgG and IgM) and extracellular matrix components (fibronectin, collagen I and laminin). The proteolytic activity of rTsCatL2 was host specific and significantly inhibited by E64. rTsCatL2 possesses the natural activity of a sulfhydryl-containing cysteine protease, and TsCatL is an important digestive enzyme that seems to be important for the nutrient acquisition, immune evasion and invasion of Trichinella in the host.

Biochemical properties of cathepsin D of porcine muscle tissue

Biochemical properties of cathepsin D of porcine muscle tissue

The effect of 3-bromopyruvate on the properties of cathepsin B in the aspect of metastatic potential of colon cancer cells.

Cathepsin B (CTSB, EC 3.4.22.1) is a protease that physiologically resides in lysosomes and whose biosynthesis, cell surface location, intracellular distribution, and enzymatic activity undergo changes during the pathogenesis of cancer; it plays an important role in metastasis. Due to its active center structure, it is theoretically susceptible to the action of 3-bromopyruvate - an analogue of pyruvic acid and an alkylator that has been studied in depth in recent years for its anti-cancer activity, mainly through the inhibition of glycolytic enzymes. To investigate the effects of 3-bromopyruvate on the tumor cell properties in selected colorectal carcinoma cell lines that are widely attributed to the dysregulation of CTSB. Moreover, the effect of direct action of 3-bromopyruvate on the CTSB molecule was investigated in vitro. The research on the effect of 3-bromopyruvate on Caco-2/HCT 116 cells and purified human CTSB included a scratch/wound healing assay, a cell invasion assay, spectrofluorimetric measurements of enzymatic activity of cathepsin B, indirect immunofluorescence and flow cytometry, zymography, and liquid chromatography/mass spectrometry methods. 3-bromopyruvate reduced the activity and secretion of active CTSB and lowered the motility and invasiveness of Caco-2/HCT 116 human colorectal cancer cells. It decreased the exposure of CTSB on the outer surface of the cell membrane in both cell lines. 3-bromopyruvate inhibited the activity of CTSB reversibly and did not alkylate the molecule of the enzyme. This is the first report on the effect of 3-bromopyruvate directly on CTSB and indirectly on the mechanisms leading to its distinct pathophysiological properties, resulting in increased metastatic potential of cancer cells, among others. Although detailed mechanisms of the interaction between 3-bromopyruvate and the active site of CTSB require further research, the results provide a new perspective from which to study the antitumor effect of 3-bromopyruvate.

The influence of linker length on the properties of cathepsin S cleavable 177Lu-labeled HPMA copolymers for pancreatic cancer imaging

N-(2-hydroxypropyl)-methacrylamide (HPMA) copolymers have shown promise for application in the detection and staging of cancer. However, non-target accumulation, particularly in the liver and spleen, hinders the detection of resident or nearby metastatic lesions thereby decreasing diagnostic effectiveness. Our laboratory has pursued the development of cathepsin S susceptible linkers (CSLs) to reduce the non-target accumulation of diagnostic/radiotherapeutic HPMA copolymers. In this study, we ascertain if the length of the linking group impacts the cleavage and clearance kinetics, relative to each other and a non-cleavable control, due to a reduction in steric inhibition. Three different CSLs with linking groups of various lengths (0, 6 and 13 atoms) were conjugated to HPMA copolymers. In vitro cleavage studies revealed that the longest linking group (13 atoms) led to more rapid cleavage when challenged with cathepsin S. The CSL incorporated HPMA copolymers demonstrated significantly higher levels of excretion and a significant decrease in long-term hepatic and splenic retention relative to the non-cleavable control. Contrary to in vitro observations, the length of the linking group did not substantially impact the non-target in vivo clearance. In the case of HPAC tumor retention, the CSL with the null (0 atom) linker demonstrated significantly higher levels of retention relative to the other CSLs. Given these results, we find that the length of the linking group of the CSLs did not substantially impact non-target clearance, but did influence tumor retention. Overall, these results demonstrate that the CSLs can substantially improve the non-target clearance of HPMA copolymers thereby enhancing clinical potential.

A Novel Approach for Characterization of Cathepsin D Protease and Its Effect on Tau and β-Amyloid Proteins

Cathepsin D is the lysosomal protease abundantly expressed in the brain. It plays an important role in the regulation of cellular apoptosis. In addition, cathepsin D has been shown to be involved in the pathogenesis of Alzheimer disease and autism. In this study, we developed a novel approach for the preparation of highly purified cathepsin D from the calf brain. This high grade purification is achieved by using DEAE-Sephacel Chromatography before the final step of applying to the Pepstatin-Sepharose 4B column. The properties of cathepsin D have also been studied. We show that cathepsin D cleaves both tau and β-amyloid precursor protein (APP). Both tau and APP are involved in the pathogenesis of Alzheimer's disease. Our findings strongly suggest a link between the lysosomal dysfunction of cathepsin D and the etiology of Alzheimer's disease. Our findings also indicate that cathepsin D could be a new approach to treating Alzheimer's disease.

Expression and characterization of cathepsin P

The mouse genome contains a family of clan C1A proteases that appear to be restricted to rodents within Eutherian (placental) mammals. mRNA analysis has shown that these genes are expressed exclusively in placenta. Sequence analysis predicts that the expressed proteins will be functional and consequently it was proposed that this family of proteases may have evolved to perform subspecialized functions of the closely related protease, cathepsin L, that is expressed in placental tissues of all mammalian species. In the present study, it was shown that cathepsin P can be expressed in Pichia pastoris as an inactive zymogen that can be activated with proteinase K, chymotrypsin or pancreatic elastase at neutral pH. Unlike other mammalian cathepsins, cathepsin P could also be autoactivated at neutral pH, but not at acidic pH. The activated enzyme was capable of hydrolysing peptidyl substrates and the protein substrates azocasein and transferrin, with optimal activity at pH 6.5-7.5. Little activity could be detected at pH 5.0 and below. Salts such as Na2SO4 and hyaluronate stimulated the activity of the protease against peptidyl substrates. The properties of cathepsin P appear to be quite distinct from those of cathepsin L, indicating that the duplication that gave rise to cathepsin P has probably not yielded an enzyme that provides a subfunction of cathepsin L in rodents. It seems probable that cathepsin P has evolved to perform a function that is performed by an evolutionarily unrelated protease in other mammalian species.

Purification and some properties of cathepsin H from rabbit skeletal muscle

Rabbit muscle cathepsin H classified as an aminoendopeptidase was purified and its properties were investigated to clarify its contribution to the proteolysis of postmortem muscle. The purification was performed by ammonium sulfate fractionation and successive chromatographies on Sephadex G-75, phosphocelluose, DEAE-Sephadex A-50 and Sephadex G-100. The purified enzyme gave a single protein band on SDS-PAGE. Its molecular mass was found to be 28 kDa by gel permeation and 30 kDa by SDS-PAGE. The optimum pHs for α- N-benzoyl- dl-arginine-β-naphthylamide (BANA)- and l-leucine-β-naphthylamide (Leu-NA)-hydrolyzing activities were 6.6 and 7.0, respectively. This enzyme was almost stable in the range of pH 4–5 and up to 50 °C at pH 5.0. The Km values of BANA- and Leu-NA-hydrolyzing activities were 0.367 and 0.203 mM, respectively. The enzyme was inhibited by monoiodoacetic acid, antipain, leupeptin, TLCK and TPCK, but was not affected by pepstatin, bestatin, puromycin, PMSF or trypsin inhibitor. This enzyme strongly acted on Arg-, Lys-, Met-, Ala-, Ser- and Leu-NAs, weakly acted on Val- and Glu-NAs, and hardly acted on Pro- and Gly-NAs. The amount of cathepsin H in muscle was estimated to be less than one-fourth of the sum of the amount of aminopeptidases C and H by the Leu-NA-hydrolyzing activity on the chromatography. This enzyme degraded myosin heavy chain, actin, tropomyosin and troponin I clearly at pH 4.0, while it slightly degraded troponin I at pH 5.0–5.6. Therefore, the contribution of cathepsin H to the proteolysis of postmortem muscle is presumed to be relatively small.

Characterization of purified cathepsin D from malignant human breast tissue.

The aspartyl protease cathepsin D (EC 3.4.23.5) appears to be found in increased amounts and/or abnormally secreted in breast cancer cells, and may contribute to the metastatic spread of malignancy. In the present study, cathepsin D was purified 4800-fold in 20% yield from malignant human breast tissue using affinity chromatography on pepstatin-agarose and DEAE-Sephadex chromatography. Slab gel SDS-PAGE of the purified cathepsin D indicated the presence of three major protein bands (31, 13, 12, kDa) and two minor protein bands (47, 29 kDa). Western blotting indicated that the 31 kDa band was the major immunoreactive species. Isoelectric focusing indicated that the purified cathepsin D consisted of three major isoforms at approximate pIs of 7.4, 7.0 and 6.6, and a possible isoform of lower activity centered around pI 3.2. The pH curve of purified cathepsin D indicated a broad optimum centered around pH 3.4. Lectin blotting suggested the presence of mannose residues but no evidence was found for lectin-available sialic acid, fucose, N-acetylglucosamine and galactose residues. The investigated properties of purified cathepsin D from malignant breast tissue are very similar, if not identical, to the properties of cathepsin D previously purified from normal human breast tissue. Our findings suggest that the elevated activity and antigenic levels of cathepsin D in malignant breast tissue are due to increased amounts of apparently normal enzyme.

The Collagenolytic Activity of Cathepsin K Is Unique among Mammalian Proteinases

Type I collagen fibers account for 90% of the organic matrix of bone. The degradation of this collagen is a major event during bone resorption, but its mechanism is unknown. A series of data obtained in biological models strongly suggests that the recently discovered cysteine proteinase cathepsin K plays a key role in bone resorption. Little is known, however, about the actual action of cathepsin K on type I collagen. Here, we show that the activity of cathepsin K alone is sufficient to dissolve completely insoluble collagen of adult human cortical bone. We found that the collagenolytic activity of cathepsin K is directed both outside the helical region of the molecule, i.e. the typical activity of cysteine proteinases, and at various sites inside the helical region, hitherto believed to resist all mammalian proteinases but the collagenases of the matrix metalloproteinase family and the neutrophil elastase. This property of cathepsin K is unique among mammalian proteinases and is reminiscent of bacterial collagenases. It is likely to be responsible for the key role of cathepsin K in bone resorption.

Properties of cathepsin B immobilized in calcium alginate beads

Cathepsin B (EC 3.4.22.1), purified from goat brain, was immobilized in calcium alginate beads in the presence of bovine serum albumin. The immobilized enzyme retained ∼63% of the original activity and could be used for seven successive batch reactions with retention of 22-30% of the initial activity. Immobilized cathepsin B hydrolysed α-N-benzoyl-D,L-arginine-β-naphthylamide (BANA) maximally at pH 5.5, exhibiting a shift of 0.5 pH unit from that of the soluble enzyme (pH optima 6.0). It showed enhanced stability in acidic as well as alkaline environments in comparison to the free enzyme. The optimal temperature and thermal stability were not altered significantly after immobilization. The K m value for the immobilized enzyme was two-fold higher than for the soluble enzyme.

Distinct Properties of Prohormone Thiol Protease (PTP) Compared to Cathepsins B, L, and H: Evidence for PTP as a Novel Cysteine Protease

The prohormone thiol protease (PTP) has been demonstrated as a major processing enzyme involved in converting the enkephalin precursor to active opiate enkephalin peptides. In this report, PTP was distinguished from other mammalian cysteine proteases, cathepsins B, L, and H, with regard to selectivity for monobasic and paired basic residue-containing peptide-MCA substrates, particular types of proteolytic activities, and sensitivity to active-site-directed peptide inhibitors. PTP cleaved at both COOH- and NH 2-terminal sides of basic residues within peptide-MCA substrates containing monobasic and dibasic sites. PTP showed greatest activity with Z-Phe-Arg-MCA and Bz-Val-Leu-Lys-MCA, and lower levels of activity with peptide substrates containing the paired basic residues Arg-Arg, Lys-Arg, and Lys-Lys. Cathepsins B and L also cleaved monobasic and paired basic residues at both COOH- and NH 2-terminal sides of basic residue(s), but differed from PTP in their preference for the same series of peptide-MCA substrates. Cathepsin H appeared to prefer cleaving at the NH 2-terminal side of basic residues. PTP showed no aminopeptidase activity which is known for cathepsin H. Also, PTP does not exhibit dipeptidylcarboxypeptidase activity, a property of cathepsin B; PTP demonstrates no elastinolytic activity, a characteristic of cathepsin L. Importantly, the sensitivity of PTP to active-site-directed peptide diazomethane inhibitors and E-64c differs from that of cathepsins B, L, and H. E-64c, a selective cysteine protease inhibitor, was the most effective inhibitor of PTP with a second-order rate constant of inactivation, k 2, of 6,710,000 M −1 s −1. These biochemical properties of PTP distinguish it from cathepsins B, L, and H, providing further support for PTP as a novel member of the family of cysteine proteases.

Binding of bovine basic pancreatic trypsin inhibitor (Kunitz) as well as bovine and porcine pancreatic secretory trypsin inhibitor (Kazal) to human cathepsin G: a kinetic and thermodynamic study.

The effect of pH and temperature on kinetic and thermodynamic parameters for the binding of the bovine basic pancreatic trypsin inhibitor (Kunitz inhibitor; BPTI) as well as bovine and porcine pancreatic secretory trypsin inhibitor (Kazal inhibitor; bovine and porcine PSTI, respectively) to human cathepsin G (EC 3.4.21.20) has been investigated. The affinity of the macromolecular inhibitors examined for cathepsin G is characterized by an endothermic, entropy-driven, behaviour, and shows the following trend: BPTI > bovine PSTI > porcine PSTI. The affinity difference of BPTI as well as of bovine and porcine PSTI for cathepsin G is mostly accounted for by changes in the values of the apparent dissociation rate constant for the proteinase:inhibitor complex destabilization. On increasing the pH from 4.5 to 9.5 (at 25.0 degrees C), the affinity of BPTI, as well as bovine and porcine PSTI for cathepsin G increases thus reflecting the acidic-pK shift of the His-57 catalytic residue from approximately 6.9 in the free enzyme to approximately 5.0 in the serine proteinase:inhibitor complexes. The BPTI as well as the bovine and porcine PSTI binding properties of cathepsin G have been analyzed in parallel with those of related serine (pro)enzyme/macromolecular inhibitor systems. Considering the known molecular models, the observed binding behaviour of BPTI as well as that of bovine and porcine PSTI to cathepsin G has been related to the inferred stereochemistry of the serine proteinase/inhibitor contact region(s).

Purification and characterization of cathepsin J from rat liver

Cathepsin J has been partially purified [Liao, J. C. R. & Lenney, J. F. (1984) Biochem. Biophys. Res. Commun. 124, 909-916], but its detailed properties are still unknown. In this study, we have purified cathepsin J completely and characterized it. It was purified to homogeneity from the mitochondrial-lysosomal fraction of rat liver by acid treatment, followed by ammonium sulfate precipitation (20-65%), and chromatographies on S-Sepharose, ConA-Sepharose, Affi-gel 501, HPLC DEAE-5PW and HPLC TSK G3000SW. Cathepsin J was found to be a lysosomal high-molecular-mass cysteine protease of about 160 kDa consisted of two different subunits. One subunit (alpha subunit) was a glycoprotein with a molecular mass of 19-24 kDa which was reduced to 19 kDa by treatment with endoglycosidase F. It has the amino acid sequence LPESWDWRNVR at its N-terminus, which was very similar to those at the N-termini of rat cathepsins B, H and L. The other subunit (beta subunit) was a glycoprotein with a molecular mass of 17 kDa, which was reduced to 14 kDa by treatment with endoglycosidase F. It had DTPANETYPDLLG at its N-terminus, which had no similarity with the N-terminal sequences of other cathepsins. Cathepsin J showed strong affinity for synthetic substrates such as N-benzyloxycarbonyl-phenylalanyl-arginine 4-methyl-coumaryl-7-amide and glycyl-arginine beta-naphthylamide. It was activated by thiol reagents and chloride ion and was inhibited by cysteine protease inhibitors. However, its initial inhibition constant Ki(initial) by N-(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucine-3- methylbutylamide (E-64-c) was 1800 nM, which was 100-500 times those of cathepsins B and L. Many properties of cathepsin J were similar to those of cathepsin C (dipeptidylaminopeptidase I) reported as a lysosomal cysteine protease with dipeptidyl-aminopeptidase activity [McDonald, J. K., Reilly, T. J. & Ellis, S. (1964) Biochem. Biophys. Res. Commun. 16, 135-140]. Furthermore, antiserum against rat liver cathepsin C reacted with rat liver cathepsin J. These findings suggested that cathepsin J is identical with cathepsin C.

Purification and properties of cathepsin B from sea urchin eggs

1. 1. Cathepsin B from sea urchin eggs was fractionated by ammonium sulfate and purified by ion exchange chromatography and affinity chromatography on organomercurial agarose. 2. 2. The molecular weight of cathepsin B from Anthocidaris crassispina and Hemicentrotus pulcherrimus, was 22,000 and 29,000, respectively, determined by gel filtration through a calibrated Sephadex G-100 column. 3. 3. Cathepsin B of sea urchins had pI values of 4.8, 5.2 and 5.7 in A. crassispina and 4.8 in H. pulcherrimus. 4. 4. Cathepsin B from both species hydrolyzed carbobenzoxy-arginyl-arginine-4-methyl-coumaryl-7-amide more efficiently than arginine-4-methyl-coumaryl-7-amide and carobbenzoxy-phenylalanylarginine-4-methyl-coumaryl-7-amide. 5. 5. The properties of sea urchin cathepsin B appeared to be similar to that of cathepsin B from other mammalian sources.

Cathepsin D-like activity in the release of Galβ 1-4GlcNAcα2-6sialyltransferase from mouse and guinea pig liver Golgi membranes during the acute phase response

1. 1. Rat Galβ 1-4GlcNAcα2-6sialyltransferase (E.C. 2.4.99.1) is released from Golgi membranes by cleavage of a portion of the enzyme containing the active site from a membrane anchor, this effect was most dramatic during the acute phase response. The enzyme that cleaved sialyltransferase had the properties of cathepsin D was most active at pH 5.6 and was likely of lysosomal origin (lammers and Jamieson, 1988). 2. 2. The acute phase response of sialyltransferase in mouse and guinea pig was previously found to differ from that in the rat. Release of sialyltransferase from mouse and guinea pig Golgi membranes has now been studied in order to make a comparison with the rat system. 3. 3. Maximum release of sialyltransferase from mouse and guinea pig Golgi occurred at pH 4.6 and 5.2, respectively; like the rat a cathepsin D-like proteinase was responsible for release of both enzymes. 4. 4. Immunoblot analysis showed that membrane-bound rat and mouse sialyltransferase had M r 49,000, whereas the guinea pig enzyme had M r 42,000. The released form of the rat enzyme had M r 42,000, but released forms of mouse and guinea pig enzymes had M r 38,000 suggesting a different cleavage site for these two enzymes compared to the rate enzyme.

Cathepsin D-like activity in neutrophils and monocytes.

Monocytes-macrophages and polymorphonuclear leukocytes contain an acid proteolytic enzyme that cleaves tritiated hemoglobin. The monocyte-macrophage-derived enzymatic activity was completely inhibited by pepstatin A, a property of cathepsin D. Monocyte-derived macrophages developed detectable cathepsin D-like activity after 5 days in culture, and this activity coincided with the appearance of other known indicators of macrophage maturation. The cathepsin D activity further increased significantly with time after day 5 of culture. The proteinase activity extracted from neutrophils was only partially inhibitable by pepstatin A, which indicates that this activity is contributed by more than one proteolytic enzyme, including cathepsin D. Cathepsin D activity demonstrated in neutrophils and macrophages may be an important marker of phagocyte function.

Purification and some properties of cathepsin B from rabbit skeletal muscle.

Cathepsin B was purified from rabbit skeletal muscle by ammonium sulfate fractionation and successive chromatographies on Sephadex G-75, phosphocellulose, peptide-conjugated Sepharose, DEAE-Toyopearl and Sephadex G-100. The purified enzyme gave a single protein band on SDS/polyacrylamide gel electrophoresis. The enzyme did not abolish the Ca sensitivity of the ATPase activity of myofibrils. The molecular mass of the enzyme was found to be 27 kDa on gel filtration and SDS/polyacrylamide gel electrophoresis. The optimum pH for the hydrolysis of N alpha-benzoyl-DL-arginine-beta-naphthylamide was 6.5. The enzyme was stable in the range of pH 4.5-5.5. Tetrathionate reacted with thiol groups of the enzyme reversibly so that it stabilized the enzyme. The enzyme was strongly inhibited by iodoacetate, HgCl2, antipain, leupeptin, N alpha-p-tosyl-L-lysine chloromethane and L-tosylphenylalanylchloromethane, but not by pepstatin or trypsin inhibitor.

Species variations amongst lysosomal cysteine proteinases

Properties of cathepsin L from rat liver lysosomes were compared with those of a similar enzyme, cathepsin S from beef spleen. Major characteristics of cathepsin L are the high activity against Z-Phe-Arg-methylcoumarylamide and sensitivity to the fast reacting irreversible inhibitor Z-Phe-Phe-diazomethane. In contrast, cathepsin S hydrolyzes Z-Phe-Arg-methylcoumarylamide only slowly and Z-Phe-Phe-diazomethane cannot be regarded as a potent inhibitor of this enzyme. The differences in the substrate specificity of cathepsin L from rat liver and cathepsin S from beef spleen are discussed in comparison with the substrate specificity of cathepsin B from rat and human liver and beef spleen.

Intracellular distribution and properties of cathepsin B in the rat retina and its inhibition by a cytosolic fraction

The intracellular distribution and properties of cathepsin b in the rat retina are similar to those of cathepsin b in other mammalian tissues. The specific activity of cathepsin b was highest in the mitochondrial-lysosomal fraction [1650±450 pmole mg −1 protein hr −1 (mean± s.d.)]. Inhibition of cathepsin b activity by a cytosol fraction of retina is also described.