Copper dependent control of the enzymic and phagocyte induced degradation of some biopolymers, a possible link to systemic inflammation

Expression of the thioredoxin-thioredoxin reductase system in the inflamed joints of patients with rheumatoid arthritis

OBJECTIVE

To examine the expression of the thioredoxin (TRX)-thioredoxin reductase (TR) system in patients with rheumatoid arthritis (RA) and patients with other rheumatic diseases.

METHODS

Levels of TRX in plasma and synovial fluid (SF) were measured using enzyme-linked immunosorbent assay. Cellular distribution of TRX was determined by flow cytometry and histochemistry. Cellular expression of TR was studied by in situ messenger RNA (mRNA) hybridization. The effect of oxidative stress and tumor necrosis factor alpha (TNF alpha) on TRX expression by cultured rheumatoid fibroblast-like synoviocytes was studied.

RESULTS

Significantly increased TRX levels were found in the SF from 22 patients with RA, when compared with plasma levels in the same patients (P < 0.001) and compared with SF TRX levels in 15 patients with osteoarthritis (P < 0.001), 13 patients with gout (P < 0.05), and 9 patients with reactive arthritis (P < 0.0001). The presence of TRX could be demonstrated within the SF-derived mononuclear cells and synovial tissue (ST) of RA patients. Concordantly, expression of TR mRNA was observed in the ST of these patients. Stimulation of synovial fibroblast-like synoviocytes with either H2O2 or TNF alpha induced an increase in the production of TRX.

CONCLUSION

The data demonstrate significantly increased concentrations of TRX in the SF and ST of RA patients when compared with the levels in patients with other joint diseases. Evidence is presented that the local environment in the rheumatic joint contributes to increased TRX production. Based on its growth-promoting and cytokine-like properties, it is proposed that increased expression of TRX contributes to the disease activity in RA.

Suppression of type II collagen-induced arthritis by N-acetyl-L-cysteine in mice

1. The antiarthritic and anti-inflammatory efficacy of N-acetyl-L-cysteine (NAC) was tested in male DBA/1 hybrid mice suffering from type II collagen-induced arthritis. Parameters including the arthritis index and the phagocytic responses recorded by chemiluminescence in unseparated blood were used for the assessment of disease activity. 2. Mice were immunized by subdermal injection of bovine type II collagen in Freund's complete adjuvant. The treatment with NAC started at day 42 after immunization and was continued over a period of six weeks: in doses ranging up to 50 mg/kg, a dose-dependent suppression of arthritis was noted; between 50 and 200 mg/kg, the inhibition curve had a plateau [ED50 = 50 mg/(kg x day)]. 3. The arthritis index correlated positively with the generation of chemiluminescence by reactive oxygen species (ROS) produced in neutrophils and monocytes activated by 12-O-tetradecanoylphorbol 13-acetate. 4. After treatment with 100 mg/kg of NAC from day 42 after immunization over a period of six weeks, the ROS production was reduced to levels occurring in whole blood of healthy animals. 5. It is concluded that low-molecular-weight antioxidants such as NAC may be adequate for controlling oxidative stress-derived damage in rheumatic diseases by modulation of ROS-dependent signal transduction pathways.

Loss of the metal binding properties of metallothionein induced by hydrogen peroxide and free radicals

The relationship between the metal-binding properties of metallothionein (MT) and its ability to interact with peroxides and free radicals was explored in vitro. The binding of 109Cd to MT and the thiol density of the protein were determined after incubation of a purified Zn/Cd-metallothionein preparation with either hydrogen peroxide alone, or with a number of free radical generating systems. Exposure of MT to H2O2, whether in the presence or absence of Fe2+, resulted in the progressive loss of the thiol residues of the protein and led to a parallel decrease of its 109Cd-binding capacity. These changes correlated with r values of 0.999 (P = 0.001) and 0.998 (P = 0.001), in the absence and presence of iron, respectively. The effects of H2O2, alone or plus Fe2+, on MT were completely prevented by catalase, but totally unaffected by superoxide dismutase or desferrioxamine. Exposure of MT to xanthine/xanthine oxidase also led to thiol oxidation and to a concomitant loss of the Cd-binding properties. In this system, both changes correlated with an r of 0.993 (P = 0.001) and were completely inhibited by superoxide dismutase. Exposure of MT to the peroxyl radical generator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH), resulted in the progressive loss of its the metal-binding properties and its thiol residues, both changes correlating with an r of 0.986 (P = 0.002). The ability of MT to bind 109Cd, lost as a result of its prior exposure to either H2O2 alone, H2O2 plus Fe2+, xanthine/xanthine oxidase, or to AAPH was, in all cases, completely recovered after incubation of the modified protein with dithiothreitol. These results indicate that H2O2 alone, and/or the oxygen-derived species, superoxide anion and peroxyl radicals, can all directly interact in vitro with MT to modify the protein oxidatively, and suggest that, under in vivo conditions, these species may be implicated as modifying factors of the metal-binding capacity of metallothionein.

Antioxidant and prooxidant roles of copper in Tween 20-induced hemolysis of hamster and pig erythrocytes containing marginal vitamin E

The concentration-dependent effects of copper acting either as an antioxidant or as a prooxidant were examined in vitro using Tween 20-induced hemolysis. When cupric ion concentration was more than 10 microM, free copper(II) acted as a prooxidant; both extensive hemolysis and production of unknown thiobarbituric acid-reactive substance occurred in hamster and pig erythrocytes irrespective of vitamin E status. However, when cupric ion concentration was 2-4 microM in the incubation medium, copper showed a clear antioxidant activity, reducing both hemolysis and malondialdehyde production induced either by diluted peroxide-containing Tween 20 with ascorbic acid and sodium azide in vitamin E-deficient hamster erythrocytes, or by peroxide-containing Tween 20 in pig erythrocytes containing marginal amounts of vitamin E. Copper(II) is taken up by the erythrocytes, where copper(I)-complexes may contribute to the protection of cells with membrane vitamin E against oxidative radical attack.

Suppression of arthritis by an active center analogue of Cu2Zn2-superoxide dismutase

The anti-arthritic and anti-inflammatory efficacy of CuPu(Py)2 ([N,N'-bis(2-pyridylmethylene)-1,4-butanediamine] (N,N',N",N"))-Cu(II), a serum-stable active center analogue of Cu2Zn2-superoxide dismutase (SOD; EC 1.15.1.1), was tested in male DBA/1 x B10A (4R) mice suffering from potassium-peroxochromate-induced (PIA) or collagen type II-induced arthritis (CIA). Parameters including the arthritis index, the plasma SOD activity, and the inhibition of phagocytic responses in unseparated blood were used for the assessment of disease activity. A dose-dependent suppression of arthritis was noted in both models. The ED50 was 2.5 +/- 0.4 mumol/kg/day of CuPu(Py)2 for PIA and 4.0 +/- 1.1 mumol/kg/day for CIA. The arthritis index correlated with both the levels of reactive oxygen species (ROS) generated by phorbol ester-activated neutrophils and monocytes in unseparated blood (r = 0.892) and the SOD-like activity in plasma (r = 0.857). CuPu(Py)2 inhibited also the lipoplysaccharide-induced release of tumor necrosis factor alpha from human monocytes and neutrophils in a dose-dependent manner. Unlike SOD, which exerts successful anti-rheumatic activity mainly upon intra-articular injection, the SOD-mimic CuPu(Py)2 can be applied systemically. Non-proteinaceous low molecular weight antioxidases may well be suited to control oxidative stress-derived damage in rheumatic diseases by modulation of ROS-dependent signal transduction pathways.

A pulse radiolytic study on the reaction of hydroxyl and superoxide radicals with yeast Cu(I)-thionein

In a pulse radiolytic study employing aqueous intact yeast copper(I)-thionein at pH 7 it was shown that both superoxide and hydroxyl radicals efficiently react with this Cu(I)- and thiolate-rich protein. The reaction constant of hydroxyl radicals with Cu(I)-thionein was determined by competition kinetics and was 2.2 x 10(11) M-1 s-1 at a rate close to a diffusion-controlled limit. The reaction of Cu(I)-thionein with superoxide was also successful and proceeded at a rate of 7.5 x 10(6) M-1 s-1. According to chiroptical and luminescence emission measurements minor oxidation of the copper(I)-thiolate oligonuclear binding centres was observed, leading to the release of some Cu(II). It is important to realise the dual reactivity of this yeast Cu(I)-thiolate protein in controlling copper transport and storage as well as its distinct role in the scavenging of free radicals.

Metal complexes of anti-inflammatory drugs. Part VIII: Suprofen complex of copper(II)

The preparation and properties of the copper(II) complex Cu(SUP)2.H2O are reported for the anti-inflammatory drug Suprofen (SUP). The diffuse reflectance spectra and magnetic moment are consistent with a dinuclear structure as found for [Cu(aspirinate)2(H2O)]2. The copper(II) complex exhibits an increased superoxide dismutase activity compared with the parent drug molecule in the nitroblue tetrazolium assay.

Reactivity of an active center analog of Cu2Zn2superoxide dismutase in murine model of acute and chronic inflammation

The antiinflammatory efficacy of CuPu(Py)2 ([[N,N'-bis(2-pyridylmethylene)-1,4-butanediamine] (N,N',N'',N''')]-Cu2+), a serum stable active center analog of Cu2Zn2superoxide dismutase (SOD), was tested in vitro and in vivo in male Wistar rats suffering from potassium peroxochromate-induced inflammation. Parameters including 99mTc gamma-scintigraphic imaging, the arthritis score, the plasma superoxide dismutase activity, the inhibition of plasma sulfhydryl depletion as well as mitogenic and phagocytic responses were used to quantify the disease activity. All parameters improved impressively during the treatment with CuPu(Py)2 and resembled those of healthy animals after 21 days. The arthritis score was inhibited by 80% (P > 0.001) and the plasma SOD activity enhanced by 380% (P > 0.001). The depletion of plasma sulfhydryls and the leukocytic responses to concanavalin A, tetradecanoylphorbolacetate, and lipopolysaccharide were significantly reduced (P > 0.001) and correlated well with the arthritis score. The collapse of antioxidant defenses in human plasma as well as the depolymerization of hyaluronic acid was mimicked in vitro and successfully inhibited by CuPu(Py)2. Oxidant-induced injury of plasma components during the aqueous decay of potassium peroxochromate were demonstrated to activate the oxidative burst of phagocytes in human blood. The role of impaired pro- and antioxidant balances in the etiology of inflammatory and autoimmune rheumatic diseases is discussed.

Elevated levels of xanthine oxidase in serum of patients with inflammatory and autoimmune rheumatic diseases

Sera of patients with various inflammatory and autoimmune rheumatic diseases were screened for the presence of xanthine oxidase (XOD) and compared to sera from healthy donors and patients with nonrheumatic diseases including AIDS, internal diseases, and different carcinomas. Up to 50-fold higher levels of XOD were detected in rheumatic sera (P < 0.001). In addition, serum sulfhydryls (SH) were determined as sensitive markers of oxidative stress. The SH status in rheumatic patients was diminished by 45-75% (P < 0.001) and inversely correlated to the concentration of serum XOD (R = 0.73), suggesting a causal interrelation. The depletion of serum sulfhydryls by the oxyradical-producing XOD/acetaldehyde system was mimicked successfully ex vivo in human serum from healthy donors. Cortisone treatment of patients suffering from systemic lupus erythematosus and rheumatoid arthritis impressively normalized elevated XOD concentrations in rheumatic sera to those of healthy controls. The participation of xanthine oxidase in the depletion of serum antioxidants in rheumatic patients is discussed in the light of substrate availability and Km values.

Reactive nitrogen intermediates, antinuclear antibodies and copper-thionein in serum of patients with rheumatic diseases

Sera from 354 patients with various inflammatory and autoimmune rheumatic diseases were screened for the presence of reactive nitrogen intermediates, antinuclear antibodies and the anti-oxidase copper-thionein (Cu-thionein), and compared to sera from healthy donors and patients with non-rheumatic diseases including AIDS, various internal as well as neurological diseases and carcinoma of different organs. When compared to healthy individuals, the levels of nitric oxides in sera from patients with autoimmune rheumatic diseases were elevated by 240-600% (P < 0.01). The status of reactive nitrogen intermediates (NOx, RNI) in sera from donors with inflammatory rheumatic diseases was increased by 170-540%, but was also significantly enhanced in sera of patients with non-rheumatic diseases, indicating a general inflammatory mechanism that is predominantly triggered by inducible nitric oxide (NO) syntheses of phagocytes. All rheumatic sera were dramatically depleted of the anti-oxidase Cu-thionein (P < 0.001), a powerful consumer of hydroxyl radicals and singlet oxygen and an efficient superoxide dismutase. The NOx levels were positively correlated with the serum titers of antinuclear antibodies (r = 0.77) and negatively correlated with Cu-thionein levels (r = 0.94), reflecting a high steady-state concentration of free radicals generated during inflammatory and autoimmune rheumatic diseases.

Copper-dependent antioxidase defenses in inflammatory and autoimmune rheumatic diseases

Gel-filtered sera of patients with various inflammatory and autoimmune rheumatic diseases (N = 354) were screened for the presence of the inflammation marker Cu-thionein. The concentrations of Cu-thionein were significantly diminished in patients with connective tissue diseases (P < 0.001). Sera of patients suffering from inflammatory rheumatic diseases were almost totally depleted of this low-molecular-weight copper protein that exerts pronounced superoxide dismutase activity and scavenges effectively hydroxyl radicals and singlet oxygen. Cortisone treatment of patients with rheumatoid arthritis, systemic lupus erythematosus, and polymyalgia rheumatica replenished impressively the serum concentration of Cu-thionein. The partial oxidation of the EPR-silent Cu(I)-chromophore to Cu(II)/Cu(I)-thionein, which is essential for the catalytic dismutation of superoxide, was monitored by electron paramagnetic resonance in the presence of activated neutrophils and monocytes. Release of Cu-thionein during the oxidative burst of peripheral blood monocytes was demonstrated in vitro. The role of prooxidant-antioxidant imbalances in the pathogenesis of rheumatic diseases is discussed.

Oxidant-induced mobilization of zinc from metallothionein

Neutrophils which accumulate at sites of inflammation secrete a number of injurious oxidants which are highly reactive with protein sulfhydryls. The present study examined the possibility that this reactivity with thiols may cause protein damage by mobilizing zinc from cellular metalloproteins in which the metal is bound to cysteine. The ability of the three principal neutrophil oxidants, hypochlorous acid (HOCl), superoxide (.O2-), and hydrogen peroxide (H2O2), to cleave thiolate bonds and mobilize complexed zinc was compared using two model compounds (2,3-dimercaptopropanol and metallothionein peptide fragment 56-61), as well as metallothionein. With all compounds, 50 microM HOCl caused high rates of Zn2+ mobilization as measured spectrophotometrically with the metallochromic indicator 4-(2-pyridylazo)resorcinol. Xanthine (500 microM) plus xanthine oxidase (30 mU), which produced a similar concentration of .O2-, also effected a rapid rate of Zn2+ mobilization which was inhibited by superoxide dismutase but not catalase, indicating that .O2- is also highly reactive with thiolate bonds. In contrast, H2O2 alone was much less reactive at comparable concentrations. These data suggest that HOCl and .O2- can cause damage to cellular metalloproteins through the mobilization of complexed zinc. In view of the essential role played by zinc in numerous cellular processes, Zn2+ mobilization by neutrophil oxidants may cause significant cellular injury at sites of inflammation.

Hypochlorous acid-induced mobilization of zinc from metalloproteins

Hypochlorous acid (HOCl), a neutrophil oxidant, can contribute to tissue injury at sites of inflammation by its reactivity with protein sulfhydryls. The present study shows that physiological concentrations (50-200 microM) of HOCl can displace Zn2+ from metalloproteins, such as metallothionein and alcohol dehydrogenase, in which the metal is bound to sulfhydryls by means of thiolate (S-Zn) bonds. No mobilization of Zn2+ was observed from superoxide dismutase in which the metal is not bound to cysteine, suggesting that HOCl reacts selectively with thiolate bonds. Zn2+ mobilization, measured spectrophotometrically with the metallochromic indicator 4-(2-pyridylazo)resorcinol, was also observed from complexes of this metal with other thiol-containing compounds such as 2,3-dimercaptopropanol and metallothionein fragment 56-61. HOCl cleavage of the thiolate bonds was confirmed by the decrease in absorbance at 250 nm. This study shows for the first time that HOCl can mobilize protein-bound Zn2+ and suggests that neutrophil oxidant injury may be partially mediated by the mobilization of cellular Zn2+.

Antiinflammatory reactivity of copper(I)-thionein

In unseparated human blood the reactivity of yeast copper (I)-thionein on TPA-activated polymorphonuclear leukocytes was evaluated and compared with low Mr copper chelates exerting Cu2Zn2 superoxide dismutase mimetic activity. Cu, 18 microM, in the form of Cu-thionein was sufficient to inhibit the superoxide production of activated human blood phagocytes by 50%. Furthermore, the scavenging of hydroxyl radicals and singlet oxygen by Cu(I)-thionein was determined, using the 2-deoxyribose fragmentation assay induced by decaying K3CrO8 and the NADPH oxidation caused by UVA illuminated psoralen, respectively. The inhibitory reactivity of Cu-thionein in both assays was compared with that of serum proteins including albumin, ceruloplasmin, transferrin, and ferritin. The galactosamine/endotoxin-induced hepatitis in male NMRI mice was used to evaluate the antiinflammatory reactivity of Cu-thionein in vivo. The serum copper, superoxide dismutase, and sorbitol dehydrogenase concentrations, as well as the activity of polymorphonuclear leukocytes in unseparated blood seemed most appropriate to quantify the protective capacity of Cu-thionein in the course of an oxidative stress-dependent liver injury. The intraperitoneal application of 32.5 mumols/kg thionein-Cu limited this damage to 45%.

Phagocytic response modifying reactivity of enzymatic cell wall digests of Nocardia opaca

Aqueous extracts (ENOCW) and enzymatic digests of purified Nocardia opaca cell wall fragments, virtually free of muramyl peptides, were monitored for their phagocytic response modifying reactivity on polymorphonuclear leucocytes, separated or unseparated in whole human blood. In the presence of ENOCW a 74% increased production of superoxide during the respiratory burst of TPA-activated polymorphonuclear leukocytes was observed, as compared to the unprimed control. Delipidation of this preparation resulted in a further increase in reactivity (144%). Even in the presence of whole human blood, as a model for competitive binding in biological fluids, an enhanced generation of superoxide by TPA activated blood phagocytes remained detectable. A 37-75% decreased phagocytic reactivity in samples of HIV-seropositive blood was considerably restored in the presence of ENOCW.

Reactivity of active center analogs of Cu2Zn2 superoxide dismutase on activated polymorphonuclear leukocytes

In unseparated human blood the Cu2Zn2 superoxide dismutase mimetic reactivity of several differently coordinated low Mr copper chelates on TPA-activated polymorphonuclear leukocytes was evaluated and compared to their apo-chelates, CuSO4, and the native enzyme. Similar to intact superoxide dismutase, 350-400 nM Cu flexibly complexed in a di-Schiff base mode in CuPu(Py)2 and CuPu(Im)2, respectively, was sufficient to inhibit the oxidative burst-dependent superoxide production of human blood phagocytes by 50%. Acetate- or biuret-type copper chelates behaved like CuSO4. The catalytic superoxide dismuting reactivity of the di-Schiff base active center analogs of SOD was confirmed using isolated porcine PMNs. Even in the presence of 600 microM albumin as a model for competitive copper chelation in biological fluids CuPu(Py)2 and CuPu(Im)2 remained active. The stability during the Cu(I)/Cu(II) redox cycling was demonstrated in the presence of activated PMNs and albumin, taking advantage of the electron paramagnetic properties of CuPu(Py)2 and CuPu(Im)2.

Effects of some non-steroidal anti-inflammatory drug copper complexes on polymorphonuclear leukocyte oxidative metabolism

Interaction between anti-inflammatory drugs and reactive oxygen metabolites must be considered in the course of pharmacological studies intended to develop new compounds. Effects of indomethacin, aspirin, and 3,5-diisopropylsalicylic acid (3,5-DIPS) and their copper complexes on PMNL oxidative metabolism and the evolution of an acute inflammatory reaction were studied in the rat. Experiments were performed in vitro by assessment of superoxide generation and reduction of chemiluminescence by PMNLs incubated or not (control) in medium containing various concentrations of these compounds. A dose-related decrease of these parameters was observed, however, copper complexes were found to be more effective than their parent drugs or Cu gluconate. Copper complexes were also more effective anti-inflammatory agents than their parent ligands or Cu gluconate when the volume of exudate and number of exudate PMNLs were assessed after induction of pleurisy in rats by injection of isologous serum. It is concluded that modulation of the PMNL oxidative burst by copper complexes offers an accounting for the anti-inflammatory activity of these compounds.

Copper-thionein in leucocytes

Upon incubation of peripheral leucocytes with copper sulphate a dramatic cellular copper uptake reaching levels of 25-50-fold compared to that of the natural copper content was measured. The orange-red fluorescence of the copper-treated white blood cells was assigned to the formation of Cu(I)-thiolate clusters in Cu(I)-thionein. A protein of 6-8 kDa was isolated from homogenized bovine leucocytes and characterized by its electronic absorption and amino acid composition to be identical to the above Cu(I)-thionein. More than 70% of the intracellular copper was attributed to this protein in its monomeric and polymeric form. Cu-thionein formation was more pronounced in monocytes than in granulocytes. As most intriguing phenomenon, the release of this Cu-thionein from leucocytes, was also noticed. The occurrence of Cu-thionein in leucocytes and the excretion of the intact Cu(I)-thiolate protein is of considerable interest with respect to the observed elevated copper levels in white blood cells and plasma during tumor malignancies and inflammatory processes.

Release of copper from yeast copper-thionein after S-alkylation of copper-thiolate clusters

Our knowledge on the release of copper from Cu-thionein in biological systems is limited. Other than oxidative cleavage or direct transfer, the possibility of an alkylation mechanism seemed attractive. Iodoacetamide and methyl methanesulphonate were successfully employed to alkylate the Cu-thiolate sulphur atom of homogeneous Cu(I)-thionein from yeast. The alkylation caused a weakening of the Cu-S bonding, which led to the release of copper. After equilibrium dialysis a proportion of the released copper was found in the dialysis buffer. When iodoacetamide was used carboxymethylcysteine was detected in the protein hydrolysate. A 10-fold molar excess over cysteine was sufficient for complete alkylation, which could be conveniently monitored by c.d. at 328 and 359 nm. The reaction proceeded under both aerobic and anaerobic conditions. E.p.r. measurements of Cu2+ revealed unequivocally the complete cleavage of the Cu-thiolate bonding in less than 5 h. It is possible that this mode of copper release might be of relevance to the molecular transport of this biochemically important transition metal.

A five-coordinate copper complex with superoxide dismutase mimetic activity from Streptomyces antibioticus

A Cu(II) complex of desferrithiocin from Streptomyces antibioticus was prepared and characterized. The first shell atoms, including one nitrogen and four oxygens, were arranged around the copper in a square-planar pyramide. Due to the axially Jahn-Teller-distorted Cu-O distance at 224.7 pm, a distinct Cu2Zn2superoxide dismutase mimetic activity was measured. The Cu-complex survived 600 microM bovine serum albumin and the thermodynamic stability (pK = 17.4) was not very different from that of Cu-EDTA. The electronic absorption properties, circular dichroism and electron paramagnetism were in accordance with those of the type-II copper species.

Copper transport from Cu(I)-thionein into apo-caeruloplasmin mediated by activated leucocytes

A study on the transfer of copper from Cu-thionein into apo-caeruloplasmin, using Cu-thionein that was previously oxidised by activated leucocytes, was performed. Cu(I)-thiolate oxidation was conveniently monitored by the progressive decline of the specific Cotton bands between 400 and 300 nm. The characteristic e.p.r. properties and NN-dimethyl-p-phenylenediamine oxidase activity indicated a successful formation of caeruloplasmin. Taking into account the simultaneous occurrence of leucocytes, apo-caeruloplasmin and Cu-thionein in blood plasma, such an interaction would favour a possible metabolic link between either copper protein.

The role of Cu(I)-thiolate clusters during the proteolysis of Cu-thionein

Rat liver Cu,Zn-[35S]thionein and yeast Cu-thionein were subjected to proteolysis in vitro using equilibrium dialysis. The partially copper-loaded vertebrate thionein (2-7 Cu/mol) was affected by different proteases including thermolysin, proteinase K, protease from Streptomyces griseus and lysosomal enzymes. Unlike the 2Cu-thionein the respective 7Cu-thiolate-centred metallothionein was hardly proteolytically digested. In contrast to fully copper-loaded native yeast Cu-thionein both the H2O2-oxidized and the metal-free protein were effectively cleaved in the presence of proteinase K. It is important to realize that the native Cu(I)-thiolate chromophore survives the proteolytic attack. When the copper-sulphur bonding is broken and the same amount of copper is unspecifically bound to the thionein portion, proteolysis proceeds identically with respect to the rate observed in the presence of the apoprotein. The unsuccessful proteolysis of native Cu-thionein is not attributable to a simple copper-dependent inhibition of the proteinases. It is suggested that prior to proteolysis the copper-sulphur clusters must be destroyed.