The effect of pH and lysosomal extract on rat liver metallothionein (Cd-thionein)

Differential sensitivity of metallothionein-1 and -2 in liver of zinc-injected rat toward proteolysis

The characterization of degradation products of metallothionein (MT) in rat liver is reported. Using gel filtration chromatography on Sephadex G-75 an unknown zinc (Zn)-containing peak was found in livers of rats injected with zinc. It contained at least 13 peptide fractions, designated as LPI-1, LPI-2, LPI-3, LPI-4, LPII-1, LPII-2, LPII-3, LPIII-1, LPIII-2, LPIII-3, LPIII-4, LPIII-5 and LPIII-6. While two of these fractions (LPII-1 and LPIII-4) were identified as the carboxyl-terminal half (alpha-domain) of rat MT-1, LPI-2 and LPI-3 corresponded to the amino-terminal half (beta-domain) of rat MT-2. In contrast, neither the beta-domain of rat MT-1 nor the alpha-domain of rat MT-2 could be recovered. The data may indicate that the two metal clusters of Zn-MT-1 and Zn-MT-2 differ in their stabilities resulting in differences in their susceptibility towards proteolytic degradation.

In vitro degradation of apo-, zinc-, and cadmium-metallothionein by cathepsins B, C, and D

Metallothionein (MT) has been extensively studied over the past several years because of its probable role in endogenous metal homeostasis and cellular protection. A large body of knowledge now exists describing the physicochemical properties of MT as well as the mechanisms involved in MT induction. It has been well established that MT protects tissues from metal toxicity by chelating metals that would otherwise be available to interact with and disrupt vital cell functions. Information on the degradation of metal-saturated MT and the fate of the metals associated with it would be extremely important in predicting metal toxicity. Lysosomes have been targeted as a possible subcellular site for the turnover of MT; however, the susceptibility of MT to degradation by specific acidic proteases (i.e., cathepsins) has not been described. Therefore, the purpose of the present study was to examine the relative abilities of cathepsins B, C, and D to degrade Zn7-MT, Cd7-MT, and apo-MT in vitro. In so doing, the effects of metal species, degree of metal saturation, and pH on the degradation processes were evaluated. Time course experiments revealed that apo-MT was rapidly degraded by all three cathepsins. Cathepsin B degraded apo-MT approximately 36-fold more rapidly than cathepsin C and 45-fold more rapidly than cathepsin D. Therefore, under the in vitro conditions used in this study, the relative potency of the cathepsins tested was cathepsin B much much greater than cathepsin C greater than cathepsin D. In comparison, metal-saturated MT was more than 1000-fold more resistant to degradation by the cathepsins tested. In order to determine how much metal was needed to protect MT against degradation, apo-MT was reconstituted with increasing molar equivalents of Zn2+. The results suggest that as metal to apo-MT ratios increase, less apo-MT substrate is available to the protease and degradation decreases.

Role of hepatic lysosomes in the degradation of metallothionein

The degradation of metallothionein (MT) by rat liver was examined. Degradation of MT by liver homogenate was greater than by cytosol. In addition, MT degradation by the homogenate at pH 5.5 was more than that at pH 7.2. Because lysosomal proteases function at acidic pH, these findings suggest the importance of lysosomes in MT degradation. The degradation by the lysosomal fraction was about 400-fold greater than that by the cytosol. Because cathepsins are the principal lysosomal proteases, we used cathepsin-specific inhibitors, such as leupeptin, E-64 and pepstatin, to determine the relative importance of different cathepsins in degrading MT. The study reveals that cathepsin B and/or L is (are) probably the most important enzyme(s) in degrading hepatic MT, because leupeptin, which blocks cathepsin B and L activity, inhibited the degradation of apo-MT by about 80%. Cathepsin D appears to be of least importance in MT degradation, because inhibition of this enzyme by pepstatin reduced degradation by only 20%. Studies on the degradation of apo-MT, ZnMT, and CdMT indicated that apo-MT is about 1500-fold more sensitive to degradation than ZnMT and CdMT. These data suggest that metals protect MT from degradation. This is further supported by a reconstitution experiment, which shows that with a progressive decrease of MT: metal ratio following titration of apo-MT by metals, there is a concomitant reduction in degradation. At a lysosomal pH of around 4.7, about 60% of Zn and 20% of Cd are displaced from MT, thereby making it susceptible to degradation. We propose, therefore, that lysosomes are probably important for MT degradation in vivo and that metal release is a prerequisite for degradation. With the release of metals, MT becomes susceptible to degradation, which is probably accomplished by the lysosomal cathepsins, in particular cathepsins B and L.

Metallothionein biodegradation in rat hepatoma cells: a compartmental analysis aided 35S-radiotracer study

Disappearance rates, synthesis and biodegradation of rat HTC cell metallothioneins (MT) were examined by measurements of [35S]MT, and expressed by their rate constants in experiments carried out under steady-state conditions, with physiological doses of zinc and copper. Overall rates of disappearance of [35S]MT did not obey first-order kinetics, apparently due to the presence of chemically and/or spatially distinguishable MT (sub)pools. Application of compartmental analysis yielded results indicating that: (a) MT in copper-treated cells is stabler than MT in zinc-treated cells, (b) apparent half-lives for MT disappearance are 22-25 h under the conditions as described, (c) addition of 0.1 mM chloroquine reduces the overall rate of MT disappearance by 25%, (d) total MT should be regarded as significantly consisting of at least two MT sub-forms, i.e., Apo-MT and M-MT and/or cytosolic MT and lysosomal MT, each being depleted and replenished at different rates, (e) mean half-lives for total MT, based on actual degradation rates, were 3-7 h, (f) addition of 0.1 mM chloroquine resulted in both increased synthesis and increased degradation of non-lysosomal MT, the latter probably due to free amino acid depletion in the cells, and (g) MT behaviour may be best examined by combination of tracer experiments (35S-labeled amino acids), simultaneous determination of MT levels in all (sub)MT pools, and application of compartmental analysis.

Effects of cadmium in freshwater clams. II. Ultrastructural changes in the renal system ofAnodonta cygnea

An ultrastructural study was made of the renal system of freshwater clams,Anodonta cygnea, that had been exposed to cadmium chloride (50 μg Cd/L) for 12 weeks. By stereological analysis an extended lysosomal system and a decreased number of mitochondria was apparent in the epithelial cells lining the proximal compartment of the kidney. The increase of the lysosomal system was mainly accountable to the appearance of a distinct type of lysosome, that accumulated in the apical cell region. The decrease of the mitochondrial population was accompanied by a considerable swelling of the individual mitochondria. Finally, a severe reduction of the glycogen stores was noticed. Similar, but less obvious, changes occurred in the distal kidney compartment. The results suggest that long-term exposure ofAnodonta cygnea to cadmium stimulates the lysosomal system and disturbs the function of organelles involved in the energy metabolism of resorptive kidney cells.

Histochemical staining of cadmium thiolate clusters in livers of rats treated chronically with cadmium

In livers of rats exposed to varying doses of CdCl2 80-90% of the cadmium content present in the fresh tissue is retained if these livers are fixed with a neutral or acid formalin fixative. Cadmium assays during different stages of the staining procedure for protein bound disulphides show the ability of this staining to demonstrate cadmium thiolate clusters next to disulphides. The methods described may also be useful in gaining more insight in the mechanism involved in fixation and staining procedure of some other metals.