Conformational changes and inactivation of rabbit muscle creatine kinase in dimethyl sulfoxide solutions.

The effects of dimethyl sulfoxide (DMSO) on creatine kinase (CK) conformation and enzymatic activity were studied by measuring activity changes, aggregation, and fluorescence spectra. The results showed that at low concentrations (< 65% v/v), DMSO had little effect on CK activity and structure. However, higher concentrations of DMSO led to CK inactivation, partial unfolding, and exposure of hydrophobic surfaces and thiol groups. DMSO caused aggregation during CK denaturation. A 75% DMSO concentration induced the most significant aggregation of CK. The CK inactivation and unfolding kinetics were single phase. The unfolding of CK was an irreversible process in the DMSO solutions. The results suggest that to a certain extent, an enzyme can maintain catalytic activity and conformation in water-organic mixture environments. Higher concentrations of DMSO affected the enzyme structure but not its active site. Inactivation occurred along with noticeable conformational change during CK denaturation. The inactivation and unfolding of CK in DMSO solutions differed from other denaturants such as guanidine, urea, and sodium dodecyl sulfate. The exposure of hydrophobic surfaces was a primary reason for the protein aggregation.     

Effects of lactic acid and NaCl on creatine kinase from rabbit muscle.

The lactic acid induced unfolding and the salt-induced folding of creatine kinase (CK) were studied by enzyme activity, fluorescence emission spectra, circular dichroism spectra, and native polyacrylamide gel electrophoresis. The results showed that the kinetics of CK inactivation was a monophase process. Lactic acid caused inactivation and unfolding of CK with no aggregation during CK denaturation. The unfolding of the whole molecule and the inactivation of CK in solutions of different concentration of lactic acid were compared. Much lower lactic acid concentration values were required to bring about inactivation than were required to produce significant conformational changes of the enzyme molecule. At higher concentrations of lactic acid (more than 0.2 mM) the CK dimers were partially dissociated, as proved by native polyacrylamide gel electrophoresis. NaCl induced the molten globule state with a compact structure after CK was denatured with 0.8 mM lactic acid, and the increasing of anions led to a tight side-chain. The above results suggest that the effect of lactic acid differed from that of other denaturants such as guanidine hydrochloride, HCI, or urea during CK folding, and the molten globule state indicates that intermediates exist during CK folding.     

Effects of aspartate on rabbit muscle creatine kinase and the salt induced molten globule state

The aspartate (Asp)-induced unfolding and the salt-induced folding of creatine kinase (CK) have been studied by measuring enzyme activity, fluorescence emission spectra, circular dichroism (CD) spectra, native polyacrylamide gel electrophoresis and ultraviolet difference spectra. The results showed that Asp caused inactivation and unfolding of CK, with no aggregation during CK denaturation. The kinetics of CK unfolding followed a one phase process. At higher concentrations of Asp (>2.5mM), the CK dimers were partially dissociated. Inactivation occurred before noticeable conformational change during CK denaturation. Asp denatured CK was mostly reactivated and refolded by dilution. KCl induced the molten globule state with compact structure after CK was denatured with 10mM Asp. These results suggest that the effect of Asp differed from that of other denaturants such as guanidine, HCl or urea during CK unfolding. Asp is a reversible protein denaturant and the molten globule state indicates that intermediates exist during CK folding.     

Lead ion effect on creatine kinase: equilibrium and kinetic studies of inactivation and conformational changes

The effects of lead ions on creatine kinase (CK) were studied by measuring activity changes, intrinsic fluorescence spectra and 8-anilo-1-naphthalenesulfonate (ANS)-binding fluorescence along with size-exclusion chromatography (SEC). Below 5 mM Pb(2+) concentration, there was nearly no change of the enzyme activity and a slight change of the ANS-binding fluorescence. The CK activity decreased significantly from 10 to 25 mM Pb(2+) concentrations. No residual activity was observed above 25 mM Pb(2+). The kinetic time courses of inactivity and unfolding were all mono-phase courses with the inactivation rate constants being greater than the unfolding rate constants for the same Pb(2+) concentration. The changes in fluorescence maximum and fluorescence intensity were relatively slow for 40-80 mM Pb(2+) as well as in the initial stage for less than 5 mM Pb(2+), showing that two transition states exist for Pb(2+) induced equilibrium-unfolding curves. The intrinsic fluorescence spectra and ANS-binding fluorescence measurements showed that even for high Pb(2+) concentrations, CK did not fully unfold. Additionally, the SEC results showed that the enzyme molecule still existed in an inactive dimeric state at 20 and 40 mM Pb(2+) solutions. All the results indicated the presence of at least one stable unfolding equilibrium intermediate of CK during Pb(2+) unfolding.     

Inactivation and conformational changes of creatine kinase at low concentrations of hexafluoroisopropanol solutions.

Using the methods of far-ultraviolet circular dichroism (CD) spectra, fluorescence spectra, and enzyme activity assays, the inactivation and conformational changes of creatine kinase (CK) induced by 1,1,1,3,3,3-hexafluoro-2-propanol (hexafluoroisopropanol (HFIP)) of different concentrations were investigated. To avoid the aggregation of CK that occurs with high HFIP, concentrations of 0%-5% HFIP were used in this study. The CD spectra showed that HFIP concentrations above 2.5% strongly induced the formation of secondary structures of CK. No marked conformational changes were observed at low concentrations of HFIP (0%-2.5%). After incubation with 0.2% HFIP for 10 min, CK lost most of its activity. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity described previously by Tsou was applied to study the kinetics of CK inactivation during denaturation by HFIP. The inactivation rate constants for the free enzyme and the substrate-enzyme complex were determined by Tsou's method. The results suggested that low concentrations of HFIP had a high potential to induce helices of protein and that the active site of the enzyme was situated in a limited and flexible region of the enzyme molecule that was more susceptible to the denaturant than was the protein as a whole.     

Effects of zinc occupancy on human O6-alkylguanine-DNA alkyltransferase

A recent crystallographic study of recombinant human O(6)-alkylguanine-DNA alkyltransferase (hAGT) revealed a previously unknown zinc atom [Daniels et al., (2000) EMBO J. 19, 1719-1730]. The effects of zinc on the properties of hAGT are reported here. In bacterial expression systems, recombinant hAGT was produced in increasingly larger quantities when growth media are supplemented with up to 0.1 mM ZnCl(2). Metal-enriched hAGT samples had a 5-fold increase in repair rate constant over conventionally purified protein samples and a 60-fold increase over metal-stripped hAGT. In addition, mutants of the zinc-binding residues had decreases in zinc occupancy that correlated with reductions in repair rate. Zinc modulation did not abolish the repair capacity of a fraction of the hAGT population, as evidenced by the stoichiometric reaction with an oligodeoxyribonucleotide substrate. Zinc occupancy had a similar effect on the rate of reaction with O(6)-benzylguanine, a free base substrate, as on the repair of methylated DNA. Differentially zinc-treated hAGTs showed the same affinity for binding to native DNA and substrate oligodeoxyribonucleotides. Metal content manipulations had little effect upon the CD spectrum of hAGT, but fluorescence studies revealed a small conformational change based upon metal binding, and zinc occupancy correlated with enhanced hAGT stability as evidenced by resistance to the denaturing effects of urea. These results indicate that the presence of zinc confers a mechanistic enhancement to repair activity that does not result from an increase in substrate binding affinity. Zinc also provides conformational stability to hAGT that may influence its regulation.     

EARLY TOXIC EFFECTS OF ZINC ON PSII OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE)1

Zinc toxicity on photosynthetic activity in cells of Synechocystis aquatilis f. aquatilis Sauvageau was investigated by monitoring Hill activity and fluorescence. The oxygen‐evolving activity decreased to about 80% of the initial value after exposure to 0.1 mM ZnSO₄ for 1 h. The PSII activity was inhibited by 40% in the presence of zinc concentrations ranging from 0.5 to 5.0 mM, suggesting that the metal effect is limited by zinc uptake. The fluorescence capacity (F_max–F/F_max) decreased from 0.57 to 0.35 and 0.20 in Zn‐treated cells for 15 and 60 min, respectively, thus providing evidence for rapid inactivation of electron transport at PSII. Zinc treatment promoted a rapid increase in PSII fluorescence that was counteracted by addition of 1,4‐benzoquinone, indicating that electron transfer at the reducing side of the PSII reaction center is arrested by zinc. Furthermore, a decline in the fluorescence yield could be observed after 1 h of zinc treatment as well as when Zn‐treated cells were excited in presence of 3‐(3′,4′‐dichlorophenyl)‐1,1‐dimethylurea. Under these conditions, zinc did not affect energy transfer from phycobilisomes to PSII, and the gradual quenching of PSII fluorescence may be due to a decrease in electron flow on the donor side of PSII. However, the 20% increase in the minimal fluorescence intensity (F_o) in parallel to the absence of changes in the maximal fluorescence intensity (F_max), observed in the first hour of zinc treatment, could also suggest a metal‐induced decline in the energy transfer from PSII‐chl a antenna to the PSII reaction center.     

Methylamine-induced conformational change of alpha 2-macroglobulin and its zinc (II) binding capacity. An X-ray scattering study

Methylamine induces a conformational change of alpha 2-macroglobulin which is very similar to that obtained by proteinase reaction and binding. This was shown by small-angle X-ray scattering at 21 degrees C in 0.03 M Hepes buffer of pH 8.0 containing 0.15 M NaCl and 0.3 mM EDTA. When alpha 2-macroglobulin reacts with methylamine the side maximum virtually disappears from the X-ray scattering curve and the radius of gyration decreases from 7.8 nm to 7.2 nm. The X-ray data of alpha 2-macroglobulin are consistent with an open shape model similar to that deduced via electron micrographs [Schramm, H. J. and Schramm, W. (1982) Hoppe-Seyler's Z. Physiol. Chem. 363, 803-812]; one projection of the model resembles the letter H; the four subunits are mainly represented as elliptical cylinders which are connected via a central, quite flat cylinder. Zinc(II) ions cause aggregation of alpha 2-macroglobulin even at such a low total zinc concentration as 12.5 microM; for 25 microM zinc(II) concentration, the average molecular mass indicates that the aggregation goes beyond the dimeric stage. Monomeric species of alpha 2-macroglobulin appear to have the capacity specifically to bind 8.0 zinc(II) ions per molecule, which corresponds to two zinc(II) ions per subunit.     

Inactivation and conformational changes of lactate dehydrogenase from porcine heart in sodium dodecyl sulfate solutions

The inactivation and conformational changes of porcine heart lactate dehydrogenase (LDH) have been studied in sodium dodecyl sulfate (SDS) solutions. Increasing SDS concentration led to a quick and concentration-dependent inhibition of the enzyme, with complete inactivation within 5 min in the presence of 1.0 mM SDS. Meanwhile, fluorescence emission and circular dichroism spectra were used to follow the conformational changes of the enzyme during this process, concurrently showing that SDS less than 1.0 mM induced only limited conformational changes to LDH. The above results are in accordance with the suggestion by Tsou (Trends Biochem. Sci. 11 (1986) 427; Science 262 (1993) 380) that the active site usually be more flexible than the enzyme molecule as a whole. Furthermore, the results of polyacrylamide gel electrophoresis (PAGE) implied that unfolding intermediates were presented in the above process. When the SDS concentration used to treat LDH was increased, the bands of native enzyme on native PAGE faded and finally almost disappeared. Meanwhile, multiple bands with lower mobility but no activity emerged behind and enhanced correspondingly. Fast protein liquid chromatography indicated that dissociation occurred during the course of denaturation. The reasons for the above phenomena have been discussed. It was suggested that SDS, binding to LDH to form different LDH-SDS complexes, conferred an array of different unfolding states over the enzyme, and in turn resulted in the formation of the multiple bands on the native PAGE.     

Effects of arginine on rabbit muscle creatine kinase and salt-induced molten globule-like state

The arginine (Arg)-induced unfolding and the salt-induced folding of creatine kinase (CK) have been studied by measuring enzyme activity, fluorescence emission spectra, native polyacrylamide gel electrophoresis and size exclusion chromatography (SEC). The results showed that Arg caused inactivation and unfolding of CK, but there was no aggregation during CK denaturation. The kinetics of CK unfolding followed a one-phase process. At higher concentrations of Arg (>160 mM), the CK dimers were fully dissociated, the alkali characteristic of Arg mainly led to the dissociation of dimers, but not denaturation effect of Arg's guanidine groups on CK. The inactivation of CK occurred before noticeable conformational changes of the whole molecules. KCl induced monomeric and dimeric molten globule-like states of CK denatured by Arg. These results suggest that as a protein denaturant, the effect of Arg on CK differed from that of guanidine and alkali, its denaturation for protein contains the double effects, which acts not only as guanidine hydrochloride but also as alkali. The active sites of CK have more flexibility than the whole enzyme conformation. Monomeric and dimeric molten globule-like states of CK were formed by the salt inducing in 160 and 500 mM Arg H(2)O solutions, respectively. The molten globule-like states indicate that monomeric and dimeric intermediates exist during CK folding. Furthermore, these results also proved the orderly folding model of CK.     

Effects of zinc on the activity and conformational changes of arginine kinase and its intermediate

The effects of zinc on arginine kinase and its collapsed-state intermediate were studied. Both arginine kinase and the collapsed-state intermediate were inactivated in the presence of zinc, following a biphasic kinetic course. The corresponding apparent rate constants of inactivation at different zinc concentrations and conformational changes in the presence of 0.5 mM zinc were obtained. The conformational changes of arginine kinase and the collapsed-state intermediate were followed by fluorescence spectra and circular dichroism spectra. Comparison of the results for arginine kinase and the collapsed-state intermediate showed that the collapsed-state intermediate was more susceptible to zinc, which indicated that the collapsed-state intermediate was more flexible and unstable than arginine kinase. The special structure of arginine kinase might explain these diverse phenomena.     

Quasi-elastic light-scattering studies of conformational states of the H,K-ATPase. Intervesicular aggregation of gastric vesicles by disulfide cross-linking

The particle size of hog gastric vesicles which contain H,K-ATPase was measured by using the method of quasi-elastic light scattering. The size of control vesicles is homogeneous as judged from its low polydispersity index. When the vesicles were treated with copper(II) o-phenanthroline (CuP), intervesicular S-S cross-linking occurred as determined by the aggregated vesicle size. The aggregation to divesicle size occurred very quickly, within 30 s, and the extent of aggregation did not depend on the extent of inactivation if the inactivation was not more than about 30%. Blocking of SH groups by 5,5'-dithiobis(2-nitrobenzoic acid) in the presence of Mg2+ prevented CuP-induced vesicular aggregation but not inactivation, indicating that S-S cross-linking rather than enzyme inactivation is the primary cause of vesicular aggregation. The presence of Mg2+ was required for the occurrence of aggregation. Nucleotides such as ADP (K0.5 = 5 microM) and 5'-adenylyl imidodiphosphate (K0.5 = 50 microM) inhibited the aggregation induced by 50 microM CuP plus 2 mM Mg2+ in a dose-dependent manner. Furthermore, K+ antagonized the effects of nucleotides. The extent of aggregation increased as the pH decreased in the pH range 6.1-7.4. Virtually no cross-linking occurred at alkaline pH (e.g., pH 8-9). These data show that vesicular aggregation can be assumed to reflect the conformational state of the responsible SH group in the native enzyme.     

Zinc can influence ornithine decarboxylase activity in rat thymus cells

Summary The thymus of young rats contained a high basal activity of ornithine decarboxylase (ODC). Treatment with zinc sulphate caused a slight increase of thymic ODC activity within 6 hours and a more marked enhancement (three-fold) in the spleen 24 h after treatment. In spite of the high activity of thymic ODCin vivo, ODC was not detectable in primary cultures of rat thymocytes, but was early and largely induced after treatment with Concanavalin A (Con A). The presence of 0.1 mM zinc in the medium increased the response of ODC to Con A. This effect of zinc in mitogen activated thymocytes may be due to the stabilization of ODC, which was found to decay with a half life of 65 min after the block of protein synthesis with cycloheximide. On the contrary in absence of zinc the half life of the enzyme was 40 min, as in the rat thymus in vivo.Zinc alone, at 0.1 mM concentration, did not affect ODC activity in resting thymocytes during the early times, but the metal was able to cause an increase of the enzyme activity after 4–6 days of culture. Other heavy metals such as mercury, cadmium and copper provoked a late increase of ODC activity, but their action was evident only at dosages which were toxic for the cells.     

SDS-induced conformational changes and inactivation of the bacterial chaperonin GroEL

The inactivation and conformational changes of the bacterial chaperonin GroEL have been studied in SDS solutions with different concentrations. The results show that increasing the SDS concentration caused the intrinsic fluorescence emission intensity to increase and the emission peak to slightly blue-shift, indicating that increasing the SDS concentration can cause the hydrophobic surface to be slightly buried. The changes in the ANS-binding fluorescence with increasing SDS concentration also showed that the GroEL hydrophobic surface decreased. At low SDS concentrations, less than 0.3 mM, the GroEL ATPase activity increased with increasing SDS concentration. Increasing the SDS concentration beyond 0.3 mM caused the GroEL ATPase activity to quickly decrease. At high SDS concentrations, above 0.8 mM, the residual GroEL ATPase activity was less than 10% of the original activity, but the GroEL molecule maintained its native conformation (as indicated by the exposure of buried thiol groups, electrophoresis, and changes of CD spectra). The above results suggest that the conformational changes of the active site result in the inactivation of the ATPase even though the GroEL molecule does not markedly unfold at low SDS concentrations.     

Kinetics of inactivation of green crab (Scylla Serrata) alkaline phosphatase during removal of zinc ions by ethylenediaminetetraacetic acid disodium

Green crab (Scylla Serrata) alkaline phosphatase (EC 3.1.3.1.) is a metalloenzyme, the each active site in which contains a tight cluster of two zinc ions and one magnesium ion. The kinetic theory of the substrate reaction during irreversible inhibition of enzyme activity previously described by Tsou has been applied to a study on the kinetics of the course of inactivation of the enzyme by ethylenediaminetetraacetic acid disodium (EDTA). The kinetics of the substrate reaction with different concentrations of the substrate p-nitrophenyl phosphate (PNPP) and inactivator EDTA suggested a complexing mechanism for inactivation by, and substrate competition with, EDTA at the active site. The inactivation kinetics are single phasic, showing the initial formation of an enzyme-EDTA complex is a relatively rapid reaction, followed a slow inactivation step that probably involves a conformational change of the enzyme. Zinc ions are finally removed from the enzyme. The presence of metal ions apparently stabilizes an active-site conformation required for enzyme activity.     

Conformational changes of lysozyme refolding intermediates and implications for aggregation and renaturation

It is believed that denatured-reduced lysozyme rapidly forms aggregates during refolding process, which is often worked around by operating at low protein concentrations or in the presence of aggregation inhibitors. However, we found that low concentration buffer alone could efficiently suppress aggregation. Based on this finding, stable equilibrium intermediate states of denatured-reduced lysozyme containing eight free SH groups were obtained in the absence of redox reagents in buffer of low concentrations alone at neutral or mildly alkaline pH. Transition in the secondary structure of the intermediate from native-like to beta-sheet was observed by circular dichroism (CD) as conditions were varied. Dynamic light scattering and ANS-binding studies showed that the self-association accompanied the conformational change and the structure rich in beta-sheet was the intermediate state for aggregation, which could form either amyloid protofibril or amorphous aggregates under different conditions as detected by Electron Microscopy. Combining the results obtained from activity analysis, RP-HPLC and CD, we show that the activity recovery was closely related to the conformation of the refolding intermediate, and buffer of very low concentration (e.g. 10mM) alone could efficiently promote correct refolding by maintaining the native-like secondary structure of the intermediate state. This study reveals reasons for lysozyme aggregation and puts new insights into protein and inclusion body refolding.     

A preferential inhibition by Zn2+ on platelet activating factor- and thrombin-induced serotonin secretion from washed rabbit platelets

Zinc ions at micromolar levels exhibited a significant inhibitory activity toward platelet activating factor (AGEPC)- and thrombin-induced serotonin release from washed rabbit platelets. In the ranges from 25 to 30 microM and 10 to 50 microM, respectively, zinc essentially prevented any serotonin release from 1.25 X 10(8) cells/microliter by 1 X 10(-10) M AGEPC and by 0.2 unit thrombin/ml. This inhibition by zinc ions, in micromolar range, occurred in the presence of 1.0 mM Ca2+. The amount of zinc needed for inhibition was inversely proportional to the amount of AGEPC present and further zinc must be added prior to or at the same time as the AGEPC to be effective. Introduction of zinc ions after the AGEPC essentially abolished the inhibitory properties of this divalent cation. Other cations such as Cu2+, La3+, Cd2+, and Mg2+ were ineffective as inhibitors at concentrations where zinc showed its maximal effects. Under conditions similar to those noted above, aggregation induced by AGEPC was blocked only to the extent of 25% of a control. No inhibitory action by zinc on thrombin-induced aggregation was noted. It is apparent that zinc ions influence a site(s) on the rabbit platelet of considerable importance to the activation (or signaling) process by AGEPC and thrombin in these cells, as expressed by serotonin release. Zinc should provide a suitable probe to explore the mechanism of action of these agonists in their interaction with sensitive cells and to define in more specific biochemical terms the putative receptor for these molecules.     

Evidence for a zinc/proton antiporter in rat brain

The data presented in this paper are consistent with the existence of a plasma membrane zinc/proton antiport activity in rat brain. Experiments were performed using purified plasma membrane vesicles isolated from whole rat brain. Incubating vesicles in the presence of various concentrations of 65Zn2+ resulted in a rapid accumulation of 65Zn2+. Hill plot analysis demonstrated a lack of cooperativity in zinc activation of 65Zn2+ uptake. Zinc uptake was inhibited in the presence of 1 mM Ni2+, Cd2+, or CO2+. Calcium (1 mM) was less effective at inhibiting 65Zn2+ uptake and Mg2+ and Mn2+ had no effect. The initial rate of vesicular 65Zn2+ uptake was inhibited by increasing extravesicular H+ concentration. Vesicles preloaded with 65Zn2+ could be induced to release 65Zn2+ by increasing extravesicular H+ or addition of 1 mM nonradioactive Zn2+. Hill plot analysis showed a lack of cooperativity in H+ activation of 65Zn2+ release. Based on the Hill analyses, the stoichiometry of transport may include Zn2+/Zn2+ exchange and Zn2+/H+ antiport, the latter being potentially electrogenic. Zinc/proton antiport may be an important mode of zinc uptake into neurons and contribute to the reuptake of zinc to replenish presynaptic vesicle stores after stimulation.     

Zinc Ions Stabilise the Association of Basic Protein with Brain Myelin Membranes

Myelin basic protein (MBP) dissociated from brain myelin membranes when they were incubated (37 degrees C; pH 7.4) at physiological ionic strength. Zinc ions inhibited, and calcium promoted, this process. Protease activity in the membrane preparations cleaved the dissociated MBP into both small (less than 4 kilodaltons) and large (greater than 8 kilodaltons) fragments. The latter were detected, together with intact MBP, by gel electrophoresis of incubation media. Zinc ions appeared to act in two distinct processes. In the presence or absence of added CaCl2, zinc ions in the range 0.1-1 mM inhibited MBP-membrane dissociation. This process was relatively insensitive to heat and Zn2+ could be substituted by either copper (II) or cobalt (II) ions. A second effect was evident only in the presence of added calcium ions, when lower concentrations of Zn2+ (less than 0.1 mM) inhibited MBP-membrane dissociation and the accumulation of intact MBP in incubation media. This process was heat sensitive and only copper (II), but not cobalt (II), ions could replace Zn2+. To determine whether endogenous zinc in myelin membranes is bound to MBP, preparations were solubilised in buffers containing Triton X-100/2 mM CaCl2 and subjected to gel filtration. Endogenous zinc, as indicated by a dithizone-binding method, eluted with fractions containing both MBP and proteolipid protein (PLP). Thus, one means whereby zinc stabilises association of MBP with brain myelin membranes may be by promoting its binding to PLP.     

Kinetic characterization of Zinc transport process and its inhibition by Cadmium in isolated rat renal basolateral membrane vesicles: In vitro and In vivo studies

We firstly characterized zinc uptake phenomenon across basolateral membrane vesicles (BLMVs) isolated from normal rat kidney. The process was found to be time, temperature, and substrate concentration dependent, and displayed saturability. Zn²⁺ uptake was competitively inhibited in the presence of 2 mM Cd with Ki of 3.9 mM. Zinc uptake was also inhibited in the presence of sulfhydryl reacting compound suggesting involvement of {–}SH groups in the transport process. Further, to elucidate the effect of in vivo Cd on zinc transport in BLMVs, Cd nephrotoxicity was induced by subcutaneous administration of CdCl₂ at dose of 0.6 mg/kg/d for 5 days in a week for 12 weeks. An indolent renal failure developed in Cd exposed rats was accompanied with a significantly high urinary excretion of Cd²⁺, Zn²⁺ and proteins. The histopathology and electron microscopy of kidneys of Cd exposed rats documented changes of proximal tubular degeneration. Notably, Cd content in renal cortex of Cd exposed rats was 215 μg/g tissue that was higher than the critical concentration of Cd in kidneys which was associated with significantly higher Zn and metallothionein (MT) contents. Zinc uptake in BLMVs isolated from kidneys of Cd exposed rats was significantly reduced. Further, kinetic studies revealed that decrease in zinc uptake synchronized with decrease in maximal velocity (V_max) and increase in affinity constant which is suggestive of decreased number of active zinc transporters. Furthermore, conformational modulation of Zn transporter in BLM was further supported by observed variation in transition temperature for zinc transport in BLMVs isolated from Cd-exposed kidney.