Multiple isoelectric variants of copper, zinc-superoxide dismutase from rat liver

Antioxidant enzymatic defenses and oxidative damage in Dentex dentex fed on different dietary macronutrient levels

A wide range of antioxidant mechanisms are present in fish maintaining an adequate "oxidative balance". When this balance tilts in favor of the oxidant agents "oxidative stress" arises with detrimental effects in molecules of great biological importance. Little has been reported about the influence of different dietary energy sources on antioxidant defenses in fish. The influence of different dietary macronutrient combinations on the key antioxidant enzyme activity, the oxidative damage to lipids and proteins and the possible modifications in the SOD isoenzymatic pattern were evaluated in liver, white muscle, heart and erythrocytes of common dentex (Dentex dentex). Four experimental diets with different protein:lipid:carbohydrate ratios (43/16/28; 43/24/4; 38/19/28 and 38/24/13) were formulated. In general, neither different dietary macronutrient levels nor the interaction among them induces substantial modifications in enzymatic antioxidant defense mechanisms. Two constitutive SOD isoforms, CuZn-SOD I and Mn-SOD, were detected in the tissues analyzed in all experimental groups, independently of diet formulation, but, a third SOD isoenzyme, CuZn-SOD II seems to be induced in white muscle by higher dietary protein levels. Densitometric analyses of western blotting membranes revealed higher CuZn-SOD expression in the heart of dentex fed on lower dietary protein levels, although these differences did not correlate with the SOD activity. Finally, a direct relation exists between the lipid or protein intake level and occurrence of oxidative damage in different tissue components.

cDNA cloning, high-level expression, purification, and characterization of an avian Cu,Zn superoxide dismutase from Peking duck

As a special species of avian, Peking duck is often used as a model for exploring effective factors against cardio-cerebrovascular diseases, and therefore investigations of antioxidant enzymes including superoxide dismutase are intriguing. By using 3(')-RACE with a gene-specific primer, a cDNA encoding duck Cu,Zn SOD was amplified from the total RNA extracted from Peking duck liver. Three free cysteine residues are found in the deduced amino acid sequence of duck SOD, among which Cys153 at the carbonyl-terminal is a distinctive feature. Production with a high yield of recombinant duck Cu,Zn SOD was achieved in Escherichia coli after the reconstituted expression vector pET-3a-dSOD was transformed into the bacterial strain BL21(DE3)pLysS. After two steps of anion exchange chromatography, a great quantity of the purified enzyme (100mg/L fermented culture) with an enzymatic activity comparable to that of native duck and bovine SOD was finally obtained. Duck SOD is a homodimer with 153 residues for each subunit. The molecular mass of the recombinant enzyme is 15,540.0Da measured by mass spectrum, which well coincides with the estimated size of the sequence but significantly differs from that of the native counterpart. Five charge isomers were observed on isoelectricfocusing (IEF). The most interesting observation is that the thermal stability of duck SOD is much lower than that of the bovine enzyme as revealed by irreversible heat inactivation at 70 degrees C. These properties are discussed in relation to the distinctive free Cys residues in duck Cu,Zn SOD.

Regional and ultrastructural immunolocalization of copper-zinc superoxide dismutase in rat central nervous system

We examined the distribution of copper-zinc superoxide dismutase (CuZnSOD) in adult rat central nervous system by light and electron microscopic immunocytochemistry, using an affinity-purified polyclonal antibody. The enzyme appeared to be exclusively localized in neurons. No immunoreactivity was seen in non-neuronal cells. The staining intensity was variable, depending on the brain region and, within the same region, on the neuron type. Highly immunoreactive elements included cortical neurons evenly distributed in the different layers, hippocampal interneurons, neurons of the reticular thalamic nucleus, and Golgi, stellate, and basket cells of the cerebellar cortex. Other neurons, i.e., pyramidal cells of the neocortex and hippocampus, Purkinje and granule cells of the cerebellar cortex, and the majority of thalamic neurons, showed much weaker staining. In the spinal cord, intense CuZnSOD immunoreactivity was present in many neurons, including motor neurons. Pre-embedding immunoelectron microscopy of the neocortex, hippocampus, reticular thalamic nucleus, and cerebellar cortex showed cytosolic and nucleoplasmic labeling. Moreover, single membrane-limited immunoreactive organelles identified as peroxisomes were often found, even in neurons that appeared weakly stained at the light microscopic level. In double immunogold labeling experiments, particulate CuZnSOD immunoreactivity co-localized with catalase, a marker enzyme for peroxisomes, thus demonstrating that in neural tissue CuZnSOD is also present in peroxisomes.

Amino acid sequence of chicken Cu, Zn-containing superoxide dismutase and identification of glutathionyl adducts at exposed cysteine residues

The copper, zinc-containing superoxide dismutase electromorphs from chicken erythrocytes have been isolated, their complete amino acid sequence determined and the identity of the protein moieties established. All electromorphs are constituted by a polypeptide chain made of 153 amino acid residues, corresponding to a molecular mass of 15,598 Da. Accurate molecular mass determination by electrospray mass spectrometry of the separated electromorphs unequivocally proved that, in the chicken superoxide dismutase, either one or two cysteine residues/subunit are involved in a mixed disulfide with glutathione. The same post-translational modification has been proven to occur in human superoxide dismutase. A different rate of S-thiolation by endogenous glutathione was also demonstrated to be responsible for charge heterogeneity in cells. Effect of this modification on the catalytic and molecular properties of superoxide dismutases, and possible mechanisms for the S-thiolation process, were also investigated and discussed.

Copper-dependent metabolism of Cu,Zn-superoxide dismutase in human K562 cells. Lack of specific transcriptional activation and accumulation of a partially inactivated enzyme

The regulation of Cu,Zn-superoxide dismutase by copper was investigated in human K562 cells. Copper ions caused a dose- and time-dependent increase, up to 3-fold, of the steady-state level of Cu,Zu-superoxide dismutase mRNA. A comparable increase was also observed for actin and ribosomal protein L32 mRNAs, but not for metallothionein mRNA which was augmented more than 50-fold and showed a different induction pattern. The copper-induced mRNAs were actively translated as judged from their enhanced loading on polysomes, the concomitantly increased cellular protein levels and an augmented incorporation of [3H]lysine into acid-precipitable material. Cu,Zn-superoxide dismutase protein followed this general trend, as demonstrated by dose- and time-dependent increases in immunoreactive and enzymically active protein. However, a specific accumulation of Cu,Zn-superoxide dismutase was noticed in cells grown in the presence of copper, that was not detectable for other proteins. Purification of the enzyme demonstrated that Cu,Zn-superoxide dismutase was present as a reconstitutable, copper-deficient protein with high specific activity (kcat./Cu = 0.89 x 10(9) M-1.s-1) in untreated K562 cells and as a fully metallated protein with low specific activity (kcat./Cu = 0.54 x 10(9) M-1.s-1) in copper-treated cells. Pulse-chase experiments using [3H]lysine indicated that turnover rates of Cu,Zn-superoxide dismutase in K562 cells were not affected by growth in copper-enriched medium, whereas turnover of total protein was significantly enhanced as a function of metal supplementation. From these results we conclude that: (i) unlike in yeast [Carrì, Galiazzo, Ciriolo and Rotilio (1991) FEBS Lett. 278, 263-266] Cu,Zn-superoxide dismutase is not specifically regulated by copper at the transcriptional level in human K562 cells, suggesting that this type of regulation has not been conserved during the evolution of higher eukaryotes; (ii) copper ions cause an inactivation of the enzyme in intact K562 cells; and (iii) the metabolic stability of Cu,Zn-superoxide dismutase results in its relative accumulation under conditions that lead to increased protein turnover.

The primary structure of turtle Cu,Zn superoxide dismutase. Structural and functional irrelevance of an insert conferring proteolytic susceptibility

A copper,zinc superoxide dismutase, has been isolated from the marine turtle Caretta caretta and the complete amino acid sequence obtained. The sequence was determined by isolation and analysis of peptides obtained after cleavage of the carboxymethylated apoenzyme with trypsin or Staphylococcus aureus protease. Turtle superoxide dismutase consists of 166 amino acid residues, which represents the largest number to date for a cytosolic copper,zinc superoxide dismutase. The comparison of this amino acid sequence with that of bovine superoxide dismutase revealed a one-residue C-terminal extension, two single residue insertions and a 12-residue insertion in the N-terminal region, in turtle superoxide dismutase. The new segment consists of a threefold repeating sequence and was found to be the site for selective proteolytic attack by trypsin under native conditions. The biochemical characteristics, the spectroscopic and catalytic properties as well as the thermal stability and the resistance to irreversible denaturation, were carefully examined and were very similar to those of other superoxide dismutases. These results indicate that the presence of the new polypeptide segment does not affect the main folding of the chain and the quaternary structure, nor the functional parameters of turtle superoxide dismutase. The possibility that the new insert constitutes a loop excluded from the protein scaffold providing the framework of the active site is also discussed.

Purification of Cu, Zn-superoxide dismutase isoenzymes from fish liver: appearance of new isoforms as a consequence of pollution

Liver cell-free extracts of fish (Mugil sp.) from polluted environments show new Cu,Zn-SOD isoenzymes when analyzed by polyacrylamide gel electrophoresis or isoelectrofocusing followed by in situ staining for SOD activity. The most active isoenzymes, with pI 6.1 and 5.1, were present both in control and problem samples while the isoenzymes of intermediate pI value showed significant differences. Fish from control areas showed three intermediate isoenzymes with pI 5.7, 5.5 and 5.4 (the last one quite faint) while polluted animals showed three bands of pI 5.9, 5.45 and 5.35, this last very intense. To further characterize their utility as biomarkers, Cu,Zn-SOD isoenzymes from polluted fish livers were purified to homogeneity. Five superoxide dismutase peaks were purified, named thereafter I (pI 6.1) to V (pI 5.1) respectively. Isoenzymes I and V displayed the highest specific activity. Upon incubation with moderate H2O2 concentrations, pure isoenzyme I yielded more acidic bands with pI 5.5, 5.45 and 5.35, this last being predominant. The pure isoenzyme V generated only a new band of pI 5.0. Concomitant with oxidation, the activity of peaks I and V was lost in a H2O2 concentration-dependent manner. The pattern of the new acidic bands generated upon the oxidixing treatment of isoenzyme I closely resembles that observed in crude extracts from polluted animals.

Characterization of the heterogeneity of polyethylene glycol-modified superoxide dismutase by chromatographic and electrophoretic techniques

Covalent attachment of polyethylene glycol (PEG) chains to the enzyme Cu,Zn-superoxide dismutase (SOD) produces a heterogeneous mixture of modified protein species. The heterogeneity of the product (PEG-SOD) derives from a variable stoichiometric combination of PEG with individual SOD molecules in addition to the polydispersity of the PEG reagent. Characterization of PEG-SOD presents significant challenges due in part to this heterogeneity in addition to the hybrid nature of the modified enzyme. The application of classical methods of protein characterization is not always successful for these PEG-proteins requiring the development of alternative or modified procedures. A series of chromatographic techniques including reversed-phase, ion-exchange, size-exclusion, and hydrophobic interaction high-performance liquid chromatography along with electrophoretic techniques including isoelectric focusing, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and capillary zone electrophoresis have been developed for assessing the degree of heterogeneity of PEG-SOD samples which encompass a range of different stoichiometries. Examples will be given demonstrating the application of these techniques to characterize PEG-SOD samples of different composition produced during the course of the reaction between SOD and an activated PEG reagent.