Nina N. Belcheva

Cadmium-Induced Oxidative Stress in the Bivalve Mollusk Modiolus modiolus

Cadmium-induced oxidative stress in the bivalve Modiolus modiolus is studied from the standpoint of the universality of the mechanism of free-radical oxidation. The kinetics of cadmium accumulation by the bivalve was revealed in a laboratory experiment. The gills accumulated higher Cd levels than the digestive gland. In the process of cadmium accumulation, there was an increase in lipid peroxidation products (malondialdehyde and lipofuscin) and a reduction in the total oxiradical scavenging capacity (TOSC). Cadmium induces oxidative stress in molluscan tissues through damage to the antioxidation system. Thus, TOSC can provide a useful biochemical indicator of early pathological changes in the cell or the organism, as well as of the environmental effects of heavy metal pollution.

The adaptation of mussels Crenomytilus grayanus to cadmium accumulation result in alterations in organization of microsomal enzyme-membrane complex (non-specific phosphatase).

The kinetic parameters (V(m), K(m) and slope) of membrane-bound microsomal non-specific phosphatase (NPase, with G6P as the substrate) from the digestive gland of unexposed and cadmium adapted (45 days for 100 µg Cd(2+)/l) mussels were investigated. In vivo and in vitro approaches were used. Adaptation of mussels (Crenomytilus grayanus) to cadmium resulted in a 1.6-fold increase in NPase activity. V(m) was increased by 1.6-fold, but K(m) was the same in terms of enzyme kinetics. This indicates that the total concentration of the enzymes in the digestive gland increased. Cd(2+) (1 mM) did not significantly alter the activity of the membrane-bound enzyme in vitro both for unexposed and for cadmium adapted mussels, meaning that cadmium ions are not a direct inhibitor of the membrane-bound enzyme in this concentration. The microsomal NPase activity in both unexposed and cadmium adapted mussels was inhibited by in vitro solubilization of microsomes with non-ionic detergent (Triton X100, 0.01%). This inhibition was uncompetitive for microsomes of unexposed mussels (K(m) decreased 3.1-fold). The most drastic events were observed in cadmium adapted mussels, where inhibition was mixed (K(m) decreased 7.2-fold). The simultaneous actions of detergent and cadmium ions did not alter NPase activity significantly in comparison with action of the detergent alone. The differences in the types and the extents of inhibition of the enzymes activity by membrane disordering agent (Triton X100) indicated that the enzyme-membrane complex (NPase) has been altered as a result of adaptation of mussels to cadmium accumulation. We conclude that the mussels produced a new enzyme-membrane complex, with the same K(m) as the previous complex, but with other detergent sensitivity and greater amounts. Thus, the adaptation capacity of this enzyme is reduced as result of adaptation of mussels to cadmium accumulation.

Using Heavy Metal Content and Lipid Peroxidation Indicators in the Tissues of the Mussel Crenomytilus grayanus for Pollution Assessment After Marine Environmental Remediation.

We examined the effects of environmental remediation on the heavy metal concentration and lipid peroxidation activity in the digestive gland and gills of the marine mussel Crenomytilus grayanus. Changes in heavy metal concentrations and lipid peroxidation biomarkers in the tissues of mussels collected at a contaminated site were compared with those obtained from a reference site. Prior to remediation the concentration of Pb, Cu, Cd, Fe and Zn and the levels of malondialdehyde, conjugated dienes and lipofuscin in mussels collected from the contaminated site were significantly increased compared with those obtained from the reference site. Three years after remediation, these parameters did not significantly exceed the reference site parameters, except Pb, whose concentration, though markedly decreased, yet was much higher than in tissues of mussels from the reference site.

Glutathione S-transferase activity in marine invertebrates from Peter the Great Bay in the Sea of Japan

A total of 23 species of marine invertebrates from Peter the Great Bay belonging to five taxonomic groups were screened for the activity of glutathione S-transferase, which is one of the main enzymes for the detoxification of organic xenobiotics. Significant variation of the enzyme activity (7.1 to 36900 nmol/min/mg of protein) was found among the different groups of organisms. The possible reasons for the differences in enzyme activity are discussed based on the example of bivalve mollusks.

Metallothionein-like proteins induced by cadmium stress in the scallop Mizuhopecten yessoensis

Organisms have evolved a cellular response called stress protein response that increases their tolerance in adverse environmental conditions. Well known stress proteins that bind essential and toxic metals are metallothionein (MT). The scallop Mizuhopecten yessoensis is the most interesting organism because it is able to accumulate toxic cadmium in its digestive gland. However, in the tissue of the digestive gland of Mizuhopecten yessoensis MT (metallothioneins) have not been found. Eastern scallops, Mizuhopecten yessoensis, were collected from two locations — one clean and one polluted site. The concentrations of cadmium (Cd), copper (Cu) and zinc (Zn) were measured in the digestive gland. There was a significant increase in Cd concentrations in this studied tissue. We found that in the presence of cadmium Mizuhopecten yessoensis can induce high molecular proteins. The results of experiments have shown that Cd-binding ligands have a number of properties similar to MT: acetone and temperature stability; the ability to bind some metals, including Cd, Cu and Zn. Protein chromatography (FPLC, Superosa 12) from the digestive gland of scallop M. yessoensis has shown that cadmium is associated with high molecular weight Cd-binding proteins (72 kDa and 43 kDa). The major cadmium-binding protein 72 kDa is glycoprotein. In experiments we have demonstrated that Cd-binding proteins can be induced when there is cadmium exposure. The results of this study strongly suggest that the far eastern scallop Mizuhopecten yessoensis has a unique and well-developed system for the detoxification of heavy metals and it allows for biochemical systems to be maintained in a relatively stable manner in the presence of heavy metals.

Seasonal Variability in the Levels of Fe, Zn, Cu, Mn, and Cd in the Hepatopancreas of the Japanese Scallop Mizuhopecten yessoensis

The relationships between concentrations (μg/g of tissue) and content (μg/organ) of the metals Fe, Zn, Cu, Mn, and Cd with seasonal changes in the weight of the hepatopancreas in the Japanese scallop Mizuhopecten yessoensis are studied. It is shown that, during a year, the weight of the hepatopancreas in the mollusk shows significant (more than twofold) changes. A positive correlation is revealed between the seasonal changes in the concentrations of the physiologically important metals Fe, Zn, Cu, and Mn and the weight of this organ, although no general consistent pattern of seasonal variability in the metal concentrations is found. For toxic Cd, a negative correlation is found between its concentration and the weight of the hepatopancreas.

The antioxidant system of the Gray’s mussel Crenomytilus grayanus (Dunker, 1853) and the Japanese scallop Mizuhopecten yessoensis (Jay, 1857) (Mollusca: Bivalvia)

This study examines the parameters of the antioxidant system (the activities of antioxidant enzymes: superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, as well as the levels of reduced glutathione and integral antioxidant activity) in the digestive gland, gills, gonads, and adductor muscles of two bivalve species: the mussel Crenomytilus grayanus (Dunker, 1853), which lives attached to the substrate, and the Japanese scallop Mizuhopecten yessoensis (Jay, 1857), which is capable of free movement. Species- and tissue-specific differences of the antioxidant system of the investigated mollusks are discussed in connection with their physiological and biochemical peculiarities.