Review Paper On Toxicological Implications Of Copper
Copper is an essential trace element required by all living organism and needed for normal human metabolism can be toxic at high concentrations. It exists naturally in the environment in the zero-valent state as copper metal (Cu), ionic copper (Cu+ and Cu 2+) as well as man -made engineered nanoparticles (Cu NP). All these forms have varying degrees of toxicity in various biological organisms. Sources of copper in the environment range from natural ores of copper in the earth’s crust to manufacturing and industrial processes like chemical, steel electroplating and wastewater treatment. Copper is required by all living organisms as a trace element and it is part of several enzymes and electron carriers in oxidative electron transport chain.
Nanoparticles are particles with size range between 1 -100 nm, having different physical, chemical and biological properties from their parent bulk materials making them widely applicable in the healthcare, agriculture and industry.
Toxic Mechanism of Copper
Organisms can protect themselves from metal-induced toxic effects through regulated mechanisms of import and export processes to ensure the maintenance of cellular homeostasis. The disruption of homeostasis during copper induced cellular toxicity occurs mainly through free radical-induced oxidative damage and several other mechanisms. Copper ion induced toxicity can also occur through the characteristic altered lipid metabolism in Wilson disease, changes in hepatic gene expression.
Human Exposure
As previously stated, the toxicity of copper varies between its different forms. The present study illustrates how onematerial (copper) can have a most different in vitro cytotoxicitydepending on its form (oxide, ionic, metal). Nanoparticles (NP) have a high potential to be deposited in the respiratory system where they target both the upper and lower respiratory tract depending on their size. Their ability to be retained in the lungs for a longer period relative to larger particles leads to stronger oxidative stress impact and inflammatory response on the human body. Copper oxide nanoparticles (CuO NP) have a wide application in antimicrobial applications and cosmetic products and are emitted from power stations and during the production of asphalt and rubber tires.
Inhalation has been established as the preferred route of exposure to CuO NPs due to their size characteristics. It was reported by Midander et al. (2009) that the cytotoxicity of CuO NP is more than that of micron-sized particles and dissolved copper ions, producing significantly higher DNA damages and subsequent cell death. Moschini et al (2013) tested the pulmonary toxicity of CuO NP on A549 cells of the human lung tissue. In this study, the cytotoxic effects and cell responses to exogenous CuO NP were used to model cell–particle interactions during cell toxicity.
The in vitro cytotoxicity and genotoxicity of CuO NPs on A549 cells has also been shown by Akhtar et al (2016). CuO NPs were shown to reduce cell viability and induce membrane damage in A549 cells in a dose-dependent manner. Tests for genotoxicity such as the comet assay and the formation of micronucleus also showed a dose-dependent increase in DNA damage and micronucleus induction. There was an observed correlation between the generation of ROS and the induction of micronucleus formation and DNA damage suggesting that oxidative stress might play a role in the genotoxic effect of CuO NPs in human lung cells.
Fate of Copper in the Environment
The unique physical and chemical properties of nanomaterials have led to their recent use in agricultural applications such as the control of plant pathogens due to their ability to bind strongly to pathogen cell wall, ultimately leading to death of the pathogen. Copper-based biopesticides such as a nanogel of copper and chitosan was found to inhibit the growth of plant pathogenic fungus Fusarium graminearum. There is an increased possibility of food chain contamination due to the use of Cu NPs in agriculture. The effect of Cu NPs and bulk copper compounds has been tested on plants such as lettuce (Lactuca sativa) and alfalfa (Medicago sativa) by Hong et al, 2015. A significant reduction in root length occurred in both plants in comparison to the controls and there was more absorption of Cu from Cu NPs treatments at high concentration compared to bulk Cu treatments. The accumulation of nutrients such phosphorus and iron were reduced in both plant tissues while there was an increased accumulation of sulfur in the shoots of alfalfa and in the root of both plant species. The stress enzyme catalase showed a reduced activity in alfalfa shoots while the activity of ascorbate peroxidase was increased in the roots of lettuce and alfalfa from all but two treatments; with Cu NPs having the highest influence thus boosting production of more reactive oxygen species (ROS).
There is an emerging concern of the ecotoxicological impacts of nanoparticles due to their widespread application. It has been predicted that the environmental concentration of Cu NPs suspension in some receiving waters is 0.06 mg Cu/L with a 95% confidence interval of 0.01–0.92 mg Cu/L.
