Abstract
Anthropogenic pollution with heavy metals is an on-going concern throughout the world, and methods to monitor release and impact of heavy metals are of high importance. With a view to probe its suitability as molecular biomarker of metal pollution, this study has determined a coding sequence for metallothionein of the African sharptooth catfish Clarias gariepinus. The gene product was recombinantly expressed in Escherichia coli in presence of Zn(II), Cd(II), or Cu, and characterised by Electrospray Ionisation Mass Spectrometry and elemental analysis. C. gariepinus MT displays typical features of fish MTs, including 20 conserved cysteines, and seven bound divalent cations (Zn(II) or Cd(II)) when saturated. Livers from wild C. gariepinus fish collected in all three seasons from four different sites on the Kafue River of Zambia were analysed for their metal contents and for MT expression levels by quantitative PCR. Significant correlations were found between Zn and Cu levels and MT expression in livers, with MT expression clearly highest at the most polluted site, Chililabombwe, which is situated in the Copperbelt region. Based on our findings, hepatic expression of MT from C. gariepinus may be further developed as a major molecular biomarker of heavy metal pollution resulting from mining activities in this region.
Highlights
Heavy metal contamination of fresh water systems is of serious concern worldwide because metals have the ability to bioaccumulate in aquatic organisms, and, in contrast to organic pollutants, are non-degradable [1,2,3]
We conclude that MT expression in C. gariepinus responds to elevated pollution with Cu resulting from mining activities in the Copperbelt region, and that MT expression may serve as a suitable biomarker of such pollution
We explored whether variations in liver metal contents are reflected in MT expression to test the utility of C. gariepinus metallothionein (cgMT) expression as a biomarker for metal pollution
Summary
Heavy metal contamination of fresh water systems is of serious concern worldwide because metals have the ability to bioaccumulate in aquatic organisms, and, in contrast to organic pollutants, are non-degradable [1,2,3] Some metals such as Fe, Cu, Co, Mn and Zn are essential to biological life forms, but become toxic above their threshold concentrations [4,5]. The primary function of MTs concerns homeostasis of essential metals such as Cu and Zn, keeping these metal ions safe but potentially available within the intracellular environment [35,36] Their high thiol content renders MTs redox-active [37] and able to respond to oxidative stress [35,38]. We conclude that MT expression in C. gariepinus responds to elevated pollution with Cu resulting from mining activities in the Copperbelt region, and that MT expression may serve as a suitable biomarker of such pollution
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