Alteration in salinity in water als

Alteration in salinity in water also affects metal availability and subsequent toxicity through competing with metal ions for binding to biological molecules (Heath, 1987). Because salinity of natural fresh waters can increase as a result of anthropogenic activities, it is important to examine more realistically the toxic effects of metals in increased salinities in order to estimate the consequences that fish face in the freshwater environments. Additionally, salinity alone can
also cause serious fluctuations in the metabolism of freshwater fish which may eventually alter their antioxidant response.
Antioxidant enzymes, such as GPX, GST, GR and SOD are important in coping with oxidative stress caused by metabolisms itself and environmental factors (Pinto et al., 2003; Tripathi et al.,2006). SOD catalyzes the reduction of superoxide radical in to hydrogen peroxide. GPX also participates in the reduction of hydrogen peroxide. On the other hand, GSTcatalyzes the con-jugation of pollutants to eliminate them from the cellular system and GR reduces the oxidized glutathione to GSH.
The Nile tilapia Oreochromis niloticus is a widely distributed freshwater fish that can persist in polluted habitats and increased salinities, and has the potential for the development as a bioindi-cator of environmental pollution. There are limited data on the combined effects of salinity and metals on the antioxidant system of freshwater fish. Therefore, the aim of this study is to investigate the influence of salinity on the effects of metals (Pb and Cr) to the
antioxidant system of freshwater fish O. niloticus.