**15. Enzymatic biomarkers in aquatic insects**

Aquatic invertebrates that accumulate the heavy metals were subjected to oxidative stress that leads to activation of the different antioxidant enzymes including acetylcholinesterase and glutathione S transferase [65]. Acetylcholinesterase, a neurotransmitter, had been observed as biomarker for detection of pollutant exposure in invertebrates [66]. Many antioxidant factors such as glutathione, catalase, lipid peroxidase and glutathione transferase were utilized as biomarkers in various living organism [67] as enzymes work with antioxidants for removal of free radical. Moreover some so-called biomarkers such as heat shock proteins and metallothioneins increase in concentration when exposed to heavy metals [68].
