Thionphosphate Inhibition Of Esterase Activity In Freshwater Snail Hellsoma Duryi
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Date
2013-03-14
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Abstract
Organophosphorus insecticides (OP's) are among the most commonly used pesticides in most African countries. These compounds are esters, amides or thiol derivatives of either phosphoric acid or thiophosphoric acids. The majority now in use such as azinphos methyl, chlorpyrifos, pirimiphos methyl and malathion contain the thiono moiety (=8) for (=0) on the phosphorus atom which increases the toxicity of the insecticide. There are six different chemical classes of OPls namely orthophosphates, thionphosphates, thiolphosphates, dithiophosphates, phosphonates and pyrophosphoramides (Hassall, 1990). When used in the vicinity of aquatic ecosystems the insecticides may enter in water bodies as a result of erosion and leaching in concentrations that I may affect non-target species such as fish and snails. Also the diversity in chemical structure of OP's that exist, results in variability in their toxicity. There is a need therefore to constantly monitor effects of these organic compounds on non-target organisms: The effects'of pirimiphos methyl, a thionphosphate on esterase activity in a freshwater snail Helisoma duryi was investigated with the aim of assessing its potential as a bioindicator of presence of OP's in water. Juvenile snails reared outdoors were exposed to pirimiphos methyl in either Matopos (pristi':le) dam water or Umguza (highly contaminated with industrial waste) dam water for 1, 7, or 14 days: Esterase activity determined in post mitochondrial supernatants was significantly depressed in',a dose dependent manner. Carboxylesterase activity measured using a-naphthyl acetate and'lpnitrophenyl acetate was reduced in the range (6-90 %) and (18-79 %) respectively while cholinesterase measured using acetylthiocholine iodide was inhibited in the range (19-85 %) depending on the water source. A decrease with time in degree of inhibition of esterase activity was also observed, suggesting a recovery with time of the snails from pesticide poisoning. This recovery of the snails was probably due to both natural and microbial decomposition of the pesticides with time. It's also possible that newly synthesized esterases replaced the inhibited ones. On comparing data from the two dams, higher inhibitions were observed in snails exposed
to Matopos dam water than those exposed to Umguza dam water. Probably the higher microbial load in the contaminated Umguza dam resulted in increased pesticide decomposition and hence reduced inhibition of the snails in those waters whel} compared to those snails in Matopos, a pristine dam. Our results have shown that esterase activity altered by up to 90% is sensitive, to presence of pesticide pollutants and hence has a potential as a bio-indicator for detecting organophosphate pollution in water samples.
Description
A paper covering Thionphosphate inhibition of esterase activity in freshwater snails.
Keywords
Thionphosphate, Esterase, Snail, Helisoma Duryi, Water Pollution, Pesticides