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A study report on a neurotoxic esterase (NTE) assay is available for zinc O,O,O',O'-tetrabutyl bis(phosphorodithioate). However, the composition of the reported test material is unclear and the impurity profile is not specified. It is thus questionable, whether the results obtained were related to the test substance as defined in the substance identity profile or to unidentified impurities in the actual test material.

In this study, the potential inhibitory effect of zinc O,O,O',O'-tetrabutyl bis(phosphorodithioate) on NTE activity was investigated in White Leghorn hens (Richardson, 1985). A group of 6 animals received a single oral dose of the test material at 2000 mg/kg bw in a gelatine capsule. Two further groups receiving corn oil (2000 mg/kg bw) and tri-o-cresylphosphate (750 mg/kg bw) served as negative and positive control groups, respectively. After 24 h, animals were sacrificed, their brains were prepared, weighed and homogenised for the biochemical assay.

The NTE assay was performed according to the protocol described in Johnson (1977). Shortly, the amount of NTE activity present in diluted whole hen brain homogenate is determined by a colourimetric reaction in which 4-amino-antipyrine (AAP) reacts with free phenol groups produced by enzymatic hydrolysis of the substrate phenyl valerate. Potassium ferricyanide is used as an oxidizing agent. If the enzyme is inhibited by neurotoxic compounds, it cannot carry out the hydrolysis of phenyl valerate, and there are no phenolic groups formed with which AAP can react (colour is less intense). NTE is assayed differentially by addition of paraoxon to the incubation media, resulting in the inhibition of the majority of esterase activity in the tissue homogenate which is not NTE.

Mean NTE activity (± standard deviation), expressed as nmol phenyl valerate/min/g wet brain weight, in the control, test and positive control groups was 2459 ± 109, 2386 ± 79 and 223 ± 90, respectively. Thus, exposure to the test material resulted in a statistically not significant inhibition of ca. 3% in the NTE activity relative to the negative control. NTE activity was statistically significantly reduced by ca. 91% in brain homogenates of the positive control group.

Under the conditions of this study, the test material did not inhibited neurotoxic esterase activity in the hen brain.

Due to unclear composition and impurity profile of the actual test material, the results of this study are doubtful and considered not reliable and insufficient for assessment.

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