Inactivation of rat brain acetylcholinesterase by pyridoxal 5'-phosphate

Effect of vitamin B6 on brain damage in valproic acid induced toxicity

Valproic acid (VPA) is an efficient antiepileptic drug widely used for the treatment of epilepsy and other seizures in both children and adults. It is also reported to have side and toxic effects on many organs and tissues. Vitamin B₆ (Vit B₆ ) is a well-described water-soluble vitamin, which has an antioxidant effect. In this study, we aimed to investigate the protective effect of Vit B₆ on VPA-induced brain injury. Male Sprague-Dawley rats were divided into four groups. Group I, control animals; Group II, Vit B₆ (50 mg/kg/day) given rats; Group III, VPA (500 mg/kg/day) given rats; Group IV, VPA and Vit B₆ given rats at same dose and time. VPA and Vit B₆ were administered intraperitoneally and orally, respectively, for 7 days. At the end of the experiments, the rats were sacrificed and brain tissues were taken. Protein carbonyl and sialic acid levels, xanthine oxidase, adenosine deaminase, acetylcholine esterase, lactate dehydrogenase, myeloperoxidase activities, total oxidant status, and reactive oxygen species levels were found to be increased, while glutathione and total antioxidant capacity levels, catalase, superoxide dismutase, glutathione-S-transferase, paraoxonase, and glutathione reductase activities were found to be decreased in the VPA group. Administration of Vit B₆ reversed these defects in the VPA group. These findings indicate that Vit B₆ has a protective effect on VPA-induced brain damage.

Inactivation of acetylcholinesterase by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride

The neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to reversibly inhibit the activity of acetylcholinesterase. The inactivation of the enzyme was detected by monitoring the accumulation of yellow color produced from the reaction between thiocholine and dithiobisnitrobenzoate ion. The kinetic parameter, Km for the substrate (acetylthiocholine), was found to be 0.216 mM and Ki for MPTP inactivation of acetylcholinesterase was found to be 2.14 mM. The inactivation of enzyme by MPTP was found to be dose-dependent. It was found that MPTP is neither a substrate of AChE nor the time-dependent inactivator. The studies of reaction kinetics indicate the inactivation of AChE to be a linear mixed-type inhibition. The dilution assays indicate that MPTP is a reversible inhibitor for AChE. These data suggest that once MPTP enters the basal ganglia of the brain, it can inactivate the acetylcholinesterase enzyme and thereby increase the acetylcholine level in the basal ganglia of brain, leading to potential cell dysfunction. It appears that the nigrostriatal toxicity by MPTP leading to Parkinson's disease-like syndrome may, in part, be mediated via the acetylcholinesterase inactivation.

Purification of soluble acetylcholinesterase from Japanese quail brain by affinity chromatography

The purification of a soluble acetylcholinesterase from Japanese quail brain using affinity chromatography on concanavalin A-Sepharose and edrophonium-Sepharose is described. The affinity matrix was synthesized by coupling an inhibitor edrophonium to epoxy-activated Sepharose. Acetylcholinesterase was purified 10,416-fold with a specific activity of 2500 U/mg protein. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and mercaptoethanol gave only one band with a molecular weight of 62.5 kDa. The molecular weight of the purified acetylcholinesterase was estimated to be 245.5 kDa by gel chromatography on Sephacryl S-200 under nondenaturing conditions. Based on the molecular weight obtained by both SDS-PAGE and gel filtration the purified acetylcholinesterase was assumed to be a tetrameric form.

Acetylcholinesterase inhibition by 1-methyl-4-phenylpyridinium ion, a bioactivated metabolite of MPTP

The effect of the neurotoxicant, 1-methyl-4-phenylpyridinium ion (MPP+) on acetylcholinesterase (AchE) activity was investigated. The MPP+ was found to inactivate the enzyme in a dose dependent manner. The kinetic parameter, Km for the substrate (acetylthiocholine), was found to be 0.216 mM and Ki for MPP+ for the inactivation of AChE was found to be 0.197 mM. It was found that MPP+ is neither a substrate of AChE nor the time-dependent inactivator. The studies of reaction kinetics indicate the inactivation of AChE to be a linear mixed-type inhibition. The inactivation of AChE by MPP+ was partially recovered by either dilution or gel exclusion chromatography. These data suggest that once MPP+ enters the basal ganglia of the brain, it can inactivate the AChE and thereby increase the acetylcholine level in the basal ganglia, leading to potential cell dysfunction. It appears likely that the nigrostriatal toxicity by MPP+ leading to Parkinson's disease-like syndrome may, in part, be mediated via the AChE inactivation.

Acetylcholinesterase is not inhibited by pyridoxal 5'-phosphate

Acetylcholinesterase activity was assayed in the absence and presence of pyridoxal 5'-phosphate. If substrate hydrolysis was measured by the pH-stat method, its rate was not significantly affected by pyridoxal 5'-phosphate. In the spectrophotometric assay, however, this compound led to an apparent decrease in rate. The discrepancy between the two assays is explained by stray-light artefacts produced by pyridoxal 5'-phosphate at the wavelengths of the spectrophotometric assay.