Fatty acids and neurodevelopment.

Knowledge of the importance of docosahexaenoic acid (DHA), arachidonic acid (AA), and long-chain polyunsaturated fatty acids (LCPUFAs) in neurodevelopment was originally obtained from animal studies. These fatty acids are rapidly accreted in brain during the first postnatal year in animal and human infants, and they are found in high concentrations in breast milk. Reports of enhanced intellectual development in breast-fed children, and reports linking LCPUFA deficiency with neurodevelopmental disorders have stressed the physiological importance of DHA in visual and neural systems. In addition to high concentrations of fatty acids in breast milk, they are also present in fish and algae oil and have recently been added to infant formulas. Esterified poplyunsaturated fatty acids act in cellular membranes, in signal transduction, in neurotransmission, and in the formation of lipid rafts. Nonesterified polyunsaturated fatty acids can modulate gene expression and ion channel activities, thus becoming neuroprotective agents. The conversion of linoleic acid and alpha-linolenic acid into ARA and DHA have led to randomized clinical trials that have studied whether infant formulas supplemented with DHA or both DHA and ARA would enhance visual and cognitive development. This review gives an overview of fatty acids and neurodevelopment, focusing on the findings from these studies.

Involvement of deacylation in activation of substrate hydrolysis by Drosophila acetylcholinesterase.

Insect acetylcholinesterase (AChE), an enzyme whose catalytic site is located at the bottom of a gorge-like structure, hydrolyzes its substrate over a wide range of concentrations (from 2 microm to 300 mm). AChE is activated at low substrate concentrations and inhibited at high substrate concentrations. Several rival kinetic models have been developed to try to describe and explain this behavior. One of these models assumes that activation at low substrate concentrations partly results from an acceleration of deacetylation of the acetylated enzyme. To test this hypothesis, we used a monomethylcarbamoylated enzyme, which is considered equivalent to the acylated form of the enzyme and a non-hydrolyzable substrate analog, 4-oxo-N,N,N-trimethylpentanaminium iodide. It appears that this substrate analog increases the decarbamoylation rate by a factor of 2.2, suggesting that the substrate molecule bound at the activation site (K(d) = 130 +/- 47 microm) accelerates deacetylation. These two kinetic parameters are consistent with our analysis of the hydrolysis of the substrate. The location of the active site was investigated by in vitro mutagenesis. We found that this site is located at the rim of the active site gorge. Thus, substrate positioning at the rim of the gorge slows down the entrance of another substrate molecule into the active site gorge (Marcel, V., Estrada-Mondaca, S., Magné, F., Stojan, J., Klaébé, A., and Fournier, D. (2000) J. Biol. Chem. 275, 11603-11609) and also increases the deacylation step. This results in an acceleration of enzyme turnover.

Exploration of the Drosophila acetylcholinesterase substrate activation site using a reversible inhibitor (Triton X-100) and mutated enzymes.

Cholinesterases are activated at low substrate concentration, and this is followed by inhibition as the level of substrate increases. However, one of these two components is sometimes lacking. In Drosophila acetylcholinesterase, the two phases are present, allowing both phenomena to be studied. Several kinetic schemes can explain this complex kinetic behavior. Among them, one model assumes that activation results from the binding of a substrate molecule to a non-productive site affecting the entrance of a substrate molecule into the active site. To test this hypothesis, we looked for an inhibitor competitive for activation and we found Triton X-100. Using organophosphates or carbamates as hemisubstrates, we showed that Triton X-100 inhibits or increases phosphorylation or carbamoylation of the enzyme. In vitro mutagenesis of the residues lining the active site gorge allowed us to locate the Triton X-100 binding site at the rim of the gorge with glutamate 107 playing the major role. These results led to the hypothesis that substrate binding at this site affects the entrance of another substrate molecule into the active site cleft.

A putative kinetic model for substrate metabolisation by Drosophila acetylcholinesterase.

Insect acetylcholinesterase, an enzyme whose catalytic site is located at the bottom of a gorge, can metabolise its substrate in a wide range of concentrations (from 1 microM to 200 mM) since it is activated at low substrate concentrations. It also presents inhibition at high substrate concentrations. Among the various rival kinetic models tested to analyse the kinetic behaviour of the enzyme, the simplest able to explain all the experimental data suggests that there are two sites for substrate molecules on the protein. Binding on the catalytic site located at the bottom of the gorge seems to be irreversible, suggesting that each molecule of substrate which enters the active site gorge is metabolised. Reversible binding at the peripheral site of the free enzyme has high affinity (2 microM), suggesting that this binding increases the probability of the substrate entering the active site gorge. Peripheral site occupation decreases the entrance rate constant of the second substrate molecule to the catalytic site and strongly affects the catalytic activity of the enzyme. On the other hand, catalytic site occupation lowers the affinity of the peripheral site for the substrate (34 mM). These effects between the two sites result both in apparent activation at low substrate concentration and in general inhibition at high substrate concentration.

Drosophila acetylcholinesterase: effect of post-translational [correction of post-traductional] modifications on the production in the baculovirus system and substrate metabolization.

Acetylcholinesterase cDNAs from Drosophila melanogaster modified on its primary sequence were cloned into baculovirus and were expressed in Sf9 cells with the aim to identify a mutant form that produces the enzyme at a high level. Directed mutagenesis was used in order to independently knockout different sites of post-translational modifications: exchange of the C-terminal hydrophobic peptide for a glycolipid molecule, dimerization by disulfide bridge, N-linked glycosylation at the five accessible sites, and subunit formation by proteolytic cleavage of a hydrophilic peptide found in the precursor. Another mutation involved the elimination of a free cysteine in the mature protein. All mutations involving post-translational modifications resulted in lower recoveries, suggesting that they are useful for maintaining high amounts of protein in the synapse. By contrast, elimination of a free cysteine in the mature protein permitted an increase in the level of production of the enzyme. These mutations did not affect specific activity of the enzyme at substrate concentrations ranging from 3 microM to 200 mM, suggesting that activation and inhibition of the enzyme activity does not originate from a polymorphism in post-translational modifications.

Engineering sensitive acetylcholinesterase for detection of organophosphate and carbamate insecticides.

High quantities of various acetylcholinesterases can now be produced following in vitro expression and it is possible to use them as biosensors to detect organophosphates and carbamates insecticides. In order to check the potentialities of acetylcholinesterase from various sources, we have studied enzyme from bovine erythrocyte, Electrophorus electricus, Drosophila melanogaster, Torpedo californica and Caenorhabditis elegans. It appears that insect acetylcholinesterase is more susceptible to a broad range of organophosphates and carbamates insecticides than the other tested enzymes. D. melanogaster is 8-fold more sensitive than E. electricus enzyme and this sensitivity has been increased to 12-fold by introducing a mutation at position 408.

Stabilization of recombinant Drosophila acetylcholinesterase.

The uses of pure and stable acetylcholinesterase can range from simple basic research to applications in environment quality assessment. In order to satisfy some of these needs its recombinant expression is routinely performed. Affinity-purified recombinant Drosophila melanogaster acetylcholinesterase proved to be instable; an apparent cause of this seemed to be the presence of contaminants with protease activity as evidenced by SDS-PAGE. The elimination of these accompanying products was achieved by anion-exchange, hydrophobic interaction, and cibacron blue affinity chromatography applied downstream from procainamide affinity chromatography. The utilization of a parallel affinity acting via an engineered histidine tail permitted the elimination of the copurified proteases as well. Despite the elimination of the contaminants, the apparently pure extracts were still unstable. It is shown that such instability can be counterbalanced by provoking protein-protein interactions, either between enzyme molecules or with other molecules such as bovine serum albumin. Another way to reduce instability is the addition of a reversible inhibitor or polyethylene glycol 3350.