Membrane ascorbate modification

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

Ascorbic acid (vitamin C) is one of the body s endogenous water-soluble antioxidants. Modifications on the ascorbic acid structure have led to some very interesting compounds, such as a novel series of 3-O-alkyl ascorbic-acid derivatives. They have been found to be inhibitors of lipid peroxidation (Nihro etal., 1991). This antioxidant activity is directly related to the lipophilicity of the alkyl chain, su esting that the lipid chain may anchor the antioxidant portion of the molecule in the membrane. 

In contrast to numerous literature data, which indicate that protein oxidation, as a rule, precedes lipid peroxidation, Parinandi et al. [66] found that the modification of proteins in rat myocardial membranes exposed to prooxidants (ferrous ion/ascorbate, cupric ion/tert-butyl-hydroperoxide, linoleic acid hydroperoxide, and soybean lipoxygenase) accompanied lipid peroxidation initiated by these prooxidant systems. 

Scheller et al. (1986a) combined polyurethane-immobilized LOD with an Au/Pd-sputtered carbon electrode. The electrode modification permits H2O2 to be electrochemically oxidized at a potential as low as +450 mV, where interferences by other anodically oxidizable compounds such as NADH and ascorbic acid are largely reduced (Fig. 57). This increased selectivity also enables the measurement of LDH activity. The sensor has been introduced in an FIA manifold. A sample frequency of 200Ai (Fig. 58) and a CV below 1% were obtained with this setup. A platinum electrode with LOD covalently bound to a nylon membrane has been employed for continuous blood lactate determination in an artificial pancreas by Mascini et al. (1985b, 1987). 

Ascorbate transport is mainly controlled by carriers availability, and thus it depends on the number of SVCT proteins present in the plasma membrane (which in turn is related to enhanced synthesis, slowed degradation, activation of nonfunctional carriers or cellular redistribution), as well as their substrate affinity. So, transcriptional, translational, and post-translational modifications of SVCTs allow a fine-tuned regulation of AA uptake (Liang et al., 2002). 

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