Antioxidant depletion

Van Der Vliet, A., Smith, D., O Neill, C.A., Kaur, H., Darley-Usmar, V.M., Cross, C.E. and Halliwell, B. (1994). Interactions of peroxynitrite with human plasma and its constituents oxidative damage and antioxidant depletion. Biochem. J. 303, 295-301. 

Minimal processing steps could be expected to induce a rapid enzymatic depletion of several natural antioxidants. Depletion of antioxidant capacity in cubed and sliced papaya fruit could be associated with depletion of both ascorbic acid and (3-carotenes, but in general, the highest loss of these important nutrients was reached before the product was unacceptable or spoiled (Rivera-Lopez and others 2005). 

Phenol-induced oxidative stress mediated by thiol oxidation, antioxidant depletion, and enhanced free radical production plays a key role in the deleterious activities of certain phenols. In this mode of DNA damage, the phenol does not interact with DNA directly and the observed genotoxicity is caused by an indirect mechanism of action induced by ROS. A direct mode of phenol-induced genotoxicity involves covalent DNA adduction derived from electrophilic species of phenols produced by metabolic activation. Oxidative metabolism of phenols can generate quinone intermediates that react covalently with N-1,N of dG to form benzetheno-type adducts. Our laboratory has also recently shown that phenoxyl radicals can participate in direct radical addition reactions with C-8 of dG to form oxygen (O)-adducts. Because the metabolism of phenols can also generate C-adducts at C-8 of dG, a case can be made that phenoxyl radicals display ambident (O vs. C) electrophilicity in DNA adduction. 

Fig. 2 presents the analysis based on OIT data and the linear extrapolation of these data to longer times. The time to reach depletion of the antioxidant system can thus be predicted even after relatively short testing times (see insert figure in Fig. 2). Data by Hassinen et al. (//) for the antioxidant concentration profiles taken from high-density polyethylene pipes exposed to chlorinated water (3 ppm chlorine) at different temperatures between 25 and 105°C followed the Arrhenius equation with an activation energy of 85 kJ mol (0-0.1 mm beneath inner wall surface) and 80 kJ mol (0.35-0.45 mm beneath the inner wall surface). It is thus possible to make predictions about the time for antioxidant depletion at service temperatures (20-40°C) by extrapolation of high temperature data. However, there is currently not a sufficient set of data to reveal the kinetics of polymer degradation and crack growth that would allow reliable extrapolation to room temperature.

Figure 3. Kinetics of antioxidant depletion. Human plasma was incubated at 37X ) with 50 mM AAPH in the absence (A) or the presence of 100 pM epicatechin (B) or catechin (C). Flavanols (squares), ascorbate (triangles), and a-tocopherol (circles). Values are mean of at least three independent experiments.SEM were...

The effects of sodium chlorite on membrane components and antioxidant depletion have been studied in rabbit corneal epithelial cells, human conjunctival epithelial cells, phospholipid vesicles prepared from egg yolk and GSH in solution. Incubation of phospholipid vesicles with 3.5 mmol sodium chlorite/l for up to 2 h had no effect, whereas incubation for 48 h resulted in lipid depletion and an increase in lipid oxidation. Sodium chlorite was found to be a very potent GSH oxidizing agent at a GSH/sodium chlorite ratio of 0.5, GSH was depleted after 5 min. GSH depletion was also seen in rabbit corneal epithelial cells and human conjunctival epithelial cells incubated with 3.5 mmol sodium chlorite/l or 0.55 mmol sodium chlorite/l. At 3.5 mmol/l, sodium chlorite caused rapid loss of cell viability in the corneal cells, as assessed by trypan blue staining and loss of adherence. At 0.55 mmol/l, sodium chlorite had very little effect over the first few hours but decreased viability after 24 h. The conjunctival cells appeared to be less sensitive than the corneal cells. No oxidatively modified lipids could be detected in the cells following sodium chlorite treatment, and no effects were seen in levels of cytosolic antioxidants (Ingram et al., 2003). 

Hsuan YG and Koemer RM (1998) Antioxidant Depletion Lifetime in High Density Polyethylene Geomembranes. Journal of Geotechiucal and Geoenviron-menlal Engineering 124 532-541... 

Under normal conditions, oxidative reactions progress extremely slowly. Correspondingly, the associated oxidative stabiliser consumption is also slow. However, not only oxidation, but other chemical degradation processes can also destroy antioxidants. For example, phosphates and other antioxidants are hydrolysis-sensitive (Gugumus 1990). In addition to such chemical antioxidant depletion , there are physical depletion processes The concentration of the added antioxidants is reduced by extraction and migration processes while the plastic is stored and used (Pfahler and Lotzsch 1988). Such processes are usually the main cause of gradual loss of stabilisers at application temperatures. All these depletion processes determine induction time t 2 and thus service lifetime of the plastic. They... 

The velocity of extraction or migration processes and consumption by oxidation or other chemical degradation processes or the antioxidant depletion rate can be approximately assumed to be proportional to the available amount of stabiliser. The antioxidant content [ 4] as a function of time can then be described by an exponential function, already introduced in Eq. 5.1 ... 

We call kefflliQ antioxidant depletion rate and its reciprocal %the depletion time constant. Both are characteristic parameters of the depletion process. Constant hff as a function of temperature T is given by the Arrhenius law Eq. 5.3. 

In this respect 6,2 is called antioxidant depletion time and E the apparent activation energy of antioxidant depletion. Again values for E in the vicinity of 50kJ/mol are experimentally found. As discussed in Sect. 5.1, the Arrhenius equation leads to the old mle of thumb that an increase in temperature by about 10 °C doubles or trebles the depletion rate. This rule can also be used as an initial guide to estimate service life. If, after ageing of geomembrane samples in water or air at 80 °C test temperature over a period of at least one year, effective stabiliser substance can still be found, for example by OIT-measurements, a service lifetime of many decades can be expected at normal ambient temperatures. 

However, none of these experiments were pursued to the end of the depletion process and oxidative degradation was not actually achieved, since this requires very long test times even at high temperatures. Therefore, the depletion rates have to be considered with caution because they have been estimated from short-term experiments, and the assumption that the antioxidant depletion time is the most relevant part of the service lifetime, needs experimental justification. 

The oxidative induction time is used to determine the quantity and types of antioxidants and to evaluate antioxidant depletion times. 

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