Redox lucigenin

The 02, radical can act as an oxidant as well as a reductant and chemical estimates of its production can also be based on its ability to oxidize epinephrine to adren-ochrome [62], These chemical methods have the additional advantage of not requiring highly specialized equipments. Also based on its redox property, the 02 radical can be determined by chemiluminescence methods through the measurement of the intensity of the fluorescence radiation emitted after chemical oxidation of 02 by, e.g., lucigenin [63-67], These methods, however, are limited by the poor selectivity and lack of capability for in-vivo performance. 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

It should be mentioned that Spasojevic et al. [57] recently determined the two-electron reduction potential of lucigenin in water as —0.14 V. As this value is close to the one-electron reduction potential of dioxygen °[02 702] = — 0.16 V, these authors regarded their finding as a support for lucigenin redox cycling. However, it has been demonstrated long ago that two-electron reduction potentials cannot be used for the calculation of equilibrium for one-electron transfer processes [58]. 

Fig. 17 Double potential step chronoamperometric results at 23°C for the reduction of lucigenin (B +) in DMF containing BU4NBF4 (0.1 M). The circles are experimental values, and the lines show the results of digital simulation for the EEC mechanism (a) including, and (b) without the homogeneous redox equilibrium (eqn 51). Potential-step times varied between 1 and 100 ms. (Ahlberg, et al., 1981)...

In alkaline solutions when suitable reducing agents are titrated with oxidizing agents such as H2O2 or a hypohalite in the presence of luminol and lucigenin, at the endpoint the redox potential of the solution suddenly moves towards a more positive value and the CL reaction occurs. In acidic solution (pH<3.5), siloxene has been used as a CL redox indicator. An outline of the use of these indicators in redox titrations is presented in Table 4B. 

Common quinone radicals Q react with O2 much faster than lucigenin, raising the possibility that some quinones could act as redox mediators in lucigenin /02 redox cycling. This is a further complication in the use of lucigenin (Wardman and VojNOVic 1999). 

Calcium oxalate monohydrate (25-250 treatment of proximal (LLC-PKl) and distal (MDCK) tubular epitheUal cell cultures increased superoxide production 3-6-fold as measured by both lucigenin chemiluminescence in permeabil-ized cells and dihydrorhodamine fluorescence in intact cells (Khand etal. 2002). The use of mitochondrial probes, substrates, and inhibitors indicated that increased Oj " production originated from mitochondria. Treatment with calcium oxalate monohydrate decreased glutathione (total and redox state), indicating a sustained oxidative insult. An increase in NADH in calcium oxalate monohydrate-treated cells suggested this cofactor could be responsible for elevating O2 generation. 

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