O

Semiquinone

FIGURE 4.13 Captodative stabilization of semiquinone radicals.

FIGURE 4.13 Captodative stabilization of semiquinone radicals.

The reversibility of semiquinone formation allows these radicals to induce one-electron reduction of oxygen molecules to superoxide radical anions, leading to an overall increase of the electron flow to oxygen derived from the activity of enzymes such as NADPH dehydrogenase, xanthine dehydrogenase, and the reductase domain of nitric oxide synthase19 (Fig. 4.14).

A competitive reaction of semiquinone 4.25 can take place, involving loss of daunosamine. Thus, two molecules of 4.25 can disproportionate to give the starting quinone and hydroquinone 4.26, which is unstable and evolves by elimination of the sugar moiety to give the anthracycline aglycon (Fig. 4.15).20 Because of their relatively high lipophilicity with regard to the glycosides, these aglycons tend to accumulate in the inner mitochondrial membrane. The oxidative deterioration of mitochondrial functions due to the formation of radicals from these aglycons is one of the factors responsible for the cardiomyopathy associated with the use of anthracyclines.20

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