Inhibition of thioredoxin reductase (E.C. 1.6.4.5.) by antitumor quinones

Thioredoxin reductase (TR) is a widely distributed flavoenzyme that provides reduced thioredoxin, a dithiol hydrogen donor for protein disulfide reduction and for the reduction of ribonucleotides to deoxyribonucleotides, the first unique step of DNA synthesis. Antitumor quinones were found to exhibit time- and concentration-dependent inhibition of purified rat liver TR that requires the presence of NADPH. Diaziquone initially shows competitive inhibition of the enzyme with 5,5'-dithiobis 2-nitrobenzoic acid as substrate with a Ki of 7.5 microM, which becomes non-competitive after 1 hour incubation with NADPH with a Ki of 0.5 microM. Doxorubicin shows non-competitive inhibition both initially and after 1 hr incubation with NADPH, with Ki values of 10 microM and 0.5 microM, respectively. Electron spin resonance spectroscopy showed the formation of semiquinone free radicals by TR incubated under anaerobic conditions with doxorubicin or diaziquone and NADPH. Redox cycling and formation of oxygen radicals does not play a major role in the inhibition of TR by antitumor quinones as shown by the minor effect on inhibition of removing O2, and the lack of effect of superoxide dismutase and catalase. Diaziquone causes time- and concentration-dependent inhibition of TR activity in intact A204 human rhabdomyosarcoma cells that is associated with growth inhibition. The results suggest that inhibition of TR by antitumor quinones could contribute to their growth inhibitory properties.