Anthracycline antibiotic reduction by spinach ferredoxin-NADP+ reductase and ferredoxin

Spinach NADPH:ferredoxin oxidoreductase (EC 1.6.7.1) catalyzes the NADPH-dependent reduction of the anthracyclines daunomycin, aclacinomycin A, and nogalamycin and their respective 7-deoxyanthracyclinones. Under anaerobic conditions, the endogenous rate of O2 reduction by NADPH catalyzed by ferredoxin reductase (0.12 s-1 at pH 7.4) is augmented by the anthracyclines and 7-deoxyanthracyclinones. The catalytic constants are approximately equivalent for this augmentation for all substrates (approximate V of 2 s-1 and KM of 75 microM). Both O2- and H2O2 are made. Under anaerobic conditions, anthracycline reduction catalyzed by ferredoxin reductase results in the elimination of the C-7 substituent to provide a quinone methide intermediate. Following tautomerization by C-7 protonation, 7-deoxyanthracyclinones are obtained. Under appropriate conditions these may be further reduced to the 7-deoxyanthracyclinone hydroquinones. For daunomycin, the quinone methide is formed rapidly after reduction and is easily monitored at 600 nm. It may react with electrophiles other than H+, as demonstrated by its competitive trapping by p-carboxybenzaldehyde. It may also react with nucleophiles, as demonstrated by its competitive trapping by N-acetylcysteine. For aclacinomycin, quinone methide formation is also rapid although no distinct transient near 600 nm occurs. In addition to protonation, it reacts with itself providing the 7,7'-dimer. With ethyl xanthate as a thiolate nucleophile, adducts derived from addition to C-7 are obtained. For nogalamycin, quinone methide formation is not rapid. Nogalamycin is reduced to its hydroquinone, which slowly converts in a first-order process [k = (1.2 +/- 0.2) X 10(-3) s-1, pH 8.0, 30 degrees C] to the quinone methide, which is then quenched by protonation. Spinach ferredoxin in its reduced form is chemically competent for anthracycline reduction. Its effect on both the aerobic and anaerobic reactions catalyzed by ferredoxin reductase is to increase severalfold the overall velocity for anthracycline reduction. In conclusion, the aerobic reaction pathways for the anthracyclines as mediated by ferredoxin reductase are remarkably similar, while the anaerobic reactions are remarkably different. If these anthracyclines exert their antitumor activity by a common anaerobic pathway, it is most likely that the pathway is determined by the properties of the anthracycline as complexed to its in vivo target. The behavior of ferredoxin further suggests that not only low-potential flavin centers but also iron-sulfur centers should be regarded as important loci for anthracycline reductive activation.