Loss of fluorescence by anthracycline antibiotics: effects of xanthine oxidase and identification of the nonfluorescent metabolites.

Rat liver cytosol and buttermilk xanthine oxidase both converted 7-deoxypyrromycinone, the 7-deoxyaglycone of marcellomycin, a new anthracycline antibiotic, to a nonfluorescent compound under anaerobic conditions and in the presence of an electron donor. Reduced nicotinamide adenine dinucleotide and reduced nicotinamide adenine dinucleotide phosphate were equally effective electron donors for liver cytosol, and xanthine was the best cofactor for xanthine oxidase. However, xanthine was inactive with liver cytosol. Reactions with xanthine oxidase obeyed Menten-Michaelis kinetics and were inhibited by allopurinol. No xanthine oxidase activity was detected in liver cytosol. Xanthine oxidase also induced a loss of fluorescence when incubated with 7-deoxydaunorubicin aglycone. The nonfluorescent metabolite of 7-deoxypyrromycinone was tentatively identified as the dihydroquinonic derivative of the parent deoxyaglycone on the basis of its spectrophotometric, fluorescent, thin layer chromatographic, and mass spectral characteristics. Our data demonstrate that more than one enzymatic activity, xanthine oxidase, and an unidentified rat liver cytosolic enzyme convert the 7-deoxyaglycones of anthracycline antibiotics to nonfluorescent metabolites.