Aglycosylation and disposition of doxorubicin in isolated rat liver nuclei and microsomes.

Although the chromatographic profile of fluorescent metabolites of doxorubicin was indistinguishable from that generated with microsomes, the rate of aglycosylation of the drug by isolated nuclei was substantially slower. Whereas microsomes consumed 90% of the doxorubicin, nuclei failed to metabolize more than 25% of the drug, even at extended incubation times. A time-dependent decrease in total measurable fluorescence recovered in butanol extracts of microsomes was accompanied by a progressive increase in drug associated with the residual pellet, suggesting the accumulation of metabolic adducts. In contrast, a large portion of the drug-related fluorescence was not extracted in butanol from nuclear reactions at all times examined, which may reflect the rapid partitioning of unmetabolized doxorubicin into nuclear DNA. The time-dependent decrease in fluorescence recovered from the reaction mixtures was dependent on metabolism and correlated with the appearance of drug-related color in the acid-insoluble precipitate following anaerobic incubation of nuclei or microsomes. The data indicate that, mechanistically, the anaerobic aglycosylation of doxorubicin by isolated nuclei resembles that catalyzed by microsomes. However, a major distinction is the association of a large portion of the drug with acid-soluble components of isolated nuclei which may restrict the accessibility of doxorubicin to the nuclear drug-metabolizing enzymes.