Reduction of 3'-azido-2',3'-dideoxynucleosides to their 3'-amino metabolite is mediated by cytochrome P-450 and NADPH-cytochrome P-450 reductase in rat liver microsomes.

Using in vitro liver systems, we previously demonstrated that 3'-azido-3'-deoxythymidine (AZT) is reduced to a highly toxic metabolite, 3'-amino-3'-deoxythymidine (AMT) through a NADPH-dependent system. This pathway also occurs for other 3'-azido-2',3'-dideoxynucleosides (3'-azido ddNs), indicating that reduction to a 3'-amino metabolite is a general catabolic route of this class of compounds. This study was undertaken to understand the enzymatic reaction responsible for this catabolic pathway. Rat liver microsomes were exposed to 1 mM [3H]AZT or 1 mM [3H]AzddU, and incubated under various conditions. Reduction to the 3'-amino derivative was enhanced 5-fold by the addition of NADPH. When FAD or FMN was combined with NADPH, AMT and AMddU formation was enhanced 2-fold. Addition of equimolar FAD and FMN enhanced azido reducing activity by 3-fold and 5-fold when compared with NADPH alone for AZT and AzddU, respectively. Exposure to carbon monoxide inhibited 3'-amino formation approximately 60%, consistent with involvement of cytochrome P-450 (P-450). This inhibitory effect was not detected in the presence of combined flavin and NADPH; in control incubations that contained these cofactors but no microsomes, AMT or AMddU formation was not observed. This suggests that a flavoprotein, possibly NADPH-cytochrome P-450 reductase (P-450 reductase), is also involved in azido reduction. Preincubation with various P-450 ligands resulted in variable inhibition; reduction of AZT and AzddU was decreased approximately 20-80%.(ABSTRACT TRUNCATED AT 250 WORDS)