Nitrofurazone-reducing enzymes in E. coli and their role in drug activation in vivo

Earlier work showed that Escherichia coli contains at least two enzymes which reduce nitrofurazone and other nitrofuran derivatives. One of these enzymes is lacking in some nitrofurazone-resistant mutant strains. We now report that there are three separable nitrofuran reductases in this organism: reductase I (mol. wt. ~50 000, insensitive to O 2 ), reductase IIa (mol. wt. ~120 000, inhibited by oxygen), reductase IIb (mol. wt. ~700 000, inhibited by O 2 ).Unstable metabolites formed during the reduction of nitrofurazone by preparations containing reductases IIa and IIb produce breaks in DNA in vitro. In vivo experiments with nitrofurazone-resistant strains, which lack reductase II but contain reductases IIa and IIb, demonstrated that lethality, mutation, and DNA breakage are all greatly increased when cultures are incubated under anaerobic conditions, i.e., conditions such that reductase II is active. These results provide further evidence for the importance of reductive activation of nitrofurazone.