Nitroreductase from Bacillus licheniformis: a stable enzyme for prodrug activation

5-aziridinyl-2,4-dinitrobenzamide (CB1954) has potential applications in enzyme/prodrug targeted anti-cancer therapies since it can be activated by nitroreductases to form a cytotoxic, bifunctional hydroxylamine derivative. A nitroreductase that can activate CB1954 has been previously isolated from Escherichia coli, but its usefulness is limited by its poor stability and low catalytic efficiency for CB1954. We now report the identification and characterization of a nitroreductase enzyme from the thermophilic bacterium Bacillus licheniformis. Although there is only 28% amino acid sequence identity between this enzyme and the previously isolated E. coli nitroreductase, the two enzymes have a number of characteristics in common. Both enzymes have been shown to reduce both CB1954 and menadione in the presence of NADH and NADPH. However, whereas E. coli nitroreductase produces equimolar amounts of the 2- and 4- hydroxylamine derivative of CB1954, the B. licheniformis enzyme produces only the desired 4-hydroxylamine derivative. It has a preference for NADPH as cosubstrate, and is also active with a range of CB1954 derivatives as substrate and reduced pyridinium cofactor analogues. Moreover, the enzyme is much more thermostable than the E. coli nitroreductase and shows maximum activity at 30 degrees C. These characteristics suggest that the B. licheniformis nitroreductase may be a possible candidate enzyme for enzyme/prodrug therapies due to its bacterial origin, the high activity observed with CB1954 and its enhanced stability.