The NADPH: sulfite reductase of Escherichia coli is a paraquat reductase.

The NADPH:sulfite reductase of Escherichia coli is a soluble enzyme that has a subunit structure alpha 8 beta 4, where the alpha subunit is a flavoprotein and the beta subunit is a metalloprotein. Overexpression of the holoenzyme in E. coli has greatly simplified the purification of this enzyme. Under aerobic conditions, recombinant sulfite reductase catalyzes the reduction of 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat) by NADPH, with Km values for paraquat and NADPH of approximately 70 microM and 80 microM, respectively. Since pure flavoprotein alpha subunit, encoded by the cysJ gene, has similar catalytic activities, it is suggested that paraquat receives electrons directly from the alpha subunit. A mutant strain lacking an active cysJ gene is resistant to paraquat. The NADPH:ferredoxin reductase of E. coli is also a paraquat reductase but with much higher Km values for paraquat and lower enzyme activities. These results suggest that the sulfite reductase is a major paraquat reductase in E. coli and is responsible for the toxic activation of the drug.