THE NADPH-SULFITE REDUCTASE OF ESCHERICHIA-COLI IS A PARAQUAT REDUCTASE

The NADPH:sulfite reductase of Escherichia coli is a soluble enzyme th at has a subunit structure alpha(8) beta(4), where the alpha subunit i s a flavoprotein and the beta subunit is a metalloprotein. Overexpress ion of the holoenzyme in E. coli has greatly simplified the purificati on of this enzyme. Under aerobic conditions, recombinant sulfite reduc tase catalyzes the reduction of 1,1'-dimethyl-4,4'-bipyridinium dichlo ride (paraquat) by NADPH, with K-m values for paraquat and NADPH of ap proximately 70 mu M and 80 mu M, respectively. Since pure flavoprotein a subunit, encoded by the cysJ gene, has similar catalytic activities , it is suggested that paraquat receives electrons directly from the a lpha subunit. A mutant strain lacking an active cysJ gene is resistant to paraquat. The NADPH:ferredoxin reductase of E. coil is also a para quat reductase but with much higher K-m values for paraquat and lower enzyme activities. These results suggest that the sulfite reductase is a major paraquat reductase in E. coil and is responsible for the toxi c activation of the drug.