Chromate reduction by Rhodobactor sphaeroides

Rhodobacter sphaeroides grew in the presence of up to 43 muM chromate and r educed hexavalent chromium to the trivalent form under both aerobic and ana erobic conditions. Reduced chromium remained in the external medium. Reduct ase activity was present in cells of Rb. sphaeroides independent of whether chromate was present or not in the growth medium. The reducing activity wa s found in the cytoplasmic cell fraction and was dependent on NADH. The chr omate-reducing enzyme was purified by anion exchange, hydroxyapatite and hy drophobic interaction chromatography, and gel filtration. The molecular wei ght of the enzyme was 42 kDa as determined by gel filtration. The optimum o f the reaction is at pH 7.0 and 30 degreesC. The enzyme activity showed a h yperbolic dependence on the concentrations of both substrates, NADH and chr omate, with a maximum velocity at 0.15 mM NADH. A K-m of 15+/-1.3 muM CrO42 - and a V-max of 420+/-50 mu mol min(-1) mg protein(-1) was determined for the enzyme isolated from anaerobically grown cells and 29+/-6.4 muM CrO42- and 100+/-9.6 mu mol CrO42- min(-1) mg protein(-1) for the one from aerobic ally grown ones.