The reactivities of CO2 and the related compounds COS and CS2 with the nickel- and iron- sulfur-containing carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum have been investigated. Both CO2 and COS were substrates for CODH in a reductant-dependent reaction resulting in the formation of CO. CO2 was reduced to CO and H2O, while COS was reduced to CO and H2S. CO was a potent inhibitor of CO2 reduction at dissolved concentrations as low as 1 microM, but this inhibition could be prevented by quantitatively trapping CO as it was formed by including reduced hemoglobin in the assays. The addition of hemoglobin to the assays also allowed the formation of CO to be monitored in real time by following the decrease in absorbance at 433 nm resulting from carboxyhemoglobin formation. A variety of low-potential reductants, including dithionite, titanium(III) citrate, and dithionite-reduced viologens (methyl and benzyl), were suitable electron donors for the reduction of CO2 and COS. Dithionite-reduced methyl viologen supported the highest rates of CO2 and COS reduction, and the stimulation of CO2 reduction (170-fold increased rate over dithionite alone) was much more dramatic than the stimulation of COS reduction (2.6-fold increased rate over dithionite alone). CO2 was reduced to CO with a Km for CO2 of 190 microM and a Vmax of 44 mumol of CO formed min-1 (mg of protein)-1, while COS was reduced with a Km for COS of 2.2 microM and a Vmax of 0.51 mumol of CO formed min-1 (mg of protein)-1.(ABSTRACT TRUNCATED AT 250 WORDS)