Growing cultures of Clostridiumpasteurianum were found to oxidize carbon monoxide to CO, at a maximal rate of approximately 15 nmol x min-l x mg protein-'. The dependence of the rate on the carbon monoxide concentration followed simple Michaelis-Menten kinetics. Half-maximal velocity was obtained with 3 pM carbon monoxide in the growth medium. The values of Vand[S],,, determined in vivo are similar to those reported for carbon monoxide oxidation with flavin nucleotides in cell-free extracts of C. pasteurianum (V = 14 nmol x min-' x mg protein-' ; [S],,, = 5 pM), indicating that in vivo and in vitro carbon monoxide is oxidized by the same system.Carbon monoxide was even removed from the medium by growing cultures of C. pasteurianum at concentrations as low as 1 nM equal to a concentration in the gas phase of approximately 1 part per million. Carbon monoxide was oxidized at a rate of 8 pmol x 1-' x h-' under these conditions. The biological significance of this finding is discussed.Cell-free extracts of NH,-grown and N,-grown Clostridium pasteurianum have been reported to catalyze the oxidation of carbon monoxide to C 0 2 under strictly anaerobic conditions. Free formate was not an intermediate. FAD, FMN, methylene blue, and viologen dyes were effective as electron acceptors, while NAD', NADPf, or oxidized ferredoxin were ineffective. Inactivation studies with cyanide and alkyl halides indicated that a vitamin B,, compound is probably involved in the catalysis of anaerobic carbon monoxide oxidation [l].The finding that cell-free extracts of C.pasteurianum catalyzed the oxidation of carbon monoxide was rather astonishing, as n o reports were available indicating that this strictly anaerobic bacterium can oxidize carbon monoxide in vivo. The physiological significance of the finding thus remained unclear.In this communication evidence is presented that carbon monoxide is oxidized to CO, by growing cultures of C.pasteurianum. The values of V and [S],,, determined in vivo are shown to be almost identical with those obtained for carbon monoxide oxidation with flavin nucleotides in cell-free extracts of the organism, indicating that in vivo and in vitro carbon monoxide is oxidized by the same system. Note. In this paper no distinctions will be made between COz, HC03 -, and CO:-.

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