We have isolated and purified an enzyme (E-2) from Klebsiella pneumoniae, which catalyzes the formation of CO from CH3-S-CH2-CH2-CO-C(OH) = CH-O- (III). This compound is an intermediate in the conversion of 5'-methylthioadenosine to methionine. Concomitant with CO formation, methylthiopropionic acid and formate are produced and O2 is consumed. E-2 also catalyzes the formation of CO, formate, and butyrate from CH3-CH2-CH2-CO-C(OH) = CH-O- (IIIa), the desthio analog of III. Experiments with isotopic IIIa have shown that formate is derived from 1-C, and CO from 2-C. E-2 has a M(r) = 18,500 and requires Mg2+, and no chromophoric cofactor has been detected.