Fractionation of the cell extract led to partial isolation of four protein fractions that are involved in the dissimilation of pyruvate into acetyl-CoA and formate: Enzyme I , molecular weight about 140000 ; enzyme 11, molecular weight about 30000, which requires an activation by ferrous ion and dithiols; enzyme 111, molecular weight 25000, which is a flavoprotein with flavin-mononucleotide as coenzyme ; and a hitherto less characterized fraction IV. The system in addition comprises the cofactors S-adenosylmethionine and thiamine diphosphate.Enzyme 111, fraction IV, and thiamine diphosphate could be replaced by the reagents cobaltous ion and thiols which form complexes of high reducing power. This part of the enzyme system obviously plays an auxiliary role, most likely by providing an appropriate redox potential via pyruvate as electron donor.The ability to dissimilate pyruvate was found to arise only and specifically from an interaction between enzyme I, ferrous ion-activated enzyme I1 and adenosylmethionine, in presence of either of t,he above auxiliary systems. Enzyme I is thereby converted into an active form, possibly by reduction, which alone is responsible for the catalysis of the pyruvate dissimilation reaction. The identification of this pyruvate formate-lyase was accomplished by sedimentation experiments in anaerobic sucrose gradients. As a substitute for the classical "clastic reaction", the semi-systematic term "pyruvate formate-lyase reaction", which will be used here, has recently been introduced to designate reaction (1) [5].I n sharp contrast to the well established mechanism and enzymology of pyruvate dehydrogenation in Escherichia coli [6], only a little information is available about the pyruvate dissimilation. The elucidation of its mechanism has been hampered mainly by the oxygen inhibition of the system [7,8] and the multiplicity of low and high molecular weight

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