Reductive dechlorination of 1,2-dichloroethane (1,2-DCA) to ethylene and chloroethane (CA) by crude cell extracts of Methanobacterium thermoautotrophicum AH with H2 as the electron donor was stimulated by Mg-ATP. The heterodisulfide of coenzyme M (CoM) and 7-mercaptoheptanoylthreonine phosphate together with Mg-ATP partially inhibited ethylene production but stimulated CA production compared Mg-ATP alone. The pH optimum for the dechlorination was 6.8 (at 60°C). Michaelis-Menten kinetics for initial product formation rates with different 1,2-DCA concentrations indicated the enzymatic character of the dechlorination. Apparent K"s for 1,2-DCA of 89 and 119 F.M and Vm,cs of 34 and 20 pmol/min/mg of protein were estimated for ethylene and CA production, respectively. 3-Bromopropanesulfonate, a specific inhibitor for methyl-CoM reductase, completely inhibited dechlorination of 1,2-DCA. Purified methyl-CoM reductase, together with flavin adenine dinucleotide and a crude component A fraction which reduced the nickel of factor F430 in methyl-CoM reductase, converted 1,2-DCA to ethylene and CA with H2 as the electron donor. In this system, methyl-CoM reductase was also able to transform its own inhibitor 2-bromoethanesulfonate to ethylene.Hydrogenotrophic and acetoclastic methanogenic bacteria reductively dechlorinate 1,2-dichloroethane (1,2-DCA) to ethylene and chloroethane (CA) (5,12,26). Corrinoids or factor F430, two cofactors present in high amounts in methanogens (10,11,20,35), catalyzed the same transformations in buffer with Ti(III) citrate as the electron donor (27).Corrinoids are found in the soluble as well as the membrane fraction of methanogens (10). Despite its high cobamide content, the role of this cofactor has not yet been fully established. Similar to already known functions of corrinoids in other organisms, cobamides in methanogens are thought to be involved mainly in methyl transfer reactions. This role was verified for the highly purified methanol:5-hydroxybenzimidazolyl-cobamide methyltransferase of Methanosarcina barkeri, an enzyme involved in methanogenesis from methanol (49-52). A possible function as a "redox protein" was proposed for a 33-kDa purified corrinoid-containing membrane protein of Methanobacterium thermoautotrophicum Marburg (44). A monospecific polyclonal antiserum against the 33-kDa corrinoid-containing membrane protein of strain Marburg cross-reacted with the 33-and 31-kDa subunits of the corrinoid-containing 5-methyl-tetrahydromethanopterin: 5-hydroxy-benzimidazolyl cobamide methyltransferase isolated from M. thermoautotrophicum AH (29,47 (17,36,48,55). The involvement of corrinoid enzymes in the methanogenesis from acetate was shown with cell extracts of M. barkeri (8,53).Factor F430, a hydrocorphinoid nickel(II) complex (39) found only in methanogens, can exist in two forms, a protein-bound form and a free form (2, 24). The free form was present only under Ni-sufficient growth conditions (2). Factor F430 is the chromophore of the methyl-coenzyme M (CoM) reductase, which catalyze...