Methane formation from methyl-coenzyme M in a system containing methyl-coenzyme M reductase, component B and reduced cobalamin.

Purified methyl-CoM reductase of Methanobacterium thermoautotrophicum was found to catalyze the reduction of methyl-coenzyme M to methane at a specific rate of 100 nmol X min-1 X mg protein-1 in a system containing partially purified component B, cob(III)alamin (B-12a), and, as electron donors, dithiothreitol or SnCl2. Under these experimental conditions B-12a was reduced to cob(II)alamin (B-12r), which is known to disproportionate to cob(I)alamin (B-12s) and B-12a to a slight extent. Methane formation from methyl-CoM was inhibited by propyl iodide, methyl iodide, chloroform and carbon tetrachloride, which were shown to react with B-12s to the corresponding light-sensitive alkyl cobalamin. Inhibition by propyl iodide was less effective in light-exposed samples. From these findings it is concluded that in this assay system B-12s might serve as electron donor for the enzymatic reduction of methyl-CoM to methane.