Demethylation and degradation of phenylmethylethers by the sulfide-methylating homoacetogenic bacterium strain TMBS 4

Biochemical studies on anaerobic phenylmethylether cleavage by homoacetogenic bacteria have been hampered so far by the complexity of the reaction chain involving methyl transfer to acetyl-CoA synthase and subsequent methyl group carbonylation to acetyl-CoA. Strain TMBS 4 differs from other demethylating homoacetogenic bacteria in using sulfide as a methyl acceptor, thereby forming methanethiol and dimethylsulfide. Growing and resting cells of strain TMBS 4 used alternatitively CO 2 as a precursor of the methyl acceptor CO for homoacetogenic acetate formation. Demethylation was inhibited by propyl iodide and reactivated by light, indicating involvement of a corrinoid-dependent ethyltransferase. Strain TMBS 4 contained ca. 750 nmol g dry mass - 1 of a corrinoid tentatively identified as 5-hydroxybenzimidazolyl cobamide. A photometric assay for measuring the demethylation activity in cell extracts was developed based on the formation of a yellow complex of Ti 3 § with 5-hydroxyvanillate produced from syringate by demethylation. In cell extracts, the methyltransfer reaction from methoxylated aromatic compounds to sulfide or methanethiol depended on reductive activation by Ti 3 +. ATP and Mg 2 + together greatly stimulated this reductive activation without being necessary for the demethylation reaction itself. The specific activity of the transmethylating enzyme system increased proportionally with protein concentration up to 3 mg ml- 1 reaching a constant level of 20 nmol min -a mg -1 at protein concentrations > 10 mg ml- 1. The specific rate of activation increased in a non-linear manner with protein concentration. Strain TMBS 4 degraded gallate, the product of sequential demethylations, to 3 acetate through the phloroglucinol pathway as found earlier with Pelobacter acidigallici