Dimethylsulphoxide and trimethylamine-N-oxide as bacterial electron transport acceptors: Use of nuclear magnetic resonance to assay and characterise the reductase system in Rhodobacter capsulatus

Nuclear magnetic resonance is established as a sensitive and specific method for following the reduction of dimethylsulphoxide and trimethylamine-N-oxide by bacteria. Using this method it has been shown that cells of Rhodobacter capsulatus reduce both dimethylsulphoxide and trimethylamine-N-oxide at linear rates at all concentrations of these acceptors that can be conveniently detected during a continuous assay. The rate of reduction of trimethylamine-N-oxide was eightfold higher than the rate of dimethylsulphoxide reduction. An upper limit of approximately 0.1 mM may be placed upon the apparent K_m value for each acceptor, but the value for dimethylsulphoxide is deduced to be lower than that for trimethylamine-N-oxide on the basis of the strong inhibitory effect of the former on the reduction of the latter. Reduction of trimethylamine-N-oxide by Rb. capsulatus was inhibited by illumination and by oxygen, but only the former effect was relieved following dissipation of the proton electrochemical gradient across the cytoplasmic membrane. Rotenone inhibited the reduction of trimethylamine-N-oxide whereas myxothiazol did not, consistent with a pathway of electrons to the reductase from NADH dehydrogenase that does not involve the cytochrome bc₁complex.