The electron transfer system catalysing reduction of fumarate by NADH under anaerobic conditions in membrane preparations of strain 4 was inhibited by 2-n-heptyl-4-hydroxyquinoline -oxide. Reduction of -type cytochrome by NADH under anaerobic conditions was much slower than reduction of fumarate by NADH, and was more sensitive to 2-n-heptyl-4-hydroxyquinoline -oxide inhibition. Thus -type cytochrome is probably not an intermediate in the reduction of fumarate by NADH under the assay conditions used. Extraction of quinone from the membrane and subsequent reincorporation showed that menaquinone (vitamin K) is an obligatory intermediate in the reduction of both fumarate and -type cytochrome by NADH, which is consistent with the 2-n-heptyl-4-hydroxyquinoline -oxide inhibition results. A tentative scheme for electron transport in is presented.


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