Fumarate reduction inProteus mirabilis

1. Proteus mirabilis formed fumarate reductase under anaerobic growth conditions. The formation of this reductase was repressed under conditions of growth during which electron transport to oxygen or to nitrate is possible. In two of three tested chlorateresistant mutant strains of the wild type, fumarate reductase appeared to be affected.
2. Cytoplasmic membrane suspensions isolated from anaerobically grownP. mirabilis oxidized formate and NADH with oxygen and with fumarate, too.
3. Spectral investigation of the cytoplasmic membrane preparation revealed the presence of (probably at least two types of) cytochromeb, cytochromea ₁ and cytochromed. Cytochromeb was reduced by NADH as well as by formate to approximately 80%.
4. 2-n-Heptyl-4-hydroxyquinoline-N-oxide and antimycin A inhibited oxidation of both formate and NADH by oxygen and fumarate. Both inhibitors increased the level of the formate/oxygen steady state and the formate/fumarate steady state.
5. The site of inhibition of the respiratory activity by both HQNO and antimycin A was located at the oxidation side of cytochromeb.
6. The effect of ultraviolet-irradiation of cytoplasmic membrane suspensions on oxidation/reduction phenomena suggested that the role of menaquinone is more exclusive in the formate/fumarate pathway than in the electron transport route to oxygen.
7. Finally, the conclusion has been drawn that the preferential route for electron transport from formate and from NADH to fumarate (and to oxygen) includes cytochromeb as a directly involved carrier. A hypothetical scheme for the electron transport in anaerobically grownP. mirabilis is presented.