Succinate-dependent NAD + Reduction by the Reversed Respiratory Electron Transport in Bradyrhizobium japonicum

Abstract

Summary Vesicles from Bradyrhizobium japonicum 122 DES catalyze a NAD + reduction which is strictly dependent on ATP and is stimulated by Succinate. NADH formation is not inhibited by KCN and antimycin A but is strongly affected by FCCP, indicating that NAD+ is reduced by complex I of the respiratory chain functioning in the reverse direction. The levels of ATP and NAD + apparently control the flow of electrons to either O 2 or NAD + . High concentrations of NAD + and ATP block NADH dependent respiratory O 2 -uptake and favour the formation of NADH by the reversed respiratory electron transport with the vesicles. NAD + affects complex I of the respiratory chain, whereas ATP blocks NADH and Succinate-dependent respiratory O 2 -uptake. NADP + is also reduced by the Succinate-dependent reversed respiratory electron transport with slightly higher activity. Reversed respiratory electron transport might have a physiological role in the generation of reductants for processes like C0 2 -fixation under lithoautotrophic growth conditions, N 2 -fixation or assimilatory nitrate reduction.