Generation of reducing power in photosynthetic bacterial chromatophores

Abstract

The mechanism of light-dependent $NAD^+$ reduction was studied with chromatophore prepared by partial purification from anaerobically grown cells of $\mbox{\underline{Rhodospirillum}}$ $\mbox{\underline{rubrum}}$. In the experimental system, key observation includes the followings: (a) Light-dependent reduction of pyridine nucleotide was specific for $NAD^+$, in contrast to that in plant system. Succinate, $DCIPH_2$, $TMPDH_2$, $PMSH_2$ and $H_2$ were effective as an electron donor for the light-dependent $NAD^+$ reduction. ; (b) In sharp contrast to plant photosynthesis, ADP, a phosphate acceptor, inhibited the light-dependent $NAD^+$ reduction and the inhibition was reversed by DCCD, an energy transfer inhibitor. A series of high-energy compounds including ATP, PPi, UTP, CTP, CP and PEP as well as light could also be used to overcome energy barrier between succinate/fumarate couple (E! = -0.031 V) and NADH/$NAD^+$ couple (E! = -0.32 V). ;(c) A component of respiratory electron transport system including NADH dehydrogenase, succinate-DCIP oxidoreductase and cytochrome oxidase, which was inhibited by KCN, was detected in chromatophore, and (d) Uncouplers, such as DNP, gramicidine and arsenate, inhibited the reactions driven by light or ATP. Antimycin A, o-phenanthroline and HOQNO inhibited the light-driven $NAD^+$ reduction and photophosphorylation, but had no effect when ATP was used as the energy source. However, both the light-and ATP-driven reduction of $NAD^+$ inhibited by rotenone. Based on all the observations, a model was proposed. In the model, high-energy intermediate, produced in the cyclic, light-dependent electron transport system, can be used to meet the requirement of various reactions ; ATP formation, PPi formation and $NAD^+$ reduction, etc. It was also shown that high-energy intermediate is a common source of energy for photophosphorylation and $NAD^+$ reduction. Inhibitions of DCCD and uncouplers were also an excellent evidence for the role...