Light-induced reduction of pyridine nucleotide and its relation to light-induced electron transport in whole cells of Rhodospirillum rubrum

Abstract

Fluorometric measurements of the light induced reduction of pyridine nucleotide (presumably NAD +) were carried out in living cells of Rhodospirillum rubrum. The rate vs. light intensity curves were different in cells from older cultures as compared with cells from young cultures. In young cells pyridine nucleotide was reduced only by higher intensities of the actinic light. The cytochrome oxidized at these intensities was cytochrome c 2 and the light-induced absorbance changes in the near-infrared spectral region showed only the photooxidation of P870. The uncoupler of photophosphorylation carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) inhibited the pyridine nucleotide photoreduction which could be restored partially by cysteine but not by O 2. In older cells pyridine nucleotide was reduced also at low intensities of the actinic light. If such cells were kept in the dark for a couple of days in a substrate-free medium, pyridine nucleotide was reduced efficiently in the light and the cytochrome predominantly oxidized, even at higher intensities, was C428. The light- minus-dark spectrum in the near-infrared spectral region showed few signs of the photooxidation of P870. Instead, a red shift of an absorption band at 880 nm was observed. FCCP inhibited the pyridine nucleotide photoreduction also under these conditions but the photoreduction could be restored by O 2. The results were interpreted as indicating that this organism uses an energy-linked reversal of electron flow mediated by a high-energy intermediate produced during a light-induced cyclic electron transport when in an early stage of their development. Older cells can use an alternative mechanism for the reduction of pyridine nucleotide. This mechanism probably is a direct light-induced electron transport from exogenous as well as from endogenous substrates to the pyridine nucleotide. The switching over from one mechanism to another is related to the internal redox potential. The present results support a different type of reaction center for each mechanism, although an interpretation based on one reaction center operating both electron transport systems cannot be ruled out.