Use of deuterium labelling from deuterium oxide to demonstrate carotenoid transformations in photosynthetic bacteria

Carotenoid biosynthesis in many purple photosynthetic bacteria of the Rhodospirillaceae is inhibited by nicotine, and biosynthetic intermediates accumulate. If the inhibitor is removed and the bacteria are then incubated in buffered 99.6% deuterium oxide, deuterium is incorporated specifically into the C-2 position in both cyclic and acyclic carotenoids that are then formed from the previously accumulated hydrocarbon precursors. The deuterated molecular species can be detected and assayed by mass spectrometry. By use of this procedure, direct proof has been obtained for the conversion of lycopene into β-carotene and rhodopin in Rhodomicrobium vannielii, of neurosporene into spheroidene in Rhodopseudomonas sphaeroides and of spheroidene into hydroxyspheroidene in Rps. gelatinosa. The results confirm the operation of the biosynthetic pathways postulated for these organisms, and prove that formation of the acyclic 1-hydroxy-1,2-dihydro end-group characteristic of the carotenoids of photosynthetic bacteria occurs by addition of water to the C-1,2 double band.