Spirilloxanthin is released by detergent from Rubrivivax gelatinosus reaction center as an aggregate with unusual spectral properties

A preparation containing spirilloxanthin has been isolated from Rubrivivax gelatinosus SC2, a mutant devoid of the reaction center-associated tetraheme cytochrome c, after solubilisation of membranes with lauryl-di-methyl-amine oxide. It was purified by ammonium sulfate precipitation and gel filtration, and analyzed by SDS-gel electrophoresis. Spirilloxanthin was shown to be aggregated in large particles (apparent M_W > 600 kDa) and was not associated with a specific protein. This aggregate was characterized by absorption, circular dichroism and resonance Raman spectroscopies. The absorption spectrum contained two UV bands at 370 and 300 nm, and did not present the visible bands of spirilloxanthin, which however reappeared when spirilloxanthin was extracted from the aggregate with organic solvents. Resonance Raman spectra indicated that at least four different populations of spirilloxanthin were present in the preparation as a mixture of different trans and cis configurations. These properties are similar to those described for a so-called carotenoprotein solubilized with sodium dodecyl sulfate from Rhodospirillum rubrum membranes Schwenker et al. (1974) Biochim Biophys Acta 351: 246-260; Kito et al. (1983) Photochem Photobiophys 5: 209-217]. We further observed absorption spectra of pure spirilloxanthin dissolved in mixtures of water, polar solvents and detergent, in the absence of protein, resembling those of the.aggregate. We conclude that the aggregate is not a carotenoprotein, but rather an artefact due to the release of spirilloxanthin from the reaction center, leading to the isomerization and association of spirilloxanthin molecules in a detergent particle. We propose the same interpretation for the complex isolated from Rhodospirillum rubrum.