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 S Delta C2, a mutant devoid of the reaction center-associated t etraheme cytochrome c, after solubilisation of membranes with lauryl-di-met hyl-amine oxide. It was purified by ammonium sulfate precipitation and gel filtration, and analyzed by SDS-gel electrophoresis. Spirilloxanthin was sh own to be aggregated in large particles (apparent M-w > 600 kDa) and was no t associated with a specific protein. This aggregate was characterized by a bsorption, circular dichroism and resonance Raman spectroscopies. The absor ption spectrum contained two UV bands at 370 and 300 nm, and did not presen t the visible bands of spirilloxanthin, which however reappeared when spiri lloxanthin was extracted from the aggregate with organic solvents. Resonanc e Raman spectra indicated that at least four different populations of spiri lloxanthin 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 R hodospirillum rubrum membranes [Schwenker et al. (1974) Biochim Biophys Act a 351: 246-260; Kito et al. (1983) Photochem Photobiophys 5: 209-217]. We f urther observed absorption spectra of pure spirilloxanthin dissolved in mix tures of water, polar solvents and detergent, in the absence of protein, re sembling those of the.aggregate. We conclude that the aggregate is not a ca rotenoprotein, 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 inte rpretation for the complex isolated from Rhodospirillum rubrum.