Light-induced oxygen uptake by photosynthetic bacterial chromatophores

Abstract

The biochemical nature of light induced oxygen uptake by the photooxidase system was studied with partially purified chromatophores isolated from photosynthetically grown cells of $\mbox{\underline{Rhodospirillum}}$ $\mbox{\underline{rubrum}}$. Such an oxygen uptake was found to be required electron donors such as TMPD/ascorbate or DCIP/ascorbate as well as illuminated light. The rate of oxygen uptake increased by the increasing amount of chromatophores, and most of the activity depends on 800-900 nm which corresponds to the peak absorption of bacteriochlorophylls. From a series of kinetic and related experiments, it was suggested that the oxygen uptake by chromatophores is mediated by the action of an enzyme, photooxidase. The activity was optimal at pH 8.0 and 35``5C, and energy of activation was 1.9 Kcal/mole. The kinetic constants were $K_{TMPD/ascorbate}=39uM/2mM$ and $K_{oxygen}=18uM$. The photooxidase activity was enhanced by the addition of antimycin A which is an electron transport inhibitor between cytochrome b and cytochrome c. This fact may indicate that the light induced oxygen uptake is due to an interaction of oxygen with some constituent above cytochrome b level in electron transport chain of chromatophore. The photooxidase appeared to be a different type oxidase from cytochrome c oxidase of terminal electron transport chain in mitichondria since its activity was not inhibited by cyanide and azide. The photooxidase reaction operated, independent of $NAD^+$ photoreduction, but in some connection with photophosphorylation. That is, photophosphorylation activity was markedly reduced when photooxidase reaction fully operated in the presence of much oxygen. Therefore it is believed that photophosphorylation and photooxidation have a common center from which electrons are drawn out to drive each reaction. It seems that the role of the photooxidase in bacterial chromatophores is to scavenge overexcited electrons beyond the capacity of photosynthetic sy...