Natural and synthetic porphyrins in acetone solution or incorporated in lipid vesicle membranes. Photosensitization of singlet oxygen and of the methyl red—ascorbate oxidation—reduction

Evidence of singlet oxygen evolution by whole living cells of Chlamydomonas reinhardtii

The oxygen evolved by Chlamydomonas reinhardtii in the light is measured simultaneously with a Clark electrode and with the nitrosodimethylaniline-imidazole colorimetric method which is specific for singlet oxygen. Experiments with wild-type and FuD7 mutant cells (unable to synthesize the D1 protein of Photosystem II), with dichlorophenyldimethylurea (which blocks electron transfer from Photosystem II to Photosystem I) and with dibromothymoquinone (which diverts electrons from their normal path between the two photosystems), as well as with hydroxylamine (an inactivator of the water-splitting part of Photosystem II and a competitor of water for electron donation to it), all point to the dependence of detected singlet oxygen on photolysis of water by Photosystem II.

The action of ascorbate in vesicular systems

Many effects of ascorbate center on its interactions with membranes from plant and animal cells. These actions can be studied using vesicles produced from phospholipid components (liposomes), by isolating naturally occurring vesicles, or by purifying particular membranes that form vesicles during the extraction process. Liposomes have provided information concerning the anti- and prooxidant properties of ascorbate and about how the water-soluble vitamin can have effects within the phospholipid bilayer. The involvement of ascorbate in transmembrane electron transport has been characterized in vesicles normally found in certain cells, such as, chromaffin granules, synaptosomes, glyoxisomes, peroxisomes, and clathrin-coated vesicles. Redox activity using reducing power associated with ascorbate/ascorbate free radical (AFR) has been characterized in some of these vesicles and it appears to be mediated by a b-type cytochrome. Ascorbate also participates in the reduction of iron within clathrin-coated vesicles. Vesicles appearing during purification of plasma membranes have transmembrane electron transport, oxidoreductase activity with ascorbate/AFR as redox agents, and an ascorbate-reducible b-type cytochrome. It is also possible that ascorbate-related redox activity exists at the tonoplast of plant cells.