| Abstract: | Abstract— PS-I particles isolated according to Shiozawa et al. (1974) show increased rates of O2-- and H +-uptake with ascorbate as electron donor upon combination with an artificial vesicular lipid membrane. The amount of increase varies depending on the reconstitution procedure used. Combination of PS-I particles with Triton X-100 micelles increases these photochemical activities even more. The observed proton uptake in PS-I lipid vesicles is not caused by the well-known proton gradient found in thylakoid membranes, since lipid vesicles containing extracted leaf Chl show the same activities and uncouplers have no effect. Because these phenomena are also caused by solubilized Chl, it is concluded that there is no obvious correlation with PS-I activity. Proton uptake most probably is caused by oxidation of ascorbate by either singlet oxygen, superoxide or OH-radicals formed in the light. Experimental results are obtained which indicate that Chl in lipid catalyzes formation of superoxide and singlet oxygen. However, it is not clear whether superoxide formation is caused by direct electron transport from excited Chl to oxygen or by a secondary reaction. Diphenylcarbazone disproportionation has been reported as a specific photosystem I reaction. However, PS-I lipid vesicles and Chl-lipid-Triton X-100 mixtures oxidize DPCN at comparable rates, showing that the reaction is not specific for PS-I. Cations stimulate DPCN disproportionation in Chl-lipid-Triton X-100 mixtures but do not affect the rate of P700 photooxidation at all. Therefore it is suggested that Gross and Greniers (1978) conclusion that cation regulation of PSI electron flow (studied by DPCN disproportionation of PS-I particles in Triton X-100 micelles) provides a fine tuning mechanism for energy transfer, has to be reevaluated. |
