Studies on the metabolism of photosynthetic bacteria. XVII. Comparative studies on simultaneous photooxidations in bacterial and plant extracts

Abstract

1. 1. Photooxidations of various redox couples by spinach leaf homogenates and sonic extracts of Rhodospirillum rubrum cell suspensions have been studied and compared. 2. 2. The most active couple found was the combination of 2,6-dichlorophenolindophenol, acting as electron carrier, and ascorbic acid, acting as ultimate electron donor. In both plant and bacterial extracts, it was found that photochemical oxygen uptake occurred until exhaustion of the excess ascorbate, after which the residual oxidation rate fell sharply to a rate equalling that in dark controls with leaf extracts, and to a rate slightly higher than dark controls in bacterial extracts. The stoichiometric ratio of two moles ascorbate oxidized for each mole oxygen taken up was found in general, although fluctuations in this ratio were noted occasionally in individual extracts. 3. 3. Addition of ethanol and catalase in large enough amounts to achieve maximal rate of the catalase-mediated oxidation of ethanol resulted in a sparing effect on ascorbate oxidation, without affecting the initial rate of oxygen uptake. This observation, together with the finding that ethanol photooxidation ceased upon exhaustion of ascorbate, indicated that the photooxidations of ethanol and ascorbate were coupled, rather than merely simultaneous. 4. 4. Measurements of the amounts of acetaldehyde produced, ascorbate oxidized, and oxygen consumed, provided confirmatory evidence that the oxidation of the DPIP-ascorbate couple was coupled photochemically with the catalase-mediated oxidation of ethanol. Hydrogen peroxide did not appear to be an intermediate in the coupling mechanism. 5. 5. The observations on the coupled photooxidation of ethanol and ascorbate in both plant and bacterial preparations were consistent with the assumption that a Hill system, modified to include oxygen as a Hill oxidant, was generated in the plant extracts, and that analogous mechanisms were available in the bacterial extracts. 6. 6. The significance of the results obtained in these researches for future experimentation is discussed.