Graduate Thesis Or Dissertation
 

Spinach peroxidase and the photo-oxidation of manganese by isolated chloroplasts

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  • Manganese has been known to be essential for photosynthesis for almost 30 years. Subsequent experimentation indicated a definite function for manganese in oxygen evolution. However, the mode of action of manganese and further localization of its site of action have not been determined. Ten years ago it was demonstrated that illuminated chloroplasts were capable of oxidizing manganese. The role of manganese in photosynthesis and the observed photooxidation of manganese by chloroplasts have often been correlated theoretically but never experimentally. Peroxidase was secondarily implicated since it is a stimulant for the manganese oxidizing reaction. The experiments presented in this thesis were conducted in order to determine more about the manganese oxidizing reaction and its relation to photosynthesis, and to characterize spinach peroxidase both as a stimulant in manganese oxidation and as a separate entity. If added, excess manganese replaces water in the photo-oxidizing system, the reaction should have properties similar to those of the Hill reaction. Results presented herein show the photo-oxidation to be sensitive to DCMU and simazine. Also the light saturation is practically identical to that of Hill reaction as is the response to temperature. Unlike the Hill reaction, the system is inhibited by acriflavin and KCN and is stimulated by FMN. Since the system requires oxygen as the oxidant and is stimulated by peroxidase, the latter results are to be expected. Additional information on the requirement for plastoquinone and other cofactors is presented. It appears that manganese is a redox intermediate acting at a site in photosynthetic electron transport between the point of oxygen evolution and the site of DCMU and simazine inhibition. Peroxidase is known to be present in almost all plants. The specific function of the enzyme is not known. It has been studied extensively in extracts from horseradish roots but not from green leaf tissue. Therefore, a preliminary study of spinach peroxidase was conducted. This study includes a purification procedure and an elementary characterization including pH optimum, substrate reactivity, and inhibitor studies. Although the results may contribute something to the multitudinous in vitro studies on peroxidase, no evidence for a specific in vivo function for this enzyme was obtained.
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