Studies on electron transport associated with photosystem I. III. The reduction sites of various hill oxidants in the photosynthetic electron transport system

Abstract 1. 1. HgCl 2 treatment of chloroplasts was effective in inducing an inhibition of 2,6-dichlorophenolindophenol (DCIP)-Hill activity in chloroplasts from several higher plants. 2. 2. The Hill reaction with various electron acceptors having a relatively high redox potential was severely, but not completely, inhibited by the HgCl 2 treatment of spinach chloroplasts. The surviving Hill activity in HgCl 2 -treated chloroplasts was somewhat higher with lipophilic electron acceptors ( e.g. 2,5-dimethylbenzoquinone) than with hydrophilic acceptors ( e.g. DCIP). 3. 3. DCIP photoreduction in Euglena chloroplasts, sonicated spinach chloroplasts, or Photosystem II particles was highly resistant to the HgCl 2 treatment. 4. 4. The HgCl 2 -insensitive part of the Hill activity showed an optimum at a more acidic pH and required higher concentrations of the electron acceptors for the maximum activity, as compared with the HgCl 2 -sensitive part of the activity. 5. 5. A high concentration of dibromothymoquinone was effective in restoring the DCIP-Hill reaction activity in HgCl 2 -treated chloroplasts. 6. 6. The results obtained indicate that the electron acceptors tested predominantly receive electrons at the reducing side of Photosystem I.