Inhibition of the photosynthetic electron transport of isolated thylakoids by hemolyzed rabbit sera. Evidence for the potential involvement of parallel electron transport in photosystem I Mehler reactions.

The inhibition patterns of rabbit sera (RS1 & RS2) from two different rabbits on the photosynthetic electron transport of isolated spinach thylakoids were studied. Fifty μl of RSI were required for 100% inhibition of a H2O → MV/O2 reaction, while only 10 μl of a 1:10 dilution of RS2 were needed for 100% inhibition. The RS2 serum was greatly hemolyzed. The γ-globulin fraction from purified rabbit serum (RS1) did not inhibit photosynthetic electron transport, indicating that the antibody fraction of the rabbit serum does not contain the inhibitor. It appears that the inhibitor is from the hemolyzed red blood cells. Rabbit sera added prior to chloroplast illumination caused no inhibition, while addition of rabbit sera during illumination inhibited a H2O → MV/O2 reaction within 1-3s. Aminotriazole, a catalase inhibitor, did not affect the efficacy of the rabbit sera indicating that the unknown rabbit serum inhibitor is not catalase. Various Hill reactions were employed to determine the site of inhibition. Rabbit sera inhibited the following reactions: DHQ/DCMU → MV/O2, DAD/Asc/DBMIB → MV/O2, and DCIP/Asc/DBMIB → MV/O2. Rabbit sera did not inhibit a H2O → DADox reaction indicating that inhibition is on the reducing side of PSI. However, a H2O → Fd/NADP(+) reaction was not inhibited by rabbit sera. NADP did not interfere with the ability of RS2 to inhibit a MV-mediated Mehler reaction. In simultaneously measured assays of Fd-mediated O2 and NADP(+) reductions, RS2 serum inhibited the reduction of O2 by ferredoxin without inhibiting the reduction of NADP(+). These results indicate the potential involvement of parallel (branched) electron transport of the reducing side of PSI in the reduction of oxygen.