PARENT SESSION
Posters P1A Proton coupled electron transport and ATPase. Abstracts (172-180)

Involvement of NADH and peripheral NADH dehydrogenase in the cyclic electron flow in higher plants. Tomoki Nishino^*,1, Yasuhiro Kashino¹, Hiroyuki Koike¹, Kazuhiko Satoh¹, ¹ Depertment of Life Science, Koto, Hyogo, Japan

ABSTRACT- In order to study involvement of NADH as well as NADPH in the photosynthetic cyclic electron flow in higher plants, we examined the effects of NADH and NADPH on the rereduction of P700 in spinach (Spinacia oleracea L.) broken chloroplasts. Rereduction rates of P700 were observed after 10 seconds of light illumination in the presence of DCMU (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea) using a PAM101/103 fluorometer. The rereduction rate of P700 was accelerated by addition of NADPH. Surprisingly, the rate was also accelerated to a similar extent by addition of NADH. The accelerating effect of NADH was inhibited by Rotenone, which is known as a specific inhibitor of complex I-type NAD(P)H dehydrogenase (NDH-1). When the diaphorase activity of the thylakoid membranes was measured, the activity that was measured using NADH as an electron donor was only about 15% of that with NADPH. Rereduction of P700 was more accelerated by simultaneous addition of ferredoxin with NADPH, and this acceleration was inhibited by Antimycin A. However, simultaneous addition of ferredoxin with NADH had no effect. From these results, it was suggested that the first electron carrier, which reacts with NADH, is not FNR (ferredoxin-NADP⁺ reductase). When the broken chloroplasts were treated with CaCl₂ or CHAPS (3-((3-Cholamidopropyl)dimethylammonio)-1-propanesulfonate), the proteins, which react with NADPH or NADH, were released from the thylakoid membranes simultaneously. It has been thought that NDH-1 is involved in cyclic electron flow. Therefore, we suggest that NADH and unknown NADH dehydrogenase are also involved in cyclic electron flow in higher plant photosynthesis.

KEY WORDS: Cyclic electron flow, NDH, NADH, Spinach