REDUCTION OF THE PLASTOQUINONE POOL BY EXOGENOUS NADH AND NADPH IN HIGHER-PLANT CHLOROPLASTS - CHARACTERIZATION OF A NAD(P)H-PLASTOQUINONE OXIDOREDUCTASE ACTIVITY

Chlorophyll fluorescence measurements were performed on osmotically ly sed potato chloroplasts in order to characterize the reactions involve d in the dark reduction of photosynthetic inter-system chain electron carriers. Addition of NADH or NADPH to lysed chloroplasts increased th e chlorophyll fluorescence level measured in the presence of a non-act inic light until reaching F-max, thus indicating an increase in the re dox state of the plastoquinone (PQ) pool. The fluorescence increase wa s more pronounced when the experiment was carried out under anaerobic conditions and was about 50% higher when NADH rather than NADPH was us ed as an electron donor. The NAD(P)H-PQ oxidoreductase reaction was in hibited by diphenylene iodonium, N-ethylmaleimide and dicoumarol, but insensitive to rotenone, antimycin A and piericidin A. By comparing th e substrate specificity and the inhibitor sensitivity of this reaction to the properties of spinach ferredoxin-NADP(+)-reductase (FNR), we i nfer that FNR is not involved in the NAD(P)H-PQ oxidoreductase activit y and conclude to the participation of rotenone-insensitive NAD(P)H-PQ oxidoreductase. By measuring light-dependent oxygen uptake in the pre sence of DCMU, methyl viologen and NADH or NADPH as an electron donors , the electron flow rate through the NAD(P)H-PQ oxidoreductase is esti mated to about 160 nmol O-2 min(-1) mg(-1) chlorophyll. The nature of this enzyme is discussed in relation to the existence of a thylakoidal NADH dehydrogenase complex encoded by plastidial ndh genes. (C) 1998 Elsevier Science B.V.