REDUCTION OF THE PLASTOQUINONE POOL BY EXOGENOUS NADH AND NADPH IN HIGHER-PLANT CHLOROPLASTS - CHARACTERIZATION OF A NAD(P)H-PLASTOQUINONE OXIDOREDUCTASE ACTIVITY
S. Corneille et al., REDUCTION OF THE PLASTOQUINONE POOL BY EXOGENOUS NADH AND NADPH IN HIGHER-PLANT CHLOROPLASTS - CHARACTERIZATION OF A NAD(P)H-PLASTOQUINONE OXIDOREDUCTASE ACTIVITY, Biochimica et biophysica acta. Bioenergetics, 1363(1), 1998, pp. 59-69
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.