Twofold effect of valinomycin on isolated spinach chloroplasts: uncoupling and inhibition of electron transport.
(1) Treatment of isolated chloroplasts with valinomycin above 10(-7) M inhibits both photophosphorylation and ms-delayed light emission, with maximum inhibition occurring at 10(-5) M. (2) In the same concentration range it stimulates electron flow, when the pH of the medium is between 7 and 8, inhibits the extent of the light-induced pH change and reverses 2,3,5,6-tetramethyl-p-phenylenediamine-quenched chlorophyll a fluorescence in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea. (3) These effects of valinomycin occur in the absence of added K+ although the presence of K+ does facilitate its action. (4) It is suggested that high concentrations of valinomycin not only induce a change in the K+ permeability but also in the H+ permeability of the thylakoid membrane and in so doing dissipate the high-energy state, i.e. uncouple. (5) However, electron transport data do not clearly demonstrate the uncoupling action of valinomycin since it also inhibits electron flow. (6) Valinomycin at concentrations above 10(-6) M inhibits electron flow and stimulates chlorophyll a fluorescence of chloroplasts uncoupled by nigericin or NH4Cl. (7) The dual effect of valinomycin accounts both for the previous suggestion that this compound inhibits photophosphorylation by acting as an energy transfer inhibitor and also explains why valinomycin modifies both normal ms-delayed light and KCl-induced ms-delayed light transients.
Telfer
A
A
Botany Department, Imperial Collage, Prince Consort Road, London, S.W.7, Great Britain.
Barber
J
J
D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism; D004579:Electron Transport / Q000502:physiology*; D007476:Ionophores / Q000378:metabolism*; D008027:Light; D009550:Nigericin / Q000378:metabolism; D018724:Spinacia oleracea / Q000166:cytology / Q000378:metabolism; D014475:Uncoupling Agents / Q000494:pharmacology*; D014634:Valinomycin / Q000494:pharmacology*
D007476:Ionophores; D014475:Uncoupling Agents; D002734:Chlorophyll; D014634:Valinomycin; D009550:Nigericin