The generally observed light-induced uptake of protons into the thylakoid lumen is diminished by adding protonophores. Instead of the H+ uptake, the release of protons was observed during illumination in the presence of various protonophores at high concentrations, namely, 1 μM nigericin, 10 μM carbonylcyanide m-chlorophenylhydrazone or 30 μM gramicidin. An uncoupler, NH4C1 (4 mM), and a detergent, Triton X-100 (0.02%), also induced the H+ release but a K+ ionophore, valinomycin, did not. The amount of H+ released reached about 100 nmol H+ (mg Chl)-1 at pH 7.5 under continuous illumination. The rate of the H+ release was similar to that of the conventional H+ uptake but its dark relaxation was much slower than that of the H+ uptake. We compared the H+ release in protonophore-added thylakoids with the previously reported H+ release in coupling factor 1 (CF1-depleted thylakoids. The H+ release in thylakoids with nigericin showed similar characteristics to that in CF1-depleted thylakoids in terms of their responses to pH, phenazine methosulfate and light intensity. Both types of H+ release were relatively insensitive to DCMU and were stimulated somewhat by DCMU at low concentrations (around 200 nM). Nigericin did not inhibit the superoxide dismutase activity of the membranes. These results indicate that the H+ release in protonophore-added thylakoids and that in CF1 depleted thylakoids involve the same mechanism and that water-derived protons from PS II that result from an impairment of the activity of superoxide dismutase, as previously proposed, are not involved. Judging from the rate of electron flow and the lumenal acidification under the illumination, we conclude that the H+ release is a light-dependent scalar process which can be observed in thylakoid membranes with high H+ permeability. The H+ release of this type was not observed in mitochondria from rat liver or in chromatophores from Rhodobacter sphaeroides.
|Number of pages||10|
|Journal||Plant and Cell Physiology|
|Publication status||Published - Oct 1991|
All Science Journal Classification (ASJC) codes
- Plant Science
- Cell Biology