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Studies on well-coupled Photosystem I-enriched subchloroplast vesicles. Neutral red as a probe for external surface charge rather than internal protonation
Ist Teil von
Biochimica et biophysica acta. Bioenergetics, 1985-01, Vol.809 (2), p.204-214
Ort / Verlag
Elsevier B.V
Erscheinungsjahr
1985
Link zum Volltext
Quelle
Elsevier Journal Backfiles on ScienceDirect (DFG Nationallizenzen)
Beschreibungen/Notizen
The occurrence of short-range proton displacements in Photosystem I (PS I) -enriched subchloroplast vesicles was studied under single-turnover conditions, using the pH indicators Cresol red and Neutral red. Bulk proton translocation was not detectable with Cresol red. Neutral red (externally added) was adsorbed to the vesicles in a salt-reversible way. It showed a flash-induced absorbance transient, recorded between 545 and 550 nm, consisting of a fast decrease (halftime < 5 ms) followed by an increase (halftime 20–40 ms). On the basis of the effects of uncoupler, ionophores, buffers and pH, it was concluded that the fast decrease represents Neutral red reduction and the following increase mainly reflects Neutral red protonation (and includes its reoxidation). The results further demonstrate that Neutral red is adsorbed to external membrane sites that have different accessibility towards various buffers. Bovine serum albumin, in a concentration at which it hardly contributed to external bulk buffering, stimulated the Neutral red response in parallel with the electric potential, indicating carotenoid response. This was presumably due to a general stabilizing effect. Our results strongly suggest that in these Photosystem I-enriched vesicles Neutral red senses flash-induced changes of external membrane surface charge, rather than intravesicular acidification, due to proton displacements across the membrane. This behavior is in many respects similar to that of the aminoacridine probes.