PROTECTIVE FUNCTION OF THE APOPROTEIN OF THE LIGHT-HARVESTING CHLOROPHYLL-A/B - PROTEIN COMPLEX IN PIGMENT PHOTO-OXIDATION

(1) When isolated under gentle conditions, the light-harvesting chlorophyll-a/b-protein complex of Photosystem II (LHC) in air-saturated solution is highly stable under strong white light (greater than 400 nm): only about 10% of the LHC-bound chlorophylls are destroyed during exposure of the LHC for 30 min to a fluence rate of 1000 W m-2. (2) Photostability of the LHC breaks down after treatment of the complex with Triton X-100 or limonene, i.e. conditions that modify the apoprotein and reduce the acid stability of the complex. (3) No coincidence is observed between photostability of the LHC and energy transfer efficiency between LHC-bound pigments, the latter being reduced by Triton X-100 but not by limonene. (4) Photodestruction of the chlorophylls in Triton X-100 or limonene-treated LHCs is completely inhibited in the absence of molecular oxygen. (5) The data show that the carotenoids present in the LHC are not sufficient to protect the chlorophylls from photo-oxidation. In addition, the intact apoprotein of the LHC is required and appears to act by limiting the access of molecular oxygen to pigment sites. © 1990.