ANALYSIS OF THE PIGMENT STOICHIOMETRY OF PIGMENT-PROTEIN COMPLEXES FROM BARLEY (HORDEUM-VULGARE) - THE XANTHOPHYLL CYCLE INTERMEDIATES OCCUR MAINLY IN THE LIGHT-HARVESTING COMPLEXES OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II

The carotenoid zeaxanthin has been implicated in a nonradiative dissip ation of excess excitation energy. To determine its site of action, we have examined the location of zeaxanthin within the thylakoid membran e components. Five pigment-protein complexes were isolated with little loss of pigments: photosystem I (PSI); core complex (CC) I, the core of PSI; CC II, the core of photosystem II (PSII); light-harvesting com plex (LHC) IIb, a trimer of the major light-harvesting protein of PSII ; and LHC IIa, c, and d, a complex of the monomeric minor light-harves ting proteins of PSII. Zeaxanthin was found predominantly in the LHC c omplexes. Lesser amounts were present in the CCs possibly because thes e contained some extraneous LHC polypeptides. The LHC IIb trimer and t he monomeric LHC II a, c, and d pigment-proteins from dark-adapted pla nts each contained, in addition to lutein and neoxanthin, one violaxan thin molecule but little antheraxanthin and no zeaxanthin. Following i llumination, each complex had a reduced violaxanthin content, but now more antheraxanthin and zeaxanthin were present. PSI had little or no neoxanthin. The pigment content of LHC I was deduced by subtracting th e pigment content of CC I from that of PSI. Our best estimate for the carotenoid content of a LHC IIb trimer from dark-adapted plants is one violaxanthin, two neoxanthins, six luteins, and 0.03 mol of antheraxa nthin per mol trimer. The xanthophyll cycle occurs mainly or exclusive ly within the light-harvesting antennae of both photosystems.