The Effect of Antibodies to Violaxanthin on Photosynthetic Electron Transport

An antiserum to violaxanthin inhibits photosynthetic electron transport between water, iodide or tetramethylbenzidine and various electron acceptors in chloroplasts from green tobacco (N icotiana tabacum var. John W illiam ’s B roadleaf). However, electron transport from manganese or diphenylcarbazide to these acceptors is not impaired. The typical photosystem I reaction from D P IP / ascorbate to anthraquinone-2-sulfonate in the presence of DCMU shows no inhibition. From this it is concluded that the effect of violaxanthin on the photosynthetic electron transport chain lies on the oxygen-evolving side of photosystem II before the site from which diphenylcarbazide or manganese donate electrons. In the presence of DCMU after preillumination we find an effect of the antiserum on fluo­ rescence. The reaction of the antibodies to violaxanthin with stroma-freed chloroplasts depends on the condition of the thylakoid membrane. Chloroplasts which are still swellable react in a bivalent manner and are agglutinated. Non sw ellable chloroplasts react only in a monovalent manner. This specific binding was demonstrated by means of the Coombs-test. From these reactions it follows that the antigenic determinants of violaxanthin are accessible to the antibodies, hence, they must be located in the outer surface of the thylakoid membrane.