Dicyclohexylcarbodiimide alters the pH dependence of violaxanthin de-epoxidation

The de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in the xanthophyll cycle of higher plants is controlled by the pH of the thylakoid lumen. The influence of N,N'-dicyclohexylcarbodiimide (DCCD) on the pH dep endence of the de-epoxidation reactions has been investigated in isolated p ea thylakoids. In the presence of DCCD, the decrease in de-epoxidase activi ty at increasing pH was found to be shifted by about 0.3 pH units to more-a lkaline pH values. This was paralleled by a less-pronounced cooperativity f or the pH dependence of de-epoxidation. Comparative studies with antenna-de pleted thylakoids from plants grown in intermittent light and with unstacke d thylakoids indicated that binding of DCCD to antenna proteins is most pro bably not responsible for the altered pH dependence. Analyses of the zeaxan thin content of different antenna subcomplexes showed that the DCCD-induced de-epoxidation at high pH leads to zeaxanthin formation in all antenna pro teins from both photosystems. Our data support the view that DCCD binding t o the violaxanthin de-epoxidase may be responsible for the altered pH depen dence.