Influence of ascorbate and pH on the activity of the diatom xanthophyll cycle-enzyme diadinoxanthin de-epoxidase

In this study, the enzyme activity of partially purified diadinoxanthin de-epoxidase (DDE) from the diatom Cyclotella meneghiniana was investigated at different ascorbate concentrations and pH values. In comparison with spinach violaxanthin de-epoxidase (VDE), we found a much higher affinity of the enzyme for the co-substrate ascorbate. The K-m value of DDE at pH 5 (0.7 mM) was significantly lower than that observed for VDE (2.3 mM). The pH-optimum of DDE activity was found at pH 5 at low ascorbate concentrations. At high ascorbate concentrations, we observed a strong shift of the pH optimum towards higher pH values, and significant DDE activity was still present at almost neutral pH values. This is in contrast to VDE, where despite a slight shift towards higher pH values, enzyme activity was never observed above pH 6.5. The pH optimum of VDE was always found in a narrow range between pH 5 and 5.2, irrespective of the presence of high or low ascorbate concentrations. The high affinity of DDE for ascorbate indicates that, even at a limited availability of reduced ascorbate, high enzyme activity is possible at low pH values. At high ascorbate concentrations, on the other hand, DDE activity can be shifted towards neutral pH values, thereby facilitating a very fast and strong response to small pH changes in the thylakoid lumen. The importance of the high ascorbate affinity of DDE for the physiology of intact diatom cells is discussed.