Post harvest improvement of zeaxanthin content of vegetables

Improvement of the Quality of Wild Rocket (Diplotaxis tenuifolia) with Respect to Health-Related Compounds by Enhanced Growth Irradiance

For healthier human nutrition, it is desirable to provide food with a high content of nutraceuticals such as polyphenolics, vitamins, and carotenoids. We investigated to what extent high growth irradiance influences the content of phenolics, α-tocopherol and carotenoids, in wild rocket (Diplotaxis tenuifolia), which is increasingly used as a salad green. Potted plants were grown in a climate chamber with a 16 h day length at photosynthetic photon flux densities varying from 20 to 1250 μmol m-2 s-1. Measurements of the maximal quantum yield of photosystem II, FV/FM, and of the epoxidation state of the violaxanthin cycle (V-cycle) showed that the plants did not suffer from excessive light for photosynthesis. Contents of carotenoids belonging to the V-cycle, α-tocopherol and several quercetin derivatives, increased nearly linearly with irradiance. Nonintrusive measurements of chlorophyll fluorescence induced by UV-A and blue light relative to that induced by red light, indicating flavonoid and carotenoid content, allowed not only a semiquantitative measurement of both compounds but also allowed to follow their dynamic changes during reciprocal transfers between low and high growth irradiance. The results show that growth irradiance has a strong influence on the content of three different types of compounds with antioxidative properties and that it is possible to determine the contents of flavonoids and specific carotenoids in intact leaves using chlorophyll fluorescence. The results may be used for breeding to enhance healthy compounds in wild rocket leaves and to monitor their content for selection of appropriate genotypes.

Violaxanthin de-epoxidase disulphides and their role in activity and thermal stability

Violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin at the lumen side of the thylakoids during exposure to intense light. VDE consists of a cysteine-rich N-terminal domain, a lipocalin-like domain and a negatively charged C-terminal domain. That the cysteines are important for the activity of VDE is well known, but in what way is less understood. In this study, wild-type spinach VDE was expressed in E. coli as inclusion bodies, refolded and purified to give a highly active and homogenous preparation. The metal content (Fe, Cu, Ni, Mn, Co and Zn) was lower than 1 mol% excluding a metal-binding function of the cysteines. To investigate which of the 13 cysteines that could be important for the function of VDE, we constructed mutants where the cysteines were replaced by serines, one by one. For 12 out of 13 mutants the activity dropped by more than 99.9%. A quantification of free cysteines showed that only the most N-terminal of these cysteines was in reduced form in the native VDE. A disulphide pattern in VDE of C9-C27, C14-C21, C33-C50, C37-C46, C65-C72 and C118-C284 was obtained after digestion of VDE with thermolysin followed by mass spectroscopy analysis of reduced versus non-reduced samples. The residual activity found for the mutants showed a variation that was consistent with the results obtained from mass spectroscopy. Reduction of the disulphides resulted in loss of a rigid structure and a decrease in thermal stability of 15 °C.

Violaxanthin de-epoxidase disulphides and their role in activity and thermal stability

Violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin at the lumen side of the thylakoids during exposure to intense light. VDE consists of a cysteine-rich N-terminal domain, a lipocalin-like domain and a negatively charged C-terminal domain. That the cysteines are important for the activity of VDE is well known, but in what way is less understood. In this study, wild-type spinach VDE was expressed in E. coli as inclusion bodies, refolded and purified to give a highly active and homogenous preparation. The metal content (Fe, Cu, Ni, Mn, Co and Zn) was lower than 1 mol% excluding a metal-binding function of the cysteines. To investigate which of the 13 cysteines that could be important for the function of VDE, we constructed mutants where the cysteines were replaced by serines, one by one. For 12 out of 13 mutants the activity dropped by more than 99.9%. A quantification of free cysteines showed that only the most N-terminal of these cysteines was in reduced form in the native VDE. A disulphide pattern in VDE of C9-C27, C14-C21, C33-C50, C37-C46, C65-C72 and C118-C284 was obtained after digestion of VDE with thermolysin followed by mass spectroscopy analysis of reduced versus non-reduced samples. The residual activity found for the mutants showed a variation that was consistent with the results obtained from mass spectroscopy. Reduction of the disulphides resulted in loss of a rigid structure and a decrease in thermal stability of 15 °C.