Interactions between cholorophyllase, chlorophyll a, plant lipids and Mg2+

An evaluation of the basis and consequences of a stay-green mutation in the navel negra citrus mutant using transcriptomic and proteomic profiling and metabolite analysis

A Citrus sinensis spontaneous mutant, navel negra (nan), produces fruit with an abnormal brown-colored flavedo during ripening. Analysis of pigment composition in the wild-type and nan flavedo suggested that typical ripening-related chlorophyll (Chl) degradation, but not carotenoid biosynthesis, was impaired in the mutant, identifying nan as a type C stay-green mutant. nan exhibited normal expression of Chl biosynthetic and catabolic genes and chlorophyllase activity but no accumulation of dephytylated Chl compounds during ripening, suggesting that the mutation is not related to a lesion in any of the principal enzymatic steps in Chl catabolism. Transcript profiling using a citrus microarray indicated that a citrus ortholog of a number of SGR (for STAY-GREEN) genes was expressed at substantially lower levels in nan, both prior to and during ripening. However, the pattern of catabolite accumulation and SGR sequence analysis suggested that the nan mutation is distinct from those in previously described stay-green mutants and is associated with an upstream regulatory step, rather than directly influencing a specific component of Chl catabolism. Transcriptomic and comparative proteomic profiling further indicated that the nan mutation resulted in the suppressed expression of numerous photosynthesis-related genes and in the induction of genes that are associated with oxidative stress. These data, along with metabolite analyses, suggest that nan fruit employ a number of molecular mechanisms to compensate for the elevated Chl levels and associated photooxidative stress.

Effect of the technological and agronomical factors on pigment transfer during olive oil extraction

The aim of this work was to determine the transfer of the chloroplast pigment fractions during the virgin olive oil extraction process, in relation to different factors: the ripening stage of the olive fruits, the irrigation water applied to the olive tree, and the addition of natural microtalc (NMT) during the oil extraction process. Results showed that the percentage of chloroplast pigments transferred from the olive paste to the oil increases with the ripening of the olive fruit (raw material). An excess of the water irrigation applied to the olive tree shows a reduction in the biosynthesis of chloroplast pigments in olive fruits, which is reflected in a low concentration in the virgin oils. Furthermore, the percentage of pigment transfer from the olive paste to the oil during the extraction process is reduced by irrigation, mainly of the chlorophyll fraction. The addition of NMT during the malaxation step produced an increase in the percentage of the total pigments transferred from the olive paste to the oil, in relation to nonaddition.

Biocatalysis of immobilized chlorophyllase in a ternary micellar system

The immobilization of chlorophyllase was optimized by physical adsorption on various inorganic supports, including alumina, celite, Dowex-1-chloride, glass beads and silica gel. The enzyme was also immobilized in different media, including water, Tris-HCl buffer solution and a ternary micellar system containing Tris-HCl buffer solution, hexane and surfactant. The highest immobilization efficiency (84.56%) and specific activity (0.34 mumol hydrolyzed chlorophyll mg protein-1 per min) were obtained when chlorophyllase was suspended in Tris-HCl buffer solution and adsorbed onto silica gel. The effect of different ratios of chlorophyllase to the support and the optimum incubation time for the immobilization of chlorophyllase were determined to be 1-4 and 60 min, respectively. The experimental results showed that the optimum pH and temperature for the immobilized chlorophyllase were 8.0 and 35 degrees C, respectively. The use of optimized amounts of selected membrane lipids increased the specific activity of the immobilized chlorophyllase by approximately 50%. The enzyme kinetic studies indicated that the immobilized chlorophyllase showed a higher affinity towards chlorophyll than pheophytin as substrate.

Inactivation of chlorophyllase by negatively charged plant membrane lipids

Chlorophyllide combines spontaneously not only with phosphatidylcholine (PC) liposomes but also with various other (plant) lipids dispersed in an aqueous medium. The lipid-associated chlorophyllide is highly fluorescent and the fluorescence yield is virtually independent of the nature of the lipid. Chlorophyllase (chlorophyll chlorophyllidohydrolase, EC 3.1.1.14) activity assays that are based on the determination of this chlorophyllide fluorescence show that phosphatidylglycerol (PG), and also sulphoquinovosyldiacylglycerol (SQDG), associate with isolated chlorophyllase, thereby inactivating the enzyme in a co-operative way. The extent of this inactivation depends on the pH and ionic strength of the reaction medium and can be completely reversed by divalent cations (Mg2+). The inhibition of chlorophyllase effected by free PG liposomes can be counteracted by electrically neutral lipids at relatively high concentration (PC and also chloroplast lipids). Digalactosyldiacylglycerol (DGDG) is not effective in this respect. When PG has been incorporated in PC or DGDG liposomes, its ability to inhibit chlorophyllase activity is reduced. Whereas the remaining chlorophyllase-inactivating effect of PG, incorporated in PC, can still be reversed by Mg2+, this is not found when enzyme inactivation is caused by PG incorporated in DGDG. The results reported here are consistent with those obtained earlier concerning the stabilization of chlorophyllase by PG and PG/galactolipid mixtures (Lambers, J.W.J., Verkleij, A.J. and Terpstra, W. (1984) Biochim. Biophys. Acta 786, 1-8). They are discussed in terms of the regulation of chlorophyllase activity by lipids surrounding the enzyme and by divalent cations.