Incorporation of plastoquinone and ubiquinone into liposome membranes studied by HPLC analysis. The effect of side chain length and redox state of quinone.

The efficiency of incorporation of plastoquinones and ubiquinones into phospholipid liposomes has been studied. The representatives of short (PQ1 and UQ1) middle (PQ4 and UQ4) and long (PQ9, UQ9 and UQ10) prenylquinones have been used to investigate the effect of quinone side chain length. The properties of hydroquinones have been also thoroughly examined in relation to the quinone forms. The extraction procedure was modified and further developed which enables removing of nonincorporated quinone by pentane washing and then determination of quinone content inside the lipid bilayer. The quantitatively evaluation of the amount of prenylquinone was assayed by means of HPLC analysis which offers much greater sensitivity and could be easily applied in case of hydroquinones. It has been found that PQ1 and UQ1 as well as their reduced forms were present mainly (about 80%) in the aqueous phase, when attempting to introduce them into phospholipid bilayer. In case of quinones having four and more isoprenyl units in side chain, a high level of quinone incorporation, ranging about 95%, was observed. The results pointed out that when comparing the effects of different exogenous quinones on membrane related processes, one has to consider the effectiveness of their incorporation within lipid bilayer.

A mathematical model describing kinetics of conversion of violaxanthin to zeaxanthin via intermediate antheraxanthin by the xanthophyll cycle enzyme violaxanthin de-epoxidase.

The xanthophyll cycle is one of the mechanisms protecting the photosynthetic apparatus against the light energy excess. Its action is still not well understood on the molecular level. Our model makes it possible to follow independently the kinetics of the two de-epoxidation steps occurring in the xanthophyll cycle: the conversion of violaxanthin into antheraxanthin and the conversion of antheraxanthin into zeaxanthin. Using a simple form of the transition rates of these two conversions, we model the time evolution of the concentration pattern of violaxanthin, antheraxanthin and zeaxanthin during the de-epoxidation process. The model has been applied to describe the reactions of de-epoxidation in a system of liposome membranes composed of phosphatidylcholine and monogalactosyldiacylglycerol. Results obtained within the model fit very well with the experimental data. Values of the transition probabilities of the violaxanthin conversion into antheraxanthin and the antheraxanthin conversion into zeaxanthin calculated by means of the model indicate that the first stage of the de-epoxidation process is much slower than the second one.

Effect of monogalactosyldiacylglycerol and other thylakoid lipids on violaxanthin de-epoxidation in liposomes.

In this study we present evidence that one of two reactions of the xanthophyll cycle, violaxanthin de-epoxidation, may occur in unilamellar egg phosphatidylcholine vesicles supplemented with monogalactosyldiacylglycerol (MGDG). Activity of violaxanthin de-epoxidase (VDE) in this system was found to be strongly dependent on the content of MGDG in the membrane; however, only to a level of 30 mol%. Above this concentration the rate of violaxanthin de-epoxidation decreased. The effect of individual thylakoid lipids on VDE-independent violaxanthin transformation was also investigated and unspecific effects of phosphatidylglycerol and sulphoquinovosyldiacyglycerol, probably related to the acidic character of these lipids, were found. The presented results suggest that violaxanthin de-epoxidation most probably takes place inside MGDG-rich domains of the thylakoid membrane. The described activity of the violaxanthin de-epoxidation reaction in liposomes opens new possibilities in the investigation of the xanthophyll cycle and may contribute to a better understanding of this process.