Pigment containing lipid vesicles. I. Preparation and characterization of chlorophyll a-lecithin vesicles

Mixing of single-chain amphiphiles in two-chain lipid bilayers. 2. Characteristics of chlorophyll a and alpha-tocopherol incorporation in unilamellar phosphatidylcholine vesicles

As a step toward the elucidation of the biological significance of the isoprenic chains found ubiquitously in single-chain lipids involved in electron and energy transfer of chloroplasts and mitochondria, we undertook a comparative study of the incorporation of chlorophyll a (Chl a) and alpha-tocopherol (alpha T) in unilamellar phosphatidylcholine (PC) vesicles. We observed that while Chl alpha is added to the PC bilayers in a simple, almost linear way, the inclusion of alpha T is a sigmoid-like function characterized first by a slow variation of alpha T incorporation followed by a rather steep increase till saturation occurs at initial mol% alpha T of approx. 5%. Owing to the likeness of the isoprenic chains of Chl a and alpha T the simplest interpretation of the data is that the different mixing behaviours of the two lipids in PC vesicles are due to the special arrangement of the tetrapyrrole macrocycle and the chromanol ring in the lipid bilayers. However, no satisfactory explanation of the mechanisms involved can be given as yet. One possibility is that the inclusion of Chl a and alpha T in the membranes results from adsorption differences brought about by the extent of penetration of the molecule into the monolayers of the unilamellar vesicles.

Transmembrane distribution of alpha-tocopherol in single-lamellar mixed lipid vesicles

A study of the molar ratio dependence of the incorporation of alpha-tocopherol into single-lamellar vesicles showed that the number of molecules which the bilayers can accommodate increased linearly with increasing alpha-tocopherol/phosphatidylcholine initial molar ratios till about 0.05, then approached a saturation limit. At 5 mol%, one alpha-tocopherol molecule per 60 phospholipids can be incorporated into the membranes. Up to this limit the distribution of alpha-tocopherol in the bilayers is uniform, while at initial molar ratios higher than 0.05 a disproportionation toward the inner monolayer of the vesicles is observed. The average outer/total ratio is found to be 0.27 +/- 0.03 at alpha-tocopherol/phosphatidylcholine molar ratios above 0.07 and is similar to asymmetrical distributions that have been reported in vesicles containing other one-chain amphiphiles (e.g., cholesterol). This large disproportionation is in contrast with the packing distribution of certain two-chain amphiphiles, and indicates that one of the driving forces for asymmetry formation in lipid bilayers might be dependent on the number of hydrocarbon chains per amphiphile molecule. A possible reason for the disproportionation effect observed in our experiments is the displacement of unsaturated phospholipids to the outer monolayer of the single-lamellar vesicles, by the more rigid isoprene units of alpha-tocopherol.

Influence of lecithin liposomes on chlorophyllase-catalyzed chlorophyll hydrolysis: comparison of intramembraneous and solubilized Phaeodactylum chlorophyllase

1.Chlorophyllase-catalyzed chlorophyll hydrolysis is greatly enhanced by the addition of divalent cations (Mg2+) combined with a reducing agent (dithiothreitol, ascorbate). A similar effect is obtained by the addition of lecithin. In the presence of lecithin, dithiothreitol has only slight or no influence on chlorophyll hydrolysis. Mg2+ eliminates the activating effect of lecithin. 2. In the absence of Mg2+ + dithiothreitol or of lecithin, Triton X-100 has a slight activating effect on chlorophyllase-catalyzed chlorophyll hydrolysis, but only at low concentrations (0.01--0.02%). In the presence of Mg2+ and dithiothreitol or of lecithin, Triton X-100 (greater than or equal to 0.02%) inhibits this reaction. 3. Whereas chlorophyllase combines with chlorophyll, no combination of chlorophyllase and lecithin could be detected. 4. Solubilized chlorophyllase is stabilized by its substrate, chlorophyll. Enzyme stabilization is eliminated by lecithin, whereas in the absence of chlorophyll, denaturation is somewhat increased by dithiothreitol. 5. No clear difference was found between the actions of intramembraneous and solubilized chlorophyllase. The results suggest that chlorophyllase is situated within membranes in such a way that the active group protrudes into the aqueous medium surrounding the membrane. 6. A hypothesis explaining the activating effects which Mg2+ combined with a reducing agent and lecithin have upon chlorophyllase-catalyzed chlorophyll hydrolysis is presented.

Pigment containing lipid vesicles. II. Interaction of valinomycin with lecithin as sensed by chlorophyll a

Valinomycin added to a suspension of chlorophyll a containing lecithin vesicles induces slight changes in the spectrum of chlorophyll a. These changes are measured as a difference spectrum between samples with and without valinomycin but of otherwise identical composition. The analysis of the experiments reveals that the effect is neither associated with the ionophoric properties of valinomycin nor due to a direct interaction of this agent with chlorophyll a. The molar ratio of valinomycin dissolved in the membrane to lecithin is found to be the relevant parameter, thus indicating an interaction between these two components. As a consequence, the aggregational state of the lecithin molecules is altered. Chlorophyll a incorporated into the membrane acts as a sensor, i.e. it reflects the alteration by a change in its spectroscopic parameters.