Solubilization of phospholipids by detergents. Structural and kinetic aspects

Hydrogenation of triton X-100 eliminates its fluorescence and ultraviolet light absorption while preserving its detergent properties

The ultraviolet-light absorption and fluorescence of Triton X-100 were virtually eliminated by hydrogenation to its reduced cyclohexyl analog, RTX-100. The critical micelle concentration of RTX-100 was 12% higher than that of Triton X-100. RTX-100 and Triton X-100 were quite similar in their abilities to extract proteins from human erythrocyte membranes.

Lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine. Effect of lipid organization and apolipoprotein C-II on enzyme activity

The effect of phospholipid organization on the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine was examined with sonicated vesicles and Triton X-100 or lysomyristoylphosphatidylcholine solubilized lipid. Triton X-100-dimyristoylphosphatidylcholine substrates were prepared at various ratios of detergent to phospholipid so as to produce lipid structures varying from bilayers to micelles. Apolipoprotein C-II, the activator protein for lipoprotein lipase, enhanced the rate of the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine for each substrate tested. Although the absolute rate of lipoprotein lipase catalysis was different for each, the factor (the ratio of lipoprotein lipase activity with apolipoprotein C-II to that without the activator protein) was nearly constant, with a value of approximately 16. We conclude that the enhancement of lipoprotein lipase activity by apolipoprotein C-II is independent of the physical form of the phospholipid substrate.

Fluorimetric determination of critical micelle concentration avoiding interference from detergent charge

Upon mixing detergent solutions with the neutral fluorescent molecule 1,6-diphenyl-1,3,5-hexatriene a large increase in fluorescence is observed if detergent exceeds the critical micelle concentration. This property has been used to determine the critical micelle concentration of anionic, uncharged, zwitterionic, and cationic detergents. Regardless of detergent charge, the critical micelle concentrations obtained agree with the values obtained by other methods. This fluorescence assay is both sensitive and rapid, and should provide a simple and general method for determination of critical micelle concentration of any detergent.

GM1 micelles modify the transport properties of the ionophore gramicidin D in artificial planar bilayers

We have analyzed the effects induced in different phospholipid planar bilayers by monosialoganglioside micelles containing the ionophore gramicidin D. The membrane conductance increases after the addition of GM1 micelles at various ionophore/ganglioside ratios. We believe this fact may be ascribed to gramicidin molecules that incorporate into the bilayer together with gangliosides. In the presence of micelles the mean lifetime and the amplitude of the gramicidin single channel did not present relevant modifications when dioleoylphosphatidylcholine or phosphatidylserine were used to form the bilayer. Calcium proved to trigger the interaction between phosphatidylethanolamine membranes and GM1 micelles containing gramicidin. In this case the ionic pore presents a longer lifetime and a lower amplitude with respect to pure gramicidin. We suggest that different properties developed by gramicidin may depend on structural organization of gangliosides when incorporated into the phospholipid bilayer.

Effect of Tween 80 on lipid vesicle permeability

As part of an examination of procedures for protein/lipid reconstitution we incubated solubilized cytochrome oxidase at different temperatures with preformed vesicles composed of saturated lipids. We observed that the 22Na permeability of these vesicles was increased following such an incubation step, with the maximal effect occurring following incubations at temperatures in the vicinity of the lipid gel to liquid-crystal phase change. However, these permeability changes could be completely duplicated by incubating similar vesicles with concentrations of Tween 80 used to solubilize the extracted oxidase. Thus detergent molecules used to solubilize extracted membrane proteins can induce significant changes in the reconstituting lipid environment and the possibility of such changes must be taken into account when interpreting the properties of the reconstituted lipid/protein system under study.