DIFFERENTIAL-EFFECTS OF PLANT STEROLS ON WATER PERMEABILITY AND ON ACYL CHAIN ORDERING OF SOYBEAN PHOSPHATIDYLCHOLINE BILAYERS

To gain some insight into the structural and functional roles of sterols in higher plant cells, various plant sterols have been incorporated into soybean phosphatidylcholine (PtdCho) bilayers and tested for their ability to regulate water permeability and acyl chain ordering. Sitosterol was the most efficient sterol in reducing the water permeability of these vesicles and stigmasterol appeared to have no significant effect. Vesicles containing 24-xi-methylcholesterol exhibited an intermediate behavior, similar to that of vesicles containing cholesterol. Cycloartenol, the first cyclic biosynthetic precursor of plant sterols, reduced the water permeability in a very effective way. Of two unusual plant sterols, 24-xi-methylpollinastanol and 14-alpha,24-xi-dimethylcholest-8-en-3-beta-ol, the former was found to be functionally equivalent to sitosterol and the latter was found to be relatively inefficient. H-2 NMR experiments have been performed with oriented bilayers consisting of soybean PtdCho with sitosterol, stigmasterol, or 24-xi-methylpollinastanol. The results provided clear evidence that sitosterol and 24-xi-methylpollinastanol exhibit a high efficiency to order PtdCho acyl chains that closely parallels their ability to reduce water permeability. By contrast, stigmasterol shows a low efficiency for both functions. These results show that sitosterol and stigmasterol, two major 24-ethylsterols differing only by the absence or presence of the DELTA-22 double bond in the side chain, probably play different roles in regulating plant membrane properties; they also may explain why 9-beta, 19-cyclopropylsterols behave as good surrogates of sitosterol.