Regulation of bilayer stability in Clostridium butyricum: studies on the polymorphic phase behavior of the ether lipids

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Abstract
Three of the major phospholipids of the cell membrane of Clostridium butyricum are phosphatidylethanolamine (PE), plasmenylethanolamine (PlaE), and the glycerol acetal of plasmenylethanolamine. When cultured in the absence of biotin in media supplemented with a cis-unsaturated fatty acid, the cellular lipids become highly enriched with the fed fatty acid. Under these conditions, the ratio of the glycerol acetal of PlaE to the sum of PE plus PlaE increases markedly over that seen in cells containing mixtures of saturated and unsaturated fatty acids [Johnston, N. C., and Goldfine, H. (1985) Biochim. Biophys. Acta 813, 10-18]. We have studied the polymorphic phase behavior of the phospholipids from C. butyricum grown on oleic acid using differential scanning calorimetry, 31P nuclear nagnetic resonance, and X-ray diffraction. The mixed PE plus PlaE fraction undergoes a transition from the gel to liquid-crystalline state at -1.9.degree. C and a lamellar to reversed hexagonal (L .fwdarw. H) transition at or near O.degree. C. The glycerol acetal of PlaE melts at 16.1.degree. C, and as predicted from lipid packing theory, the lamellar phase is stabilized, up to 50.degree. C. Addition of the oleate-enriched glycerol acetal of PlaE to dioleoylphosphatidylethanolamine, or the PE plus PlaE fraction from oleate-grown cells, stabilized the lamellar arrangement of the mixtures. A ratio of glycerol acetal of PlaE to total PE (PE plus PlaE) of 0.5, which is close to that found in cells grown on palmitic plus oleic acid, 0.6-0.7, did not produce a lamellar phase at 37.degree. C when the lipids enriched with oleic acid were tested, but a 1:1 mixture of these lipids were sufficient to produce the lamellar arrangement. In cells grown on oleic acid, the ratio is close to 2.0. It appears that these cells are capable of regulating the stability of the bilayer arrangement of the cell membrane by altering the ratio of the glycerol acetal of PlaE to the total PE fraction in response to changes in membrane lipid unsaturation.

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