Metabolic changes of membrane lipid composition in Acholeplasma laidlawii by hydrocarbons, alcohols, and detergents: arguments for effects on lipid packing

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Abstract
The packing of lipids into different aggregates, such as spheres, rods, or bilayers, is dependent on (i) the hydrophobic volume, (ii) the hydrocarbon-water interfacial area, and (iii) the hydrocarbon chain length of the participating molecules, according to the self-assembly theory [Israelachvili, J. N., Marcelja, S., and Horn, R. G. (1980) Q. Rev. Biophys. 13, 121-200]. The origin of the participating molecules should be of no importance with respect to their abilities to affect the above-mentioned parameters. In this investigation, Acholeplasma laidlawii, with a defined acyl chain composition of the membrane lipids, has been grown in the presence of three different classes of foreign molecules, known to partition into model and biological membranes. This results in an extensive metabolic alteration in the lipid polar head group composition, which is expressed as changes in the molar ratio between the lipids monoglucosyldiglyceride (MGDG) and diglucosyldiglyceride (DGDG), forming reversed hexagonal and lamellar phases in excess water, respectively. The formation of nonlamellar phases by A. laidlawii lipids depends critically upon the MGDG concentration [Lindblom, G., Brentel, I., Sjolund, M., Wikander, G., and Wieslander, .ANG., (1986) Biochemistry (preceding paper in this issue)]. The foreign molecules tested belong to the following groups: nonpolar organic solvents, alcohols, and detergents. Their effects on the gel to liquid crystalline phase transition temperature (Tm), on the order parameter of the acyl chains, and on the phase equilibria between lamellar and nonlamellar liquid crystalline phases in lipid-water model systems are known in several instances. The observed alterations in the ratio MGDG/DGDG can only be explained as a response to changes of the geometrical packing parameters i-iii caused by the foregin molecules, and hence as a response to changes of the equilibria between lamellar and nonlamellar phases. No consistent correlation was found between the lipid ratio and the Tm or the order parameter. Our conclusions lend further support to the applicability of the self-assembly theory for explaining lipid regulation in A. laidlawii membranes.

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