Pestic. Sci. 1973,4, 839-842

Lipid Dynamics in Cell Membranesa

Dennis Chapman

Department of Chemistry, University of Shefield (Manuscript received 3 April 1973 and accepted 26 July 1973)

Membranes are composed primarily of lipids and proteins; other constituents include water, cholesterol, metal ions-and the last being found in some, but not all, types of membranes. The first significant chemical fact about membranes is that the proportion of protein varies greatly from one to another. Thus in the myelin membranes sheathing the nerve fibres, the proportion of lipid to protein is something like 9: 1, while at the other extreme with the mitochondrial membrane, the ratio is about 1 : 1. The most probable explanation of this difference is that the myelin membrane is prim- arily an insulator, its rBle being to exclude substances that would interfere with the transmission of nerve impulses; as we know from the demyelinating diseases, damage to these membranes produces severe impairment of nerve function. The mitochondrial membrane, on the other hand, is thought to serve the function of organising-in space and perhaps in time-the action of the many enzymes associated with the mitochondrial system.

Nearly all membranes contain a variety of lipids. Thus myelin membrane lipid, for example, has as major constituents cerebroside, phosphatidylethanolamine, and the non-polar molecular cholesterol, plus smaller amounts of lecithin, sphingomyelin, phosphatidylserine, and other substances. Erythrocyte membrane lipid contains many of the same constituents, though in somewhat different proportions-notably, much more sphingomyelin. Mitochondria1 membrane lipids, consist almost entirely of leci- thin, phosphatidylethanolamine, and cardiolipin, with little cholesterol and no phos- phatidylserine, sphingomyelin, or cerebroside. These differences in lipid composition clearly reflect differences in the physiological functions of the membranes in question.

Cell membranes contain different lipid classes and each of the lipid classes has not one but a variety of discrete fatty acids associated with it. Lecithin, for example, is actually not a compound but is rather a group of compounds, all of which contain the (polar) phosphatidyl choline group but different fatty-acid chains.

Some clues to the significance of these variations in membrane fatty acids have come from model experiments with purified phospholipid preparations. For example, with pure dipalmitoyl lecithin, the lipid which plays such a predominant rBle in alveolar membranes, in the anhydrous condition the hydrocarbon chain portion of the molecule melts at a lower temperature than the rest of the crystal. This transition temperature,

a A summary of the paper presented at a symposium Biological Interfaces on 30 January 1973 organised by the Physicochemical and Biophysical Panel of the Pesticides Group, Society of Chemical Industry.

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840 D. Chapman

does not involve the change of state from solid crystal to “normal” liquid usually implied by the term “melting”, but rather a shift from a crystal to a “liquid crystal”. In this state the glycerol and polar groups retain a fairly regular organisation although the polar group does have considerable mobility, and the fatty-acid chains melt and acquire considerably more mobility, with the methyl end of the chain having greatest motion.

Experiments with a variety of phospholipids that differ only in the nature of their fatty-acid chains have established that the transition temperature is dependent on chain length and degree of saturation. Thus the transition temperature of hydrated lecithin, for instance, can be raised as high as 60 “C by increasing the length of its fatty-acid chains, or it can be lowered to temperatures below 0 “C by shortening the chains or by incorpor- ating unsaturated chains.

The same phenomena have also been observed in natural membranes from bacteria (e.g., Escherichia coli membranes and Achole plasma laidlawii membranes). The transi- tion temperature can be most easily observed and measured in a calorimeter, which gives sensitive readings on heat absorption by a preparation of natural membranes. Since the shift from gel to liquid crystal is endothermic-i.e. relatively large quantities of heat must be absorbed in order to break the bonds that hold the molecules in the rigid “gel” structure-a sharp peak in heat absorption at a given temperature signals that the change of state has in fact occurred, By growing certain bacteria on media containing different sorts of fatty acids, one can vary the composition of the membrane and observe the effect of chain length and saturation on the transition temperature. The same experiments, incidentally, also give evidence that the membrane lipids are at least partially arranged in a bilayer structure, since otherwise one would not expect them to behave in this way.

Similar experiments, with both artificial systems and natural membranes, have identi- fied yet another variable affecting the transition temperature : the presence of cholesterol. Addition of this substance appears to abolish the transition entirely, in the sense that the lipids retain the loose, liquid-crystal organisation even when cooled well below the physiologic range of temperatures. In essence, the cholesterol molecules interpose themselves between the lipid chains and prevent them from assuming the orderly crystalline-gel configuration.

Thus, a membrane-or portion of a membrane-may exist in the liquid-crystal form at physiologic temperatures, depending on (1) the length of its fatty-acid chains, (2) the degree of saturation of those chains and (3) the presence-and the proportion- of cholesterol. One obvious fact is that in the liquid-crystal state the membrane will interpose far less of a barrier to the passage across it of organic molecules, which can more readily “dissolve” and penetrate the membrane. Furthermore, small polar mole- cules-notably, for instance, water molecules-can also more readily cross the mem- brane. If, for example, the transition behaviour of mitochondria1 and myelin mem- branes is compared, it is found that both are in the liquid-crystal state at physiological temperatures (37”), but for quite different reasons : in the mitochondria, this is because the fatty acids are relatively unsaturated, whereas in myelin it is because of the high cholesterol content (around 25 %). The effect of cholesterol is not only to keep the lipids in a fluid condition. This can be seen when the organisation is considered of a number of lipid molecules arranged side by side, as they are in bilayer membranes. The motion in

Lipid dynamics in cell membranes 841

one fatty-acid chain is to some extent transmitted to the chains next to it, and so on down the line in a cooperative manner. Cholesterol can prevent this cooperativity.

Transition temperatures and hence lipid fluidity can be shifted in temperature by ions binding to the polar groups of membrane. The interaction of protein with the lipid polar group can also affect membrane permeability. Certain drugs can also exert similar shifts on the lipid transition temperature-and thereby, evidently, on membrane permeability.

Mitochondria1 membranes

Term ino I CH,s ,

Erythrocyte ghosts

Lecithin and sphingomyelin

200 I50 I00 50 0 Paris/miilion from external TMS

( b )

Figure 1. 25.2 MHz 13C n.m.r. spectra of (a) mitochondrial membranes 37°C and (b) erythrocyte ghosts. (After Keough et al., 1973.)

It seems clear that temporary alterations of membrane characteristics by exogenous and endogenous substances are potentially a most important area of research. This area is already being studied by some of the most sophisticated techniques for probing molecular structures without disrupting them, such as i.r. spectroscopy, Raman spectro- scopy measurements of nuclear magnetic resonance, electron spin resonance, etc.

The high degree of mobility of the lipid chains in some membranes is such that high resolution signals can be observed using nmr spectra. This is seen with proton resonance as well as using 13C resonance. This is shown in Figure 1 which shows the 13C spectrum of mitochondrial membranes. The considerable mobility of the groups associated with the chains and the polar parts of the lipid molecules is evident from the signals associated with these groupings. Signals from the protein material are not so

842 D. Chapman

evident in the spectrum. Chloroplast membranes also give rise to sharp signals associ- ated with the lipid material of the membrane.

Physical techniques have also established that the concept of membrane lipids being fixed in place is only relatively true. Individual lipid molecules can migrate or diffuse from one part of the membrane to another, and can even (albeit rarely) “flip over” from the outside to the inside of the bilayer and vice versa.

Reference 1. Keough, K. M.; Oldfield, E. ; Chapman, D. ; Beynon, P. Chem. Phys. Lipids 1973,10,37.

General Bibliography Chapman, D. In Membranes and Ion Transport, Vol. 1, Wiley-Interscience, New York. 1970, p. 23. Chapman, D. Sci. J . 1968, p. 55. Biological Membrunes (Chapman, D., ed.) Vol. 1. Academic Press, London, 1968. Biological Membranes (Chapman, D.; Wallach, D. F. H., eds) Vol. 2. Academic Press, London, 1973. Chapman, D. An Introduction ofLipids McGraw Hill, New York. 1968. Oldfield, E.; Chapman, D., Fedn Eur. Biol. SOC. Lett. 1972,23, 285.