Structure of Cellular Membranes and Regulation of Their Lipid Composition
Two lipid class substitution groups have been defined. Members of a group can replace each other in membranes. Similar substitution groups are found for animal cells, fungi, and bacteria, although a lipid class that occurs in many organisms may be completely absent from others. Myelin is a membrane that has the maximum amount of lipid (fully packed) and essentially all of the protein appears to bind polar lipid. Other membranes contain less lipid and more protein, some of which does not appear to bind lipid. In membranes, lipid is postulated to lie upon the protein to which it is attached by apolar bonding of carbon chains as well as ionic and hydrogen bonds through polar groups. Lipid molecules lie close to each other and interact through their carbon chains (apolar bonding). In myelin, cholesterol probably binds to acidic lipids. The maximum cholesterol level is seen in myelin and the human red blood cell in which one-half the molar amount of cholesterol equals the amount of acidic lipid (acidic phospholipids, phosphatidyl ethanolamine, sulfatide, and ganglioside). It is thus postulated that two molecules of cholesterol can bind to one of acidic lipid. The cross section of the membrane is postulated to consist of four layers of lipid and two of protein in the sequence lipid-protein-lipid-lipid-protein-lipid. In myelin which is maximally packed with lipid, the four layers are completely filled. In other membranes with less lipid and lipid-binding protein, the lipid layers are incompletely filled. In the human red blood cell, lipid appears to cover only about one-third of the protein surface. Membrane permeability to many ionic substances appears to be related to the total amount of lipid, those membranes having the most lipid being the least permeable. It is suggested that the blood-brain barrier exists because plasma membranes of cells of the nervous system contain more lipid than those of other organs and are thus more slowly penetrated by some ionic substances. The different lipid compositions of subcellular particulates of the same cell type and from different organs is correlated with the relative proportions of the different types of membranes. It is postulated that, during differentiation, levels of each type of subcellular particulate are set and that thereafter lipid composition changes follow a similar course in all organs. Some changes in membrane lipids in abnormal states are discussed.
KeywordsLipid Class Phosphatidyl Choline Phosphatidyl Ethanolamine Substitution Group Acidic Phospholipid
Unable to display preview. Download preview PDF.
- (3).Rouser, G., Yamamoto, A., and Kritchevsky, G. This Volume.Google Scholar
- (4).Rouser, G., Nelson, G.J., Fleischer, S. and Simon, G. in Biological Membranes (Ed. D. Chapman), Academic Press (1968) p. 5–69.Google Scholar
- (9).Opden Kamp, J.A.F., van Deenen, L.L.M., and Tomasi, V. in Structural and Functional Aspects of Lipoproteins in Living Systems (Eds. E. Tria and A.M. Scanu), Academic Press (1969) p. 227–325.Google Scholar
- (12).Chapman, D., and Salsbury, N.J. in Adv. Surface Science, Vol. 3 (Eds. J.F. Danielli, A.C. Riddiford, and M. Roseberg), Academic Press (1970), p. 121–168Google Scholar
- (14).Benedetti, E.L., and Emmelot, P. in The Membranes, Vol. 4 of Ultrastructure in Biological Systems (Eds. A.J. Dalton and F. Haguenan), Academic Press (1968), p. 33–121.Google Scholar
- (18).Eylar, E.H., Madoff, M.A., Brody, O.W., and Oncley, J.L. J. Biol. Chem. 237: 1992 (1962).Google Scholar
- (19).Winzler, R.J. in Red Cell Membrane (Eds. G.A. Jamieson and T.J. Greenwalt), J.B. Lippincott Co. (1969), p. 157–228.Google Scholar
- (20).Rosenberg, S. A., and Guidotti, G. in Red Cell Membrane (Eds. G. A. T. J. Greenwalt), J. P. Lippincott Co. (1969), p. 93–109.Google Scholar
- (23).Turner, J. D., and Rouser, G., unpublished results.Google Scholar
- (28).Rouser, G., unpublished results.Google Scholar
- (30).Therriault, D., and Rouser, G., unpublished results.Google Scholar
- (31).Rouser, G., and Yamamoto, A. in Handbook of Neuro chemistry (Ed. A. Lajtha), Plenum Press (1969), p. 121–169.Google Scholar