Lipids, in myelin

The concept of ion-dipole interaction between lecithin and cholesterol has been suggested by many workers for the packing of these lipids in myelin or in the cell membrane (18, 19, 52). This concept is not supported by the surface potential measurements of mixed monolayers of lecithin and cholesterol. In contrast to dicetyl phosphate-cholesterol... 

Other abundant lipids in myelin are galactosylcerebrosides (Gal-C) and their sulfated derivatives (sulfatides). GalC represent 20% lipid dry weight in mature myelin. Immunological and chemical perturbation studies indicate that these lipids are involved in oligodendrocyte differentiation, myelin formation and myelin stability. These galactolipid-deficient animals exhibit severe tremor, hindlimb paralysis and display electro-physiological deficits in both CNS and PNS (Baumann and Pham-Dinh, 2001). 

Vitamin E is found in the diet in the lipid fractions of some vegetable oils and in liver, egg yolks, and cereals. It is absorbed together with lipids, and fat malabsorption results in symptomatic deficiencies. Vitamin E circulates in the blood in lipoprotein particles. Its deficiency causes neurologic symptoms, probably because the polyunsaturated lipids in myelin and other membranes of the nervous system are particularly sensitive to free radical injury. 

Since organelles and their membranes carry out such a wide range of different functions, it is scarcely surprising that their lipid compositions vary. Moreover, the percentage of lipid and protein in different membranes can range from 80% lipid in myelin to only about 25% in mitochondria... 

Figure 41-1. Ratio of protein to lipid in different membranes. Proteins equal or exceed the quantity of lipid in nearly all membranes. The outstanding exception is myelin, an electrical insulator found on many nerve fibers.

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

The function of the ALD protein is not fully understood, and knockout mice lacking it do not exhibit the severe CNS neurological deficits commonly associated with the human disease despite a similar accumulation of VLCFAs [26], Furthermore, the clinical variability in human patients cannot be accounted for by the severity of the biochemical abnormality or the nature of the gene defect. These observations, plus other data from mice with defects in VLCFA metabolism, raise the issue of whether the accumulation of VLCFAs in myelin is crucial to the pathological mechanisms or is an epiphenomenon. Unlike most other lipid-storage diseases, active ALD brain lesions are characterized by perivascular accumulation of lymphocytes. For this reason, it has been hypothesized that the severity of CNS pathology may relate to an autoimmune reaction that varies from patient to patient and... 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

Membranes consist largely of protein and lipid. The ratio (by weight) of protein to lipid varies from 0.25 in myelin to 3.0 in bacterial membranes. In membranes of erythrocytes it is about 1.2 and a ratio of about 1.0 may be regarded as typical for animal cells. Small amounts of carbohydrates (<5%) are present, as are traces of RNA (<0.1%). 

Glycolipids are important constituents of the plasma membranes, of the endoplasmic reticulum, and of chloroplasts. The cerebrosides and their sulfate esters, the sulfatides, are especially abundant in myelin. In plant membranes, the predominant lipids are the galactosyl diglycerides.29 74 The previously described ether phospholipids (archaebacteria), ceramide arnino-ethylphosphonate (invertebrates), and sulfolipid (chloroplasts) are also important membrane components. 

Selivonchick, D.P. and Roots, B.J. (1976). Variation in myelin lipid composition induced by change in environmental temperature of goldfish. Journal of Chemical Biology 1,131-135. 

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