CHEMICAL ARCHITECTURE OF THE CENTRAL NERVOUS SYSTEM 



'797 



by having little, if any, neutral or free fat under 

 normal circumstances. Most of the constituent units 

 of cerebral lipids are found in smaller amounts in 

 one or another of the body's organs. The classification 

 and relative amounts of the components of cerebral 

 lipids are given in table 2, and the basic structures of 

 the various classes are shown in figure 2. 



From table 2 the distinction between lipid com- 

 position of gray matter and white matter can be 

 readily discerned. Concentrations of cholesterol, 

 sphingomyelin and cerebrosides in white matter are 

 several times those found in gray matter. From the 

 similarity in composition of both cerebral white 

 matter and the peripheral myelin sheath, Johnson 

 et al. (115-118) have concluded that these three are 

 typical of myelin and myelinated structures, whereas 

 the phosphatides are more typical of cellular areas. 

 Since there is still some analytical confusion about 

 the cephalin group of phosphatides and about the 

 glycolipids, the individual percentages listed in the 

 table do not entirely correspond to the totals deter- 

 mined independently. This situation is due to the 

 facts that it has only recently been recognized that 

 the cephalin fraction is composed of four related 

 compounds (13, 14, 66, 71, 75, 78, 134, 183), one of 

 which, the plasmalogens, may actually be a group 

 (13, 134, 183), and that the ganglioside structure is 

 still not completely worked out (22, 23, 25, 38, 45, 

 74, 93, 1 28-1 31). As a result there are overlaps in 

 quantities determined by various methods. 



The structural formulas in figure 2 represent the 

 basic building blocks of cerebral lipids. How these 

 are formed into macromolecules such as gangliosides 

 and myelin is still poorly understood. Recent studies 

 by Cornforth et al. (45) and Bogoch (25) have clarified 

 the structure of gangliosides. Bogoch has proposed 

 the following unit structure for brain ganglioside: 



r-° 



®< 



c— ! 



o-r 



PROTEI 



LIPID 



LIPID 



H 2 



PROTEIN 



H 

 2 



ONE "PERIOD" 



PROTEIN 



FIG. 3. Proposed models for the macromolecular structure 

 of the myelin sheath. Structure A is that suggested by Schmitt 

 (203), and H is the modification suggested by Finean (62). 

 The latter incorporates cholesterol into the scheme, as indi- 

 cated by the solid bars. 



the Structure of which has been determined by 

 Cornforth et al. (45). Studies by Gottschalk (93), 

 Klenk (130) and Bogoch (25) suggest that ganglio- 

 sides may function as specific receptors (e.g. for 

 viruses) \i.i the neuraminic acid "surface' groups. 

 From the work of Folch and his collaborators, it 

 is apparent that much of the cerebral lipid is associ- 

 ated with protein as lipoprotein or proteolipid (68, 

 72, 77) as discussed in the next section. An indication 

 of how such structures may be built up may be 

 inferred from the isolation by Folch & LeBaron (70) 

 of phosphatidopeptides from brain white matter. A 

 different type of complex, called strandin, has been 

 isolated from gray matter (69) and is apparently a 

 macromolecular aggregation of gangliosides or 

 ganglioside-like components (74). 



[Cell Surface] 



[Cell Interior] 



Neuraminic Acid 



I 

 Galactosamine 



I 

 Galactose 



I 

 Glucose 



I 

 Sphingosine 



I 

 Fattv Acid 



Neuraminic Acid 

 Galactose^ 



Neuraminic Acid 



Galactosamine 



I 

 Galactose 



Glucose 



I 

 Sphingosine 



I 

 Fatty Acid 



X 70-80 



Some 70 to 80 of these units appear to be polymerized 

 together as a macromolecule, arranged along the 

 cell membrane in the orientation indicated. The 

 neuraminic acid is present as the N-acetyl derivative, 



Some idea of the organization of the final structure 

 lias been provided by polarized light, x-ray diffraction 

 and electron microscope studies of myelin sheaths 

 (60-64, J 34. '7 2 > ! 97» 201-203). On the basis of 



