12 METABOLISM IN THE NORMAL FUNCTIONAL STATE 



part of these consists of phospholipids. A Hst of the quantities of 

 the various groups of components found in the residue is found in 

 Table 4. Of the total phosphorus present some 80-90% consists 

 of phospholipid phosphorus, the remainder coming from nucleic 

 acids, phosphoproteins and a complex residue. The values given 

 for most of the phospholipids are representative only and are 

 included to illustrate the preponderance of this group over the 

 other phosphates. More extensive summaries of the quantities and 

 types of phospholipids in brain have been given by Sloane- Stanley 

 (1952), Deuel (1955), Folch and Le Baron (1956, 1957) and Rossiter 

 (1957). Two groups of lipids may be singled out for comment. 

 Sphingomyelin constitutes part of the " myelin lipids " (Johnson 

 et al.j 1954) which include cholesterol and cerebrosides from which 

 phosphorus is absent. It is believed that these form the major 

 lipid components of the myelin sheaths of the axons both in the 

 central nervous system and in the peripheral nerve. Evidence for 

 this has been summarized by Johnson et al. (1954) and is based 

 partly on the finding that sphingomyelin exists in greatest quantity 

 in white matter, containing the bulk of the myelinated fibres of the 

 brain, and least in the grey matter. The quantities of the inositol 

 phospholipids are approximate only and have been calculated on 

 the basis of information available. This group of lipids are 

 extremely active as regards phosphorus metabolism but detailed 

 studies have so far been hampered by the lack of a suitable method 

 of analysis. 



Similar considerations apply to the quantities of phosphorus 

 found in the fractions containing nucleic acids, phosphoproteins 

 and the residual organic phosphorus (see p. 14). Probably the 

 most reliable figures are given by the methods of Strickland (1952) 

 and Logan et al. (19526). Quantities of phosphoprotein phos- 

 phorus are about 1-0 jitmole phosphorus/g wet wt. and values in 

 excess of this can be attributed to contamination with part of the 

 residual organic phosphorus fraction (Logan et al., \9S2a\ LeBaron 

 and Folch, 1956). 



With the exception of certain of the phospholipids, the isolation 

 or even partial purification of the majority of the components 

 listed in Table 4 has not been achieved. The difficulties attendant 

 upon any such separations are well illustrated both by the com- 

 plexities already encountered with the phospholipids and by the 

 demonstration that brain nucleic acids show numerous differences 



