1808 HANDBOOK OF PHYSIOLOGY — NF.rROPHYSIOLOCY III 



table 6. Cill Fractions* 



* References: 2-5, 27, 28, 33, 34, 39, 48a, 83, 90, 102, 1 10, 156, 171 , 181 , 186, 195, 208, 210, 235, 243. 

 t Estimated from specilic activities of proteins during incorporation of radioactive amino acids. 

 I No quantitative data; relative distribution indicated by 4- signs. 

 § Lipid ' , dry \vt. of fraction for nuclei 27; for microsomes 57. 



It is probable th;it the values for phosphatides 

 reflect the distribution of total lipids since the phos- 

 phatides make up the hulk of cellular lipids, with the 

 possible exception of the supernatant fraction. In 

 liver the lipid of this fraction is mostly neutral fat 

 (210), and in brain preparations this fraction tended 

 to separate into lipid and nonlipid portions (3). It is 

 also probable that the values for total nitrogen 

 reflect protein distribution, although no direct data 

 are available. One other point of interest is the high 

 concentration of potassium in mitochondria. It has 

 been suggested that this may account for the non- 

 diffusible fraction of potassium in brain tissue en- 

 countered by a number of investigators (no). When 

 these results on neural tissue fractions are compared 

 with those for liver, there is in general a close cor- 

 respondence (19, 109, 136, 171, 181, 205, 206, 210, 

 an, 217). 



The distribution of enzyme systems, summarized 

 in table 6, also resembles that for liver cell fractions 

 The glycolytic system (converting glucose to lactic 

 acid) resides principally in the supernatant fraction, 

 although it requires ATP generated by mitochondria 

 lor iis lull activity (3, 48a, 83, 102, 139, 156). In 

 contrast iii liver, brain mitochondria possess con- 

 siderable glycolytic activity of their own in addi- 

 tion to thai in the supernatant (3, 39, 48a, 83, 

 10.', 156). However, the mitochondria are pre- 



disrupting mitochondria and find thai the enzyme assembly 



■ ■ arj 1 mplete substrate oxidation is retained intact 



.mil firmi) bound to relatively small fragments ol the mito- 

 chondi 1.1I structure 1 a istac 



eminent in being the principal site of oxidative 

 enzymes (concerned with the complete oxidation of 

 glucose and its associated generation of ATP) (2-4, 

 33> 34. 39> 48a, 83, 102). Brain mitochondria differ 

 in another respect from liver mitochondria in pos- 

 sessing all links in the oxidative chain (34, 83). 

 Brain mitochondria also appear to be the site of 

 production and metabolic utilization of 7-amino- 

 butyric acid, as well as a major site for its storage 

 (154, 230, 231; unpublished observations). It is 

 possible that brain cells also differ from liver cells 

 in their site of DPN production, which in liver is a 

 nuclear function (109), since added DPN has been 

 reported unnecessary for brain mitochondria (33). 

 This point is not yet clear, however. 



Lipid synthesis by neural cell fractions appears 

 to conform to that found in liver cell fractions (27). 

 Mitochondria and microsomes are the primary sites 

 of such activity, supplemented to varying degrees 

 by requisite contributions from the supernatant 

 fraction (6, 28, 37, 196). Because of the complexity 

 of neural lipids, requiring carbohydrate and amino 

 acid components as well as the more usual lipid 

 units, it is understandable that lipid synthesis should 

 involve several fractions and be dependent upon the 

 particular type of lipid being synthesized. The brain 

 also conforms to liver in respect to protein synthesis 

 In liver, the microsomal fraction is primarily re- 

 sponsible lor incorporating amino acids into proteins, 

 Supplemented bv supplies of cofactors (such as ATP) 

 from mitochondria and supernatant fractions (121, 

 140, 208). Microspectrographic analyses of neurons 

 suggested thai they utilized these same principles 



