

ii Winn K ik i ii physiology 



NEUROPHYSIOLOGY III 



oxidase, were considered on various grounds to be 

 ruled out in the production of ammonia (222). De- 

 amination of amino acids was found not to occur with 

 cerebral suspensions (222 I. It was suggested thatammo- 

 nia arises from some reaction linked to proteolysis but 

 .is yet no convincing evidence has been produced in 

 support of the view. Several active proteinases and 

 peptidases exist in cerebral tissue (6, 7, 93, 164). 



Phosphates 



Although studied in vivo in relation to functional 

 activity since the 1940's, the investigation of phos- 

 phate metabolism in cerebral tissue in vitro is largely 

 of more recent origin (76, 128). It is convenient here 

 to consider some of the various aspects of metabolism 

 under two headings, in tissue slices and in disinte- 

 grated preparations. Phosphate intermediates in rela- 

 tion 10 carbohydrates will not be considered. 



mi i vboi.ism in tissue slices. Cerebral tissues have 

 been shown to contain a large number of phosphorus 

 derivatives, some of which can be extracted by re- 

 agents such as trichloroacetic acid and include 

 creatine phosphate, adenosine triphosphate and in- 

 organic phosphate as the major constituents. Others 

 insoluble in trichloroacetic acid include all the phos- 

 pholipids, nucleic acids, phosphoprotein and phos- 

 phoinositides. Derivatives of cytidine, uridine and 

 guanosine phosphates have also been isolated and 

 characterized (75, 76, 182, 207). 



When freshly cut, cerebral slices contain low levels 

 of creatine and adenylphosphates and a high level of 

 inorganic phosphate; but on incubation in a suitable 

 oxygenated saline containing glucose, levels of 

 creatine phosphate and adenosine triphosphate rise 

 while the level of inorganic phosphate decreases 

 1 inn, [38). Although under such conditions the levels 

 "l * nergy-rich' phosphates do not rise to more than 



■>n per cent of the levels normally found in run, the 

 level nl lie nine phosphate can be increased to 70 per 

 cent of tin- in vivo level in 2 hr. by including creatine 

 in tin- incubation medium (206). Maintenance of 

 adequate levels of creatine phosphate in the slice is 

 dependent upon a supply of oxygen and glucose 

 , ui of pyruvate (76), Other substrates such as 

 ■ m. ite, succinate, linn. irate and glutamate failing to 

 support resynthesis. I he failure of glutamate, even in 



the presence of glucose, to maintain the level of 

 creatine phosphate is curious in view of its effect in 



maintain the potassium concentration of tissue 



slurs .Mid ui iis existence free in the tissue /" vivo al 



approximately 10 mmole (220). It has been shown that 

 the 7-phosphorus of adenosine triphosphate is the pre- 

 cursor of the phosphorus of phosphocreatine, the latter 

 having a turnover rate of about 160 to 170 /jmole per 

 gm wet wt. tissue per hr. (76). Attempts to promote 

 the synthesis of phosphopyridine clinucleotide by in- 

 clusion of possible precursors in the medium were 

 unsuccessful (65). 



Metabolism of other phosphate derivatives such as 

 phospholipids, nucleic acids and phosphoprotein has 

 been shown to be an active process. Thus radioactive 

 phosphate is incorporated into cerebral phospho- 

 lipids (46, 57, 197) in the presence of oxygen and 

 glucose but not in the absence of glucose or under a 

 nitrogen atmosphere in the presence of glucose. 



The in vitro phosphate metabolism of nucleic acids 

 and phosphoproteins has been investigated (33, 75, 

 197). Incorporation of radioactive phosphate into 

 phosphorus considered to arise from phosphoprotein 

 was shown to proceed at a rate greater than incor- 

 poration into other groups of acid-insoluble phos- 

 phates. Incorporation into nucleic acids was slow and 

 took place almost exclusively into ribose nucleic acid, 

 little or no activity occurring in deoxyribose nucleic 

 acid. It is reported (179) that the inositol phosphorus 

 of the tissue residue after removal of acid-soluble 

 phosphates and phospholipids also exhibits a marked 

 turnover. As with energy-rich phosphates, substrates 

 such as succinate, malate, glutamate and a-oxoglu- 

 tarate failed to support incorporation of radioactive 

 phosphorus into phospholipids, nucleic acids and 

 phosphoprotein of cerebral slices. Pyruvate and lac- 

 tate supported incorporation to a degree less than 

 glucose. Anerobiosis abolished incorporation of phos- 

 phate into any fraction. 



METABOLISM IN DISINTEGRATED PREPARATIONS. Disrup- 

 tion of cellular structure permits the intermingling of 

 cellular components and consequend) man) phos- 

 phates essential to metabolism are readily degraded. 

 Thus adenosine triphosphate is rapidly converted to 

 adenylic acid and inorganic phosphate by the com- 

 bined action of adenosine triphosphatase and myo- 

 kinase (66, IIQ, 150, 160). The adenosine triphos- 

 phatase sv stem involves at least two enzymes, one 

 activated by magnesium ions and inhibited by 

 calcium ions and the other activated bv calcium ions 

 (155). It is not identical with the inorganic pyro- 

 phosphatase system of brain (66, 165). Di- and 

 triphosphopyridine nucleotides are degraded by an 

 enzyme specific lor the oxidized forms (14")), the 

 breakdown being inhibited bv nicotinamide (121). 



