CENTRAL NERVOUS SYSTEM METABOLISM IN VITRO 



1835 



Degradation of creatine phosphate does not occur in 

 dilute homogenates of cerebral tissues by a simple 

 hydrolytic mechanism but requires the addition of 

 adenylic acid or adenosine diphosphate (157, 160). 

 Under these conditions adenosine diphosphate is 

 converted to the triphosphate while creatine phos- 

 phate is degraded to creatine. 



PHOSPHATE SYNTHESIS AND OXIDATIVE PHOSPHORYLA- 

 TION. It was early recognized (12) that inorganic 

 phosphate and adenylic acid were essential for the 

 oxidation of pyruvic acid by dialyzed pigeon brain. 

 Further examination by Ochoa revealed that during 

 such oxidation considerable quantities of labile phos- 

 phate, presumably adenosine triphosphate, were 

 formed. In the presence of sodium fluoride to inhibit 

 adenosine triphosphatase, some two to three atoms 

 of phosphorus were esterified for each atom of oxygen 

 consumed, yielding P/O ratios of two to three (25, 

 no, 159). 



Since oxidative phosphorylation is a primary step 

 in aerobic cellular metabolism, it is to be expected 

 that factors inhibiting or accelerating it would also 

 affect the metabolism of the integrated tissue. Among 

 such factors are levels of inorganic phosphate and 

 creatine phosphate. Literature relating to the effects 

 of different levels of inorganic phosphate has been 

 summarized elsewhere (76, 128) and parallelism 

 shown to exist between the concentrations of inor- 

 ganic phosphate inducing marked oxygen uptake in 

 homogenates and those existing in cerebral tissues 

 in vivo in different physiological stales. Levels of phos- 

 phate acceptors such as adenine derivatives, creatine 

 or glucose (12, 64, 159) have similarly been shown to 

 affect the rate of oxygen uptake. 



As might be expected by analogy with othei 

 tissues, mitochondrial preparations from cerebral 

 tissues carry out oxidative phosphorylation with a 

 number of substrates (1, 22) which include the 

 intermediates of the tricarboxylic acid cycle and 

 glutamic acid. With such substrates the majority of 

 P/O ratios were 2.0 or greater. The phosphorylative 

 activity is more stable at 18 than at 37°C (22), and 

 attempts have been made to find the causes of insta- 

 bility at the higher temperature (24, 59). 



Lipids 



The state of knowledge of the metabolism of brain 

 lipids in vitro up to the autumn of 1951, was covered 

 by an earlier review from this laboratory (185). 

 Later discoveries were described by Rossiter (180), 



Lynen (120) and Klenk (95). The synthesis of some 

 highly unsaturated fatty acids (196) and the oxidation 

 of carboxyl-labeled octanoate, laurate (8, 61, 84) and 

 palmitate added in vitro have by now been demon- 

 strated with some certainty in brain slices and 

 homogenates with the aid of C 14 ; the /3-ketoacvl- 

 thiolase, acylcoenzyme A-deacylase and 0-hydroxy- 

 acyl dehydrogenase activities of brain have been 

 detected and measured (95); but the evidence for the 

 oxidation of intrinsic fatty acids by brain slices or 

 homogenates, whether in the form of C 14 2 from fatty- 

 acids given ante mortem by stomach tube or of a 

 special breakdown product of unsaturated fatty 

 acids cannot be considered quite as significant (29). 

 Work with C M has also shown that added fatty acids 

 are slowly incorporated into the lipids of respiring 

 brain slices and homogenates in the presence of 

 coenzyme A (84). The metabolism of cholesterol in 

 the central nervous system is confined, so far as is still 

 known, to the brains of embryos or young animals 

 during myelination (185), and the metabolism of 

 cerebrosides and gangliosides in the central nervous 

 system remains largely unknown. Strandin (52) is 

 now believed to be a mixture of gangliosides with a 

 sin. ill proportion of mucopolysaccharides (178, 201, 

 202). Unpublished work in these laboratories by 

 Sloane-Stanley has not confirmed an earlier announce- 

 ment (205) of a 'cerebrosidase' in brain. Nothing 

 appears to be known of the metabolism of sulpha- 

 tides in villi). 



The breakdown and synthesis of phospholipids, 

 or at least the removal and reattachment of their 

 polar groups, have been studied more thoroughly 

 (cf. 5). Perhaps the only enzymic reaction of a brain 

 lipid known to be as rapid as, say, brain respiration 

 (60 pinoles per hr. per gm fresh tissue) is still the 

 hydrolysis of diphosphoinositide into organic phos- 

 phate, inositol monophosphate and acid-insoluble 

 residues (184, t8r>). This reaction has now been 

 shown to occur in at least two independent steps, one 

 of which is activated by calcium and can proceed at a 

 rate of 300 pinoles per hr. per gm fresh tissue or more 

 (176). The synthesis of diphosphoinositide in brain 

 slices and homogenates has also been detected by 

 following the incorporation of P 32 into the "inositol 

 diphosphate' isolated by paper chromatography from 

 the products of alkaline hydrolysis of the lipids 

 extracted from the tissues after precipitation with 

 trichloroacetic acid, or from an alkaline or acid 

 hydrolysis of the precipitate (31, 32). The incorpora- 

 tion was shown to need the energy supplied by respira- 

 tion or glycolysis, and to be very much more rapid 



