FACTORS AFFECTING METABOLISM in Vttro 105 



in Table 17 and those enabling slices to maintain adequate levels 

 of phosphocreatine suggest a common factor, the simplest of 

 which would be a shared precursor such as adenosine triphosphate. 

 Nevertheless, differences exist. Thus pyruvate and lactate support 

 incorporation into phosphoprotein to a markedly less degree than 

 does glucose but support incorporation into nucleic acids to an 

 extent similar to glucose. Fructose does not support phosphate 

 incorporation into the nucleic acids but does so into the other 

 fractions though to a less degree than glucose or mannose. 



Differences here may relate partly to the concentration of 

 substrate in addition to its ability to maintain adequate levels of 

 energy-rich phosphates. Thus, in contrast to the above, Schachner 

 et al. (1942) found that glucose, mannose, fructose and galactose 

 were all equally effective in supporting the incorporation of 

 phosphate into rat cerebral phospholipids. It has been suggested 

 (Strickland, 1954) that the difference between the results with 

 fructose (Table 17) and those found by Schachner et al. may be 

 explained by the inhibition of fructose metabolism by traces of 

 glucose present in the tissue slices, though it was admitted that this 

 did explain why the addition of fructose increased the oxygen 

 uptake of the slice. Schachner et al. used substrate concentrations 

 of 20 mM whereas Strickland used concentrations of 10 mM. 

 Such differences can be important for it has been shown (Mcllwain, 

 1953Z>) that whereas glucose at 10 mM was adequate to support 

 the respiratory response of cerebral slices to applied stimuli, 

 fructose at 10 mM was not, and higher concentrations were 

 required before results comparable with glucose were obtained. 

 Similar considerations might apply to the effects of pyruvate and 

 lactate, though here a simpler correlation exists in relation to 

 maintenance of levels of energy-rich phosphates. This would 

 imply that quantity as well as rate of metabolism of, say, adenosine 

 triphosphate is important in introducing the labelled phosphate 

 into the phospholipids but it mu^t be admitted that information 

 relating to equilibria in these systems does not exist. 



The decreased ability of fructose metabolism to support the 

 incorporation of phosphate into the phosphoprotein fraction 

 (Lissovskaya, 1956; see also review, Stekol, 1957) has been con- 

 sidered to be related to the lowered glycolysis with this substrate 

 rather than to the levels of adenosine triphosphate which decreased 

 to some 60% of the quantities found in the presence of glucose. 



8— PMB 



