174 F. Dickens, G. E. Glock and P. McLean 



that in these liver tumours, at any rate, a smaller proportion 

 of the respiratory CO2 is derived from the pentose P pathway 

 than in normal liver. 



Some very striking alterations in the pattern of enzyme 

 activity, particularly among enzymes concerned with G6P 

 metabolism, have been demonstrated in liver tumours. 

 Weber and Cantero (1957) have shown that the activity of 

 G6P dehydrogenase in Novikoff hepatoma is five times higher 

 than that of normal rat liver. A similar increase in the G6P 

 dehydrogenase activity has been found in the liver tumours 

 of DAB-treated rats while, in marked contrast to this, the 

 activity of 6-PG dehydrogenase remained unchanged and thus 

 becomes the limiting enzyme (Glock and McLean, 1958, un- 

 published). The reduced overall activity of this pathway 

 cannot, therefore, be explained by these alterations in enzyme 

 activity and it is perhaps due to the very low TPN content of 

 the tumours, which may become rate-limiting (Glock and 

 McLean, 1957). The supply of TPN+ may be even further 

 limited by their very low levels of TPNH-cytochrome c 

 reductase and pyridine nucleotide transhydrogenase (Reina- 

 farje and Potter, 1957). 



There is a progressive decrease in the G6Pase activity of 

 liver which occurs quite early during the induction of DAB 

 tumours (Spain, 1956) and in certain liver tumours no G6Pase 

 activity could be detected (Weber and Cantero, 1957). 

 Another function of the normal liver cell which is lost in 

 neoplastic tissue is the storage of glycogen. The related 

 enzymic changes are particularly clearly seen in Novikoff 

 hepatoma where there is a very marked decrease in phospho- 

 glucomutase activity (to less than 10 per cent of that in 

 normal liver) and a complete absence of fructose diphosphatase, 

 an enzyme of importance in the synthesis of glucose from 

 pyruvate and lactate (Weber and Cantero, 1957). 



Synthesis of ribose 



The two alternative pathways for ribose synthesis can be 

 assessed in vivo by measurements of the incorporation of ^*C 



