216 E. Racker and R. Wu 



were simply hydrolytic and apparently wasteful, it could 

 represent a most vital mechanism by allowing continuous 

 operation of the Krebs cycle, though at a reduced rate. 

 Perhaps it is superfluous to remind this group that the 

 Chairman of this symposium has frequently emphasized that 

 the tricarboxylic acid cycle has biosynthetic functions (gluta- 

 mic acid, porphyrins, etc.) in addition to its role in oxidation. 



The second example concerns an ATPase activity which we 

 have encountered in the presence of highly purified glycolytic 

 enzymes. Dr. Krimsky made the observation that both 

 transphosphorylating enzymes of glycolysis can be readily 

 reversed provided the ATP/ADP ratio is maintained very 

 high. Thus, phospho^no/pyruvate can be formed from 

 pyruvate and 1 : 3-diphosphoglycerate (1 : 3-diPGA) from 

 PGA in large enough amounts to be amenable for preparative 

 procedures. However, 1 : 3-diPGA is hydrolysed quite 

 rapidly even at neutral pH at 37°. Thus, starting with ATP 

 and catalytic amounts of PGA in the presence of a system 

 which regenerates ATP (e.g., PGA with PGA mutase, enolase 

 and pyruvate kinase or phospho^noZpyruvate with pyruvate 

 kinase), one can observe an ATPase activity which is quite 

 appreciable. Indeed, it hindered early attempts to demon- 

 strate reversal of glycolysis. 



Although there is no evidence that a PGA-dependent 

 ATPase activity is functional in intact cells, it may serve as a 

 model for a self ad justing metabolic mechanism which would 

 allow for a limited rate of glucose breakdown in Pi-deficient 

 cells. 



There follows a brief discussion of a second metabolic 

 feature of the intact ascites cells. It has been known for over 

 30 years that tumour cells glycolyse aerobically, but there is 

 no adequate explanation for this phenomenon. Straub and 

 his collaborators (Acs et al., 1955) have proposed that hexoki- 

 nase associated with mitochondria of ascites cells may 

 account for the high glycolysis. Though we have confirmed 

 their finding that a considerable proportion of tumour 

 hexokinase appears in the mitochondrial fraction, we are not 



