TPN + , and ATP or IT? is probably at least -7 kcal. The 

 ratio of the forward reaction to the back reaction, given by 



DT-i /forward rate' 

 V back rate 



/back rate plus net rate\ 

 \ back rate J 



would thus be 10^ or greater. Since the rate of labeling of 

 malic acid is measurable and gives the net rate by a simple 

 calculation, the back reaction, and hence the exchange label- 

 ing, can be shown to be of negligible importance. 



This type of calculation is of considerable importance in 

 in vivo steady-state kinetic calculations. Another example is 

 the conversion of malic acid to fumaric acid. In this case, the 

 actual free energy change is small; the two acids are essen- 

 tially in equilibrium with respect to C^*-labeling. Thus the 

 sum of the pools of the two acids can be treated from a 

 labeling standpoint as a single entity. 



In any event, if malic acid is not labeled by exchange 

 and is not converted to succinic acid yet is being formed at 

 a rapid rate under steady-state conditions, it must undergo 

 some as-yet-unknown conversion. One possibility might be 

 that it is split to give glyoxylic acid and free acetate. The 

 actual free energy change for such a reaction under steady- 

 state conditions would be negative, whereas the reaction to 

 give glyoxylic acid and acetyl Co A would probably be posi- 

 tive and the latter reaction would not occur. Acetate could 

 be converted to acetyl phosphate with ATP and then to 

 acetyl CoA. The acetyl CoA thus formed could be used in 

 fatty acid synthesis and other biosynthetic reactions. The 

 glyoxylic acid could be used in the synthesis of glycolic acid, 

 glycine, and possibly, as suggested in the previous section, 

 glutamic acid. 



The synthesis of labeled malic acid could occur via 

 condensation of glyoxylate with acetyl CoA, provided there 

 is some other route for the labeling of these two-carbon 

 acids (such as are suggested later). It is quite likely that malic 



38 



