EVALUATION OF METABOLIC PATHWAYS 41 



apparatus, in which the reaction flask and CO2 receiver are 

 constructed as shown in Fig. 2.7. It may be noted that the 

 manometer o£ the usual apparatus has been exchanged 

 for a system for trapping respiratory CO^; by proper manip- 

 ulation of the stopcock above the CO2 trap (G in Fig. 2.7), 

 shifts can be made to alternate receivers. (For operating in- 

 structions, see reference 11.) Shifts to new receivers permit 

 analysis of the KOH solution holding the recovered CO2, 

 and thus the CO2 obtained in a desired interval of time may 

 be recorded; the apparatus becomes an interval radio- 

 respirometer. Plots of the interval yields of Ci^02 from the 

 labeled substrate obtained automatically display the kinet- 

 ics of the process. It will also be noted that several simul- 

 taneous experiments may be carried out; by using differ- 

 ently labeled substrates in each flask, a battery of experi- 

 ments may be conducted that will give a simultaneous pic- 

 ture of the kinetics of oxidation of each of several carbon 

 atoms of a given substrate. The uncontrollable complex- 

 ities of metabolism that cause dilution of individual carbon 

 atoms and spuriously influence specific activities may be ig- 

 nored since the instrument permits a measure of the total 

 yield of CO2 from a given labeled position of a substrate 

 molecule. 



It is reasoned that oxidation of glucose exclusively at 

 carbon 1 is indicative of phosphogluconate cleavage. Any 

 entrance of carbon 1 into CO2 via the Embden-Meyerhof 

 route followed by the TCA cycle should also be reflected in 

 equal C^ oxidation; so the difference between the Ci and 

 C(> yield in respiratory CO2 should give a measure of the 

 extent of phosphogluconate cleavage. The per cent of the 

 total metabolism of glucose that this process supports, on 

 the other hand, will be chiefly influenced by the appearance 

 of carbons 3 and 4 in CO2 (indicative of glycolysis). The 



