38 METABOLIC PATHWAYS IN MICROORGANISMS 



since the ratio of Ci^02 production, Ci/Cq, does not carry 

 any quantitative significance unless the actual amounts of 

 substrate glucose are known. Moreover, the evolution of 

 COo from carbons 3 and 4 (reflecting glycolysis) was not 

 taken into account. 



Major objections to the use of specific activities of C^^Oo 

 to measure pathway participation are two: (a) whenever 

 more than one pathway is operative, as is usually true, 

 regeneration of hexoses by the pentose cycle results in 

 unpredictable dilution of the respiratory COo, from glucose 

 carbon atoms that undergo glycolysis or other breakdown; 

 (b) there is a dilution of all C^-^ atoms (5) by endogenous 

 metabolites, which are oxidized and enter the total CO2 

 pool. 



The latter objection may also be raised against methods 

 that employ measurement of the specific activity of such 

 intermediates as pyruvate, lactate, or alanine. Weinhouse 

 and co-workers (3) have met this by the ingenious device 

 of using glucose-U-C^* in concurrent experiments as a ref- 

 erence standard, so that the dilution may be recognized 

 and compensated. However, this method also assumes that 

 there exists no drainage of pentose cycle intermediates for 

 synthetic purposes, and it assumes further that pentose P 

 does not re-form hexose P. The error caused by this last 

 assumption will be small if the contribution that is made 

 by the pentose cycle to total metabolism is small. Dawes 

 and Holms (6-8) have assumed, correctly, I believe, that 

 any regenerated hexose will be catabolized by botJi gly- 

 colysis and the pentose pathway; however, they have also 

 assumed no drainage of pentose cycle intermediates for syn- 

 thesis. 



In a comprehensive, thoughtful analysis of this subject, 

 Katz and Wood (9) have recognized the dilution problem 



