142 CONTROL MECHANISMS IN CELLULAR PROCESSES 



nism by which this regulation is mediated, Httle that is definitive 

 can be said about the phenomenon at the present time. 



In order to discuss this assertion clearly, one should define the 

 terms to be used. A great deal of confusion exists due to the mean- 

 ing attached to the term glycolysis. In this paper glycolysis will be 

 used as defined by Hatch and Turner (1959): "Glycolysis is the 

 breakdown of carbohydrate to pyruvate by the Embden-Meyerhof- 

 Parnas pathway, irrespective of the subsequent fate of pyruvic acid." 



The regulatory phenomena referred to are generally known as the 

 "Pasteur effect ( reaction ) " and the "Crabtree effect." The difficulty 

 of defining the Pasteur effect has not been solved since the initial 

 observation by Pasteur himself. Many definitions in the literature 

 are chosen to accommodate the experimental conditions. The Pas- 

 teur effect will be defined here as "the steady state inhibition of 

 glycolysis through the utilization of oxygen." The Crabtree eftect 

 will be defined as "the inhibition of endogenous respiration by the 

 utilization of exogenous carbohydrates." 



These definitions were chosen among the many variants available 

 not because an explanation will be oftered to fit the effects as de- 

 fined, but because they allow discussion of the widest possible variety 

 of experimental phenomena. 



Pertinent Metabolic Reactions. It is inherent in any discussion 

 of metabolic interrelationships and metabolic control mechanisms 

 that we assume that the sequence of reactions involved are at least 

 qualitatively known and understood. It seems worthwhile to in- 

 vestigate this assumption for a moment. 



We are interested in the catabolism of hexose to pyruvate and 

 lactate (or ethanol) and the further oxidation of pyruvate to CO- 

 and water. On tliis second stage of carbohydrate metabolism, we can 

 be confident. While variants of the tricarboxylic acid cycle do exist 

 in nature, we know that the cycle is the main pathway and are rea- 

 sonably certain that all steps in the oxidation are known. Of course 

 we are not sure how such oxidation traps energy in the form of ATP, 

 but we are certain that it does. 



While the sequences of glucose to pvruvate are usually considered 

 to be equally well known, these need some further discussion. The 

 Embden-Meyerhof sequence is firmlv established and so is the hexose 

 monophosphate shunt, but there is evidence that these two do not 

 quantitatively account for carbohydrate fermentation and oxidation. 



