CELLULAR REGULATION 285 



During mitochondrial respiration, on the other hand, the turnover of 

 phosphate in the adenine nucleotides is more rapid than the diffusion of 

 ATP to the outside of the mitochondria or of phosphate acceptors to the 

 inside. 



But the concentrations of AMP, ADP and ATP remain constant in the 

 stationary state in mitochondria during active oxidative phosphorylation. 

 So apparently there is a control of oxidative phosphorylation by means of a 

 regulation of the entry of nucleotide phosphate acceptors. According to 

 Siekevitz and Potter, this entry is under the control of an enzyme situated 

 in the outer region of the mitochondria, this enzyme, adenylic kinase, 

 catalyses the establishment of equilibrium between AMP, ADP and ATP. 



H. Stationary States 



The intervention of the various factors responsible for intracellular 

 regulation leads to the persistence over an extended period of concen- 

 trations of the various substances which compose the cell. The stationary 

 states themselves also serve to regulate the orientation of a reaction chain 

 where alternative pathways are possible. 



II. THE PASTEUR EFFECT 



From the results of experiment it has been possible to deduce that the 

 control of cellular oxidation depends upon two factors : the concentration 

 of inorganic phosphate and the concentration of phosphate acceptors. An 

 example of the regulatory influence of these concentrations is the so-called 

 Pasteur Effect. First observed by Pasteur, as indicated by the name, this 

 phenomenon consists of the fact that when a cell is using oxygen glycolysis 

 proceeds less rapidly than in the absence of oxygen. Although glycolysis 

 is not a process in which oxygen actually plays a part, yet it is partially 

 inhibited by the presence of oxygen. In a cell lacking oxygen a much greater 

 number of sugar molecules undergo glycolysis than in the presence of 

 oxygen. Yet, in the second case, the amount of useful energy, obtained in 

 the form of energy-rich bonds, is much greater. From a fundamental point 

 of view the Pasteur Effect is of great interest, but it is its mechanism, as an 

 example of intracellular regulation, which interests us here. 



The Pasteur Effect can be described by saying that "aerobic glycolysis" 

 is weaker than "anaerobic glycolysis". Also, in cells during growth and in 

 cancer cells, aerobic glycolysis is greater than in resting cells, hence the 

 statement of Warburg; "No growth without glycolysis". The explanation 

 of the Pasteur Effect has been provided by Lynen who has shown that the 

 intensity of glycolysis depends on the concentration of inorganic phosphate 

 available at the stage of the dehydrogenation of phosphoglyceraldehyde 

 with production of 1,3-diphosphoglyceric acid. 



When the oxidative phosphorylations of respiration take place, the 



