396 2. ANALOGS or enzyme reaction components 



by very low levels of P^. During the oxidation of ethanol, the ATP/ADP 

 and P,/ADP ratios are high; when 2-DG is added it temporarily augments 

 respiration by lowering these ratios, but within a minute enough phosphate 

 has been trapped in 2-DG-6-P to cause a marked fall in the Pj/ADP ratio 

 (perhaps from 17 to 1.5). The P, may now be so low that it limits the respi- 

 ration. 



The respiration of certain tissues is diminished by the addition of glucose, 

 a phenomenon often called the Crabtree, or reversed Pasteur, effect; it 

 is particularly evident in Ehrlich ascites carcinoma cells and most of 

 the studies of the mechanisms involved have been on these cells. It has 

 been stated that an acceleration of glycolysis inhibits the oxidation of 

 pyruvate, but there was no real evidence to link the entire EM pathway 

 with respiratory control. The effects of 2-DG are thus of particular impor- 

 tance, since it is phosphorylated but not further metabolized to any extent. 

 It was shown that 2-DG inhibits respiration to about the same degree as 

 glucose (Ibsen et al., 1958). Respiration and pyruvate decarboxylation are 

 reduced 50% by 10 milf 2-DG (Ram et al, 1963). There has been dis- 

 agreement as to whether glucose and 2-DG act by the same mechanism or 

 differently. Let us briefly compare the responses to these sugars. (1) 2-DG 

 depresses the respiration more slowly than does glucose. Yushok (1964) 

 has shown in a group of sugars that the rate of respiratory inhibition is 

 correlated with the rate of phosphorylation. One would thus expect 2-DG 

 to act more slowly than glucose, so this does not constitute a real difference 

 in action. (2) The inhibition by glucose is released when it is all glycolyzed 

 but the inhibition by 2-DG remains (Ibsen et al., 1962; Hofmann et al., 1962). 

 This does not seem to me to be valid evidence for different mechanism of 

 action. (3) The addition of glucose leads to the formation of lactate whereas 

 2-DG does not (Ram et al., 1963). This is what would be expected, of course, 

 but emphasizes that glycolysis, as defined classically, is not necessary for the 

 effect. (4) Glucose at 10 mM inhibits the respiration 40%, 2-DG at 20 mM 

 inhibits it 48%, and both together inhibit it only 23% (Wenner and Cereijo- 

 Santalo, 1962). This was interpreted to mean that the inhibitory mechanisms 

 are quite different. (5) It has been stated that amobarbital prevents the 

 inhibition of respiration by 2-DG but not by glucose (Wenner and Cereijo- 

 Santalo, 1962). This is true, however, only in the presence of succinate, 

 since the endogenous respiration is not affected by either glucose or 2-DG 

 in the presence of amobarbital (there is very little to be affected). (6) The 

 respiratory inhibition by glucose is released by 2,4-dinitrophenol, but 

 there is some disagreement as to the effect of the uncoupler with 2-DG, 

 Ibsen et al. (1962) stating that it releases the inhibition and Ram et al. 

 (1963) stating that it does not. The latter workers, however, did not feel 

 that this is evidence for different mechanisms and were inclined to attrib- 

 ute the differences to the availability of glycolytic intermediates. (7) Both 



