REGULATORY MECHANISMS IN ENERGY METABOLISM 159 



oxidation is explained by the fact that acetate oxidation does not re- 

 quire TPN, while higher fattv acids do. However, the observations 

 bv Kvamme (1958c) that a-ketoglutarate oxidation is subject to a 

 Crabtree effect may not fit this theory. On the otlier hand, if am- 

 monia were present reduction to glutamate might occur. Holzer and 

 Witt (I960) found that the addition of ammonia to yeast cells 

 Sreatlv accelerated the shunt. 



Another objection might be that it is very likely that TPNH is 

 equalh' compartmentized, so that TPNH in the mitochondria is not 

 TPNH in the c\'toplasm. It must be mentioned, however, that iso- 

 lated mitochondria are more permeable to TPNH than they are to 

 DPNH (Birt and Hartley, 1960). However, a point for considera- 

 tion is that dinitrophenoi, when it releases the Crabtree effect, does 

 not raise the TPN level (Ibsen et al, 1958). 



Hydrogen Ion Concentration. One of the characteristic features 

 of the ascites tumor cell is the extremely sharp pH optimum exhib- 

 ited by the endogenous respiration (McKee et al, 1953). It is 

 therefore obvious that the pB. requires control, and therefore control 

 of hydrogen ions has remained a constant worry for Crabtree in- 

 vestigators. An explanation of the Crabtree effect based on hydro- 

 gen ion concentration was first offered by Tiedemann ( 1952 ) . 



Arguments against a pH effect have been repeatedly brought 

 forth. There is no need to list the large number of experiments used 

 to indicate that extracellular pH is no factor in the Crabtree effect. 

 There are indications however that the extracellular lactate is not 

 a true reflection of intracellular lactate, so that hydrogen ion con- 

 centration as a factor in intact cells is not definitely ruled out 

 ( Dewey and Green, 1959 ) . 



Obviously the intracellular pH is the factor of interest. To study 

 this effect homogenates are required. As far as is known the only 

 study of the Crabtree effect in homogenates is that of Cereijo- 

 Santalo (1960). Ascites tumor cell homogenates show a Crabtree 

 eflFect which is largely similar to that of intact cells with two major 

 exceptions: iodoacetic acid abolishes the effect and 2-deoxy glucose 

 is not a strong Crabtree effect inducer. However, all other sugars, 

 glucose, fructose, mannose, which induce a Crabtree effect in intact 

 cells, induce the same effect in homogenates. The effect is seen 

 whether an oxidizable substrate, such as glutamate, is present or 

 endogenous substrates are used ( Cereijo-Santalo and Park, 1960). 



