REGULATORY MECHANISMS IN ENERGY METABOLISM 153 



wlien pyruvate levels are lowered by the addition of hexokiiiase and 

 2-deoxyglucose or glucose. Tliis is not necessarily an indication that 

 microsomal cytochrome-c-reductase is the pliysiological mediator. 

 However, the glycerol-1-phospliate cycle is not the mediator. In 

 spite of the fact that DPNH-can l^e catalytically oxidized through 

 the cycle by liver mitochondria (Ciaccio and Keller, 1960), the rate 

 is far too low, due to the insoluble enzyme, for the requirement im- 

 posed by the rate of ghcolvsis. Furthermore, the preferential utiliza- 



TABLE 5-3 



The Effect of Microsomes and Cytochrome c on Coupling 

 Respiration and Glycolysis 



C, Uptake Lactate Fomiation 

 Experiment Conditions (/xmole) (/^mole) 



The incubation system described by van Eys and Warnock ( 1959 ) was used; 30 

 fxmole 2-deoxyglucose were included. Where reconstituted homogenates were used, 

 the amounts of tissue fractions correspond to the original homogenate. 



tion of glycerol-1-phosphate hydrogen for glycogen biosynthesis in 

 liver and kidne\' tends to argue against a glycerol- 1-phosphate cycle 

 in these tissues (Bloom, 1959). 



This may be different in the brain, where the glycerophosphate 

 cycle is possible, due to the high rate of insoluble a-glycerophos- 

 phate-dehydrogenase (Green, 1936; Ringler and Singer, 1959; Zebe 

 et al., 1959). It may be pertinent that the soluble a-glycerophos- 

 phate-dehydrogenase is low in tumors (Boxer and Shonk, 1960; 

 Delbriick et al., 1959) and that glycolysis in tumors fails to give rise 

 to a-glycerophosphate (Ciaccio et al., 1960). Furthermore, the in- 

 soluble a-glycerophosphate-dehydrogenase is under control of thy- 

 roxine (Lee et al., 1959); thus the glycerol-1-phosphate cycle may 

 become operational in liver in hyperthyroid states. 



