600 DOWSE, NEUMAN, LANE, AND NEUMAN 



equivalence between the two is obtained. Though this is not con- 

 ckisive proof, it is suggestive evidence that glycogen is indeed the 

 principal, and possibly the only, endogenous substrate present. 



Attempts were made to deplete the endogenous glycogen to per- 

 mit a studv of the utilization patterns of exogenous substrates. How- 

 ever, aeration for a period of up to 4 hours, with medium changes to 

 eliminate the lactate formed, did not accomplish this. Longer periods 

 of aeration may well be adequate, but deterioration of the tissue 

 under such circumstances raises questions concerning the validity 

 of the data so obtained. 



Exogenous glucose uptake is also related to glycolysis ( Table V ) . 

 Uptake was determined by difference after a 2-hour incubation, and 

 the fact that an equivalent amount of lactic acid is produced in- 

 dicates that exogenous glucose is metabolized entirely to lactic acid. 



lodoacetate, at a concentration of 2 X 10~^ m (sufficiently high 

 to inhibit completely lactic acid production), reduces the Qon to 

 zero. This would suggest, as a first approximation, that oxygen up- 

 take is completely supported by the utilization of a carbohydrate 

 substrate. Conclusions from data obtained by the use of iodoacetate 

 must be interpreted with caution, however. In any event, no con- 

 clusion may be drawn as to the sequences involved. lodoacetate may 

 well restrict carbohydrate oxidation in the hexose monophosphate 

 shunt shown to be present in bone (Cohn and Forscher, 1962fl, 

 1962b) if the cell is dependent for reoxidation of the reduced TPN 

 formed in the first two stages upon coupling the reoxidation with 

 either the DPN formed in the pyruvate-to-lactate step or the TPN 

 formed in the malic enzvme reaction. Ability of the cell to utilize 

 the TPN formed in synthetic mechanisms or transamination reac- 

 tions would, of course, relieve it of dependence on later glycolytic 

 intermediates as electron acceptors. 



Uncoupling 



Uncoupling of oxidative phosphorylation in a tissue can be accom- 

 plished with 2,4-dinitrophenol (DNP) at a concentration of 2 X 

 10~^ M. With DNP, there was an increase in the Q0.2 of some 50 

 per cent and an increased accumulation of lactic acid under aerobic 

 conditions ( Table VI ) . Both endogenous and exogenous lactate pro- 



