RESPIRATION 111 



vertebrates, on the other hand, depresses the amount of a- 

 ketoglutaric acid present, pyruvic acid is 200% of the normal value, 

 suggesting an inhibition of the mechanism by which pyruvate is 

 metabolized into the early stages of the citric acid cycle. The 

 inhibition of triosephosphate dehydrogenase (Hawke, Oser, and 

 Summerson, 1954) evidently does not occur here since this would 

 lead to a depression in the amounts of pyruvic acid rather than an 

 accumulation. 



The citric acid cycle and the ability of tissue to metabolize the 

 intermediate substances produced is also studied by observing 

 whether or not the addition of these substrates to tissue preparations 

 is followed by an increase in the oxygen uptake of the tissue. 

 Petrucci (1954) added succinate to homogenates of P. velutinus 



Table 17 



Levels of Pyruvic and a-KETOGLUTARic Acid in the Tissues 



OF P. velutinus and E. foetida (in ixgjg Fresh Weight) 



(From Petrucci, 1952) 



tissues and observed that a large increase in the oxygen con- 

 sumption occurs, as also happens when pyruvic and a-ketoglutaric 

 acid is provided. Mechanisms for the metabolism of the substances 

 do, therefore, exist. Arsenite as mentioned before appears to block 

 the citric acid cycle since the addition of pyruvic acid causes 

 respiratory increase, but a-ketoglutaric acid does not. Somewhere 

 in the stages between these compounds a blockage occurs. 

 Malonate, which inhibits succinodehydrogenase in vertebrates, 

 causes only a 12% drop in oxygen uptake, but completely 

 depresses the increase normally noted on addition of succinate. 

 The comparatively small depression of tissues untreated with 

 succinate, due to malonate, is explained by the endogenous 

 substrate (succinate) being normally present in only small quan- 

 tities (Fig. 34). 



