ENDOGENOUS RESPIRATION 211 



the terminal system. Were this so, not 50 per cent but 75 per cent of 

 the total metabolism would be substrate metabolism; put another way, 

 the endogenous system would be suppressed 50 per cent. 



Barker (11) proposed a method to determine whether or not sub- 

 strate utilization does suppress endogenous respiration. The method 

 relies essentially on the assumption that the oxygen uptake per mole of 

 substrate consumed is not affected by the concentration of substrate. 

 Application of this technique to Penicillium chysogeniim indicates 

 that glucose does not suppress endogenous respiration (272). In yeast, 

 on the other hand, suppression of endogenous respiration by acetate 

 occurs with agar-grown cells but not with cells grown in a liquid 

 medium (322). 



A second approach to this problem is to use cells the endogenous 

 reserves of which have been labeled by cultivation on a carbon- 14 

 substrate (37, 244). It is essential that all of the reserves be labeled 

 by growth from a very early stage on the isotopic substrate; a short 

 exposure to labeled acetate, for example, may not succeed in marking 

 all of the reserves. This method has received only a limited trial to 

 date (25, 63, 101, 128, 203), and it is, of course, open to the criticism 

 that not enough is known of the composition and dynamics of the 

 many metabolic pools involved. Experimentally, it is only necessary 

 to determine w r hat the total activity of carbon dioxide from labeled 

 cells is and whether or not this activity is affected by the simultaneous 

 oxidation of substrate. 



Amino acids have been reported as affecting endogenous respiration 

 both positively (17) and negatively (93). These findings cannot at 

 present be interpreted, since the presence of amino nitrogen in the 

 system and the possibility of amino acid toxicity unduly complicate the 

 picture. 



Experimentally, it is often desirable or even essential to reduce the 

 level of endogenous respiration. Starving the cells for a period of 

 time prior to measuring their respiratory response to an exogenous 

 substrate has been successful with some fungi and actinomycetes (34, 

 70, 316, 320). Others, however, lose substrate respiratory capacity 

 during the starvation period (14, 134, 195). There is also the danger 

 that the starvation period may result in developmental changes in the 

 organism (145). Since endogenous respiration is affected by the com- 

 position of the growth medium (254), the better approach would seem 

 to be an attempt to find a medium in which the cells do not accumu- 

 late reserve materials in large amounts (102). Many of the media com- 

 monly used for fungi are so concentrated that accumulation of such 

 reserves is almost forced upon the organism. 



