DISCUSSION ON ANIMAL TISSUE RESPIRATION 277 



dence for the normal activity of the cell. Attempts to make the condi- 

 tions more "normal" by adding physiological salt solutions to tissue 

 suspensions is essentially paradoxical, for it is known that the ionic 

 environment inside the cell diflFers markedly from that outside, and 

 thus, for the study of intracellular enzymes, physiological salt solu- 

 tions are unphysiological. If better results are obtained by the addi- 

 tion of such solutions to broken cell suspensions, it may simply mean 

 that the salts are acting on the residual penneabilities of partially 

 damaged cells rather than on the enzymes directly. 



The physiological significance of results obtained in vitro must be 

 obtained by physiological means, i.e., by experiments on the whole 

 animal. The classical method for studying intermediary metabolism 

 in which substance x is fed and substance y is isolated in the urine 

 does not ordinarily give enough detailed information about the fate 

 of the substance. Correlation with the data obtained with isolated 

 tissue suspensions is therefore often impossible. With the increasing 

 use of labeled atoms animal experiments are yielding more precise 

 results and thus offer promise of better correlation between in vitro 

 and in vivo results. The story of sarcosine illustrates this point. When 

 it is fed to a rabbit with benzoic acid, an increased amount of hip- 

 puric acid is excreted. This shows that sarcosine in some way gives 

 rise to glycine. When sarcosine with labeled nitrogen is fed, the 

 hippuric acid excreted contains the labeled nitrogen. This shows 

 that sarcosine must be demethylated in the body but not deaminated. 

 When sarcosine is added to liver suspensions an extra oxygen uptake 

 occurs and glycine is formed. The sarcosine undergoes an oxidative 

 demethylation. In this series of experiments the in vitro results serve 

 to elucidate the mechanism of a reaction known to occur in the intact 

 animal. Only in this way is the physiological significance of results 

 with tissue experiments established. 



Workers in tissue respiration tend to look for reactions of general 

 significance. Results obtained with yeast or hashed pigeon breast 

 muscle are applied with great facility to mammalian tissue. The 

 omnipresence of the cytochrome oxidase and succinoxidase encour- 

 ages this attitude, but does not justify it. Animals that differ so 

 profoundly, physiologically, pharmacologically, and in all other 

 ways, undoubtedly show differences in their respiratory mechanisms. 

 Perhaps in the future more information will be obtained if these 

 differences between animals are emphasized rather than minimized 

 in an attempt to provide universal mechanisms that may have only 

 superficial similarity. 



