372 RESPIRATORY METABOLISM 



However, because of the small differences in the ratios and because of 

 the complications of the technique of heat measurement, the calorific 

 quotient has not been found to be very useful in determining the energy 

 source for Metazoa (Needham, 1931), and apparently has not been 

 tried for Protozoa. 



Investigations Which Concern the Mechanism of Respiration 



1. general theory 



For a general consideration of the mechanism of respiration, the 

 reader is referred to the monographs of Meldrum (1934) and Holmes 

 (1937), to standard textbooks of general physiology, to several excel- 

 lent discussions in recent volumes of the Annual Revietv of Biochemistry, 

 and to the forthcoming volume of the Cold Spring Harbor Symposia in 

 Quantitative Biology (Vol. VII). The present discussion of the mecha- 

 nism of respiration will include only those portions of a bare outline 

 which are necessary for an understanding of the data and interpretations 

 which are to follow. 



The first step in oxidation of a substrate is the removal of hydrogen 

 from the substrate molecule, and the addition of this hydrogen to any 

 other molecule which will serve as a hydrogen acceptor. After de- 

 hydrogenation is accomplished, the resulting molecule is supposed to be 

 very unstable and easily undergoes oxidation by molecular oxygen, to 

 form CO2 and water. The enzymes necessary for these final stages in 

 respiration are not well known, but the enzymes and respiratory pig- 

 ments responsible for dehydrogenation and the subsequent transfer of 

 the hydrogen to Oo with formation of water are listed below. (Any 

 distinction between respiratory enzymes and pigments is purely arbi- 

 trary. ) 



( 1 ) Dehydrogenases are enzymes which bring about activation of the 

 substrate, so that it may be oxidized by oxygen or intermediate hydrogen 

 acceptors such as cytochrome. These enzymes are highly specific, in that 

 they react with only one or a few substrates. Dehydrogenases are divided 

 into two groups : anaerobic dehydrogenases, which cannot reduce molecu- 

 lar O2 in the presence of their substrates, and aerobic dehydrogenases 

 which can do so. Cytochrome and cytochrome oxidase are important 

 factors in the completion of oxidation by anaerobic dehydrogenases. 



