124 A SYMPOSIUM ON RESPIRATORY ENZYMES 



this way by Warburg and Christian. They showed that diphospho- 

 glyceraldehyde rather than triosephosphate is the substrate of 

 cozymase in fermentation and perhaps in glycolysis (44). The main 

 purpose of giving, in Table 7, the degree of purity of the apoenzymes 

 obtained thus far is to afford some idea of the reliability of our 

 knowledge respecting the reactions in question. 



The most noteworthy fact regarding the apoenzymes is that they 

 are responsible for the specificity of the dehydrogenase, whereas 

 codehydrogenases I and II combined with different proteins form 

 a relatively large number of dehydrogenases. The protein moiety 

 is therefore more specific than the prosthetic group. 



It was found that both codehydrogenase I and codehydrogenase 

 II can act as coenzymes for glucose apodehydrogenase (45). This 

 apoenzyme, however, has not yet been sufficiently purified, and 

 therefore it is not impossible that a transformation of codehydro- 

 genase I into codehydrogenase II or vice versa may occur in this 

 system. 



In connection with alcohol apodehydrogenase it should be men- 

 tioned that methyl, propyl, and amyl alcohol can also serve as sub- 

 strate (46). This seems to indicate that the specificity of the 

 apodehydrogenases even toward the substrate is not always an 

 absolute one. 



Warburg has claimed that the protein part of a given dehydro- 

 genase probably differs for each type of cells. Robison ester apode- 

 hydrogenase from yeast and from rat blood were found to be 

 different, their isoelectric points being at pH 4.8 and 5.8 respectively 

 (lb). In some cases the difference is so great as to suggest that the 

 specificity for the codehydrogenases depends on the source of the 

 apoenzyme. Thus glutamic acid apodehydrogenase from liver was 

 found by Adler to require codehydrogenase II; the apoenzyme 

 from plants uses codehydrogenase I as a prosthetic group (47). 

 Besides the cozymase-dependent glycerophosphate apodehydro- 

 genase, a glycerophosphate dehydrogenase was found to occur in 

 muscle tissue which does not require a coenzyme (48). Similar 

 findings have been reported for formic dehydrogenase. Whereas the 

 dehydrogenase preparations from seeds consist of cozymase plus 

 apoenzyme (49), the corresponding enzyme from Bacterium coli 

 does not need cozymase (50). 



If there are actually as many variations of apodehydrogenases as 

 there are different types of cells, a further important specificity may 

 be based on this difference. To obtain more evidence, the apode- 



