NICOTINAMIDE NUCLEOTIDE ENZYMES 125 



hydrogenases must be prepared from different sources and the re- 

 sulting products compared with one another. It is probable that the 

 differences will consist in a slight variation in the pH for optimum 

 activity or a change in the equilibrium between protein, substrate, 

 and coenzyme rather than in a completely different mode of action. 



The preparation of some apodehydrogenases in crystalline form 

 has permitted an estimation of the amount present in the cell. They 

 are found in much smaller concentration than the codehydrogenases. 

 O. Meyerhof (Ic) has pointed out that if each cozymase molecule 

 in rabbit muscle or yeast were accompanied by one molecule of 

 each of the apodehydrogenases, the protein of the cells would con- 

 sist exclusively of the apodehydrogenases. 



The apodehydrogenases combine with the nicotinamide nucleo- 

 tides, the reduced nucleotides, the substrate, and the reaction 

 product. The extent to which the complexes thus formed are dis- 

 sociated detennines the direction and the rate and equilibrium of the 

 reaction. As an example may be mentioned the following system: 

 alcohol, acetaldehyde, cozymase, dihydrocozymase, and apodehy- 

 drogenase. This system was investigated by Negelein and Wulff, 

 who used the pure components (51). They found that under their 

 experimental conditions the concentration of each substance at 

 which the apodehydrogenase is half saturated with it is as follows: 

 cozymase, 0.0001 M.; dihydrocozymase, 0.00003 M.; acetaldehyde, 

 0.0001 M.; and ethyl alcohol, 0.024 M. The reduced coenzyme and 

 acetaldehyde are bound to a much greater extent than cozymase and 

 alcohol, and under normal conditions the reversible reaction acetal- 

 dehyde + dihydrocozymase ^ alcohol + cozymase proceeds in the 

 direction of alcohol formation. Experiments carried out on other 

 pyridine dehydrogenase systems have yielded similar results. The 

 dehydrogenases requiring codehydrogenase II, however, seem to be 

 less dissociated. 



The high degree of dissociation between the pyridine nucleotides 

 and their protein parts corresponds to that of the cophosphorylases 

 and their specific proteins. M. Dixon and L. G. Zerfas have pointed 

 out the differences between this group of enzymes and the other 

 type, in which the prosthetic group is in a relatively stable linkage 

 to the protein and the ratio between prosthetic group and protein 

 is 1:1 (52). On the basis of interesting experimental results with 

 artificial hydrogen acceptors they state in their discussion that the 

 use of the term "pyridine-proteid" is misleading and should be dis- 

 continued. It would seem, however, that this suggestion is too radi- 



