DISCUSSION ON HYDROGEN TRANSPORT 39 



One molecule of aldehyde undergoes an oxidation to the corre- 

 sponding acid at the expense of another molecule of aldehyde which 

 is reduced to alcohol. An enzyme which catalyzes this type of re- 

 action, called aldehyde mutase, has been found in hver by Dixon 

 and Lutwak-Mann (1). It requires as a coenzyme diphosphopyridine 

 nucleotide, which thus appears to function in the role of carrier of 

 electrons and hydrogen ions from one aldehyde molecule to another. 

 Another example that might be given is the dismutation of triose- 

 phosphate. 



The role of a carrier in a so-called coupled oxidoreduction diflFers 

 from that in a dismutation reaction only in that electrons and 

 hydrogen ions are transferred between molecules of two different 

 substances. A well-known example of such a reaction (alcohoHc 

 fermentation) may be written as follows if we omit the coupled 

 phosphorylation steps that accompany it: 



( 3 ) CHs • CHO -H 2e + 2H" -> CH3CH2OH 



(4) CHO COOH 



I I 



CHOH -1- H2O - 2e - 2H^ -> CHOH 



HaC-OPOaH, H2COPO3H2 



The carrier of the electrons and hydrogen ions between these two 

 aldehyde molecules is also diphosphopyridine nucleotide. The reac- 

 tion is not classed as a dismutation simply because the aldehyde mole- 

 cules involved are not identical. One could give other examples of 

 coupled oxidoreductions in which the reacting molecules are more 

 dissimilar and involve diphosphopyridine nucleotide as a carrier. 



Such reactions constitute the main type of energy exchange in 

 anaerobic processes. Under aerobic conditions that half of the re- 

 action which involves the loss of electrons and hydrogen ions to the 

 pyridine nucleotide may still occur. The reduced pyridine nucleo- 

 tide, however, under aerobic conditions, loses its electrons and 

 hydrogen ions to a flavoprotein rather than to another substrate 

 molecule. This type of reaction, then, might be hsted as still another 

 class of coupled oxidoreductions. 



As compared with the pyridine nucleotides, the flavoproteins par- 

 ticipate in only a few direct reactions with substrate molecules. 

 Examples of dismutations or coupled oxidoreduction reactions such 

 as have been given for the pyridine nucleotides are less plentiful 

 for the flavoproteins. One example of a dismutation reaction which 

 may be classed as involving a flavoprotein is that for the substrate 



