FUNCTION 



contains about 0-5 g. per kg. and the heart muscle of man and the 

 rabbit 0-4 g. per kg. It also occurs in micro-organisms (see page 282). 



Function of Coenzyme II 



Coenzyme II also brings about the conversion of glucose into 

 gluconic acid ^^ and of glutamic acid into a-ketoglutaric acid,^^ but it 

 also effects certain changes not brought about by coenzyme I. Thus 

 glucose-6-phosphoric acid is converted into 6-phosphogluconic acid,^* 

 6-phosphogluconic acid into phosphoketohexonic acid, and citric 

 acid into D-ketoglutaric acid.^^ Coenzyme II also catalysed the de- 

 carboxylation of oxaloacetic acid by oxaloacetic carboxylase from 

 pigeon liver, but not the exchange reaction between Ci^02 and the 

 ^-carbonyl carbon atom of oxaloacetic acid.^^ 



Codehydrogenase II (or coenzyme II) appears to occur in all cells 

 in association with codehydrogenase I, but the ratio of the two co- 

 enzymes varies greatly in different tissues, e.g. yeast contains very 

 little codehydrogenase II, whereas animal tissues may contain 40 to 

 80 /zg. per g.27 



In effecting these transformations, the codehydrogenases are 

 reduced to the dihydro-forms, which are themselves dehydrogenated 

 by flavine enzymes. As already explained in the chapter on ribo- 

 flavine (page 191) the nucleotides are reduced in the process to dihydro- 

 compounds, which are re-oxidised by the cytochrome-cytochrome 

 oxidase system. This is re-oxidised in turn by molecular oxygen 

 with the ultimate formation of water as the end-product. The transfer 

 of hydrogen from the substrate at one end of the scale to oxygen at 

 the other can be represented schematically as follows : 



Substrate ^- > Oxidised substrate 



\ H2 

 Codehydrogenase ^ ^ ^ Dihydro-codehydrogenase 



Flavoprotein ^ ^ ^ Dihydro-flavoprotein 



Cytochrome b ^ ^ > Reduced cytochrome b 



Cytochrome c ^ ^ > Reduced cytochrome c 



Th^ ~~ 

 Cytochrome a ^ ^ > Reduced cytochrome a 



Cytochrome oxidase ^ ^ ■ > Reduced cytochrome oxidase 



Oxygen ^^ ► Water 



Many of the dehydrogenations effected by codehydrogenase I are 

 reversible. For example, acetoacetic acid can be converted by dihydro- 

 codehydrogenase I into j8-hydroxybutyric acid, pyruvic acid into 



277 



