202 BACTERIOLOGICAL CHEMISTRY 



The reduction of oxidised cytochrome to reduced 

 cytochrome can be catalysed by a number of dehydro- 

 genases specific for the substrate which is the hydrogen 

 donator. Thus the dehydrogenases for the oxidation of 

 « -glycerophosphate to glyceraldehyde phosphate, of 

 succinic acid to fumaric acid, of lactic acid to pyruvic 

 acid and of formic acid to carbon dioxide, all transfer 

 hydrogen to cytochrome. Some of them can make use 

 of acceptors other than cytochrome ; for instance methy- 

 lene blue or pyocyanine are acceptors for glycerophosphate 

 dehydrogenase but riboflavin, the f la vo -proteins, gluta- 

 thione or ascorbic acid cannot serve this purpose ; 

 methylene blue can also accept hydrogen from succinic, 

 lactic and formic dehydrogenases. 



Aerobic bacteria contain all the cytochrome com- 

 ponents, the facultative anaerobes one or two of them, 

 whilst the strict anaerobes contain no cytochrome at 

 all. The respiratory activity of aerobic organisms is 

 proportional to the amounts of cytochrome and cyto- 

 chrome-oxidase which they contain. Almost all aerobic 

 respiration takes place through the cytochrome system. 



In addition to the cytochrome system there are other 

 systems which have a similar function in acting as 

 intermediaries in hydrogen transfer reactions. Thus in 

 alcoholic fermentation acetaldehyde is reduced to ethyl 

 alcohol and phosphoglyceraldehyde is oxidised to phospho- 

 glycerate by a pair of coupled reactions in which co- 

 enzyme I, diphosphopyridine nucleotide, acts as hydrogen 

 carrier : — 



(1) 3-Phosplioglyceraldehyd3 + phosphate + co-enzyme I 

 Triose phosphoric enzyme 



1 : 'i (liphosplmglycerate + dihydroco-enzymo T 



(2) Diliydroco-cJizyme J + ('IfyCilO > ( 'ociizyii-c i i (Uy)H. 



