MECHANISM OF HYDROGEN TRANSPORT 49 



the heemochromogen precursor to cytochrome may act as carriers between hydrogen 

 donators and molecular oxygen. They may also act as direct catalysts, promoting 

 the oxidation of substrates that are not activated by specific dehydrogenases. 

 The various hsematin compounds of the cell are also responsible for the catalase 

 and peroxidase reactions that occur in the presence of HjOj. The H2O2, which 

 is an end-product in a number of bacterial oxidations, is reduced in the presence 

 of catalase. Catalase production is probably limited to bacterial species capable 

 of aerobic respiration (Kluyver 1924). Callow (1923, 1924) found oxygen utiliza- 

 tion minimal in bacteria producing no catalase (see section on Aerobiosis and 

 Anaerobiosis, p. 70). 



Substituting cytochrome for methylene blue in the succinic dehydrogenase 



system mentioned above, we may represent the oxidation of succinic acid in two 



stages, the first catalysed by the dehydrogenase, the second by cytochrome oxidase. 



Succinic acid and cytochrome — > reduced cytochrome and fumaric acid. 



Reduced cytochrome -|- O2 — > cytochrome + water. 



This reaction has not been directly demonstrated in bacterial cells, nor can 

 many of the dehydrogenase systems directly reduce oxidized cytochrome. But 

 it appears that the dehydrogenase systems capable of reacting directly with reduced 

 cytochrome may act as intermediate links between cytochrome and other dehydro- 

 genase systems, by which they are themselves reduced. 



Many of the dehydrogenase systems, in addition to hydrogen donator, hydrogen 

 acceptor, dehydrogenase, water, inorganic ions and the correct pH, require the 

 presence of co-enzymes. A co-enzyme may be defined as a thermostable substance 

 necessary in addition to enzyme and substrate to initiate a reaction. Co-enzymes 

 are usually organic in nature, and their molecules are small enough to pass through 

 semi-permeable membranes that hold back the larger enzyme molecules. Hence 

 existence of a co-enzyme is usually demonstrated by the inactivation of a natural 

 enzyme preparation after it has been dialysed, and its reactivation by addition 

 of the dialysate. Co-enzyme I, which has been identified as pyridine nucleotide 

 diphosphate, takes part in many enzyme reactions. Co-enzyme I and its dehydro- 

 genase constitute one of the dehydrogenase systems capable of direct reaction with 

 cytochrome. Its relation to the cytochrome system may be represented (Dewan 

 and Green 1938) as follows : 



A H2 + Co-enzyme I dehydrogenase for A ^ ^ Hj— co-enzyme I 

 H2— co-enzyme I + oxidized cytochrome dehydrogenase for co-enzyme I 



co-enzyme I + Hj — cytochrome 



The following diagram of the general oxidative mechanisms as they involve 

 cytochrome itself have been taken, with slight modification, from that given by 

 Keilin (1928-29). 



Substrate — Dehydrogenase — H^- > n>\ r \ <-^ — Oxidase — 0, 



L a, (6), c J 2 



\ 



PHiEMATIN ~| 



Substrate < Cytochrome h' W O^ 



as direct catalysts Lh^mochromogen-I 



Substrate < 77- 1 n™^^'L,„ ..' u ^ J^ H2O2 



FH^MATIN 

 peroxidase LCytochrome a', h 



