48 METABOLISM 



may take place during the experiment, the enzymic activities displayed are not 

 necessarily those of proliferating cells, nor even of Uving cells (Cook and Stephenson 

 1928, Sandiford and Wooldridge 1931). In the Thunberg tube, Bacterium coli, 

 for example, may be shown to possess an enzyme that is capable of oxidizing succinic 

 acid to fumaric acid in the presence of methylene blue (Quastel and Whetham 

 1924, Quastel, Stephenson and Whetham 1925). A reversible equilibrium is set up. 

 Succinic acid and methylene blue ±5; fumaric acid and leuco-methylene blue. 



The enzyme responsible, succinic dehydrogenase, activates the hydrogen. 

 The methylene blue acts as a hydrogen acceptor, and in doing so is reduced to 

 the colourless compound leuco-methylene blue. It will be noted that the methy- 

 lene blue does not require activation to become a hydrogen acceptor. In the 

 dehydrogenase systems occurring in nature, it is probable that most of the hydrogen 

 acceptors are activated by enzymes. The natural hydrogen acceptors may be 

 either intermediate products of carbohydrate or protein dissimilation, or molecular 

 oxygen itself. In the first case we have a biological oxidation in the absence of 

 air, which we call an anaerobic oxidation, and in the second case an aerobic oxida- 

 tion, in which oxygen is acting as the hydrogen acceptor. Dehydrogenases may 

 catalyse the transfer of hydrogen from the donator directly to oxygen as the 

 acceptor, but this mechanism appears to be relatively rare. 



The transport of oxygen in the cell is usually brought about by a complex 

 system of oxygen carriers, and the oxygen is activated by enzymes which receive 

 the general name of oxidases. Warburg (1925a, b) pictures the molecular oxygen 

 uniting in the cell with some complex organic substance containing iron in the 

 reduced, or ferrous state, and converting it into ferric iron. In the presence of 

 an oxidizable organic molecule and a suitable oxidase oxygen is transferred and 

 the iron returns to the ferrous condition. A natural carrier of oxygen, cytochrome, 

 has been demonstrated in the cells of animals, yeasts and bacteria by Keihn (1925, 

 1926, 1928-29, 1930), and Keihn and Hartree (1938a, b, c). Cytochrome is a 

 respiratory pigment, made up of a number of related heematin compounds, which 

 plays an imj)ortant part in cell respiration. Keilin's studies supply the liuk 

 between the Wieland hypothesis of hydrogen transport and the Warburg hypo- 

 thesis of oxidation by iron-containing compounds. 



Keilin's view of cellular oxidations may be briefly summarized as follows : 

 Organisms whose respiration demands molecular oxygen contain a widely dis- 

 tributed respiratory pigment cytochrome composed of three main heematin com- 

 pounds, the components a, b and c. The pigment also contains an unbound 

 haematin compound similar to the protohaematin of haemoglobin, and a haemochro- 

 mogen precursor of cytochrome. Of these, the b component of cytochrome, the 

 protohaematin and the haemochromogen precursor are autoxidizable. The a and c 

 comj)onents are oxidized in the presence of a thermostable enzyme cytochrome 

 oxidase ; all factors that destroy or inhibit this oxidase diminish the oxygen 

 uptake of the cell. Some bacterial cells, e.g. B. svbtilis, have the full complement 

 of cytochrome components, and a fourth component a^. Bad. coli, on the other 

 hand, has no a, a^, c or cyto-oxidase, but only b, and another component a^, which 

 may possibly act as an oxidase to 6 (Keihn and Harpley 1941). Cytochrome 

 therefore acts as a carrier between two activating mechanisms of the cell, the 

 cytochrome oxidase activating molecular oxygen, and the dehydrogenase activating 

 the hydrogen of various organic substrates, metabolic intermediaries and co- 

 enzymes. The autoxidizable haematin compound b, the unbound haematin, and 



