332 



ANIMAL METABOLISM 



18). Ten atoms of hydrogen (two each in reactions 16, 20, 22, 23, 25) 

 and three moles of carbon dioxide (reactions 16, 21, 22) are also removed. 

 The net result, therefore, is: 



CH3COCOOH + 3HoO -> 10(H) + SCOo 



Since at the end of the cycle another molecule of oxalacetic acid is formed, 

 more pyruvic acid can at once be catabolized. The citric acid cycle 

 may be regarded as a sort of machine for metabolizing pyruvic or acetic 

 acids, or any other substance which can be converted into one of the 

 compounds involved in the cycle (e.g., glutamic acid, p. 343). 



Oxalacetic acid occupies a position of special importance, since it is 

 the substance with which the incoming stream of acetic acid molecules 

 must combine in order to set the cycle in operation. Although oxalacetic 

 acid is regenerated at each "turn of the wheel," it is obvious that at 

 least a small amount must be present before the cycle can start at all. 

 In other words, there must be some source of oxalacetic acid other than 

 that regenerated by the cycle itself. This other source is pyruvic acid, 

 which can combine with carbon dioxide to give oxalacetic acid directly 

 (reaction 14) or with carbon dioxide and hydrogen (from TPN • Ho) to 

 form malic acid, which then goes to oxalacetic (reactions 15 and 25). 

 It is probable that the latter pathway is quantitatively the more im- 

 portant in animal tissues. 



Cytochrome System. The only oxidative processes shown in Fig. 13-4 

 are indirect ones consisting of the addition of water and removal of 

 hydrogen. Thus succinic acid, for example, is converted into oxalacetic 

 acid, which contains one more oxygen atom. This indirect method of 

 oxidation is a very common biochemical process. 



The hydrogen so produced is never present in the free state in the 

 tissues. It forms reduced coenzymes [e.g., DPN • H2) and from them 

 is passed through the cytochrome system to combine with the oxygen 

 brought to the muscles by the blood stream. It is important to note 

 that, of the two metabolic end products — carbon dioxide and water — 

 only the latter comes from a direct union with the inhaled oxygen. The 

 carbon of the original sugar is never oxidized directly to carbon dioxide. 

 Likewise, the bulk of the energy derived from the metabolism of fats 

 and carbohydrates comes from the oxidation of hydrogen (p. 422). 

 , The most important coenzymes which receive hydrogen from metabo- 

 lites and transfer it to cytochrome are the pyridine nucleotides, DPN 

 and TPN, and the flavin nucleotides, FAD and FMN (p. 277). In most 

 cases the hydrogen from the metabolite first passes to one of the pyridine 

 coenzymes, which is thereby converted into the reduced form, DPN • H2^ 



^ These abbreviations are used merely for convenience. In reality, one of tlie two 

 extra hydrogens is ionized : 



DPN-Ho^ (DPX.H)- + H^ 



