PRIMING REACTIONS 223 



oxidizes to SO4 — . The cysteine-sulphinic acid can also be oxidized to 

 cysteic acid, or decarboxylated to form taurine. 



(k) Alanine 



It is deaminated to pyruvic acid which enters the cycles of priming 

 reactions. 



C. The General Scheme 



The overall plan in Fig. 55 shows the general pathways for the complete 

 oxidation of the amino acids. This does not necessarily mean that the 

 amino acids always follow these paths. There may exist, and do exist, 

 particularly in animals, numerous interrelations between the metabolic 

 paths of the amino acids. Figure 55 shows the most important ways in 

 which the carbon chains of the amino acids can be oxidized completely. 



VI. INTERRELATIONS BETWEEN PRIMING REACTIONS 



These are represented as shown in Fig. 56. Certain aspects, such as the 

 relation between pyruvate and oxaloacetate, or malate, will be explained later. 



Figure 56 shows the interrelations between glycolysis and the hexo- 

 semonophosphate shunt at the level of G — 6 — P and F — 6 — P, between 

 glycolysis, the fatty acid cycle and the tricarboxylic acid cycle at the level 

 of acetyl-CoA, between the tricarboxylic acid cycle and the succinate- 

 glycine cycle at the level of succinyl-CoA. 



Figure 56 also shows the points of entry of the various amino acids into 

 the series of priming reactions. 



VII. ENERGETICS OF THE PRIMING REACTIONS 



A. Glycolysis 



In Fig. 57 are shown the values of —AFq in kilocalories per mole for the 

 conversion of glucose into alcohol or into lactic acid and for the conversion 

 of glycogen into lactic acid. A part of this free energy is lost as heat but the 

 remainder is retained in reserve in the form of ATP energy-rich bonds. 

 A balance-sheet can be drawn up for the free energy and for the phosphate 

 bonds and from it we can determine the efficiency of the process. In the 

 course of the phosphorylation of glucose and of the phosphorylation of 

 F — 6 — P, in each case a mole of ATP has been used up. On the other hand, 



REFERENCES 



Meister, a. (1955), Transamination, Advanc. Enzymol. 16, 185-246. 



McElroy, W. D. and Bentley Glass H. (Editors), 1955.^ Symposium on Amino 

 Acid Metabolism. Johns Hopkins Press, Baltimore. 



Fromageot, C. (1955), The metabolism of sulfur and its relations to general meta- 

 bolism. Harvey Lectures 49, 1-36. 



