10 BASIC ASSUMPTIONS 



Krebs in 1948 discussed the universality of the tricarboxylic 

 cycle in cells and tissues. He stated, " there is no doubt that yeast 

 cells can synthesise succinate in the presence of glucose, and 

 citrate in the presence of acetate, but none of the strains of 

 baker's and brewer's yeast tested at the Sheffield laboratory was 

 found capable of oxidising succinic or citric acids at a significant 

 rate under whatever conditions these substances were tested." 

 In 1954 he was of much the same opinion: " thus all the enzyme 

 systems required for the tricarboxylic cycle are present in yeast 

 cells and there can be no doubt that the cycle can take place. . . . 

 However, these findings are not decisive evidence for the assump- 

 tion that the cycle is the main terminal respiratory process in 

 yeasts. ... In many other organisms another terminal oxidation 

 mechanism seems to play a major role. Its nature is unknown in 

 the case of yeast. It may be a dicarboxylic acid cycle in certain 

 bacteria." It now appears that the events that suggested the 

 existence of a dicarboxylic acid cycle in bacteria may be better 

 explained in terms of a divergence from the tricarboxylic acid 

 cycle (Romberg 1958). The system of terminal oxidation in 

 yeasts is still obscure. 



In effect, then, the situation in bacteria, yeasts, plants and the 

 lower animals is not as simple or clear cut as might be imagined. 

 There is more than one pathway for the breakdown of carbo- 

 hvdrates, and the glycolysis cycle and the citric acid cycle are but 

 two of many that are in the process of elucidation. Thus recently 

 a hexose monophosphate shunt has been described as an alterna- 

 tive method by which bacteria and many animal tissues break down 

 glucose. This, combined with the possibility of an alternative 

 terminal oxidation system, enables us to postulate two more 

 systems that may be active in tissue metabolism. This view is 

 supported by Cohen (1955a), who in his account of alternative 

 pathways in carbohydrate metabolism states, " the time is past 

 when we uncritically ascribe phenomena in carbohydrate metabol- 

 ism to variations in the Embden-Meyerhof scheme." Cohen 

 (1955b) also suggests that at least six major pathways for glucose 

 metabolism are known and several may exist simultaneously in 

 the same organism. 



When one considers the various animals and bacteria that have 

 been studied, it becomes quite clear that what we have so far 



