CARBOHYDRATE METABOLIC PATHWAYS 75 



the consumption of this metabohte is assured ii transaldo- 

 lase is also functional. Although very recent years have 

 seen the determination that triose phosphate may feed into 

 either the Krebs cycle or the pentose cycle or both (Fig. 

 2.10), it is clear from Figs. 2.2 and 2.3 that these cycles need 

 not be interdependent; operation of the pentose pathway 

 can under certain circumstances lead to quantitative pro- 

 duction of CO2, as discussed in Chapter 2. 



In attempting to assess the over-all importance of the 

 pentose cycle, a logical first question might be raised re- 

 garding the ubiquity of this complex of enzymes in living 

 tissue. Several laboratories, including our own, have sought 

 to demonstrate its activity in a variety of organisms, with 

 generally complete success. In addition to the isotopic 

 evidence cited in the previous chapter, enzymes of the pen- 

 tose cycle have been found in higher animals (18-32), insects 

 (33-37), marine organisms (38, 39), worms (40, 41), higher 

 plants (42-48), yeast and other fungi (49-58), as well as 

 bacteria (59-63). The enzymes of this complex seem, in 

 short, to be at least as widespread as those of the Krebs 

 cycle. 



The demonstration of a metabolic pathway in an organ- 

 ism is fairly complicated. To carry out a complete search 

 for the Krebs cycle in a new organism, for example, would 

 probably involve at least five sets of operations: (a) attempts 

 to obtain complete oxidation of each intermediate; (b) 

 search for the individual enzymes, with (c) enough purifi- 

 cation of each so that the suspected intermediates may 

 accumulate and their formation be proved; (d) use of 

 certain inhibitors, e.g., malonate, to permit the recognition 

 of intermediates; and (e) the use of one or more isotopically 

 labeled substrates to determine whether the intramolecular 

 distribution of products is in agreement with expectations. 

 Likewise, a search for the pentose cycle would follow this 



