AEROBIC METABOLISM OF CARBOHYDRATE 129 



occur without influencing this alternate route for gkicose oxidation. There 

 is also good evidence, as we shall see, that the pentose-P cycle can operate 

 well in iodoacetate-treated cells. 



Following the demonstration by Warburg and his co-workers in 1935 that 

 a yeast extract can oxidize glucose-6-P to gluconate-6-P and then metab- 

 olize this product, Lipmann (1936) showed that bromoacetate at a con- 

 centration which inhibits fermentation completely has little effect on this 

 pathway. The oxygen uptake from ribose-5-P is not significantly inhibited 

 in extracts of horse liver (Dickens and Glock, 1951), and the utilization of 

 ribose-5-P by Lactobacillus brevis is slightly stimulated by iodoacetate (Eltz 

 and Vandemark, 1960). The oxidation of ribose-5-P by Streptomyces is not 

 inhibited by even 10 mM iodoacetate when NADP-dependent systems are 

 used (Cochrane et al., 1953; Cochrane and Hawley, 1956), but in other cases 

 the oxidation pathway may be through 3-PGDH, as in Pseudomonas hydro- 

 pJiila (Stone and Hochster, 1956), so that appreciable inhibition is exerted. 

 Ordinarily it may be a mixed type of oxidation so that inhibition is partial, 

 as in Streptomyces avreofaciens, where 2 mM iodoacetate inhibits 27%, the 

 amount of sedoheptulose formed remaining constant but pyruvate forma- 

 tion being completely blocked (Sang-ch'ung and Yung-p'u, 1960). The evi- 

 dence from fragmentary studies on the oxidation of gluconate-6-P and 

 pentose-P thus supports the results on isolated enzymes, and indicates an 

 insensitivity of the pentose-P pathway to iodoacetate. 



We shall now turn to data obtained by the recovery of C^^Og from glu- 

 cose- 1-C^^ and glucose-6-C^^, this providing information on the relative im- 

 portance of the two pathways and the effects of iodoacetate on the pattern 

 of glucose oxidation. The results in Table 1-20 illustrate the variable re- 

 sponses observed, which is not unexpected since the patterns of glucose 

 oxidation in different cells are not uniform. Exclusive metabolism through 

 the EM pathway should lead to a C-l/C-6 ratio of 1; values greater than 1 

 are generally considered to indicate the operation of the pentose-P i^ath- 

 way. The incubation times are quite important since it is a matter of the 

 relative rates at which C^^Oa is formed from labeled glucose. For example, 

 the increased glucose utilization and respiration associated with initiation 

 of phagocytosis in leucocytes are accompanied by an increase in the contri- 

 bution from the pentose-P pathway, as shown by the rise in the C-l/C-6 

 ratio (Table 1-20). If iodoacetate blocks the EM pathway at 3-PGDH spe- 

 cifically, one might expect the EM pathway contribution to be reduced, 

 the formation of C^^Og from glucose-6-C^* inhibited more than from glucose- 

 l-C^*, and an increase in the C-l/C-6 ratio. Actually this is not commonly 

 observed; except for the preparation of beef heart mitochondria and super- 

 nate in the presence of NADP or NADPH, where the formation of C^^Og 

 from glucose-6-Ci* is blocked completely, the C-l/C-6 ratio tends either to 

 remain fairly constant or to decrease. 



