REGULATORY MECHANISMS IN ENERGY METABOLISM 143 



First of all, there is the recently recognized glucuronic acid path- 

 way (Touster, 1959; Burns, 1959). Glucose metabolized through 

 this pathway would yield xylulose-5-phosphate, utilizing rather than 

 generating TPNH. Furthermore, in contrast to the shunt, after glu- 

 cose-6-phosphate ATP is utilized. To operate, the presence of mito- 

 chondria is required. These differences could becloud interpretation 

 of Pasteur phenomena. However, experiments by Hiatt and Lareau 

 (1958) indicate that very little glucose is metabolized through this 

 pathwav. It is therefore probably justified to ignore it in the present 

 discussion. 



TABLE 5-1 

 Accumulation of Methylglyoxal 



MiTiole Lactate ixmole Methylglyoxal 



Honiogenates were used, incubated in the system described by van Eys and Warnock 

 (1959), with the addition of 30 ,umole of 2-deo.xyglucose. In the H\er homogenate, 

 12 /imole fructose diphosphate were used; in the brain homogenate, 20 A^mole fructose 

 diphosphate were initiahx- present. Incubation of 40 minutes. Methylglyoxal was 

 measured by the method of Dische and Robbins (1934). 



A second possible pathwav poses more prol^lems. Meyerhof 

 (1934) dismissed methylglyoxal as an artifact. Since that time the 

 participation of methylglyoxal as an intermediate in carbohydrate 

 metabolism has been generally discarded as a possibility. Yet, at 

 regular intervals the possible role of methylglyoxal in carbohydrate 

 breakdown has been suggested. Recently, McKinney and co-work- 

 ers claim such a role in leukocyte glycolysis ( McKinney and Martin, 

 1956; McKinney and Bundles, 1956). ' Salem (1954,' 1955) is the 

 most recent investigator who found that methylglyoxal accumulates 

 in thiamine-deficient rats. On the other hand, Weinhouse (Lewis 

 et al, 1959) found in tumor tissues little evidence of methylglyoxal 

 as an intermediate in the conversion of glucose to lactic acid. 



Consideration of methylglyoxal may appear to be reviving closed 

 issues. However, the troublesome fact exists that, when one looks 

 for methylglyoxal in tissue homogenates, which are incubated with 

 substrates, one invariably finds its formation ( Table 5-1 ) . Even if 

 this formation of methylglyoxal is an artifact, it still could give rise 

 to considerable lactic acid and thus complicate the interpretation of 

 lactic acid analyses. 



