218 RESPIRATION 



tion 7 is probably tbe sum of two reactions. It is assumed that the 

 aerobic reoxidation of reduced coenzyme (TPNH) occurs and that in 

 the process the energy of the oxidation is utilized for synthesis of 

 adenosine triphosphate or related compounds. The phosphoglu- 

 conate pathway is an aerobic system; evidence for an analogous 

 fermentative pathway is summarized by Gunsalus et al. (118). 



The first pentose phosphate formed, i.e., the product of Equation 

 7, is ribulose-5-phosphate; this rapidly comes into enzymatic equilib- 

 rium with two other pentose phosphates, ribose-5-phosphate and 

 xylulose-5-phosphate. 



The second phase of the sequence is a complex series of non-oxida- 

 tive conversions of the pentose phosphates into other phosphorylated 

 sugars. Direct intermediates in this phase include sedoheptulose-7- 

 phosphate, glyceraldehyde-3-phosphate, a tetrose-4-phosphate (prob- 

 ably erythrose-4-phosphate), and fructose-6-phosphate. Since glucose-6- 

 phosphate is regenerated, this second phase may be summarized: 



6 Pentose phosphate -> 5 glucose-6-phosphate (8) 



The key enzymes in this phase are transketolase and transaldolase. 

 In the presence of enzymes of the Embden-Meyerhof sequence, still 

 other sugar phosphates, fructose- 1,6-diphosphate and sedoheptulose- 

 1,7-diphosphate, appear; in addition a greater or less amount of the 

 triose phosphate formed may be converted to pyruvic acid and thereby 

 removed from the system under discussion. 



It was natural to investigate the fungi for an aerobic system of this 

 type, but so far comparatively little progress has been made toward 

 an organized and coherent picture of the role of this cycle in the 

 oxidative metabolism of the fungi. The evidence at hand includes 

 three types of study: demonstration of particular enzymes, demonstra- 

 tion of one or more of the characteristic sugar phosphates, and study 

 of the distribution of isotope from specifically labeled glucose. 



The two dehydrogenases of the oxidative phase of the pathway have 

 been identified in Neurospora crassa (213), Tilletia caries (212), and 

 Streptomyces coelicolor (66). Extracts of Aspergillus niger contain 

 glucose-6-phosphate dehydrogenase (147). 



The conversion of pentose phosphate to hexose phosphate and the 

 formation of sedoheptulose phosphate and other known intermediates 

 are easily demonstrated in cell-free preparations of both fungi and 

 actinomycetes (64, 212, 263). 



A variety of methods depending on the formation of metabolites 

 from carbon-14 labeled glucose have been devised in an attempt to 

 assay the quantitative importance of the phosphogluconate oxidation 



