DEWnTT STETTEN, JR. 



most of the dietary carbohydrate of mammals is polysaccharide, 

 this implies a preliminary digestion. Glucose enters the hepatic 

 vein by the hydrolysis of glucose-6-phosphate and this com- 

 pound may in turn arise ( 7) from glucose itself, (2) from glucose- 

 1 -phosphate derived from glycogen or (3) from hexosediphos- 

 phate arising in turn from triose phosphate, etc. These processes 

 undoubtedly occur also in tissues odier than the liver, notably 

 the kidney. Glucose may also arise hydrolytically within cells, 

 and as an example of this process may be cited the action of 

 amylo-l,6-glucosidase in muscle. This latter contribution is 

 probably quite small. 



Glucose is utilized, so far as is known, by every mammalian 

 cell. For blood glucose to be utilized, it must first cross cell 

 membranes. It is generally held that simultaneously with or 

 immediately succeeding such passage, glucose undergoes hexo- 

 kinase-catalyzed phosphorylation. From the hexose phosphate 

 thus produced at least four distinct metabolic sequences may 

 arise. 



(1) Glucose-6-phosphate — ^ glucose- 1 -phosphate -^ glucosides 

 (e.g., glycogen) or rearrangement products (e.g., galactose- 

 1 -phosphate). 



(2) Glucose-6-phosphate -^ 6-phosphogluconate, initiating the 

 "oxidative pathway." 



(3) Glucose-6-phosphate — > fructose-6-phosphate, initiating 

 the "glycolytic pathway." 



(4) Glucose-6-phosphate — ^ glucose + inorganic orthophos- 

 phate. 



Of interest in this regard is the question of the possible 

 utilization of glucose without preliminary phosphorylation. 

 This view rests largely upon the demonstration of a glucose 

 dehydrogenase activity in mammalian liver extracts which 

 forms gluconic acid from glucose in excellent yield (6). Attempts 

 to demonstrate the occurrence of this particular transformation 

 in the intact animal have thus far yielded negative results (20). 

 Provisionally this enzyme activity must be considered inoperative 



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