Cell Structure and Metabolic Regulation 37 



is that a barrier to glucose penetration exists deep within the 

 cell itself, and that the purpose of the glucose-6-phosphatase 

 localized in the ER may be not only to secrete glucose to the 

 exterior but also to remove glucose from contact with the 

 glycolytic enzymes in the cytoplasmic matrix, thus in efTect 

 regulating, possibly one way among many, the pathways of 

 glucose metabolism in the cell. As a corollary, it is suggested 

 that the nucleus can grab its glucose out of the hungry mouth 

 of the glycolytic enzymes in the cytoplasmic matrix by 

 virtue of its sitting enclosed by the membranes of the ER 

 (Fig. 7). I am in the dark as to whether a scheme of this sort 

 can work in the manner pictured by Mitchell (1957), in which 

 carrier compounds, which can be group-transferring enzymes, 

 are located in the membrane and are thought to move com- 

 pounds through by their own actual movements within the 

 membrane. 



The possible existence of a barrier within the cell has 

 already been mentioned; some recent results have dramatic- 

 ally indicated the actuality (Lajtha, Berl, and Waelsch, 1959). 

 Previously Waelsch (private communication) had found that 

 the synthesis of glutamine from glutamic acid takes place in 

 the liver and brain but not in the blood and, in the liver and 

 brain, the microsome fraction is the most active in this 

 synthesis. The surprising finding, related in Table VI, is that 

 if radioactive glutamic acid is injected into the blood, and the 

 liver and plasma quickly separated, the specific activity of the 

 glutamine isolated from the plasma is higher in more than 

 half the cases than is the specific activity of the glutamine in 

 the liver. The same results appear even when non-radio- 

 active glutamine is injected with the radioactive glutamic 

 acid. The authors' tentative explanation is that the glutamic 

 acid gets into the liver cell, is converted to glutamine in some 

 compartment, and the glutamine gets out again without 

 mixing with the rest of the glutamine in the liver cell. Since 

 synthesis takes place mostly in the microsome fraction, they 

 visualized the ER to be the site of this synthesis and to be the 

 barrier between plasma glutamine and liver glutamine. There 



