CELLULAR BIOCHEMISTRY 



is similar to that in intestine and kidney is an open question. 

 In intestine the more rapid absorption of galactose and glucose 

 as compared with pentoses is attributed to an "active" process, 

 specifically phosphorylation of the hexoses, whereas the absorp- 

 tion of the pentoses is attributed to a simple process of dif- 

 fusion. This phosphorylation (and by implication) -dephos- 

 phorylation mechanism is also supposed to operate in the prox- 

 imal convoluted tubules of the kidney. Without attempting to 

 examine the evidence for this hypothesis in detail, certain diffi- 

 culties may be pointed out at this time. Carefully controlled 

 experiments carried out by Sols (18) with extracts of rat intes- 

 tinal mucosa have failed to show the presence of enzymes capable 

 of phosphorylation of galactose and 3-methyl glucose (contrary 

 to statements found in the literature). Yet these two sugars 

 are absorbed as rapidly as glucose. A hexokinase capable of 

 phosphorylating glucose, fructose, and mannose and having 

 a specificity similar to that of brain hexokinase was always found 

 in these extracts. On the dephosphorylation side there is the 

 diflSculty that glucose-6-phosphatase is missing in the kidney of 

 children with severe glycogen storage disease of liver and kidney, 

 but these children do not have glycosuria (5). 



Keston (14) has recently described the occurrence of a 

 mutarotase in kidney, an enzyme which has previously been 

 found in molds and which is specific for certain sugars. On the 

 assumption that a form of low abundance, presumably an open- 

 chain form of sugar, penetrates preferentially into cells, a role is 

 ascribed to this enzyme in sugar transport in all tissues. Muta- 

 rotase, according to this theory, would control an important 

 rate-limiting step in carbohydrate metabolism, the rate at 

 which sugar enters the cell, and would also be implicated in 

 diabetes and the action of insulin. Levine's (16) theory of 

 insulin action is also based on the assumption that the rate of 

 glucose utilization in tissues is controlled by the rate of sugar 

 transport across the cell membrane. An alternative hypothesis 

 is that the rate of glucose metabolism is controlled by intracel- 

 lular enzymes. Important as a decision between these two 



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