264 2. ANALOGS OF ENZYME REACTION COMPONENTS 



port systems presumably involving carriers in the membranes. The pat- 

 terns of competition between various sugars have often led to the assumption 

 of two or more different transport mechanisms for carbohydrates and that 

 these systems can operate simultaneously and independently. Competition 

 for transport can be demonstrated in rat diaphragm muscle for members 

 of the group including D-glucose, D-mannose, D-xylose, D-arabinose, l- 

 arabinose, D-lyxose, and 3-0-methyl-D-glucose, but not between these 

 and D-galactose, D-fructose, maltose, a-methyl-D-glucoside, and /?-methyl- 

 D-glucoside (Battaglia and Randle, 1960), so that different sites for entry 

 were postulated. A number of sugars are transported into erythrocytes: 

 all the aldoses penetrate by a transport system characterized for glucose, 

 while the ketoses penetrate according to a pattern of passive diffusion 

 (LeFevre and Davies, 1951). The aldoses compete with each other, e.g., 

 the uptake of glucose is strongly inhibited by mannose, but any aldose 

 delays the entrance of a ketose, e.g., the uptake of fructose is prevented 

 by glucose and galactose, while the ketoses do not perceptibly alter the 

 transport of the aldoses. One carrier system in the erythrocyte has been 

 characterized as reacting only with those monosaccharides in which the 

 pyranose ring tends to assume the "chair" configuration, which illustrates 

 a unique type of stereospecificity (LeFevre and Marshall, 1958). On the 

 other hand, in hamster intestine only a single transport mechanism or car- 

 rier seems to be present, mutual inhibition occurring between D-glucose 

 and D-galactose, D-glucose and 1,5-anhydro-D-glucitol, D-galactose and 1,5- 

 anhydro-D-glucitol, D-glucose and 6-deoxy-D-glucose, and 6-deoxy-D-glucose 

 and 1,5-anhydro-D-glucitol, whereas sugars that are not transported do not 

 interfere with those that are (R. K. Crane, 1960). 



Amino Acid Transport 



The situation here is very much the same as in sugar transport and there 

 is good evidence for stereospecific systems and multiple pathways. There 

 is competition between L-leucine and DL-isoleucine for renal tubular resorp- 

 tion in the dog, these being well resorbed amino acids, and there is also 

 competition between the poorly resorbed L-arginine and L-lysine (Beyer 

 et at., 1947). However, no interference is observed between L-leucine and 

 L-arginine. It is possible to classify the amino acids into groups with respect 

 to their mutual interference in resorption. In the rat kidney, dibasic amino 

 acids (arginine, lysine, and cystine) are actively accumulated and there 

 is mutual inhibition of the transport (Rosenberg et al., 1962). Monobasic 

 amino acids (alanine, phenylalanine, and histidine) do not interfere with 

 the uptake of the dibasic amino acids, nor does arginine depress the uptake 

 of the monobasic amino acids. It seems likely that separate transport sys- 

 tems are present. The transport system for basic amino acids in the hamster 

 intestine is distinct from that for other amino acids and similar to the renal 



