BIOLOGICAL TRANSPORT 



everted-sac preparation. Intestinal transport of sugars requires a 

 D-pyranose structure with a free hydroxyl group at position 2 in the 

 same steric position it occupies in D-glucose, and a methyl or sub- 

 stituted methyl group representing carbon 6 (Wilson and Crane, 

 1958; Jorgensen et al., 1961). The hydroxyl group at carbon 2 is 

 the only one that is specifically necessary for affinity, since the 

 others could be omitted or replaced one at a time. (The student 

 may consider whether this means that mediation of transport would 

 occur for an analog simultaneously lacking all four hydroxyl groups 

 at carbons 1, 3, 4, and 6.) The way in which the transport system 

 recognizes that the sugar is a pyranose of 6 carbons and has the 

 required configuration has not been ascertained. 



Two valuable muscular tissues for study of transport in vitro 

 are the rat diaphragm, especially in the so-called "intact" form [in- 

 cluding a section of the rib cage until incubation is complete 

 (Kipnis and Cori, 1957)] and the perfused rat heart (Fisher and 

 Lindsay, 1956). These two show very similar specificities for sugar 

 transport, identical neither with the concentrative systems of the 

 intestine or of the kidney nor with the passive uptake by the human 

 erythrocyte, but rather more like the last. Affinity is shown for 

 D-glucose, D-mannose, 2-deoxyglucose, and both d- and L-arabinose, 

 among other sugars (Kipnis and Cori, 1959; Battaglia and Randle, 

 1960). 



The technique of using solutes not easily metabolized in the 

 tissue in question has also been useful in studying sugar transport, 

 as well as for amino acid transport, as already described. The follow- 

 ing may serve as examples: 3-O-methylglucose transport by various 

 tissues introduced by Csaky (1938; 1942) in an early study of this 

 kind (although at the time he sought only to prevent phosphoryla- 

 tion at position 3 by the presence of the methyl group); galactose 

 uptake by kidney slices and into everted intestinal sacs (both trans- 

 ports concentrative in nature); ribose penetration into muscle; and 

 thiogalactoside transport by E. coli. 



This use of solutes obstructed to certain metabolic changes has 

 also served to exclude several important suggestions as to how trans- 

 port is produced. For example, the classic view that a given sugar 

 transport occurs by phosphorylation at a certain position on the 

 sugar molecule can be excluded by showing that transport occurs 

 for a sugar lacking a hydroxyl group at the position in question. 

 As another example, a few years ago uphill sugar transport was 



68 



