SPECIFICITY OF TRANSPORT 



A priori this type of specificity apparently could have three 

 origins: 



1. A mere equatorial orientation of a sufficient number of 

 hydroxyl -f hydroxymethyl groups about a chair ring could produce 

 affinity. LeFevre and Marshall's study shows, however, that this 

 factor cannot be sufficient, since, in contrast to the D-pyranoses 

 most highly stabilized in the Cl conformation, their mirror-image 

 l enantiomorphs are the sugars showing the lowest affinities. (Con- 

 fusingly, these "all-equatorial" mirror-image forms are said to have 

 the 1C conformation.) 



(In the case of arabinose, it is the l form that shows the higher 

 apparent stability of binding to the transport site, this being the 

 enantiomorph having one more equatorial than axial substituents in 

 the Cl conformation. Either enantiomorph of fucose is transported, 

 the d form showing the higher affinity, in agreement with the same 

 general criterion.) 



2. Instead, an equatorial orientation of a sufficient number of 

 hydroxyl -f hydroxymethyl groups about one of the two possible 

 chair rings (the one designated Cl) seems to produce high affinity; 

 around the other chair ring, low affinity. 



3. Since a direct relationship has, however, not been estab- 

 lished between transport affinity and the fraction of the pyranose 

 existing in the favorable conformation, the factor unfavorable to 

 transport could instead be a conformational flexibility, i.e., an ease 

 of departure from the Cl conformation; or the structural features 

 tending to stabilize the proper conformation could otherwise facili- 

 tate the interactions involved in transport. 



To account for the observed preference, the cell membrane 

 must have, in the words of these authors, "a systematic assembly 

 of at least three contact points within a few Angstrom units of each 

 other, in a fairly rigid pattern." Faust (1960) calls attention to a 

 faster rate of passage at pH 7 and 8 for /?-D-glucose than for «-d- 

 glucose and urges that various sugars must be compared in their 

 mutarotated state. Comprehensive reviews of monosaccharide trans- 

 port into red blood cells (LeFevre, 1961a), across the intestinal wall 

 (Crane, 1960), and of sugars into microorganisms (Cirillo, 1961) are 

 available. 



The uphill transports of the intestine and the kidney discrimi- 

 nate more closely among monosaccharides than the red blood cell 

 system does. The former have usually been explored with the 



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