CLUES FROM ASSOCIATED EVENTS 



Previously we have encountered also the observation that the 

 oxygen at C2 of monosaccharides is not displaced during their in- 

 testinal transport, even though oxygen must be present at this posi- 

 tion for mediation to occur. The remarkable fact is that not one 

 case has been found of the persistence of an essential mark on a trans- 

 ported molecule to serve as a clue to the nature of the transport in- 

 termediate. This fact may well mean that binding to the transport 

 site involves only displacement of atoms already so rapidly ex- 

 changed with the environment that we cannot expect to detect any 

 acceleration of exchange. Hydrogen bonding and electrovalences 

 may play a large part in the bonding of solutes to their transport 

 sites. The peculiar flexibility in the structural requirement for sugar 

 transport suggests somewhat indiscriminate hydrogen bonding to 

 several of the hydroxyl groups (cf. Rosenberg, 1961). The proba- 

 bility that coenzymes or other small transferable groups act as trans- 

 port carriers appears to be minimized by the absence of the pre- 

 dicted atom exchanges. 



Clues to the character of the intermediation needed for trans- 

 port may also come from incidental fates of a transported solute. 

 If the solute is activated in a particular way, it may incidentally 

 become more likely to form certain derivatives. The formation of 

 N-acyl derivatives, perhaps peptides (Gale and Van Halteren, 1952), 

 during amino acid transport, even if it occurs only in some organ- 

 isms, might be a clue indicating that activation at the amino group 

 occurs in the transport. This result might indicate, for example, that 

 an N-phosphoryl derivative is formed. A hazard to be avoided is 

 that an activation quite irrelevant to transport may occur; here again 

 the use of transportable solutes escaping one or another enzymatic 

 activation has helped to avoid erroneous conclusions. 



An interesting case illustrating the isolation of a derivative 

 formed incidentally to transport has been reported by Zabin and 

 associates (1959; 1962). Isopropylthiogalactoside had previously 

 been shown to be converted partially to the 6-O-acetyl derivative 

 by Herzenberg, a result not published by him until 1961. Zabin 

 and associates (1959) report that this acetylation is catalyzed by 

 extracts of E. coli strains that have the specific galactoside transport 

 system, but only to a small extent by the strains that lack the trans- 

 port system. The acetyl derivative has been shown, however, not 

 to be a possible intermediate in the transport. Its formation may 

 nevertheless represent a clue to the nature of the intermediation. 



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