\J\J\3vJs3V?Os^N>0\>>3s3\3 



Clues from 

 associated events 



Persisting evidence of transient binding 



In transport, as we have defined it throughout these discussions, 

 the same apparently unchanged molecule that disappeared from 

 phase 1 reappears in phase 2. Yet we must believe that this molecule 

 has entered into a chemical reaction with a site of limited capacity, 

 to account for the susceptibility of the migration to saturation or 

 to inhibition by analogs. Many of the interactions that have been 

 considered for transport might be expected to replace an atom 

 of the transported solute with an identical (or an isotopic) atom 

 from the environment. For example, if an amino acid forms an 

 adenylic acid anhydride, in the instances studied, cleavage of this 

 anhydride leaves the oxygen that was originally part of the carboxyl 

 group of the amino acid, in the adenylic acid molecule, and a new 

 oxygen is received by the amino acid from the environment (Hoag- 

 land et al, 1957). If the cleavage occurs in this characteristic way 

 in transport, O 18 initially present in the amino acid should be dis- 

 placed by O 16 . The finding that very little O 18 loss occurred during 

 the transport of a-aminoisobutyric acid (Christensen et al, 1958) 

 raises great doubt that an AMP anhydride arises as an intermediate. 

 Binding of the carboxyl group of the amino acid in a peptide link 

 or as a thio ester is, at the same time, apparently excluded. 



7 6 



