BIOLOGICAL TRANSPORT 



be relieved by free histidine, Stein (1958) argued that the binding 

 site could not be a sulfhydryl group, as had been suggested. Instead 

 he has reported in a preliminary paper (Stein, 1958) the detection 

 of an N-terminal histidine whose reaction with phenylisothiocyanate 

 can be specifically blocked by the presence of 1,3-propanediol. If 

 this approach proves to have broad applicability, it should help very 

 much in the identification of transport sites and the discovery of 

 their function. 



A series of fixed sites leading through the membrane and re- 

 ceiving the solute in turn, one from the other, with complete re- 

 versibility should yield the same behavior as a single fixed site. Fig- 

 ure 8 shows Danielli's visualization of a polar pore in a lipid barrier 

 through which a solute could migrate, by hydrogen bonding, across 

 the interval between pairs or sets of polar groups. Danielli (see the 

 quotation, page 28) also considered the possibility that oscillations 



Protein molecule 



Lipoid molecule 



Polar pore 



Figure 8 Diagram of a polar pore in a lipid membrane, as visualized 

 by Danielli. The pore consists of a succession of polar sites on polypep- 

 tide chains extending through the bimolecular layer of lipid. \Fro?n 

 Danielli, J. F. (1954), Proc. Symposium Colston Research Soc, 7, 1; 

 with permission.'] 



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