BIOLOGICAL TRANSPORT 



Stein has considered cases where two dissimilar sugars may 

 dimerize and enter the cell together more readily than one of the 

 sugars does alone (1961b). In this event, we should expect to see the 

 influx of one sugar increased by the addition of the other. Accelera- 

 tions of influx of one solute on the addition of an analogous solute 

 have indeed been reported (Kepes, 1960; Jacques, 1961). These 

 may also be produced by a stimulation of exchange. Stein (1961b) 

 has also considered the situation in which the proposed mixed 

 complex of two sugars is unsuitable to transport, but each of the 

 "pure" dimers is suitable. 



Associated chemical fluxes 



Although secondary physical migrations might be expected to 

 present more characteristic clues to the nature of transport, ac- 

 celeration of chemical reactions may also point to the identity of a 

 carrier. Hokin and Hokin (1958; 1959a; 1959b) have observed that 

 P 32 enters phosphatidic acids and certain other phospholipids more 

 rapidly in a number of tissues when secretion is stimulated. The 

 salt glands of the albatross or of the goose, which secrete hypertonic 

 NaCl solutions, have been useful in this exploration. The Hokins 

 have proposed a cycle of alkali metal transport whereby sodium ion 

 leaves the cell through the lipid barrier as sodium phosphatidate. 

 Phosphatidic acid is taken to be generated by action of ATP at the 

 inside face of the membrane and is split by the action of a phospha- 

 tase to release the sodium ion from the outside face. This cleavage 

 leaves phosphate as the not-too-plausible candidate for the role of 

 carrier for potassium-ion uptake. Phosphatidate was chosen as 

 sodium-ion carrier and phosphate as potassium-ion carrier because 

 the phosphatidate-synthesizing system lies inside the cell, and not 

 because of demonstrated differences in their affinity for the alkali 

 metals. More recently the Hokins (1962) have entertained the hy- 

 pothesis that the phosphatidic acid whose cleavage is associated with 

 sodium-ion transport is part of a lipoprotein; the tertiary structure 

 and alkali metal-binding properties of this lipoprotein may undergo 

 a critical oscillative change when the phosphoester bond is split and 

 reformed. Whatever the validity of either of these transport cycles, 

 the association between the transport of hydrophilic solutes, on the 

 one hand, and phosphatidate turnover, on the other, is an extremely 



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