ANALOG INHIBITION OF MEMBRANE TRANSPORT 263 



a simple competitive formulation. Indeed, the exact site of inhibition has 

 not been established in any case. The entrance of L-arabinose into rat 

 heart cells is inhibited 92% by glucose at equimolar concentration and, 

 since L-arabinose is not metabolized, the inhibition is presumably on some 

 phase of membrane transport (Morgan and Park, 1958). In the same tissue 

 the competition between glucose and 3-methylglucose at the outer but 

 not the inner surface of the cell membrane leads to a net outward transport 

 of 3-methylglucose against a concentration gradient, and it was concluded 

 that there are stereospecific combining sites at both surfaces. A compari- 

 son between cellular and subcellular preparations can occasionally indicate 

 a membrane site for an inhibition. Galactose markedly inhibits the utili- 

 zation of fructose by intact ascites tumor cells but not in homogenates, 

 pointing to a competition before the hexokinase step and probably in 

 transport (Nirenberg and Hogg, 1957). The effects of 2-deoxy-D-glucose 

 on the fermentation of glucose, fructose, and mannose by yeast (to be 

 discussed in more detail on page 391) led Scharff (1961) to assume a trans- 

 port system which is stable, since it still functions in acetone-dried cells, 

 and perhaps bound to a complex or "bundle" of fermentation enzymes 

 somewhere in the outer regions of the yeast cells. 



Although the nature of the transport mechanisms and the site of inhi- 

 bition are generally not known, it is clear that the interference is quite 

 specific and dependent on the molecular configurations of the sugars. The 

 transport of D-galactose (5 mM) by hamster jejunum is inhibited to varying 

 degrees by other sugars and derivatives at 25 nxM (see tabulation) (Wilson 



Sugar % Inhibition 



D-Mannose Stim 4 



D- Xylose 7 



3-0-Methyl-D-glucose 59 



a-Methyl-D-glucoside 95 



et al, 1960), and the uptake of glucose by lymph node cells is likewise 

 inhibited differently by several sugars at 9 mM (see tabulation) (Helm- 

 reich and Eisen, 1959), both observations pointing to stereospecific trans- 



Sugar % Inhibition 



D-Arabinose 7 



D-Galactose 9 



D-Fructose 40 



D-Mannose 61 



