52 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



potassium and sodium movements subsequent to the addition of iodoace- 

 tate (38). 



The experiments with iodoacetate and phenylurethane support the conclu- 

 sion that iodoacetate penetrates the cells in the light. Phenyl urethane was 

 selected as the agent to inhibit photosynthesis in these experiments since the 

 margin of safety between 50% inhibition of photosynthesis and 50% inhibition 

 of respiration was shown to be largest of a series of urethanes studied by 

 Warburg (45). In actual practice, however, it was impossible to find a concen- 

 tration which effectively blocked photosynthesis which did not at the same 

 time cause some loss of potassium and gain of sodium presumably from some 

 inhibition of cell respiration. These ion movements cannot be the result of 

 stopping photosynthesis, since, although darkness causes some net ion changes, 

 these occur only after a relatively long period of time and are not nearly so 

 large as those with phenyl urethane. At any rate, iodoacetate markedly en- 



Table 4. Combined effects of io^''' m/l. iodoacetate and io"^ m/l. phenyl urethane on 



POTASSIUM and sodium CONTENTS 



hances the potassium loss immediately after addition to samples in phenyl 

 urethane for 11.5 hours, and, after several hours, increases the sodium uptake. 



Another possible interpretation of the action of iodoacetate is that light 

 produces sulfhydryl groups which prevent the inhibitor from reacting with the 

 enzyme. Three pieces of evidence, however, support the first interpretation: 

 i) the higher potassium and lower sodium in illuminated iodoacetate-samples 

 than in controls, as already discussed. 2) The experiments reported in which 

 potassium and sodium immediately begin to move with their chemical gradients 

 when samples were transferred to inhibitor-free sea water in the dark after 12 

 hours in the light with 2 X io~^ m/1. iodoacetate. Had the iodoacetate not been 

 tied up by the enzyme but rather by other sulfhydryl groups within the cell, 

 these ion shifts would not have been observed, since the reaction between 

 iodoacetate and sulfhydryl groups (alkylation) is essentially irreversible (2, 9). 

 j) Finally, the experiments discussed above, in which iodoacetate is efifective in 

 the light in the presence of an inhibitor of photosynthesis, phenyl urethane. 



Exogenous Phosphogly cerate and Pyruvate Given With Iodoacetate. To 

 evaluate further the interpretation of the action of iodoacetate (in the dark) 



