GEORGE T. SCOTT AND HUGH R. HAYWOOD 5 1 



In the complementary study of sodium movements in these same samples 

 quite different behavior was observed. As in the experiment when arsenate and 

 iodoacetate were given simultaneously, the arsenate added after iodoacetate 

 (fig. 12) offered no protection in the dark. In the light, on the other hand, a 

 fairly rapid active resecretion of sodium occurred so that it reached the control 

 level in several hours. A similar resecretion of sodium was observed when 

 samples were transferred to light and running sea water, without arsenate 

 (35). The presence of arsenate, then, as regards the sodium secretion in the 

 light app:)ears to be quite incidental. The fact that sodium is not resecreted in 

 the dark is further evidence that the procedures employed involved no 'washing 

 out' of the inhibitor. 



It is probable that this effect of light on the sodium pump is quite apart 

 from the generation of carbohydrate intermediates through photosynthesis 

 (which appears to be the chief effect of light as regards the potassium-pumping 

 mechanism), since under these conditions light and darkness have a much 

 greater effect on sodium than on potassium — the difference between no pro- 

 tection and complete resecretion. From the existing data it is not possible to 

 postulate the exact nature of this effect, but it might be suggested, since the 

 reducing properties of light are well known, that it is associated with a redox 

 mechanism of some sort. In this connection it should be mentioned that Con- 

 way (5, 6) has postulated a redox pump for potassium in the yeast cell and 

 Lundegardh (24) a redox mechanism for anion absorption in plant roots. 



These experiments with arsenate serve to indicate that iodoacetate acts on 

 the potassium mechanism in the same manner as already proposed (8, 14, 16, 

 35, 39) (i.e. inhibition of 3-phosphoglyceraldehyde dehydrogenase with a 

 resulting stoppage of energy-yielding reactions in the cell), that it acts on the 

 sodium pump in a different manner, and that light, perhaps through its reducing 

 power, has a primary action on the sodium-pumping mechanism (fig. 21). 



Synergistic Effects of Iodoacetate and Phenyl Urethane. Iodoacetate causes 

 a loss of potassium and a gain of sodium in Ulva in the dark, but fails to do so 

 in the light (35, 36). This protection by light was interpreted as resulting from 

 the photosynthetic production of phosphoglyceric acid (product of the reaction 

 inhibited by iodoacetate) in the light, but not in the dark. To evaluate this 

 interpretation iodoacetate and phenyl urethane, a photosynthetic as well as a 

 metabolic inhibitor, were added together to samples in the light. Results of a 

 typical experiment are shown in table 4. Thus io~'^ m/1. iodoacetate has no 

 appreciable effect on cellular sodium and potassium concentrations. Phenyl 

 urethane in io~^ m/1. concentration alone causes some potassium loss and 

 sodium gain, but iodoacetate and phenyl urethane together cause considerably 

 greater ion shifts. An examination of the kinetics of potassium loss and sodium 

 gain caused by the addition of io~^ m/1. iodoacetate to samples previously 

 maintained in io~^ m/1. phenyl urethane in the light reveals a difference in 



