CONCENTRATION OF NARCOTICS 959 



cence intensity) indicate that this poison does not, or not merely, interfere 

 with the enzj^matic supply to the photosynthetic apparatus of the oxidant 

 (carbon dioxide), or of the reductant (such as hydrogen), but affects the 

 primary photocomplex, X- Chi -HZ, directly, possibly by displacing the 

 reductant HZ in this complex. Such a close association with the photo- 

 complex could be expected to lead to the decomposition of the azide in 

 light, an expectation which might, perhaps, be tested by experiments. 



Urethans. In chapter 12 (Vol. I, page 321) the "narcotizing" effect of 

 phenylurethan on photosynthesis of Chlorella, which appears to be equally 

 strong at all light intensities, was illustrated by figure 30, taken from War- 

 burg's early work on Chlorella (1920). This figure showed half inhibition 

 at about 2 X 10~* mole/1.; Table 12.VIII, taken from the same paper, 

 indicated hah inhibition at 5 X 10~^ mole/1, for phenylurethan and 0.22 

 mole/1, for ethylurethan. 



An inhibition curve for Chlorella, given by Wassink and co-workers 

 (1938) (fig. 31, page 323) indicated half inhibition by about 2.5 per cent, 

 or about 0.02 mole/1, ethylurethan, and the measurements of Wassink, 

 Katz and Dorrestein (1942) on Chromatium (fig. 27.19) gave about 1.3%, 

 or 0.01 mole/1., as half-inhibiting concentration, at 29° C. Wassink and 

 Kersten (1945) found about 2% as half -inhibiting concentration for the dia- 

 tom Nitzschia dissipata. 



One peculiar characteristic of the effect of urethan on Chromatium — 

 similar to that of azide — is the enhanced inhibition at low light intensities, 

 leading to a more pronounced sigmoid shape of the light curves of photo- 

 reduction in inhibited cells (fig. 28. HE). 



Theoretically, a weaker inhibition at the higher light intensities {i. e., 

 light curve systems of "type 3"; cf. chapter 26) could be explained if one 

 would assume that the narcotic partially covers the chlorophyll-bearing 

 "photocomplex," but leaves free the enzyme that determines the limiting 

 yield of photosynthesis in strong light. In this case, the decrease in in- 

 hibition would be brought about by continued increase, with increasing 

 light intensity, of the rate of photosynthesis of narcotized cells — in the in- 

 tensity range in which photosynthesis in noninhibited cells is light-saturated. 

 Figure 28. HE shows, however, that light saturation occurs at the same in- 

 tensity on both curves, so that this explanation appears inadequate. 



In Nitzschia, according to Wassink and Kersten (1945), the inhibition 

 by urethan is somewhat less strong in weaker than in stronger light. 



The influence of ethylurethan on fluorescence of Chromatium is like 

 that of azide. As with azide, addition of urethan causes the fluorescence 

 to decrease in intensity in the absence of reductants (fig. 28.50A). 

 When reductants, such as hydrogen or thiosulfate, are present, and the 

 fluorescence of nonnarcotized cells is thus considerably weakened, addi- 



