312 



CATALYST POISONS AND NARCOTICS 



CHAP. 12 



comparative indifference of the last-named process to hydroxylamine 

 clearly indicates that the hydroxylamine-sensitive enzyme participates 

 only in the oxygen-liberating stage of photosynthesis. 



Weller and Franck (1941) measured the hydroxylamine inhibition of 

 photosynthesis of Chlorella in continuous light of varying intensity. 

 Figure 28 shows the unexpected result: The inhibition was found to be 



600 



0400 





200 



— ■- ' 



20 40 60 



Relative light intensity 



80 



100 



Fig. 28. — Continuous light saturation curves of 

 Chlorella with and without hydroxylamine hydrochloride 

 (after Weller and Franck 1941). o: not inhibited; •: 

 inhibited by 2.5 X IQ-" m./l. of NH2OHCI. 



independent of light intensity, and thus similar to the effect of narcotics 

 (of. page 320 et seq.) rather than to that of typical "enzyme poisons." 

 This confronted one with the alternatives: either to consider hydroxyl- 

 amine as a "narcotic," thus renouncing the simple interpretation of its 

 ineffectiveness in photoreduction, or to find an explanation of the way 

 in which an enzyme poison can affect photosynthesis in weak light. 

 Weller and Franck (1941) suggested that such an explanation is possible 

 if one assumes that the available quantity of the hydroxylamine-sensitive 

 enzyme is proportional to the intensity of illumination. In other words, 

 this enzyme must be continuously produced (or activated) by light, and 

 inactivated by a dark reaction, so that its stationary concentration is 

 proportional to the intensity of illumination. This assumption is made 

 less arbitrary by the fact that Gaffron had previously arrived at the 

 same conclusion while attempting to explain the induction phenomena 

 (c/. Vol. II, Chapter 33). 



We thus assume, with Weller and Franck, that, in the presence of 

 hydroxylamine, the rate-limiting catalyst is not the one which causes 

 light saturation in nonpoisoned cells ("catalyst B" in Franck's theory), 

 and not the one which limits the over-all reaction in the presence of 



