156 1. lODOACETATE AND lODOACETAMIDE 



site is the formation of porphobilinogen, but that in cells some depression 

 may derive from ATP depletion, or possibly even the availability of suc- 

 cinyl-CoA. Effects on hematopoiesis in intact animals must also be taken 

 into account. 



EFFECTS ON PHOTOSYNTHESIS 



The effects of iodoacetate and iodoacetamide on photosynthesis have been 

 studied intensively since the original demonstration by Kohn in 1935 that 

 a very potent inhibition is exerted, and these inhibitors have proved val- 

 uable in the analyses of the photosynthetic pathways and mechanisms. 

 Results have often been conflicting and the site of the block has evaded 

 certain localization, but the recent techniques for tracing the distribution 

 of C^* throughout the photosynthetic carbon-reduction cycle (Calvin cycle) 

 following illumination have provided the means for obtaining some under- 

 standing of how these inhibitors act. Photosynthesis may be roughly divid- 

 ed into two phases. The light reactions comprise the photon absorption by 

 chlorophyll, the initiation of electron flow, the formation of ATP, the re- 

 duction of NAD(P) to NAD(P)H,* and the evolution of O2. The dark reac- 

 tions include the fixation of CO2 and the complex sequence of chemical 

 reactions whereby carbohydrate is synthesized. An inhibitor of total photo- 

 synthesis may act primarily on either the light or dark reactions. In the 

 case of iodoacetate the important site of attack is within the photosynthetic 

 carbon cycle, and the light reactions are significantly affected only at rela- 

 tively high concentrations. 



That the inhibition is primarily on the dark reactions was indicated in 

 the early work of Kohn (1935) on Chlorella pyrenoidosa. Although O2 evolu- 

 tion was measured, it was assumed that the rate is limited by the dark 

 reactions (Blackman reaction) with continuous illumination, and by the 

 light reactions (photochemical reaction) with flashing light. Inasmuch as 

 inhibition in continuous light is greater than in intermittent light, Kohn 

 concluded that the inhibition is exerted on the dark reactions (it may be 

 noted that his definition of dark reactions does not exactly coincide with 

 modern terminology). Iodoacetamide at low concentrations (0.006-0.01 

 mM) stimulates photosynthesis as much as 30%, an effect assumed to be 

 on the photochemical mechanism. No reaction of iodoacetate with chloro- 

 phyll can be spectroscopically detected. Finally, Kohn noted the marked 

 differential effect on photosynthesis and respiration, it being possible to 

 inhibit the former while respiration is unaffected or actually stimulated, 

 an observation repeatedly reported and considered by some to be of sig- 

 nificance. 



* The abbreviation NAD(P) will be used to designate both NAD and NADP in 

 processes where either or both may be involved. 



