EFFECTS ON PHOTOSYNTHESIS 157 



Photochemical Light Reactions and Phosphorylation 



The Hill reaction (pliotocliemical decomposition of water acconripanied by 

 evolution of 0.^ and reduction of some substance such as quinone or an indo- 

 phenol) is not very sensitive to iodoacetate (lA). It requires 10 mM to in- 

 hibit 50% in spinach chloroplasts (Macdowall, 1949) while the Hill reaction 

 in CJdorella is not affected by 2.7-7.4 mM iodoacetamide (lAM) (Fraser, 

 1954; Whittingham, 1956), although 20 mM may inhibit nearly completely 

 (Ehrmantraut and Rabinowitch, 1952). Since photosynthesis is strongly in- 

 hibited at concentrations far below these, one can eliminate the initial light 

 reaction as an important site for the inhibition. The reduction of NADP is 

 likewise resistant, no inhibition being observed in spinach grana at 0.6 mM 

 iodoacetate (San Pietro and Lang, 1958) or in spinach chloroplasts at 0.05 

 mM iodoacetamide (Gibbs and Calo, 1960 a), at which concentrations the 

 fixation of CO2 is markedly depressed. In fact, iodoacetamide at 10 mM and 

 20 mM inhibits only 30% and 40%, respectively (J.S.C. Wessels, 1959). 

 Photosynthetic phosphorylation seems to be the least sensitive of all the 

 light reactions. Iodoacetamide at 10 mM has no effect in spinach chloro- 

 plasts (Arnon et al, 1956; Avron et at., 1957; J.S.C. Wessels, 1959) or in 

 dahlia, datura, and tomato leaves (Massini, 1957). These results, taken 

 with the failure of iodoacetamide to inhibit the burst in O2 evolution upon 

 illumination (Whittingham, 1956), all indicate strongly that the light reac- 

 tions are resistant to these inhibitors and, at low concentrations inhibiting 

 total photosynthesis, no effect on these reactions is to be expected. 



Photosynthetic Carbon Cycle (Dark Reactions) 



Total photosynthetic formation of carbohydrate as influenced by iodo- 

 acetate has seldom been measured and where it has been the results are 

 difficult to interpret, although incorporation of C^^'Og into various fractions 

 has been studied thoroughly (page 158). Photosynthetic activity in most 

 instances has been determined by measuring COg utilization. The results 

 are variable because in cellular preparations the penetration of iodoacetate 

 may limit its action. For this reason, iodoacetamide has often been used 

 and generally is more potent than iodoacetate in the pH range above 6. 

 Extremely potent inhibition has been observed in some cells; indeed, the 

 inhibition is often as marked as that of the EM pathway, as seen in the 

 accompanying tabulation. The much smaller inhibition reported by Arnon 

 et al. (1956) in spinach chloroplasts — 17% by 1 mM and 79% by 5 mM 

 iodoacetamide — is unexplained. Perhaps under certain conditions there are 

 two or more pathways for CO2 fixation, only the photosynthetic one being 

 extremely sensitive. 



Following illumination for 4-30 sec in the presence of C^^Og the first stable 

 intermediate (i.e., with the highest specific activity) was found to be 3-P- 



