1910 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



marized them as showing an "approximately equal" inhibition of both 

 processes; but the reproduced inhibition curves indicate (in agreement 

 with Kohn's earlier data) a considerably stronger effect of iodoacetate on 

 photosynthesis than on respiration. Interesting is the strong transient 

 enhancement of oxygen consumption after a period of illumination in the 

 presence of iodoacetate, illustrated by fig. 37D.8B; it seems to indicate 

 rapid re-oxidation of intermediate products of photosynthesis (such as 

 PGA), accumulated in light in front of the iodoacetate block. (However, 

 no accumulation of tagged PGA was found by Calvin et at. when iodoacet- 

 amide was used as inhibitor of C(14)-fixation.) 



A difference in the effects of iodoacetic acid and of its amide was noted 

 already by Kohn, and attributed to differences in membrane permeability 

 (c/. p. 319). Holzer (1954) reported that the amide (8 X 10"* m./l.) 

 inhibits photosynthesis of Chlorella at pH 9 almost completely, while leav- 

 ing respiration almost unaffected. 



2,4-Diniirophenol. This inhibitor is known to uncouple the autoxidation 

 of pyridine nucleotides, through the cytochrome system, from the produc- 

 tion of high energy phosphates (ATP). Its effect on photosynthesis was 

 mentioned on p. 319. Holzer made more systematic experiments on the 

 inhibition of photosynthesis by dinitrophenol, first (1951) with relatively 

 high DNP-concentrations in alkaline solution, and then (1954) with much 

 lower concentrations in acid solution (acid reaction increases the concen- 

 tration of the cell-penetrating, undissociated species of DNP). Fig. 

 37D.8C shows that photosynthesis is almost completely inhibited by 5 X 

 10 ~^ M DNP — an amount which affects respiration only very slightly. 

 The observed effect is compatible with the hypothesis (Ruben, van der 

 Veen) that photosynthesis utilizes energy produced by re-oxidation of a 

 part of its intermediates — perhaps TPNH2 — by oxygen, or by oxidized 

 intermediates such as a f erricytochrome ; and that this energy is made 

 available in the form of high energy phosphates, whose formation is 

 coupled with this partial re-oxidation. Since the rate of re-oxidation must 

 be, in strong light, considerably in excess of normal respiration, it is not 

 inplausible that DNP inhibition markedly affects photosynthesis at lower 

 concentrations than are needed to affect respiration. 



"Vitamin K Antagonists.'^ Wessels (1954) noted that the photoreduc- 

 tion of 2,6-dichlorophenol indophenol by chloroplasts is 50% inhibited by 

 5 X 10-^ M 2,4-dinitrophenol (cf. above), 10"^ M 2-Me-l,4-naphthaqui- 

 none, 6 X 10 ~^ M phthiocol, and 10 ~^ M dicoumarol-soluble substances 

 related to, and known to act as antagonists of, the (water-insoluble) vita- 

 min K. (This relation was first pointed out by Gaffron, in the study of the 

 effects of these substances on photosynthesis and photoreduction in algae, 

 cf. p. 314.) 



