KINETICS 1009 



CO2 with an acceptor (A or RH) prior to reduction. The "stabilizing" 

 catalyst, Eb, whose existence was postulated by Franck and Herzfeld in 

 their interpretation of the kinetics of photosynthesis, and which they made 

 responsible for the light saturation of photosynthesis under normal condi- 

 tions in constant as well as in flashing light, should not be involved in Hill 

 reaction if its function is to stabilize the intermediate reduction products 

 of ACO2 (symbolized by AHCO2, AH2CO2. . .), but it should be required if 

 it acts on oxidation intermediates (or on reduction intermediates not de- 

 rived from carbon dioxide). In Franck and Herzf eld's scheme 7.VA 

 (Vol. I, page 164) this catalyst was supposed to act on both the reduction 

 and the oxidation intermediates; but we have argued that this is implaus- 

 ible because of the well-laio\vn specificity of enzymes. The experiments of 

 Clendenning and Ehrmantraut on Hill reaction in live cells (c/. part C) 

 indicate the probability that the same catalyst limits the rate in strong 

 light of both photosynthesis and the Hill reaction ; this makes it likely that 

 this catalyst operates on the primary photochemical oxidation product (or 

 a primary reduction product not derived from CO2), and not on a reduction 

 intermediate of carbon dioxide. 



We expect the Hill reaction to be sensitive to inhibitors which affect 

 the enzymes Eb, Ec, Eq, but not to inhibitors which act on enzymes, such as 

 Ea, involved only in the transformation of carbon dioxide. 



Cyanide was characterized in chapter 12 (Vol. I, page 309) as a specific 

 poison of the' "carboxylase," E^. Hill and Scarisbrick (1940) found that 

 the oxygen liberation of isolated chloroplasts in the presence of ferric salts 

 is not inhibited by cyanide. Warburg and Luttgens (1946) said that they 

 were unable to study the effect of cyanide on the Hill reaction with quinone 

 as oxidant because (at 20° C. and pH 6.5) quinone oxidized HCN directly 

 in the dark, with liberation of carbon dioxide. Aronoff (1946) found no ef- 

 fect of cyanide on the photoreduction of quinone in "grana preparation" 

 from spinach leaves. Ehrmantraut and Rabinowitch (1952) noted no 

 inhibition of the quinone reaction in live cells at 10° C. and pH 6.5 when 

 0.005 M HCN was added immediately after the beginning of illumination 

 (cf. part C below) . 



Macdowall (1949) found that >0.01 M KCN were needed to produce 

 50% inhibition of Hill reaction with phenol indophenol (Table 35.XI). 



Wessels (1954) found no inhibition of potentiometrically recorded Hill 

 reaction (with quinone as oxidant) by 0.01 M KCN. 



Gorham and Clendenning (1952) noted that the presence of 10"' 

 mole/1. KCN affects the i^ = / (pH) curves of the Hill reaction. It 

 stimulated the activity of washed chloroplasts at pH 6.3-7.3, but inhibited 

 it at other pH values. This influence is similar to that of other anions 

 {cf. section 3(c)). 



