PHOTOCHEMISTRY OF LIVE CELLS 1621 



which seem to support the concept of a common rate-Hmiting enzymatic 

 process in Hill reaction and photosynthesis — was discussed in chapter 34C. 



Induction. Under the conditions when photosjaithesis in continuous 

 as well as in flashing light had the often observed induction period of about 

 5 min., fully developed after about 5 min. of darkness (c/. chapter 33), 

 oxygen liberation with cjuinone was found to begin instantaneously. This 

 finding of Clendenning and Ehrmantraut (1951) already was described in 

 chapter 33 (section A8) and its possible implications for the theory of the 

 induction phenomena were discussed there (section C4). 



The same difference between induction in photosynthesis and quinone 

 reduction by Chlorella was found also in flashing light. 



Inhibition. The Hill reaction with quinone in whole Chlorella cells is 

 not inhibited by cyanide (0.0005 mole/liter) — a result that agrees with pre- 

 viously reported observations on chloroplast preparations (section B5(d) 

 above) , but which is nevertheless important because it proves that the Hill 

 reaction in live cells does not proceed via the formation of free carbon di- 

 oxide, followed by normal photosynthesis. 



With Hill's mixture, oxygen evolution by Chlorella cells in dark was 

 inhibited by cyanide as effectively as photosynthesis — a confirmation of 

 the suggestion (cf. below) that this evolution results from photosynthesis 

 at the cost of carbon dioxide produced by photolysis of oxalate. 



The Hill reaction in live cells was found by Clendenning and Ehrman- 

 traut to be inhibited by hydroxylamine (3 X 10"*^ gave 100% inhibition), 

 fluoride (0.02 M, 65% inhibition), ethylurethan (1% solution gave 33% 

 inhibition), iodoacetate (0.02 M, 100% inhibition) and — rather unexpect- 

 edly — malonaie (35% inhibition at 6 X 10 ~^ M). No chloride effect was 

 noted by comparing the Hill reaction in Chlorella cells grown with or with- 

 out potassium chloride, or by adding chloride to the reaction mixture. 



Preilluminaiion Effect. The quinone reduction by Chlorella is inhibited 

 by preillumination of cells in the absence of oxidant (Fig. 35.27). The 

 inhibition occurs in red light as well as in light of shorter wave lengths, 

 indicating that it is a chlorophyll-sensitized process. (It is not a direct 

 photochemical transformation of chlorophyll — or, at least, does not reveal 

 itself as such by a change in color.) 



The preillumination effect reminds one of a similar phenomenon ob- 

 served by Warburg and Liittgens (1946) and Arnon and Whatley (1949) 

 with isolated chloroplast preparations. (We recall, however, that no in- 

 hibition by preillumination of chloroplasts was found by Holt and French, 

 1948.) It also makes one think of the observation reported in chapter 19 

 (Vol. I) that photoxidation can be induced in cells by illumination in the 

 absence of carbon dioxide. However, inhibition by preillumination was 

 observed by Clendenning and Ehrmantraut in nitrogen as well as in air. 



Clendenning and Ehrmantraut noted that preillumination of Chlorella, 

 which reduced the rate of quinone reduction by 50%, did not affect the 



