KINETICS 



1607 



The pyrocatechol thus serves as a catalyst; the accumulated oxidation 

 product is p-quinone, which is less toxic than o-quinone. The oxygen up- 

 take continues uniformly until all hydroquinone is used up (fig. 35.24). 

 The production of carbon dioxide (which, without added substrate, is about 

 equivalent to the consumption of oxygen) is reduced by about 50% by the 

 presence of pyrocatechol, and is close to zero if both pyrocatechol and hy- 

 droquinone are added. This shows that hydroquinone has completely dis- 

 placed the endogenous respiration substrate. When the suspended ma- 

 terial was separated from the liquid by centrifuging (at pH. 6.5), oxygen 



300 



10 



20 30 40 



TIME IN DARK, minutes 



Fig. 35.24. Oxygen consumption by spinach chloroplast preparations 

 in darkness (after Warburg and Liittgens 1948). 



consumption continued in both phases; but after centrifuging at pH 5 

 the "polyphenol oxidase" of the liquid phase was coprecipitated with the 

 cliloroplast material (since now only the precipitate showed autoxidation) . 



Arnon (1949) studied the polyphenol oxidase activity of leaf material. 

 He found that chloroplastic matter, separated from the cytoplasmic fluid, 

 retains all the polyphenol oxidase activity of the original cellular material. 



Clendenning and Gorham (1950^ noted that in crude leaf macerates, 

 freed only of heavy particles (by slow centrifugation), acid production upon 

 addition of Hill's mixture occurs also in the dark. In some species the rate 

 of this dark reaction was quite high — for example, in material from spruce 

 and other gymnosperms it reached 4 moles acid per minute per mole 



