1748 CHLOROPLASTS, CHROMOPLASTS AND CHROMATOPLASM CHAP. 37A 



responsible for the "respiration" of mashed spinach leaves remained, after 

 centrifugation, in the cytoplasmic fluid; only at pH 5 was the enzyme 

 precipitated with the chloroplast fragments. Similarly, Bonner and 

 Wildmann (1947) noted that, after grinding spinach leaves in a colloid mill 

 all polyphenoloxidase stayed in the colorless fluid, and none was precip- 

 itated with the chloroplast fragments. 



In a later paper, Wildmann and Bonner (1947) described fractionation 

 of cytoplasmic proteins from spinach leaves (probably including some of 

 the chloroplast stroma proteins) into two main fractions. A large electro- 

 phoretically homogeneous fraction (70-80% of the total) was found to 

 contain auxin and a phosphatase, while a smaller, inhomogeneous fraction, 

 contained polyphenoloxidase, as well as four different dehydrogenases, 

 a peroxidase and a catalase. 



McClendon (1953) measured the polyphenol oxidase activity of the 

 several fractions he obtained from homogenized tobacco leaves (cf. above), 

 and found no significant concentration in any one of the three precipitates; 

 about one-half of the enzyme was left in the colorless supernatant. Much 

 of the enzyme could be easily washed out of the chloroplast fragments, but 

 some seemed to be bound to these fragments more tightly. 



Smirnov and Pshenova (1941) described a special kind of polyphenol oxidase they 

 found in tobacco leaves which oxidized hydroquinone. Laties (1950) observed a new 

 cyanide-insensitive oxidation enzj-me in spinach chloroplasts. 



Catalase cap. be considered as an oxidase because it, too, catalyzes a 

 reaction which involves molecular oxygen (usually as a product rather than 

 as a reactant, because of the irreversibility of the dismutation of hydrogen 

 peroxide). In Chapter 14 (page 379) we noted that, according to Neish, 

 all leaf catalase is present in chloroplasts ; but Krossing found it also in the 

 cytoplasm. In the four fractions obtained by McClendon (1953) by cen- 

 trifugation of tobacco leaf homogenates, about one-half of the enzyme was 

 in the chlorophyll-free supernatant, the other half in the three precipitates, 

 its amount being about proportional to that of total protein in each frac- 

 tion. However, since washing the precipitates by centrifugation extracted 

 a large part of catalase, it is quite possible that before fractionation much 

 more than one-half of catalase was associated with the plastides. 



Glycolytic ejizymes are responsible for phosphorylation and cleavage of 

 hexoses, and hydrogen transfer from trioses and the acids of the Krebs 

 cycle, to TPN, DPN (co-enzymes I and II) and lipoic acid (which in turn 

 transfer them to flavins, cytochromes, and ultimately to oxygen). Since 

 many of the partial reactions in glycolysis are reversible, the same enzymes 

 could conceivably participate also in the reverse process of sugar formation 

 in photosynthesis; their occurrence in green plant cells — in particular that 



