COMPOSITION OF CHLOROPLASTS 1749 



of triose phosphate dehydrogenase — has been touched upon m chapter 36. 

 In respiration, this enzyme oxidizes phosphoglyceraldehyde to phosphogly- 

 ceric acid, transferring hydrogen to a pyridine nucleotide; it could con- 

 ceivably catalyze the reverse process in photosynthesis. 



A controversy has arisen about the presence of triose dehydrogenase in green plants 

 in connection with observations of the hick of iiiliibition of their respiration by iodo- 

 acetate; however, as described in chapters 36, page 1687, and 'S7D (section 2), Arnon was 

 able to demonstrate this inhibition, and thus to confirm the occurrence in plants of 

 the respiration path via triose and glycerate. More recently, Arnon, Rosenberg and 

 Whatley (1954) described a new kind of TPN— specific triose phosphate dehydrogenase 

 in green cells, not requiring inorganic phosphate as co-factor. 



Sisakyan and Shamova (1949) identified several known dehydrogenases 

 in chloroplast material. Sisakyan and Kobjakova (1949) found phospho- 

 glucomutase. Holzer and Holzer (1952) (of. also Holzer 1954) demonstrated 

 the presence, in plasmolyzed Chlorella cells and in acetone powder extracts 

 from these cells, of hexokinase, phosphofructo-kinase, aldolase, and triose 

 phosphate dehydrogenase. Inhibition of the latter with iodoacetic acid pro- 

 duced, in glucose medium, the same accumulation of fructosediphosphate 

 in Chlorella as it did in yeast. 



Chloroplasts must also be the sites of carbohydrate transformations by 

 which hexose are converted to sucrose and starch. Nezgovorov (1940, 

 1941) found in chloroplasts small amounts of free amylase; Sisakyan and 

 Kobjakova (1949), inveriase and amylase. 



For more detailed compilations of the enzymatic components of chloro- 

 plasts (and other plant cell structures) we must refer here to the reviews by 

 Sisakyan (1951), Van Fleet (1952) and Weier and Stocking (1952). 



t 



(c) Heavy Metals 



The paper by Liebich (1941) on the iron content of chloroplasts was sum- 

 marized in Volume I (p. 378) on the basis of an earlier note by Noack and 

 Liebich. A new study of mineral components of chloroplast ash was made 

 by Vecher (1947), who added a few new elements to those identified 

 earlier by Neish and Menke (Vol. I, p. 376). Whatley, Ordin and Arnon 

 (1951) found that, of the elements Fe, Cu, Mn, Zn and Mo, the first two 

 are present in chloroplasts in higher concentrations than in the (sugar 

 beet) leaves as a whole. 



{d) Ascorbic Acid 



In chapters 10 (part E) and 14, we summarized briefly the evidence 

 for the occurrence of ascorbic acid in the chloroplasts, and in the cyto- 

 plasm of green leaves, and for the increase of its concentration in light. 

 A participation of ascorbic acid as intermediate hydrogen carrier in photo- 



