1746 CHLOROPLASTS, CHROMOPLASTS AND CHROMATOPLASM CHAP. 37A 



B.4, Chapter 35), malic enzyme must be looked upon as the port of entry of 

 carbon dioxide into photosynthesis. However, this conclusion is not sup- 

 ported by other evidence, in particular by the more recent radiocarbon 

 studies described in Chapter 36. These studies have rather conclusively 

 eliminated malic acid as the primary carboxylation product in photosyn- 

 thesis. Ribulose diphosphate has emerged as the most likely primary car- 

 bon dioxide acceptor {cf. Chapter 36, section A7). Its carboxylation, 

 combined with hydrolytic splitting of the carbon chain, leads to two mole- 

 cules of phosphoglyceric acid (eq. 36.8). Weissbach, Smyrniotis and 

 Horecker (1954) found that an enzyme catalyzing this reaction can be ex- 

 tracted from spinach leaves; Quayle, Fuller, Benson and Calvin (1954) 

 found it also in ChloreUa. Thermochemical calculations {cf. Chapter 36, 

 Section A12) made it plausible that the equilibrium lies, in this case, far on 

 the side of carboxylation, as required by the kinetics of photosynthesis. 

 It will be interesting to know whether this enzyme is cyanide sensitive. 



To sum up, it seems unlikely, at present, that any of the common de- 

 carboxylases found in plants are directly involved in photosynthesis; and 

 the same probably is true of malic enzyme. 



Boichenko (1948) asserted that clover-leaf chloroplasts contain an 

 enzyme (formic dehydrogenase or hydrogenlyase) capable of combining 

 CO2 with H2 to formic acid ; it was mentioned above that this enzyme was 

 not found in ChloreUa cells by Clendenning and co-workers. 



Coming now to enzymes engaged in the transfer of hydrogen from 

 organic substrates to oxygen (which conceivably could also be useful in 

 the reverse process), we note that cytochrome c was repeatedly reported 

 in plants, but first successfully extracted from them by Hill and Scaris- 

 brick (1951) using both chlorophyllous and nonchlorophyllous plant 

 tissues. They also discovered two new plant hemoporphyrins, which 

 they designated as cytochrome h and cytochrome f. The second of these 

 new cytochromes was found in chlorophyllous tissues only, not alone in 

 leaves, but also in some species of algae. One molecule of cytochrome / 

 was present per about 400 molecules of chlorophyll. Cytochrome / was 

 found by Davenport and Hill (1951) to have a more negative potential 

 than cytochrome c {E= -0.365 volt vs. -0.260 volt). 



Hill, Northcote and Davenport (1953) observed the characteristic 

 cytochrome / absorption band at 555 m/x also in acetone-extraction residue 

 from ChloreUa cells. The ratio (chlorophyll) : (cytochrome /) seemed to be 

 of the order of 500. 



Cytochrome oxidase, which mediates between cytochrome c and molecu- 

 lar oxygen, was reported as present in chloroplasts by Rosenberg and 

 Ducet (1949) and Sisakyan and Filipovich (1949). Baghvat and Hill 

 (1951) demonstrated the occurrence of the complete cytochrome system. 



