COMPOSITION OF CHLOROPLASTS 1745 



It is generally assumed that carbon dioxide enters photosynthesis 

 through carboxylation of an organic molecule. Vennesland and co- 

 workers (1947, 1949, 1950) have identified a number of carboxylases in plants, 

 using mostly wheat germ and other nonchlorophyllous tissues. How- 

 ever, oxalacetic carboxylase was identified also in spinach leaves. 



Waygood and Clendenning (1950) found that, in leaves, the carboxylases 

 (pyruvic, oxalacetic, oxalsuccinic, ketoglutaric, and glutamic) were present 

 in cytoplasmic rather than in chloroplastic material. 



The so-called malic enzyme, which simultaneously carboxylates and 

 reduces pyruvic acid, also was found in non-chlorophyllous plant tissues 

 (and, in smaller amounts, in spinach leaves) by Conn, Vennesland and 

 Kraemer (1949) and Ceithaml and V^ennesland (1949) ; cf. also Vennesland 

 (1950). Arnon (1951) extracted the same enzyme from beet leaves (it 

 could be obtained from leaf mash from which chloroplasts had been re- 

 moved, and thus seemed to be located in the cytoplasm). Combining the 

 extract with a chloroplast suspension from the same plant, Arnon was able 

 to photosynthesize malate and oxygen from carbon dioxide, pyruvate and 

 water, with only TPN and Mn++ added from non-plant sources. He de- 

 scribed this as an "extracellular photosynthetic reaction"; the limitations 

 of this analogy have been pointed out in Chapter 35 (section B.4f). 



Clendenning, Waygood and Weinberger (1952) inquired into the exist- 

 ence in leaves of a carboxylase abundant enough to "bear the traffic" of 

 photosynthesis, and sensitive to cyanide (since it has been suggested that 

 the carboxylation reaction is the main locus of cyanide sensitivity in 

 photosynthesis, cf. Chapter 12, p. 307, and Chapter 37D, section 2). In 

 Chlorella extracts, they found only a low activity of oxalacetic and a- 

 ketoglutaric carboxylases, and practically none of pyruvic and glutamic 

 carboxylases; also no evidence of malic enzyme or hydrogen lyase. In 

 wheat leaf extracts, the glutamic carboxylase content was high (Wein- 

 berger and Clendenning, 1952); of the known carboxylases (leaving aside 

 the hydrogen lyase) , it alone is cyanide sensitive. However, this carboxylase 

 is found in only few species, and the reaction it catalyzes is practically 

 irreversible and therefore unsuitable for photosynthesis. Oxalacetic 

 carboxylase and malic enzyme were found by Clendenning and co-workers 

 to be more concentrated in the extract from parsley leaves than in the 

 extract from the roots of the same plant. They estimated that the malic 

 enzyme concentration in parsley leaf macerate was sufficient to maintain 

 the high rate of carboxylation needed to keep up with the Hill reaction as 

 observed in the same material. Although — according to Clendenning 

 et al. — this enzyme is cyanide-sensitive "only at very low enzyme concen- 

 trations," it was suggested by them, that in consideration of the observa- 

 tions of Ochoa and Vishniac, Tolmach, and Arnon (described in section 



