660 



N. E. Tolbert 



mechanism from the chloroplasts to the cytoplasm. The glycerate 

 may be converted to sugars or metabolized to phosphoenol pyruvate 

 and on to malate and aspartate. Added glycolate-C-"-"^ results in 

 considerable synthesis of malate-C-'-'* and when the glycolate path- 

 way in tobacco leaves was blocked by the a-hydroxysulf onate in- 

 hibitors, C''^^ labeling of a large isocitrate reservoir was com- 

 pletely blocked (4). These findings suggest that further search 

 for glyoxylate cycle enzymes for isocitrate synthesis in tobacco 

 leaves is merited. 



Permease system : The excretion of glycolate by algae or by 

 chloroplasts has the characteristics of a permease system in which 

 the action of phosphoglycolate phosphatase would provide direc- 

 tion. This theory requires that phosphoglycolate but not glycol- 

 ate might cross the membrane. Phosphoglycolate is observed to 

 come out of the chloroplasts. Glycol ate -C'"'^ is not absorbed by 

 chloroplasts at significant rates (16). A slow, light-stimulated, 

 oxidation of it to CO2 has been reported (41). Moses and Calvin 

 (42) extended this theory when they reported that glycolate con- 

 tains a major portion of the tritium incorporated from TgO during 

 photosynthesis. They suggested that the glycolate pathway serves 

 to carry not only carbon but hydrogen from the photosynthetic 

 apparatus. (However, clarification of their results are needed 

 since the apparent high tritium content of glycolate could have 

 resulted from glycolate sublimation from the paper chromatogram 

 onto the film.) Such a system would be analogous to the role of 

 the glycerol phosphate dehydrogenases in transport of reducing 

 power into the mitochondria. Glycolate would be oxidized in the 

 cytoplasm to glyoxylate. Glyoxylate, unlike glycolate, can re- 

 enter the chloroplasts (16) where there is a NADPH linked glycol- 

 ate reductase to reduce it back to glycolate with photosynthetic 

 assimilatory power. A major fault of this scheme or any other 

 scheme involving a combination of glycolate oxidase and glyoxyl- 

 ate reductase is that glycolate oxidase has not been coupled to 

 oxidative phosphorylation (32) or NAD reduction. If phosphogly- 

 colate ware synthesized from phosphoglycolaldehyde , as mentioned 

 previously, the diose phosphate dehydrogenase system would create 

 sufficient assimilatory power for the resynthesis of siogars from 

 glycolate. 



Glycolate metabolism and 0^ evolution : Several similarities 

 exist between O2 evolution and glycolate accumulation or excre- 

 tion during photosynthesis vhich taken all together suggest that 

 they may be related. Both processes are inhibited by Mn defici- 

 ency and by CMU. Glycolate oxidase contains FMN and it will re- 

 duce DCPIP. Both these factors have been associated with the Hill 

 reaction. Homann and Gaffron (43) have indicated that CMV changes 



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