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N, E. Tolbert 



could not have been formed directly from phosphoglycerate , but 

 could have been formed from glycolate. The glycolate pathway re- 

 sults in the production of uniformly labeled glycerate (1), which 

 in turn, produces uniformly labeled hexoses , instead of 3,4-labeled 

 hexoses as expected by the photosynthetic carbon cycle. The rapid 

 incorporation of specific C^^-labeled glycolate, glycine and ser- 

 ine into glycerate and sugars and the corresponding labeling pat- 

 terns in these products have been sufficiently documented to es- 

 tablish this part of the sequence (1, 7, 8, 9). Sucrose synthe- 

 sis by this pathway occurs in the light; in the dark glycolate is 

 metabolized to malate. However several anomalies still exist. 

 Free glycerate is produced by the glycolate pathway. Plant enzymes 

 are known for this synthesis, since a glyoxylate-serine transam- 

 inase produces hydroxypyruvate (E. R. Waygood , personal communica- 

 tion) which in turn is reduced by D-glycerate dehydrogenase (10). 

 Nevertheless, the bulk of the free glycerate in all plants exam- 

 ined has been carboxyl labeled during photosynthesis (1, 11), even 

 though the large serine reservoir is uniformly labeled. Thus the 

 free glycerate pool in some plants is derived mostly from phospho- 

 glycerate, although uniformly labeled glycerate is produced by the 

 glycolate pathway. Wang and Burris (8) have also observed con- 

 siderable phosphoserine whose function is unknown except that it 

 could lead to uniformly labeled phosphoglycerate. 



An exception to the above C^"* distribution patterns was the 

 label found in serine produced by isolated chloroplasts. Only a 

 small amount of serine was formed during CO2 fixation by whole 

 chloroplasts, but it was carboxyl labeled and probably came di- 

 rectly from phosphoglycerate (W. Chang and N. E. Tolbert, unpub- 

 lished). The same isolated chloroplasts produced more glycine 

 than serine, and the glycine was uniformly labeled as if it came 

 from glycolate. Glycine conversion to serine did not occur read- 

 ily in chloroplast preparations. Thus, there seems to be two ser- 

 ine pools, one in the chloroplast which is formed from phosphogly- 

 cerate and one elsewhere which is formed by the glycolate pathway. 



Origin of Glycolate 



Phosphogl ycol ate : The amount of phosphoglycolate is difficult 

 to determine for it nearly cochromatographs with phosphoglycerate 

 (12), Another reason for inaccurate estimation of phosphoglycol- 

 ate is the presence of an active phosphatase which hydrolyzes this 

 ester (13). This phosphatase is stable in dilute methanol (14), 

 acetone and ethanol , and often it may not be destroyed during kill- 

 ing procedures. When isolated chloroplasts were treated to pro- 

 duce glycolate products, a large amount of C^'* was found in phos- 

 phoglycolate (15, 16) in part because the chloroplasts had lost 

 much of the phosphatase. Because phosphoglycolate is a C-'^'^Oa 



