6o INTERMEDIARY METABOLISM AND GROWTH I 



Glucose llsocitrate] 



:» |PhosphoglycerQte| ~g =- Phosphohydroxypyruvate -= ^ Phosphoserine 



ATP 



succinate 



r-\ i. -2H r- — T^ -=r, — -. — I Glutamic ^, 



Glycerate-= =^Hydroxypyruvate,= — ^ [Serine^. — ^Glycine| -= =- Glyoxylate 



acetyl 

 phosphate 

 CoA 



Glycolate 



"Glycolaldehyde" V 



Dk ^ I t ! iMalatel 

 — Phosphoenolpyruvate^;^ I ' ' 



'1 ^"^^Pyruvate | Ribulose-5-phosphQte| 



Oxalccetate^*- — 



Fig. 2 1 . Proposed pathways of serine and glycine biosynthesis. 



tissues by a pathway in which the glycolytic intermediate, phosphoglycerate' 

 is a proximal precursor (Fig. 21) (Black et al., 1955). When uniformly labelled 

 glucose was fed to young rats, it was found that both the serine and the glycine 

 became labelled (Arnstein and Keglevic, 1956; Arnstein and Stankovik, 1956). 

 The feeding of glucose- i-^'^C led to the labelling of serine but not of glycine, 

 a result to be anticipated, since the first carbon of glucose is converted via glycolysis 

 to the beta carbon of phosphoglycerate and to the beta carbon of serine. The 

 latter carbon atom is lost when serine is converted to glycine. However, when 

 labelled glucose was fed to folic acid deficient rats, the labelling of glycine was 

 greatly inhibited while serine radioactivity was not reduced. Since the deficiency 

 of folic acid inhibits the interconversion of glycine and serine, it was evident that 

 the serine was formed prior to glycine from the labelled glucose. Radioactive 

 alanine was a much poorer precursor of rat serine and glycine than of alanine, 

 aspartic, and glutamic acids, suggesting that the pyruvate which is derived from 

 alanine was not the immediate precursor of serine. 



The synthesis of the beta carbon of serine from glucose- i-'^^C and from glucose- 

 6-^'*C was also demonstrated in vitro in tumors and in various normal tissues of 

 the mouse and rat (Kit, 1955). From the results with glucose- i-^'^C the conclusion 

 could be drawn that serine could be synthesized by a mechanism which did not 

 necessitate the direct oxidative pathway of glucose metabolism since the labelled 

 carbon would have been lost as CO2 if glucose- i-^'^C had been metabolized via the 

 latter pathway. When glucose-2-^'*C or uniformly labelled glucose were used as 

 substrates, radioactivity was found in both glycine and serine. This result is also 

 consistent with the postulated pathway via phosphoglycerate since phosphogly- 

 cerate-2-^'*C and serine-2-''*C are presumably formed from glucose-2-^'^C and, 

 therefore, the label is retained in the glycine which is formed from serine. The 

 formation of serine and of glycine from glycerol- i-^'*G was also demonstrated in 

 lymphatic tissues and tumors (Kit and Graham, 1956b). 



Serine and glycine biosynthesis was not inhibited when fluoride and non-labelled pyruvate 

 were added to the incubation flasks, although the labelling of alanine, aspartate, und gluta- 

 mate was greatly inhibited. This result supports the idea that phosphoglycerate is a more 

 likely serine precursor than phosphoenolpyruvate or pyruvate. Serine was also formed prior 



