IV BIOSYNTHESIS OF AMINO ACIDS 6l 



to glycine in the tumor cells since the addition of non-labelled serine to the medium 

 reduced the conversion of labelled precursors to glycine but increased the radioactivity 

 of serine. 



Enzymes effecting the conversion of glyceric-3-i-'C to serine have been partially purified 

 from rat liver (Ichihara and Greenberg, 1955). ATP, DPN*, and an amino donor such as 

 glutamic acid or alanine, stimulate serine synthesis. The conversion of phosphoglyceratc 

 and phosphohydroxypyruvate to serine could also be shown in the same enzyme system. 

 The formation of labelled phosphoserine and serine from glycerol or glyceric acid has been 

 demonstrated in experiments with intact tumor cells and with tumor homogenates 

 (Kit, unpublished). 



The above results are all consistent with a pathway of serine and glycine bio- 

 synthesis via phosphoglycerate, phosphohydroxypyrtivate, and phosphoserine. 

 It is also possible, however, that serine may be formed by a pathway involving 

 hydroxypyruvate and D-glycerate as direct precursors. Free glycerate occurs in 

 tobacco leaves and in cress seedlings (Palmer, 1956; Isherwood et al., 195413). 

 Free hydroxypyruvate and also glyoxylate have been observed in rabbit kidney, 

 liver, and tumor tissue (Linko and Virtanen, 1955). Plant tissues contain an enzyme 

 which catalyzes the reduction of hydroxypyruvate by DPNH to D-glycerate 

 (Staflford et al., 1954). The hydrolysis of phosphoglycerate to glyceric acid as well 

 as of phosphoserine to serine is apparently catalyzed by non specific enzymes. The 

 transamination of hydroxypyruvate and alanine to serine and pyrtivate by en- 

 zymes from liver and kidney has also been shown (Sallach, 1954)- However, 

 hydroxypyruvate-2-^'*C is a relatively poor serine precursor in the green algae, 

 Scendesmus (Milhaud et al., 1956) possibly due to the fact that the exogenous 

 hydroxypyruvate is rapidly catabolized (Schweet et al., 1951; Horecker, 1954). 

 In yeast cells, glycine and serine biosynthesis has been studied with pyruvate-2- 

 ^'^G as substrate (Wang et al., 1954). Although the alanine which was isolated 

 was labelled primarily on the alpha carbon atom, it was observed that serine and 

 glycine were equally labelled on the carboxyl and alpha carbons. This result 

 suggested that pyruvate was not directly converted to serine and glycine, but that 

 pyruvate-2-''*C was first carboxylated to malate-2-^''C by the malic enzyme. As 

 a result of the oxidation of the latter substance through the tricarboxylic acid 

 cycle, the malate-2-''*C would be converted to oxalacetate-i, 2-^^*0 which in 

 turn could be decarboxylated by the enzyme, oxalacetic carboxylase, to phos- 

 phoenolpyruvate-i,2-^'*C. This stibstance could then give rise to the serine and 

 glycine compounds with the observed pattern of labelling. 



The formation of glycine from pentose. A "serineless" E. coli mutant is known in 

 which the serine requirements cannot be supplied by glycine. When this strain is 



H'^CO hJ^COH /h'^COhN Hi-'COH H'^CO hI^CNH, 



HCOH ^ C=0 ^ y CHOy COOH COOH COOH 



HCOH HCOH "Active Glycoiote Glyoxylote Glycine 



I I glycolaldehyde" 



HCOH HCOH 



I I 



HjCOPOjHj HjCOPOaHj 



Ribose- 5- phosphate Ribulose- 5 -phosphate 



Fig. 22. Postulated mechanism of glycine synthesis from ribose-5-phosphate. 



Literature p. 124 



