62 



INTERMEDIARY METABOLISM AND GROWTH 



grown in the presence of uniformly labelled glucose, glycine but not serine be- 

 comes labelled (Meinhart and Simmonds, 1955). Since this mutant is still capable 

 of synthesizing glycine, it is apparent that mechanisms of glycine formation must 

 exist in which serine is not an obligatory precursor of glycine. The probable 

 glycine precursors in this instance are glyoxylate, glycolate, and glycolaldehyde. 

 Since the latter substances are rapidly converted to glycine by animal and plant 

 tissues (Weinhouse and Friedmann, 1951 ; Friedmann et al., 1956), it is of interest 

 to inquire as to the possible sources of these two carbon compounds (Friedmann 

 et al., 1956; Tolbert and Cohan, 1953). 



The "active glycolaldehyde" donor, ribulose phosphate is an interesting poten- 

 tial precursor of glycine. Weissbach and Horecker have demonstrated glycine 

 formation from i-''*C -ribose-5-phosphate in spinach leaf extracts (Fig. 22; Weiss- 

 bach and Horecker, 1955). 



It was observed that 80 % of the glycine radioactivity was attributable to the 

 alpha carbon. Little label was found in serine. The distribution of radioactivity 

 ruled out the possibility that fructose-6-phosphate and phosphoglycerate were 

 intermediates, even though the conversion of i-''*C-ribose-5-phosphate to 1,3- 

 ^'*C-fructose-6-phosphate, occurs in spinach extracts. The latter substance woixld 

 have been metabolized to glycine- i-''*C instead of to glycine-2-^'*C. 



Isocitric lysaseK The tricarboxylic acid cycle intennediate, isocitric acid is also a 

 potential glycine precursor. The following aldolose type of reaction is catalyzed 

 by enzymes from yeast, Aspergillus, Rhizopus, and Pseudornonas (Smith and Gunsa- 



COOH 

 HO— C — H 



CH — COOH 

 CH2 

 COOH 

 Isocitric 



Mg^* 



COOH 



I 

 I 

 CH2 



CH2 



COOH 



Succinic 



-h 



COOH 

 CHO 



Glyoxylate 



Fig. 23. Isocitric lysase reaction. 



lus, 1954; Olson, 1954; Saz and Hillary, 1956) (Fig. 23). It is to be noted that 

 since this reaction is reversible, it also represents an alternative mechanism for the 

 formation of the tricarboxylic acid cycle intermediate, the isocitrate. 



Glyoxylate from malate^. An enzyme purified from E. coli G-26 catalyzes the fol- 

 lowing reaction (Wong and Ajl, 1956) : 



Acetyl phosphate + glyoxylate 



CoA 



) 



Mg- 

 cysteine 



malate 



^ See Addendum, Note 3, p. 123. 



