BIOSYNTHESIS OF PENTOSES 263 



h. Synthesis of Deoxyribose-5-phosphate 



Racker''^ has shown that extracts of E. coli catalyze the following re- 

 versible reaction: 



glyceraldehyde-3-P + acetaldehyde ^ deoxyribose-5-P 



Deoxyribose phosphate aldolase (DR aldolase), which catalyzes this re- 

 versible aldol condensation, is widely distributed in microorganisms and 

 animal tissues, thymus and liver being particularly rich in this enzyme. 

 It has been purified from E. coli extracts and found to be unaffected by 

 NaF (10-2 M) and iodoacetate (IQ-" M) but inhibited by octyl alcohol, 

 chloral hydrate, and propionaldehyde. Enzymically formed deoxyribose- 

 5-phosphate has been isolated and characterized and found to be utilized 

 by crude bacterial extracts for the formation of nucleosides. Deoxyribose- 

 5-phosphate can also be formed from ribose-5-phosphate and more readily 

 from ribulose-5-phosphate in the presence of acetaldehyde and partially 

 purified enzymes from baker's yeast and E. coli. The yeast enzyme cata- 

 lyzes the formation of glyceraldehyde-3-phosphate from ribulose-5-phos- 

 phate and the E. coli enzyme the subsequent condensation with acetalde- 

 hyde. 



Since DR aldolase has a very low affinity for acetaldehyde, this may not 

 be the natural substrate for this enzyme. Racker suggests that in the cell 

 an aldehyde linked to a purine or pyrimidine precursor may normally 

 condense with glyceraldehyde-3-phosphate, thus accomplishing a direct 

 synthesis of deoxyribose nucleotide. Hammarsten et alP have suggested 

 that deoxyribose may arise from ribose while the latter is in glycosidic 

 linkage. 



c. Synthesis of Rihulose-5 -phosphate 



Akabori et al}^ demonstrated the formation of a ribose phosphate, be- 

 Heved to be ribose-5-phosphate, on incubating fructose- 1,6-diphosphate 

 and dihydroxymaleic acid with minced rabbit muscle. It was suggested 

 that carboligase catalyzed the condensation of glyceraldehyde-3-phosphate 

 and hydroxypyruvic acid (formed by decarboxylation of dihydroxymaleic 

 acid) with elimination of CO2 and formation of ribulose-5-phosphate, in 

 equilibrium with ribose-5-phosphate, according to Fig. 6. Dihydroxymaleic 

 acid could not be replaced by glycolaldehyde. 



This somewhat novel method of synthesis of pentose phosphate was con- 

 firmed by Racker et al.^^ Using a crystalline enzyme from baker's yeast, 

 which catalyzes the cleavage of ribulose-5-phosphate, they obtained de- 

 carboxylation of hydroxypyruvic acid in the presence of an "acceptor 

 aldehyde." With d- or DL-glyceraldehyde-3-phosphate as acceptor, decar- 



^6 E. Racker, Nature 167, 408 (1951); J. Biol. Chem. 196, 347 (1952). 



" E. Hammarsten, P. Reichard, and E. Saluste, /. Biol. Chem. 183, 105 (1950). 



