BIOSYNTHESIS OF PENTOSES 265 



adapted E. coli.^^ Cells adapted to either glucuronic acid or galacturonic 

 acid could oxidize or ferment both uronic acids but not D-xylose, L-arabi- 

 nose, or D-ribose. It was concluded that, at least in the K 12 strain of E. 

 coll, the metabolism of uronic acid does not proceed by the direct decar- 

 boxylation of uronic acid to pentose. An alternative method of uronic acid 

 metabolism was suggested, namely, phosphorylation of glucuronic acid 

 followed by decarboxylation to D-xylose phosphate, which might be the 

 intermediate in pentose formation. The findings of Heald*^ that D-glucuro- 

 nate and D-xylose gave similar feraientation products with mmen bacteria 

 from the sheep may, perhaps, be taken as indirect evidence for the forma- 

 tion of D-xylose by decarboxylation of D-glucuronic acid. 



The only other experimental evidence is that of Enklewitz and Lasker,^* 

 who reported that whereas administration of D-glucuronic acid to normal 

 subjects produced no pentosuria, there was a marked increase in the ex- 

 cretion of L-xyloketose in pentosurics. These results, however, are not very 

 convincing and require confirmation preferably using labeled glucuronic 

 acid. 



3. In Vivo Synthesis from Noncarbohydrate Sources 



Evidence is gradually accumulating to indicate extensive formation of 

 PNA ribose from sources other than carbohydrate. Using C^^-labeled sub- 

 strates, it has been shown that significant amounts of PNA ribose can be 

 formed in the chicken from glycine,^^ acetate,^' and formate.*^ When doubly 

 labeled acetate (C^^ in the methyl group and C'^ in the carboxyl group) was 

 fed to rats,*^ there was preferential incorporation of the a-C indicating that 

 acetate is probably not an immediate precursor of ribose but enters it by 

 some indirect route. 



4. Miscellaneous 



In the preliminary experiments of Charalampous^^ on the incorporation 

 of formaldehyde into phosphorylated sugars, he found that a liver enzyme, 

 distinct from aldolase, catalyzed the condensation of formaldehyde with 

 triose phosphate. The reaction products were separated bj'' ion -exchange 

 chromatography and, although the main component was a tetrose phos- 

 phate (later identified as erythrulose-1-P),^^ approximately 10% was a 



«' S. S. Cohen, J. Biol. Chem. 177, 607 (1949). 



82 P. J. Heald, Biochcm. J. 50, 503 (1952). 



83 M. Enklewitz and M. Lasker, J. Biol. Chem. 110, 443 (1935). 

 8< B. Low, Acta Chem. Scand. 4, 294 (1950). 



85 I. A. Bernstein, Federation Proc. 11, 187 (1952). 



86 B. Low, Acta Chem. Scand. 6, 304 (1952). 



87 F. Charalampous, Federation Proc. 11, 196 (1952). 



88 F. C. Charalampous and G. C. Mueller, /. Biol. Chem. 201, 161 (1953). 



