HANS KLENOW 



and still others may be found. The transketolase reaction then provides us with a 

 system by which pentoses can be formed by condensation between a two-carbon 

 and a three-carbon fragment. A third mechanism for pentose formation is suggested 

 by work of Hough and Jones (1953) who found xylulose phosphate to be formed from 

 triose phosphate and dihydroxymaleic acid in the presence of an enzyme from peas. 

 The details of this mechanism seem, however, not to be entirely clear yet. 



We have now accounted for some enzyme reactions for pentose formation. But 

 how are the pentoses actually formed in the intact organism ? By which mechanism 

 are the pentoses in the nucleic acids formed ? The best tool for getting such informa- 

 tion is obviously ingestion of isotopically labelled compounds, the fate of which can 

 be followed. In the case of ribose the pattern of labelling of the carbon atoms of the 

 pentose of the nucleic acids obtained in this way may give valuable information. 



Table I 



'Active glycolaldehyde' donors and acceptors 



a Horecker et al. (1953). b Racker et al. (1953). 



Such experiments have been performed by Bernstein (1953). The concept which 

 led to these experiments was the following: If the pentoses were formed by the oxi- 

 dative breakdown of glucose-6-phosphate by removal of number one carbon or 

 possibly by removal of number six carbon by decarboxylation of a hexuronic acid, 

 the distribution of the tracer in the ribose should be similar to that of the remaining 

 five carbons of the hexose. A deviation from this picture would indicate the involve- 

 ment of some other synthetic mechanism. The liver glycogen and the ribose from the 

 nucleic acids of the internal organs of chicks were therefore isolated after feeding with 

 different 14 G-labelled compounds. The glycogen and the ribose were degraded bio- 

 logically and chemically, and the specific activity of each carbon was determined. 

 Assuming that the 14 C pattern of glycogen corresponds to that of glucose-6-phosphate 

 during the experiment, and that the 14 G pattern of the ribose is not altered when the 

 nucleic acids are formed, Bernstein compared the relative pattern of 14 C labelling 

 in the glucose and in the ribose. As can be seen from Table II the pattern of labelling 

 of the ribose does not in any case correspond with that of the 5 carbons in succession 

 of the glucose derived from glycogen. However, pentose formation by a condensation 

 of a two-carbon with a three-carbon compound is consistent with the results obtained. 

 C( 3 ), C(4) an d C (5 ) of the pentose should then arise from the same triose which is the 

 precursor of glycogen. The C (1 > and C( 2 > of the pentose could be derived from C (1) 



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