LUIS F, LELOIR 



activated with magnesium ions and diphosphothiamine, it cata- 

 lyzes the transference of the CHOH — CO — group from ketoses 

 to aldoses, as shown in equation (7). 



Instead of D-ribulose-5-phosphate as donor of the glycolalde- 

 hyde group, the enzyme can use hydroxypyruvic acid, L-erythru- 

 lose, or sedoheptulose-7-phosphate. Several substances can 

 serve as acceptors. D-Ribose-5-phosphate and D-glyceraldehyde- 

 3-phosphate yield sedoheptulose-7-phosphate and D-ribose-5- 

 phosphate, respectively. The products formed with other ac- 

 ceptors have not been identified : L-glyceraldehyde-3-phosphate 

 would probably yield L-ribulose-5-phosphate; D-glyceraldehyde 

 would give D-ribose; and glycolaldehyde would form L-erythru- 

 lose. 



Thus, transketolase can give rise to the most varied prod- 

 ucts. When the enzyme is added to the donor in the absence of 

 acceptor, no chemical change can be detected. Diphosphothi- 

 amine appears to act as intermediary acceptor and donor of gly- 

 colaldehyde. 



Transfer of Three-Carbon Units 



It is generally considered that the only function of aldolase 

 is to catalyze the conversion of fructose-l,6-diphosphate into 

 dihydroxyacetone phosphate and glyceraldehyde phosphate, as 

 shown in equation (8). 



CH9OPO3-- 



CO 



I 

 HOCH 



I ^=^ CO + HCOH 

 HCOH 1^^ 1^ (8) 



HCOH 



CH20P03— 



(X) 

 Fructose- 1,6- 



diphosphate 



Undoubtedly, this reaction is a part of the main metabolic 



594 



