616 2. ANALOGS OF ENZYME REACTION COMPONENTS 



isocitrate dehydrogenase (Ruffo et al., 1962 b; Ruffo and Adinolfi, 1963). 

 If no oxalacetate is present, glyoxylate directly inhibits the oxidations of 

 a-ketoglutarate and succinate, so that two sites of cycle inhibition can occur, 

 one by glyoxylate alone and one by its condensation product with oxalace- 

 tate. Payes and Laties (1963) have claimed that the of-hydroxy-/?-oxalosuc- 

 cinate (oxalomalate) initially formed in the condensation reaction is rapid- 

 ly decarboxylated to y-hydroxy-or-ketoglutarate, which is the actual in- 

 hibitor. y-Hydroxy-a-ketoglutarate competitively inhibits yeast aconitase 

 [K^ = 0.14 mM), potato a-ketoglutarate dehydrogenase {K^ = 0.7 mM), 

 and isocitrate dehydrogenase. It is also moderately inhibitory to the res- 

 piration of potato slices, 35% depression being observed with 2 mM and 

 71% with 5 mM. It is rather surprising that it does not serve as a sub- 

 strate for a-ketoglutarate dehydrogenase and enter into the sequence of 

 reactions involving coenzyme A, as does y-methyl- y-hydroxy-or-ketogluta- 

 rate (parapyruvate). 



