INHIBITORS OF CARBOHYDRATE METABOLISM 409 



manner as the substrate, forming a stable complex with the enzyme, but is 

 unable to complete the sequence. 



The use of D-threose-2,4-diP to block glycolysis in intact cells or tissues 

 will probably not meet with general success due to the poor penetration. 

 Extracts of ascites tumor cells are inhibited readily, but glycolysis in intact 

 ascites or HeLa cells is unaffected (Racker et al., 1959). Preparations of 

 glycolaldehyde-2-P, containing the tetrose-diP, inhibit the growth of some 

 bacteria and not others, probably depending on the degree of penetration. 

 The possibility was considered that the inhibitor, can be formed intracellu- 

 larly but so far appropriate precursors have not been found. 



If photosynthesis involves the reversal of the glycolytic sequence, 

 D-threose-2,4-diP should inhibit, and it has been found that the total C^^Og 

 fixation in sonically ruptured spinach chloroplasts is reduced 57% by this 

 analog at 0.1 mM (Park et al., 1960). The photoreduction of 3-phospho- 

 glycerate is inhibited and this substance accumulates in the presence of the 

 inhibitor. This inhibition is due primarily to an action on glyceraldehyde-3-P 

 dehydrogenase leading to a deficiency of ribulose-l,5-diP, the CO2 acceptor. 

 Carboxy dismutase is inhibited only slightly by 0.1 mM D-threose-2,4-diP 

 but higher concentrations inhibit appreciably. This inhibitor may thus play 

 a role in photosynthetic studies. 



Keleti and Telegdi (1959) examined various glyceraldehyde-3-P dehy- 

 drogenases and found inorganic phosphate to stimulate the activity at low 

 concentrations but to inhibit progressively above 20-30 mM. The inhi- 

 bition of the yeast enzyme is competitive with both glyceraldehyde-3-P 

 and NAD. Taylor et al., (1963) also found phosphate inhibition of the 

 hydrolysis of p-nitrophenylacetate by glyceraldehyde-3-P dehydrogenase, 

 the muscle enzyme being much more sensitive than the yeast enzyme. 

 Indeed, the muscle enzyme is 56% inhibited already at 10 mM phosphate. 

 Phosphate inhibitions of the glycolytic enzymes have seldom been consid- 

 ered as playing a role in the regulation of carbohydrate metabolism. 



Enolase 



Enolase catalyzes the reaction 



D-2-phosphoglycerate :^ phosphoenolpyruvate 



and various analogs of these substances inhibit competitively (see tabula- 

 tion) (Wold and Ballou, 1957). The following substances do not inhibit: 

 D-lactate, D-glyceraldehyde-3-P, dihydroxyacetone-P, and glycerol-2-P. An 

 inhibitor must have a carboxyl and a phosphate group, and the distance 

 between them is of some importance, although not critical. The 3-methyla- 

 tion of 2-phosphoglycerate does not lead to much reduction in binding, but 

 3-methylation of the 3-phosphoglycerate reduces the afiinity by approxi- 



