INHIBITION OF ENZYMES 



21 



15-30 min at room temperature or above will inactivate completely. The 

 inhibition is due to a reaction of the iodoacetate with the apodehydrogenase 

 rather than with NAD, since preincubation of enzyme with the inhibitor 

 leads to initial inhibition when substrate and NAD are added, whereas 

 preincubation of NAD with the inhibitor is ineffective (Adler et al, 1938). 

 The sequence of reactions catalyzed by 3-PGDH is written by Racker 

 (1961) as: 



Step 1: 



E-SH + NAD+ 



/NAD 

 ^S 



H^ 



Step 2: 



/NAD 

 ^S 



R— CHO 



E— S— C-R + NADH 



Step 3: 



E— S-C— R + HPO/ 



E-SH 



O 



II 

 C-0-PO = 



where R — CHO represents 3-phosphoglyceraldehyde and R — CO — — P03= 

 represents 1,3-diphosphoglycerate. This is based on the evidence that NAD 

 is bound to the enzyme in part through an SH group and that an acyl en- 

 zyme intermediate is formed. Inasmuch as NAD is bound quite tightly to 

 the enzyme, at least in the case of muscle 3-PGDH (dissociation constant 

 around 7 X 10-« M) — the 3-PGDH crystallized from rabbit muscle still 

 retains 2 moles of NAD per mole of enzyme — it is possible that the follow- 

 ing scheme, which is analagous to that of Segal and Boyer (1953), may 

 represent the situation more closely in vivo: 



Step 1: 



/NAD 

 E I + R-CHO 



^S 



'NADH 



Step 2: 



.NADH 



S-C 



II 

 O 



+ NAD 



.NAD 



S-C 



II 



o 



NADH 



Step 3: 



.NAD 



^s-c- 



II 

 O 



HPO42 



,NAD 



+ R-C— O— PO^- + H 



