INHIBITION OF ENZYMES 27 



limited a variety of sources for alcohol dehydrogenase has been examined, 

 it is impossible to draw conclusions about the sensitivity of the enzyme 

 throughout the plant and animal kingdoms, but there is no doubt that the 

 plant enzyme is reasonably easily inhibited. One must conclude, at least in 

 most work where iodoacetate has been used in concentrations greater than 

 are required to block 3-PGDH effectively, that the alcohol dehydrogenase 

 must be inhibited to some extent. 



Little is known about the detailed mechanism of the inhibition. The yeast 

 alcohol dehydrogenase is protected from iodoacetate by both ethanol and 

 NAD (Barron and Levine, 1952), indicating the inhibition to be dynamically 

 competitive, and the SH groups attacked to be at or near the active center. 

 It is also possible that some of the inhibition observed with iodoacetamide 

 may not be entirely related to carbamylmethylation of SH groups, since 

 Woronick (1961) found various acetamides to inhibit horse Hver alcohol 

 dehydrogenase competitively with respect to acetaldehyde, forming com- 

 plexes of the EI and E-NAD-I types (see accompanying tabulation). Yet 



McKinley-McKee (1963) claim that neither alcohol nor acetaldehyde pro- 

 tects horse liver alcohol dehydrogenase against iodoacetamide. On the other 

 hand, NADH protects 2 SH groups per mole of enzyme from attack by 

 36 mM iodoacetate, and the loss of enzyme activity can be attributed to 

 the carboxymethylation of these SH groups when the enzyme is not com- 

 plexed with NADH (Li and Vallee, 1963). Treatment of the enzyme with 

 iodoacetate-1-C^^ followed by trypsin digestion allowed the isolation of a 

 single labeled polypeptide of the structure: 



Met— Val— Ala— Thr—Gly— lieu - Cys — Arg 



CH2 

 COO" 



suggesting that the apodehydrogenases contains two identical chains (Li 

 and Vallee, 1964). Zn++ is necessary for the activity but this peptide does 

 not bind Zn++, so this ion must be attached to an adjacent chain. 



