D-AMINO ACID OXIDASE 



345 



and some of the other inhibitors may be written as: 



,/0 



Nicotinate 



^O 



2-Furancarboxylate 



HOH,C 



W A-""-""=V 



HCH^=CH-CH=C:f - 

 O 



O 



Cinnamate 



Crotonate 



Indole - 2 - carboxylate 



Such structures would be less possible or impossible for hydrocinnamate, 

 l,4-pyrone-2,6-dicarboxylate, cyclohexanecarboxylate, phenylacetate, and 

 some of the other weaker inhibitors. The orientation relative to the sub- 

 strate, according to Klein, would be represented as: 



+ 

 /C-CC 





Substrate 



Inhibitor 



where X is carbon, oxygen, or nitrogen. 



(3) Frisell and his co-workers, on the other hand, emphasized the impor- 

 tance of a double bond near the carboxylate group. Saturation of crotonate, 

 cinnamate, dimethylacrylate, and benzoate definitely reduces the inhibition. 

 They postulated that this double bond might correspond in position on the 

 enzyme to the double bond of the iminoketonic form of the dehydrogenated 

 product, and thus according to their theory the structural correspondence 

 would be: 



Product 



— C> 



^c-c: 



,o 



Benzoate 



-'^--c-cr" 



Aliphatic carboxylate 



(4) These two theories are not incompatible, since we have seen that 

 the presence of a positive charge generally depends on resonance, which in 

 turn requires double bonds and either conjugation or hyperconjugation. 



