362 



2. ANALOGS OF ENZYME REACTION COMPONENTS 



they readily bound to the enzyme since they inhibit weakly (Zeller, 1940). 

 The short aliphatic diamines are very poor substrates but inhibit quite 

 well; thus both ethylenediamine and trimethylenediamine inhibit the 

 oxidation of cadaverine by pig kidney diamine oxidase (Zeller, 1938). 

 If the amino groups of these inhibitors react with the same anionic enzyme 

 sites as does cadaverine, these anionic groups must be fairly close so that 

 cadaverine would have to assume a very bowed configuration. On the other 



NH3+ 

 \ ^ 

 CH, 

 \ 



CH, 



/ ' 



NH+ 



H,C- 

 ^H,N 



-CH, 



NH, 



(a) 



(b) 



(c) 



hand, the anionic groups may be separated by a distance corresponding 

 to the amino groups in cadaverine and the inhibitors react with only one 

 of the anionic sites ,the other amino group interacting with some anionic 

 site outside the active center (as in (c)). 



Certain guanidine derivatives are more potent inhibitors. Guanidine 

 itself is rather weak, inhibiting cadaverine (2 mM) oxidation 42% at 10 

 mM, but methylguanidine inhibits 63% at 1 mM (Zeller, 1938). Although 

 Zeller noted that in the pig kidney preparation methylguanidine did not 

 inhibit histamine oxidation very well, Waton (1956) found marked inhibi- 

 tion of histaminase activity in cat kidney, 0.01 mM inhibiting 42% and 

 0.1 mM 75%. The oxidations of putrescine and agmatine are both well 

 inhibited by methylguanidine (Zeller, 1940). An even more potent inhibitor, 

 however, is aminoguanidine. Apparently the diamine oxidases differ in 

 sensitivity to aminoguanidine; 50% inhibition is given by 0.00005 mM for 

 the enzyme from pig kidney (Schuler, 1952), by 0.001 mM for the enzyme 

 from cat kidney (Waton, 1956), by 0.01 mM for the enzyme from rabbit 

 liver (Kobayashi, 1957), and the enzyme from mouse liver is not inhibited 

 even by 0.1 mM. The nature of the inhibition is not clear, inasmuch as 

 aminoguanidine is also a derivative of hydrazine and might act by attacking 

 carbonyl groups: hydrazine and semicarbazide are, indeed, potent inhibitors 

 of diamine oxidase (Schuler, 1952; Waton, 1956). A further complication is 

 that aminoguanidine hydrolyzes to form semicarbazide and eventually 

 hydrazine. It behaves chemically more like a hydrazine than a guanidine, 

 and reacts with carbonyl groups without being hydrolyzed (Lieber and 

 Smith, 1939). It is also possible that the NHNHg group simulates the 

 CH2NH2 substrate group, as in the monoamine oxidase inhibitors, and forms 

 a tight bond to the enzyme. In vivo inhibition of diamine oxidase by amino- 



