INHIBITION OF XANTHINE OXIDASE 

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287 



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Xanthopterin Isoxanthopterin Leucopterin 



Actually all the photolytic oxidation products of folate are inhibitory 

 but pterin-6-aldehyde, the primary product: 



Folate -> pterin-6-aldehyde -> pterin-6-carboxylate -> pterin -> isoxanthopterin 



is by far the most potent; indeed, it is one of the most potent inhibitors 

 known. 



Inhibition of xanthine oxidase by pterin-6-aldehyde is observed at a 

 concentration of 2 X 10^* //g/ml or roughly 10"^ M (Lowry et at., 1949 a). 

 Competitive inhibition with respect to both xanthine and pterin has been 

 demonstrated, and a /iC, of 6 x 10"' mM calculated for the milk enzyme 

 (Lowry ef al., 1949b). It was shown that 35% inhibition occurs when enzyme- 

 FAD = 9.3 X 10-6 jnM, the substrate pterin = 78 x 10-« mM, and pte- 

 rin-6-aldehyde = 2.26 X 10-« mM, this indicating that 2.26 X 10-« mM 

 inhibitor completely blocks 3.3 X 10-^ mM enzyme (on the basis of FAD 

 content). It may have been that all the FAD was not catalytically active 

 or that more than one FAD molecule was associated with one enzyme 

 molecule. However, there is no doubt that this is a mutual depletion system 

 and that pterin-6-aldehyde titrates the enzyme. A few other reports will 

 be mentioned to illustrate the potency. Both the milk and rat liver enzymes 

 are inhibited, 40-50% inhibition occurring at 5-8 X 10"^ mM when xan- 

 thine is 0.07 mM (Kalckar et al, 1950). The xanthine oxidase from Clostri- 

 dium cylindrosporum is potently inhibited by 0.0002 mM (Bradshaw and 

 Barker, 1960). Milk xanthine oxidase is completely inhibited by pterin-6- 

 aldehyde at 0.0033 mM when hypoxanthine is 3.33 mM (Petering and 

 Schmitt, 1950), and the rat intestine enzyme is inhibited completely by 

 0.067 mM when hypoxanthine is 6.6 mM (Westerfeld and Richert, 1952). 

 In many experiments the substrate concentrations have been unnecessarily 

 high since maximal rates are usually obtained at concentrations well below 



