INHIBITION OF XANTHINE OXIDASE 281 



analogs, are oxidized by the enzyme in the 8-position (Shaw and Woolley, 

 1952). Eqiiimolar concentrations of 2-azaadenine prolong the formation of 

 urate from xanthine 2-fold and from hypoxanthine 4-fold, the inhibition 

 being competitive. The kinetics in such situations may be complicated 

 by two factors: (1) the disappearance of the inhibitor (Shaw and Woolley 

 found, for example, that the azaadenine essentially all disappeared before 

 much urate was formed), and (2) the inhibition produced by the product 

 of the inhibitor oxidation. 8-Azapurine and all of its monohydroxyl and 

 monoamino derivatives are oxidized by xanthine oxidase and the products 

 are frequently inhibitory not only to xanthine oxidase but to other enzymes. 

 2-Amino-8-azapurine is converted to 8-azaguanine and hence can be used 

 as a precursor of this inhibitor (Bergmann et al., 1959). 



Certain inhibitions of xanthine oxidase by purine compounds are sum- 

 marized in Table 2.2 The inhibitions are not always competitive despite 

 the close similarity of substrate and inhibitor structures. Some of the simple 

 analogs are bound more tightly to the enzyme than are the normal sub- 

 strates. 6-Chloropurine and pyrazoloisoguanine are bound particularly well 

 and this brings up questions regarding the forces involved. Very little is 

 known about these forces. Ionic forces must be unimportant and it is pos- 

 sible that hydrogen bonds, coupled with an appropriate fit of the bonding 

 groups, play a major role. PjTazoloisoguanine is the 4-amino-6-hydroxy 

 derivative of pyrazolopyrimidine, and it is interesting to note that the 

 4-amino derivative is a very weak inhibitor relatively, as are the 4-methyl- 

 amino and l-methyl-4-amino derivatives (Feigelson et al., 1957). It has 

 been stated that there is some correlation between the potency of the xan- 

 thine oxidase inhibition and the carcinostatic activity of these and related 

 compounds. 



When inhibitory purine analogs are administered to animals it is often 

 difficult to determine the toxic mechanisms because of the multiple possible 

 sites for interference. The biological effects of 6-mercaptopurine seem to 

 be related to its conversion to the ribonucleotide, which inhibits inosinic 

 acid metabolism, rather than to any direct enzyme inhibition (Silberman 

 and Wyngaarden, 1961). On the other hand, it has been postulated that 

 8-azaguanine induces a guanine deficiency by inhibiting xanthine oxidase, 

 which operates in one guanine biosjoithetic pathway (i.e., hypoxanthine 

 -^ xanthine -^ guanine) (Feigelson and Davidson, 1956 a). It has been 

 shown in one instance that purine metabolism can be inhibited in vivo. 

 6-Chloropurine given to rats at 80 mg kg inhibits the formation of C^^Og 

 from xanthine-6-C^* about 40% when administered 20 min before the xan- 

 thine (Duggan et al., 1961). This is probably not due to a direct action 

 on xanthine oxidase but to the formation of 6-chlorourate and the resulting 

 inhibition of uricase. 6-Chloropurine also depresses the conversion of ace- 

 tate to lipid, of glycine to protein, and nucleic acid synthesis. 



