INHIBITION OF XANTHINE OXIDASE 289 



ertheless effective inhibitors of xanthine oxidase. The following tabulation 

 shows the inhibitions after 20 min incubation at pH 8.5 when the substrate 



concentration is 0.063 mM (Hofstee, 1949). The potency relative to pterin- 

 6-aldehyde is not seen here since 0.001 mM inhibits 82% under these con- 

 ditions. From the K„, for xanthine of 0.02 mM (Hofstee, 1955), a K^ for 

 pterin-6-aldehyde of 5.1 X 10"^ mM may be calculated, which is higher 

 than was obtained by Lowry et al. (1949 b). The inhibition by xanthopterin 

 appears to be completely competitive (i^,„ for xanthine 0.0053 milf , and K, 

 = 0.0016 mM), and xanthopterin-7-carboxylate is of comparable potency 

 (Krebs and Norris, 1949). Bovine serum xanthine oxidase is inhibited 94% 

 by 0.052 mM pterin-6-aldehyde but only 3% by xanthopterin at a com- 

 parable concentration (Villela et al., 1956). There is no doubt that the al- 

 dehyde group at the 6-position confers a strong affinity for the enzyme, 

 since when it is reduced to a CHgOH group, oxidized to a COO" group, 

 or altered to a CHg group, the inhibitory activity is markedly reduced (Pe- 

 tering and Schmitt, 1950). Pterin-6-aldehyde is bound to xanthine oxidase 

 approximately 4.9 kcal/mole more tightly than xanthopterin, and it would 

 appear that pterin itself is bound somewhat less tightly than xanthopterin. 

 It is tempting to relate the augmenting action of the aldehyde group to 

 the fact that xanthine oxidase oxidizes simple aldehydes. There must be 

 a site on the enzyme capable of reacting with aldehyde groups, and it is 

 possible that the pterin-6-aldehyde is bound in a configuration such that 

 the aldehyde group interacts in this manner. Support for this interpretation 

 comes from the observation by Lowry et al. (1949 b) that pterin-6-aldehyde 

 is slowly oxidized to pterin-6-carboxylate by xanthine oxidase. 



