376 4. ALLOXAN 



Reactions with Thiols 



Alloxan can oxidize cysteine to cystine and GSH to GSSG, being simul- 

 taneously reduced to dialurate. The dialurate may complex with alloxan 

 to form alloxantin or, under aerobic conditions, may be reoxidized to al- 

 loxan. Reduction of alloxan by HgS yields dialurate when there is excess 

 H2S but alloxantin when the H2S is nearly equimolar with the alloxan 

 (Tipson and Cretcher, 1951). The reaction with cysteine anaerobically may 

 be written as: 



2 Alloxan + 2 cysteine -^ alloxantin + cystine 



It is interesting that Lieben and Edel (1932) found cysteine to be the most 

 reactive amino acid in producing the red-purple color with alloxan. They 

 concluded that the color reaction observed with proteins is due mainly 

 to the SH groups, inasmuch as it parallels the nitroprusside reaction in 

 various materials. Furthermore, urea-denatured ovalbumin reacts more 

 readily than the native protein, although this does not prove participation 

 of the SH groups. Actually, reduction of alloxan by SH groups alone would 

 not yield a colored product (unless perhaps NH4+ is present) and it is thus 

 doubtful if Lieben and Edel were measuring the reaction with SH groups. 

 The problem was made more complex, but also more interesting, by the 

 finding of Lazarow et al. (1948) that the reaction between alloxan and 

 glutathione gives rise to products with absorption maxima at 270 m// 

 and 305 m//; these peaks are not related to the same substance and can 

 vary independently. The peak at 305 m// occurs when equimolar amounts 

 of the reactants are mixed, even at concentrations as low as 0.2 mM. The 

 rate of the reaction is rapid, being 75% complete in 3 min at pH 7.4 and 

 370. Alloxan does not react with GSSG and dialurate does not react with 

 GSH. Thus, in addition to the oxidation of SH groups, there is another 

 reaction whereby a complex is formed. Complex formation also occurs with 

 cysteine when it is present in an equimolar ratio with alloxan, but if cysteine 

 is in excess, reduction of alloxan may be dominant (Patterson et al., 1949 b). 

 The product with the peak at 305 m// was called Complex 305 and its struc- 

 ture was represented by Lazarow as: 



HN 



OH 



^ 



O^ ^N" ^N^^^CO— glycine 

 glutamate 



Complex 305 



this being analogous to the reaction of alloxan with o-phenylenediamine 

 (see page 377). This structure was questioned by Resnick and Wolff (1956) 



