EFFECTS ON ENZYMES 395 



The similar distribution of the 2-C''* and 5-C^^ alloxans shows that the ureide 

 and malonic acid portions of the molecule are not split apart. Again, only 

 low activity was found in the pancreas. No evidence has emerged from any 

 of this work that alloxan is accumulated in the pancreas, but gross analyses 

 may not reveal the levels in the /5-cells. However, Landau and Kenold 

 (1954) obtained autoradiographs of tissues from rats given alloxan-2-C^^ 

 and could find only a rather uniform distribution in the tissues, the islets 

 containing no more than lung, liver, and spleen. Since the animals were 

 sacrificed 5 min after the alloxan injection, at a time when alloxan is pre- 

 sumably exerting its action on the /5-cells, it would appear that this is valid 

 evidence against selective uptake by the islets. 



EFFECTS ON ENZYMES 



Many of the results on enzyme inhibition are difficult to interpret be- 

 cause insufficient attention' has been paid to (1) the rapid disappearance 

 of alloxan during the incubation with the enzyme, (2) the fall in the pH 

 accompanying the formation of alloxanate, and (3) the possibility of in- 

 hibition by the products of alloxan breakdown. Under the usual conditions 

 of enzyme study, and at concentrations of alloxan below 5 mM, it is likely 

 that the action of alloxan on the enzyme is confined to a period not longer 

 than 2-3 min. Increases of inhibition after this could be due to secondary 

 changes in the enzyme or to effects exerted by the alloxan products. Al- 

 loxan may react with enzymes in the ways outlined for proteins (page 

 379) and, in addition, may inhibit by inactivating cofactors or by competing 

 with substrates or coenzymes because of structural similarity. Interest in 

 the inhibition of enzymes by alloxan lies either in the characterization of 

 functional SH groups or in the attempt to find a susceptible enzyme whose 

 inactivation would explain the diabetogenic activity. Some inhibitions are 

 summarized in Table 4-2. Probably some of these inhibitions would be 

 much greater if care had been taken to expose the enzymes to freshly 

 dissolved or neutralized alloxan. 



Alloxan can presumably either oxidize enzyme SH groups or react with 

 them to form a complex. If the inhibition is entirely due to oxidation, 

 it should be reversible upon adding cysteine or glutathione, provided that 

 no secondary inactivation has occurred subsequent to the formation of di- 

 sulfide bonds. In the earliest work on enzymes. Purr (1935) claimed that 

 the SH groups of papain and cathepsin are oxidized without affecting the 

 protein structure, since activity could be restored upon reduction. In 

 most instances, reactivation is partial — as for succinate dehydrogenase 

 (Hopkins et al., 1938), fructose- 1,6-diphosphatase (Walsh and Walsh, 

 1948), and the liver acetylating enzyme (Cooperstein and Lazarow, 1958) — 

 so it is difficult to interpret the results. That part of the inhibition due to 



