Inhibition of urease by copper salts in the presence of ascorbic acid and related substances.

Elson (1943) found that urease is inhibited by small amounts of ascorbic acid and that this inhibition is eliminated in the presence of cysteine. Quastel (1943) suggested that the inhibition was due to a reaction between dehydroascorbic acid and an -SH group of the enzyme, and that the protective effect of thiol compoundcls was due to the fact that they reduced dehydroascorbic acid. Giri & Seshagiri Rao (1944), however, did not find that dehydroascorbic acid inhibited urease, and since they observed that there was no inhibition of the enzyme by ascorbic acid in the presence of substances which form complexes with copper (8-hydroxyquinoline and diethyl-dithio-carbamate), they suggested that the inhibition might be due to the formation of intermediary substances like Cu2O, produced during the oxidation of the vitamin. Our experiments show that neither ascorbic, nor its oxidation products, dehydroascorbic acid or 2:3-diketo-gulonic acid, themselves inhibit urease. Ascorbic acid and 2:3-diketo-gulonic acid, however (but not dehydroascorbic acid), act as inhibitors in the presence of Cu++. Cu+ inhibits urease to a much greater extent than does Cu++, and the increased inhibition observed when ascorbic acid is added to a solution of the enzyme containing Cu++ is due, it is therefore concluded, to the reduction of Cu++ to Cu+.