Imidase, a Dihydropyrimidinase‐Like Enzyme Involved in the Metabolism of Cyclic Imides

Imidase, which preferably hydrolyzed cyclic imides to monoamidated dicarboxylates, was purified to homogeneity from a cell‐free extract of Blastobacter sp. A17p‐4. Cyclic imides are known to be hydrolyzed by mammalian dihydropyrimidinases. However, imidase was quite different from known dihy‐dropyrimidinases in structure and substrate specificity. The enzyme has a relative molecular mass of 105000 and consists of three identical subunits. The purified enzyme showed higher activity and affinity toward cyclic imides, such as succinimide (Km= 0.94 mM; Vmax= 910 μmol · min‐1· mg‐1) glutarimide (Km= 4.5 mM; Vmax= 1000 μmol · min‐1· mg‐1) and maleimide (Km= 0.34 mM; Vmax= 5800 μmol · min‐1· mg‐1), than toward cyclic ureides, which are the substrates of dihydropyrimidinases, such as dihydrouracil and hydantoin. Sulfur‐containing cyclic imides, such as 2,4‐thiazolidinedione and rhodanine, were also hydrolyzed. The enzyme catalyzed the reverse reaction, cyclization, but with much lower activity and affinity. The enzyme was non‐competitively inhibited by succinate, which was found to be a key compound in cyclic‐imide transformation in relation with the tricarboxylic acid cycle in this bacterium, suggesting that the role of imidase is to catalyze the initial step of cyclic‐imide degradation.