Enzymatic Synthesis and Breakdown

Considerable evidence has accumulated from studies in vivo and with tissue slices indicating that erotic acid (4-carboxyuracil), or a closely related compound, is a precursor of the pyrimidines in nucleic acid (l-6). In order to study the pathway of erotic acid metabolism at the enzymatic level, a source material capable of metabolizing large amounts of erotic acid was sought. Such a system was found in an erotic acid-fermenting bacterium, Zymobacterium oroticum, isolated from mud by enrichment culture (7). Although fermentation studies with growing or resting cultures indicated a fairly extensive degradation of the pyrimidine to ammonia, CO?, acetic acid, and a dicarboxylic acid (or acids), the use of enzyme fractions from broken cell preparations of the organism resulted in the accumulation of more complex derivatives. With cell-free extracts and partially purified enzyme preparations, evidence has been obtained to support the accompanying scheme for erotic acid breakdown and synthesis. Reaction 1, the reversible reduction of erotic acid, is catalyzed by dihydroorotic dehydrogenase, the purification and properties of which have been described (7). The reversible hydrolysis of L-dihydroorotic acid to yield the acyclic L-ureidosuccinic acid (Reaction 2) is mediated by an enzyme to be referred to as dihydroorot,ase. In Reaction 3, L-ureidosuccinic acid is recyclized to L-5carboxymethylhydantoin; the enzyme responsible for this reaction will be referred to as 5-carboxymethylhydantoinase. The purpose of this report is to present evidence for Reactions 2 and 3 and to describe some of their properties.