Bovine liver dihydropyrimidine amidohydrolase: purification, properties, and characterization as a zinc metalloenzyme

Beef liver dihydropyrimidine amidohydrolase has been purified to homogeneity using both an electrophoretic and a hydrophobic chromatographic method. The enzyme is a tetramer with a molecular weight of 226,000 g mol-1, a subunit molecular weight of 56,500 g mol-1, and contains 4 mol of tightly bound (Ks greater than or equal to 1.33 X 10(9) M-1) Zn2+ per mole of active enzyme. The enzyme appears to be a true Zn2+ metalloenzyme because there exists a direct proportionality between enrichment of Zn2+ and active enzyme during purification, there is an almost quantitative relationship between the loss of both enzyme activity and Zn2+ during 8-hydroxyquinoline-5-sulfonic acid treatment to form apoenzyme, Zn2+ and Co2+ reactivate dipicolinic acid-inhibited enzyme, and saturating concentrations of a substrate, dihydrothymine, protect against 8-hydroxyquinoline-5-sulfonic acid inhibition. EDTA does not inhibit the enzyme; however, 8-hydroxyquinoline-5-sulfonic acid, o-phenanthroline, and 2,6-dipicolinic acid cause a time-dependent loss in activity which follows pseudo-first-order kinetics. Analysis of the resulting kinetic data for these three chelators indicates that the reaction pathway involves the formation of an enzyme-Zn2+-chelator ternary complex which then dissociates to form apoenzyme and a Zn2+-chelator complex. Like other Zn2+ metalloenzymes, the enzyme is inhibited by a number of substituted sulfonamides. In the case of p-nitrobenzenesulfonamide, this inhibition is competitive in nature. Using the purified enzyme, kinetic constants were determined for a variety of dihydropyrimidines, ureidocarboxylic acids, and hydantoin substrates. Normal hyperbolic kinetics were observed for the hydrolysis of the cyclic compounds, but the cyclization of the ureidoacids showed biphasic kinetics and different values of Km can be estimated at either high or low concentrations of these substrates.