In enzymology, a dihydropyrimidinase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are 5,6-dihydrouracil and H<sub>2</sub>O, whereas its product is 3-ureidopropanoate.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amides. The systematic name of this enzyme class is 5,6-dihydropyrimidine amidohydrolase. Other names in common use include hydantoinase, hydropyrimidine hydrase, hydantoin peptidase, pyrimidine hydrase, and D-hydantoinase. This enzyme participates in 3 metabolic pathways: pyrimidine metabolism, beta-alanine metabolism, and pantothenate and coa biosynthesis.
As of late 2007, 10 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , and .