Catalyzes the second step of the reductive pyrimidine degradation, the reversible hydrolytic ring opening of dihydropyrimidines. Can catalyzes the ring opening of 5,6-dihydrouracil to N-carbamyl-alanine and of 5,6-dihydrothymine to N-carbamyl-amino isobutyrate. Tissue specificity:Liver and kidney. Involvement in disease: Defects in DPYS are the cause of dihydropyrimidinase deficiency (DHPD). DHPD is an autosomal recessive disorder characterized by dihydropyrimidinuria and associated with a variable clinical phenotype: epileptic or convulsive attacks, dysmorphic features and severe developmental delay, and congenital microvillous atrophy.