Converts D-glucuronic acid residues adjacent to N-sulfate sugar residues to L-iduronic acid residues, both in maturing heparan sulfate (HS) and heparin chains. This is important for further modifications that determine the specificity of interactions between these glycosaminoglycans and proteins. [heparosan-N-sulfate](n) = [heparan-N-sulfate](n) Glycan metabolism; heparan sulfate biosynthesis. Glycan metabolism; heparin biosynthesis. Homodimer (By similarity). Interacts with HS2ST1 (PubMed:11687650). Widely expressed with highest levels in lung and lowest levels in spleen. Perinatal lethality. Pups die shortly after birth, apparently from respiratory failure. Mice show immature lung structure, lack kidneys, have abnormally small thymus and spleen, defects in inguinal lymph node development and in blood vessel formation proximal to the inguinal lymph nodes, and display important skeletal deficiencies. Mutant mice have heparan and heparin chains that are deficient in iduronic acid residues, and that therefore have altered O-sulfation patterns. Belongs to the D-glucuronyl C5-epimerase family.