Converts D-glucuronic acid to L-iduronic acid (IdoUA) residues. Plays an important role in the biosynthesis of the glycosaminoglycan/mucopolysaccharide dermatan sulfate. chondroitin 4'-sulfate = dermatan 4'-sulfate Also has weak activity in the presence of Mg(2+) or Ca(2+) ions. Glycan metabolism; chondroitin sulfate biosynthesis. Glycan metabolism; heparan sulfate biosynthesis. N-glycosylated. Glycosylation is important for enzymatic activity. Belongs to the dermatan-sulfate isomerase family.