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 (PubMed:16156897, PubMed:25568314). Plays a role in dorso-ventral axis specification during early development, together with glcea, where it may potentiate signaling via the BMP pathway (PubMed:16156897). [heparosan-N-sulfate](n) = [heparan-N-sulfate](n) Inhibited by O-sulfated heparin, which is produced by a downstream step in the heparin biosynthesis pathway. Glycan metabolism; heparan sulfate biosynthesis. Glycan metabolism; heparin biosynthesis. Homodimer. Detected from 0.5 to 48 hours post-fertilization (hpf) (PubMed:16156897). At 24-36 hpf, expressed in adaxial mesoderm, somites, notochord, floorplate, hypochord, tail bud, eye, telencephalon, diencephalon, midbrain, hindbrain and otic vesicle (PubMed:17195182). Double morpholino knockdown of both glcea and glceb results in dorsalized embryos, with reduced ventral tail fin, kinked tail and an enlarged heart cavity. In severe cases there is significant shortening of the body and tail coiling. Epimerase activity is reduced. Expression of the dorsalizing factor chordin is expanded, whereas expression of bmp2b is reduced, notably in the tail. Expression of gata1 in blood islets is also reduced. Belongs to the D-glucuronyl C5-epimerase family. It is uncertain whether Met-1 or Met-2 is the initiator. Truncated N-terminus. Truncated N-terminus.