Nucleotide sugar hydrolase that catalyzes the fourth step in the biosynthesis of pseudaminic acid, a sialic-acid-like sugar that is used to modify flagellin. Mediates the removal of UDP from C-1 of UDP-2,4-diacetamido-2,4,6-trideoxy-beta-L-altropyranose forming 2,4-diacetamido-2,4,6-trideoxy-beta-L-altropyranose. H2O + UDP-2,4-diacetamido-2,4,6-trideoxy-beta-L-altrose = 2,4-diacetamido-2,4,6-trideoxy-beta-L-altrose + H(+) + UDP kcat is 27 sec(-1) (PubMed:16728396). kcat is 25 sec(-1) (PubMed:19483088). Monomer. Belongs to the PseG family.