Catalyzes the four-electron oxidation of UDP-N-acetyl-D-mannosamine (UDP-ManNAc), reducing NAD(+) and releasing UDP-N-acetylmannosaminuronic acid (UDP-ManNAcA). H2O + 2 NAD(+) + UDP-N-acetyl-alpha-D-mannosamine = 3 H(+) + 2 NADH + UDP-N-acetyl-alpha-D-mannosaminouronate Homodimer. The protomer folds into three distinct domains: an N-terminal nucleotide binding domain, a central helical domain involved in dimerization and a C-terminal substrate binding domain. Belongs to the UDP-glucose/GDP-mannose dehydrogenase family.