Catalyzes the oxidation of L-aspartate to iminoaspartate, the first step in the de novo biosynthesis of NAD(+) (By similarity). Can use either oxygen or fumarate as electron acceptors, which allows the enzyme to be functional under aerobic and anaerobic conditions (By similarity). L-aspartate + O2 = H2O2 + iminosuccinate fumarate + L-aspartate = iminosuccinate + succinate Binds 1 FAD per subunit. Cofactor biosynthesis; NAD(+) biosynthesis; iminoaspartate from L-aspartate (oxidase route): step 1/1. Belongs to the FAD-dependent oxidoreductase 2 family. NadB subfamily.