Catalyzes the oxidation of L-aspartate to iminoaspartate, the first step in the de novo biosynthesis of NAD(+) (PubMed:18959769). Can use either oxygen or fumarate as electron acceptors, which allows the enzyme to be functional under aerobic and anaerobic conditions (PubMed:18959769). L-aspartate + O2 = H2O2 + iminosuccinate fumarate + L-aspartate = iminosuccinate + succinate Binds 1 FAD per subunit. kcat is 0.18 sec(-1) for L-aspartate:oxygen oxidoreductase activity. kcat is 15.4 sec(-1) for fumarate reductase activity. kcat is 0.50 sec(-1) for L-aspartate:fumarate oxidoreductase activity. Cofactor biosynthesis; NAD(+) biosynthesis; iminoaspartate from L-aspartate (oxidase route): step 1/1. Monomer. Homodimer. Expression is repressed in the presence of nicotinic acid. Mutant requires exogenous nicotinic acid for growth. Belongs to the FAD-dependent oxidoreductase 2 family. NadB subfamily.