Catalyzes the oxidation of L-aspartate to iminoaspartate, the first step in the de novo biosynthesis of NAD(+) (PubMed:18226609, PubMed:23371294). Can also use L-asparagine, but not L-phenylalanine, L-glutamate, glycine, L-proline, L-alanine and D-aspartate (PubMed:23371294). L-aspartate + O2 = H2O2 + iminosuccinate Binds 1 FAD per subunit. Optimum pH is 8.0 (for L-aspartate oxidation) (PubMed:18226609). Optimum pH is about 10.0. Stable between pH 7.0 and 10.0 (PubMed:23371294). Optimum temperature is 65 degrees Celsius. Thermostable up to 80 degrees Celsius. Cofactor biosynthesis; NAD(+) biosynthesis; iminoaspartate from L-aspartate (oxidase route): step 1/1. Monomer. Folds into three domains: the FAD-binding domain, the capping domain and the C-terminal helical domain. Belongs to the FAD-dependent oxidoreductase 2 family. NadB subfamily.