Reductase transferring electrons from NADH to cytochrome b5. Required for the NADH-dependent electron transfer involved in the desaturation and hydroxylation of fatty acids and in the desaturation of sterol precursors. No activity with NADPH as electron donor. 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + H(+) + NAD(+) Monomer. Interacts with AKR2A (PubMed:20215589). Expressed in roots, stems, flowers and siliques. Detected in leaves. Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.