Reduces ferric leghemoglobin (Lb) to ferrous Lb. 2 Fe(III)-[leghemoglobin] + NADH = 2 Fe(II)-[leghemoglobin] + H(+) + NAD(+) 2 Fe(III)-[leghemoglobin] + NADPH = 2 Fe(II)-[leghemoglobin] + H(+) + NADP(+) Binds 1 FAD per subunit. kcat is 3.1 sec(-1). Optimum pH is 6.5. Homodimer. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.