The mature form of rat adrenodoxin (Ad) was purified from a heterologous direct expression system in Escherichia coli with a yield of 56 mg/l culture. The purified Ad showed a A414/A280 ratio of 0.91 and the sequence of 10 amino terminal residues was identical with that of authentic rat Ad. By Time of Flight/Mass spectrometry, the molecular mass of purified Ad was identical to that calculated from the cDNA sequence and the carboxy terminal residue was estimated to be Ser, which was also as expected from the cDNA. These results indicate that the purified recombinant Ad is a precise mature form. In measurements of NADPH-cytochrome c reductase activity reconstituted with bovine adrenodoxin reductase (AdR), the apparent Km value for rat Ad was 46.9+/-2.5 nM, indicating a somewhat lower affinity for rat Ad to bovine AdR than for bovine Ad. On the other hand, the spectral Kd value for rat Ad to bovine cytochrome P-450scc was 0.46+/-0.05 microM, a value which was almost identical with that of the bovine counterpart.