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Structure and Properties of the Recombinant NADH–Cytochrome b 5 Reductase of Physarum polycephalum

A cDNA for NADH–cytochrome b 5 reductase of Physarum polycephalum was cloned from a cDNA library, and the nucleotide sequence of the cDNA was determined (accession no. AB259870). The DNA of 943 base pairs contains 5′- and 3′-noncoding sequences, including a polyadenylation sequence, and a coding sequence of 843 base pairs. The amino acid sequence (281 residues) deduced from the nucleotide sequence was 25 residues shorter than those of vertebrate enzymes. Nevertheless, the recombinant Physarum enzyme showed enzyme activity comparable to that of the human enzyme. The recombinant Physarum enzyme showed a pH optimum of around 6.0, and apparent K m values of 2 μM and 14 μM for NADH and cytochrome b 5 respectively. The purified recombinant enzyme showed a typical FAD-derived absorption peak of cytochrome b 5 reductase at around 460 nm, with a shoulder at 480 nm. These results suggest that the Physarum enzyme plays an important role in the organism.