Purification and characterization of NAD-dependent morphine 6-dehydrogenase from hamster liver cytosol, a new member of the aldo-keto reductase superfamily

Morphine 6-dehydrogenase, which catalyzes the dehydrogenation of morphine to morphinone, was purified 815-fold to a homogeneous protein from the soluble fraction of hamster liver with a yield of 15%. The enzyme was a monomeric protein with a molecular weight of 38 kDa and an isoelectric point of 5.6. Although both NAD and NADP served as cofactors, the enzyme activity with NADP was less than 5% that found with NAD at pH 7.4. With NAD, the enzyme gave the maximal activity at pH 9.3, and the K(m) and V(max) values toward morphine were 1.0 mM and 0.43 unit/mg protein, respectively. Among morphine congeners, normorphine exhibited higher activity than morphine, but codeine and ethylmorphine were poor substrates, and dihydromorphine and dihydrocodeine showed no detectable activity. The enzyme also exhibited significant activity for a variety of cyclic and alicyclic alcohols. In addition to xenobiotics, the enzyme catalyzed the dehydrogenation of 17beta-hydroxysteroids with much higher affinities than morphine. In the reverse reaction, the enzyme exhibited high activity for o-quinones, but morphinone, naloxone, and aromatic aldehydes and ketones were reduced at slow rates. Sulfhydryl reagents and ketamine strongly inhibited the enzyme, whereas pyrazole, barbital, and indomethacin had little effect on enzyme activity. 17beta-Hydroxysteroids inhibited the enzyme in a competitive manner against morphine. A total of 302 amino acid residues, which comprised approximately 94% of whole protein, were identified by sequencing of the peptides obtained by proteolytic digestion. This amino acid sequence of the enzyme showed significant homology to members of the aldo-keto reductase (AKR) superfamily and shared 63-64% identity with members of the AKR1C subfamily. These findings indicate that the enzyme is a new member of the AKR superfamily that is involved in steroid metabolism as 17beta-hydroxysteroid dehydrogenase as well as xenobiotic metabolism.