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Rabbit liver cytosol produced approximately equal amounts of 6alpha-naloxol and 6beta-naloxol from naloxone in the presence of NADPH at pH 7.4, and contained at least four forms of naloxone reductase. The two major forms, NR1 and NR2, which catalyze the stereospecific reduction of naloxone to 6alpha-naloxol and 6beta-naloxol, respectively, were purified to apparent homogeneity by various chromatographic techniques. Both enzymes are monomeric proteins with similar molecular weights of 35000-36000, but NR1 is a basic protein with an isoelectric point (pI) of 9.3, while NR2 is an acidic protein (pI of 5.9). NR1 and NR2 gave the maximal activities at pH 8.0 and 6.1, respectively. NR1 exhibited considerable activity with NADH as well as with NADPH, whereas NR2 showed highly restricted specificity for NADPH. The Km and Vmax values of NR1 and NR2 for naloxone were 1.0 and 0.06 mM, and 76 and 162 munits/mg, respectively. In addition to naloxone, naltrexone and dihydromorphinone served as good substrates for NR2 but were poor substrates for NR1. Both enzymes reduced aliphatic and aromatic aldehydes, cyclic and aromatic ketones, and quinones at higher rates. The two enzymes catalyzed the dehydrogenation of 17beta-hydroxysteroids with low Km values, and NR2 showed an additional 3alpha-hydroxysteroid dehydrogenase activity. Amino acid sequence data of NRI (99% of whole protein) and NR2 (66%) showed that both enzymes belong to the aldo-keto reductase (AKR) superfamily and can be classified into the AKR1C subfamily. These findings therefore indicate that they are new members of the AKR superfamily and may be involved physiologically in the steroid metabolism as well as in the detoxification of xenobiotic carbonyl compounds.

Purification and Characterization of Two Major Forms of Naloxone Reductase from Rabbit Liver Cytosol, New Members of Aldo-Keto Reductase Superfamily.