Characterization of rabbit morphine 6-dehydrogenase and two NAD(+)-dependent 3α(17ß)-hydroxysteroid dehydrogenases.

Mammalian morphine 6-dehydrogenase (M6DH)(1) converts morphine into a reactive electrophile, morphinone. M6DH belongs to the aldo-keto reductase (AKR) superfamily, but its endogenous substrates and entire amino acid sequence remain unknown. A recent rabbit genomic sequencing predicts three genes for novel AKRs (1C26, 1C27 and 1C28) that share >87% amino acid sequence identity and are similar to the partial sequence of rabbit liver M6DH. We isolated cDNAs for the three AKRs, and compared the properties of their recombinant enzymes. Like M6DH, only AKR1C26 that shares the highest sequence identity with hepatic M6DH oxidized morphine. The three AKRs showed NAD(+)-dependent dehydrogenase activity towards other non-steroidal alicyclic alcohols and 3α/17ß-hydroxy-C(18)/C(19)/C(21)-steroids, and their mRNAs were ubiquitously expressed in rabbit tissues. The kinetic constants for the substrates suggest that at least AKR1C26 and AKR1C28 act as NAD(+)-dependent 3α/17ß-hydroxysteroid dehydrogenases. AKR1C27 differed from AKR1C28 in its high K(m) values for the substrates and low sensitivity towards competitive inhibitors (ikarisoside A, hinokitiol, hexestrol and zearalenone), despite their 95% sequence identity. The site-directed mutagenesis of Tyr118 and Phe310 in AKR1C27 to the corresponding residues (Phe and Ile, respectively) in AKR1C28 produced an enzyme that was similar to AKR1C28, suggesting their key roles in ligand binding.

Modulation of activity and inhibitor sensitivity of rabbit aldose reductase-like protein (AKR1B19) by oxidized glutathione and SH-reagents.

Rabbit aldo-keto reductase (AKR) 1B19 is an ortholog of human aldose reductase-like protein (ARLP), AKR1B10, showing 86% amino acid sequence identity. AKR1B19 exhibits the highest catalytic efficiency for 4-oxo-2-nonenal, a major product of lipid peroxidation, compared to known reductases of this aldehyde. In this study, we found that the reductase activity of AKR1B19 was activated to about 5-fold immediately after the addition of 10 µM SH-reagents (p-chloromercuriphenylsulfonic acid and p-chloromercuribenzoic acid) in the absence or presence of NADPH. In addition, a maximum of 3-fold activation of AKR1B19 was induced by incubation with glutathione disulfide (GSSG) for 1h. The activated enzyme was converted into the native enzyme by further incubation with dithiothreitol and glutathione. The activation was abolished by the C299S mutation of AKR1B19, and the glutathionylated Cys299 was identified by mass spectrometry analysis. The Cys299-modified enzyme displayed different kinetic alterations depending on substrates and inhibitors. In the reduction of 4-oxo-2-nonenal, the catalytic efficiency was increased. Thus, AKR1B10 may be modulated by cellular ratio of GSSG/glutathione and more efficiently act as a detoxifying enzyme for the cytotoxic aldehyde under oxidatively stressed conditions. Furthermore, such an activity alteration by GSSG was not detected in AKR1B10 and rat ARLPs, suggesting the presence of a GSSG-binding site near Cys299 in AKR1B19.

Characterization of rabbit aldose reductase-like protein with 3ß-hydroxysteroid dehydrogenase activity.

In this study, we isolated the cDNA for a rabbit aldose reductase-like protein that shared an 86% sequence identity to human aldo-keto reductase (AKR)(1) 1B10 and has been assigned as AKR1B19 in the AKR superfamily. The purified recombinant AKR1B19 was similar to AKR1B10 and rabbit aldose reductase (AKR1B2) in the substrate specificity for various aldehydes and α-dicarbonyl compounds. In contrast to AKR1B10 and AKR1B2, AKR1B19 efficiently reduced 3-keto-5α/ß-dihydro-C19/C21/C24-steroids into the corresponding 3ß-hydroxysteroids, showing K(m) of 1.3-9.1 µM and k(cat) of 1.1-7.6 min(-1). The stereospecific reduction was also observed in the metabolism of 5α- and 5ß-dihydrotestosterones in AKR1B19-overexpressing cells. The mRNA for AKR1B19 was ubiquitously expressed in rabbit tissues, and the enzyme was co-purified with 3ß-hydroxysteroid dehydrogenase activity from the lung. Thus, AKR1B19 may function as a 3-ketoreductase, as well as a defense system against cytotoxic carbonyl compounds in rabbit tissues. The molecular determinants for the unique 3-ketoreductase activity were investigated by replacement of Phe303 and Met304 in AKR1B19 with Gln and Ser, respectively, in AKR1B10. Single and double mutations (F303Q, M304S and F303Q/M304S) significantly impaired this activity, suggesting the two residues play critical roles in recognition of the steroidal substrate.