Human NRDRB1, an Alternatively Spliced Isoform of NADP(H)-Dependent Retinol Dehydrogenase/Reductase Enhanced Enzymatic Activity of Benzil

Abstract

Aims: Human NRDRB1, a 226 amino acid alternatively spliced isoform of the NADP(H)- dependent retinol dehydrogenase/reductase (NRDR), lacks the complete coding region of exon 3, but preserves all the important functional motifs for NRDR catalytic activity. Nevertheless, its tissue distribution and physiological function remain to be elucidated. Methods: Expression of NRDRB1 and NRDR in cells and tissues was analyzed by semi-quantitative polymerase chain reaction (PCR) and western blot. NRDRB1 was expressed as a His₆ fusion protein and subjected to kinetics assays. Results: Recombinant NRDRB1 had 1.2 to 8.6 fold higher k_cat/K_m values than recombinant NRDR, depending on the substrate. NRDRB1 catalyzed the NADPHdependent reduction of α-dicarbonyl compounds, such as isatin, 9,10-phenanthrenequinone, and especially benzil. The significantly high catalytic activity and the relatively high expression in human liver of NRDRB1 conferred cellular resistance to benzil-induced cell toxicity and over-expression of NRDRB1 in low expressing Ec109 cells significantly enhanced cell tolerance toward benzil. Conclusions: Based on its substrate specificity, catalytic activity and relatively high expression in human liver tissue, our results suggest that NRDRB1, an alternatively spliced isoform of NRDR in vivo functions better than NRDR as a dicarbonyl reductase for xenobiotics containing reactive carbonyls. Our study is the first reporting this phenomenon of the enzymes involved in biochemical reactions.