Characterization of human DHRS6, an orphan short chain dehydrogenase/reductase enzyme -: A novel, cytosolic type 2 R-β-hydroxybutyrate dehydrogenase

Human DHRS6 is a previously uncharacterized member of the short chain dehydrogenases/reductase family and displays significant homologies to bacterial hydroxybutyrate dehydrogenases. Substrate screening reveals sole NAD(+)-dependent conversion of (R)-hydroxybutyrate to acetoacetate with K-m values of about 10 mM, consistent with plasma levels of circulating ketone bodies in situations of starvation or ketoacidosis. The structure of human DHRS6 was determined at a resolution of 1.8 angstrom in complex with NAD(H) and reveals a tetrameric organization with a short chain dehydrogenases/ reductase-typical folding pattern. A highly conserved triad of Arg residues (triple R motif consisting of Arg(144), Arg(188), and Arg(205)) was found to bind a sulfate molecule at the active site. Docking analysis of R-beta-hydroxybutyrate into the active site reveals an experimentally consistent model of substrate carboxylate binding and catalytically competent orientation. GFP reporter gene analysis reveals a cytosolic localization upon transfection into mammalian cells. These data establish DHRS6 as a novel, cytosolic type 2 ( R)hydroxybutyrate dehydrogenase, distinct from its well characterized mitochondrial type 1 counterpart. The properties determined for DHRS6 suggest a possible physiological role in cytosolic ketone body utilization, either as a secondary system for energy supply in starvation or to generate precursors for lipid and sterol synthesis.