CARKD

Abstract: Hydration of NAD(P)H to NAD(P)HX, which inhibits several dehydrogenases, is corrected by an ATP-dependent dehydratase and an epimerase recently identified as the products of the vertebrate Carkd (Carbohydrate kinase domain) and Aibp (Apolipoprotein AI binding protein) genes, respectively. Our purpose was to assess the presence of these enzymes in mammalian tissues and determine their subcellular localisation. The Carkd gene encodes proteins with a predicted mitochondrial propeptide (mCARKD), a signal peptide (spCARKD), or neither of them (cCARKD). Confocal microscopy analysis of transfected CHO cells indicated that cCARKD remains in the cytosol, whereas mCARKD and spCARKD are targeted to the mitochondria and the endoplasmic reticulum, respectively. Unlike the other two forms, spCARKD is N-glycosylated, supporting its targeting to the endoplasmic reticulum. The Aibp gene encodes two different proteins, which we show to be targeted to the mitochondria (mAIBP) and the cytosol (cAIBP). Quantification of the NAD(P)HX dehydratase and epimerase activities in rat tissues, performed after partial purification, indicated that both enzymes are widely distributed, with total activities of ≈ 3-10 nmol/min/g tissue. Liver fractionation by differential centrifugation confirmed the presence of the dehydratase and the epimerase in the cytosol and in mitochondria. These data support the notion that NAD(P)HX repair is extremely widespread.