Can the DCoHα isozyme compensate in patients with 4a-hydroxy-tetrahydrobiopterin dehydratase/DCoH deficiency?

4a-Hydroxy-tetrahydrobiopterin dehydratase/DCoH is a bifunctional protein. In the cytoplasm it is an enzyme required for the regeneration of tetrahydrobiopterin, an essential cofactor for phenylalanine hydroxylase. In the nucleus it functions as a transcriptional coactivator by forming a 2:2 heterotetramer with the hepatic nuclear factor HNF1α (HNF1). Patients with a deficiency of dehydratase activity have elevated levels of phenylalanine, and accumulate 7-pterins due to degradation of its substrate 4a-hydroxy-tetrahydrobiopterin. Curiously, the hyperphenylalaninemia is transient, and no defects in the transcriptional coactivator function have been reported. Recently, a human isozyme, dehydratase/DCoHα, has been detected which shares 60% identity with dehydratase/DCoH. This investigation was undertaken to ascertain if dehydratase/DCoHα has the pre-requisite properties to compensate in individuals lacking an active form of DCoH. DCoHα demonstrated the ability to quantitatively alter HNF1-dependent DNA-binding in vitro whereas DCoH was ineffective in vitro. This characteristic, due to the presence of dimeric DCoHα, demonstrates that DCoHα does not require any additional mammalian regulation process to alter DNA binding and therefore, may be more effective than DCoH at low concentrations. The dehydratase activity of each isoform was measured by a direct spectrophotometric assay. Km and Vmax for DCoHα were both 2-3 times higher than for DCoH, thus leaving the catalytic efficiency (Vmax/Km) the same for both enzymes. In conclusion, the properties of dehydratase/DCoHα are consistent with the hypothesis that the activity of this isozyme could account for the relatively mild symptoms reported for patients with a defect in dehydratase/DCoH.