title: Catalytic characterization of 4a-hydroxytetrahydropterin dehydratase.
abstract: The cofactor product of the aromatic amino acid hydroxylases, 4a-hydroxy-6(R)-tetrahydrobiopterin, requires dehydration before tetrahydrobiopterin can be regenerated by dihydropteridine reductase. Carbinolamine dehydration occurs nonenzymatically, but the reaction is also catalyzed by 4a-hydroxytetrahydropterin dehydratase. This enzyme has the identical amino acid sequence to DCoH, the dimerization cofactor of the transcription regulator, HNF-1 alpha. The catalytic activity of rat liver dehydratase was characterized using a new assay employing chemically synthesized 4a-hydroxytetrahydropterins. The enzyme shows little sensitivity to the structure or configuration of the 6-substituent of its substrate, with Km's for 6(S)-methyl, 6(R)-methyl, 6(S)-propyl, and 6(R)-L-erythro-dihydroxypropyl all between 1.5 and 6 microM. Turnover numbers at 37 degrees C are 50-90 s-1 at pH 7.4 and 2.5-3-fold lower at pH 8.4. Both 4a(R)- and 4a(S)-hydroxytetrahydropterins are good substrates. The quinoid dihydropterin products are strong inhibitors of the dehydratase with KI's about one half of their respective Km's, but no inhibition was observed with 7,8-dihydropterins or tetrahydropterins. The enzyme contains no metals and no phosphorus. Reaction mechanisms which involve either acid and/or base catalysis are discussed. 4a-Hydroxy-6(R)-tetrahydrobiopterin was determined not to be a product inhibitor of phenylalanine hydroxylase. It is concluded that the dehydratase (which was found to be 6 microM in rat liver) is essential in vivo to prevent rearrangement of 4a-hydroxy-6(R)-tetrahydrobiopterin and to maintain the supply of tetrahydrobiopterin cofactor for the hydroxylases under conditions where the nonenzymatic rate would be inadequate.
authors.affiliation: Department of Pharmacology, College of Medicine, University of South Alabama, Mobile 36688, USA.
authors.forename: I
authors.initials: I
authors.lastname: Rebrin
authors.forename: S W
authors.initials: SW
authors.lastname: Bailey
authors.forename: S R
authors.initials: SR
authors.lastname: Boerth
authors.forename: M D
authors.initials: MD
authors.lastname: Ardell
authors.forename: J E
authors.initials: JE
authors.lastname: Ayling
mesh_terms: D000818:Animals; D001708:Biopterins / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D002384:Catalysis; D002417:Cattle; D006801:Humans; D006836:Hydro-Lyases / Q000032:analysis / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008670:Metals / Q000032:analysis; D009243:NAD / Q000494:pharmacology; D009994:Osmolar Concentration; D010651:Phenylalanine Hydroxylase / Q000037:antagonists & inhibitors / Q000378:metabolism; D010758:Phosphorus / Q000032:analysis; D011622:Pterins / Q000378:metabolism; D051381:Rats; D013379:Substrate Specificity; D013816:Thermodynamics
chemical_list: D008670:Metals; D011622:Pterins; D009243:NAD; C011552:tetrahydropterin; D001708:Biopterins; D010758:Phosphorus; C017226:7,8-dihydrobiopterin; C093282:4a-hydroxytetrahydrobiopterin; D010651:Phenylalanine Hydroxylase; D006836:Hydro-Lyases; C076392:pterin-4a-carbinolamine dehydratase