title: The time-dependent inactivation of human brain dihydropteridine reductase by the oxidation products of L-dopa.
abstract: Dihydropteridine reductase (DHPR) was irreversibly inactivated in a time-dependent way by incubation with 3,4-dihydroxyphenylalanine (L-dopa). The inactivation was oxygen-dependent; incubation under nitrogen gave partial protection. The inactivation was stimulated by the presence of horse-radish peroxidase/hydrogen peroxide. L-Dopa itself was not an inhibitor of DHPR although dopachrome, the aminochrome formed following oxidation of L-dopa, was a reversible inhibitor of DHPR with an I50 of 0.60 mM. The quinone products of oxidation of L-dopa were responsible for the time-dependent inactivation of DHPR. Adrenochrome also demonstrated a time-dependent inactivation of DHPR. Inactivation by adrenochrome demonstrated a saturation effect suggesting the reversible formation of a complex preceding inactivation. No radiolabel was incorporated into DHPR following inactivation by L-[14C]-dopa. Sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated the presence of a dimer of DHPR. A mechanism of inactivation involving the oxidative coupling of essential thiol groups was proposed to explain inactivation.
mesh_terms: D000323:Adrenochrome / Q000494:pharmacology; D001921:Brain / Q000201:enzymology*; D004093:Dihydropteridine Reductase / Q000037:antagonists & inhibitors*; D004220:Disulfides / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D045563:Indolequinones; D007211:Indoles / Q000494:pharmacology; D007980:Levodopa / Q000378:metabolism / Q000494:pharmacology*; D009243:NAD / Q000378:metabolism; D009247:NADH, NADPH Oxidoreductases / Q000037:antagonists & inhibitors*; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000494:pharmacology; D011809:Quinones / Q000494:pharmacology; D013997:Time Factors
chemical_list: D004220:Disulfides; D045563:Indolequinones; D007211:Indoles; D011809:Quinones; D009243:NAD; C001123:dopachrome; D007980:Levodopa; D000323:Adrenochrome; D006861:Hydrogen Peroxide; D010544:Peroxidases; D004093:Dihydropteridine Reductase; D009247:NADH, NADPH Oxidoreductases