title: Peroxidase catalysed aerobic degradation of 5,6,7,8-tetrahydrobiopterin at physiological pH.
abstract: The aerobic degradation of 5,6,7,8-tetrahydrobiopterin at neutral pH is catalysed by peroxidase (EC 1.11.1.7) and provides quinonoid 7,8-dihydro(6H)biopterin which readily loses the side chain to yield 7,8-dihydro(3H)pterin. The latter is in equilibrium with trace amounts of 6-hydroxy-5,6,7,8-tetrahydropterin (covalent hydrate) which is irreversibly oxidised to quinonoid 6-hydroxy-7,8-dihydro(6H)pterin, and this finally rearranges to 7,8-dihydroxanthopterin. Spectroscopic evidence (ultraviolet, 1H NMR and 13C NMR) is presented for the reversible addition of water across the 5,6-double bond of 7,8-dihydro(3H)pterin. The intermediate quinonoid 6-hydroxy-7,8-dihydro(6H)pterin is a good substrate for dihydropteridine reductase (EC 1.6.99.7) with a Km of 16.3 microM and kcat of 22.5 s-1. The rate of aerobic degradation (oxidation and loss of the side chain) of natural (6R)-5,6,7,8-tetrahydrobiopterin is several times slower than the rate for the unnatural (6S) isomer. By using a modified assay procedure the kinetic parameters for dihydropteridine reductase are as follows: with (6R)-7,8-dihydro(6H)biopterin Km = 1.3 microM and kcat = 22.8 s-1; with (6S)-7,8-dihydro(6H)biopterin Km = 13.5 microM and kcat = 51.6 s-1; and with (6RS)-7,8-dihydro(6H)neopterin Km = 19.2 microM and kcat = 116 s-1.
mesh_terms: D000332:Aerobiosis; D001708:Biopterins / Q000031:analogs & derivatives; D002384:Catalysis; D055598:Chemical Phenomena; D002621:Chemistry; D004093:Dihydropteridine Reductase / Q000378:metabolism; D006735:Horseradish Peroxidase; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000201:enzymology; D010084:Oxidation-Reduction; D010544:Peroxidases; D011621:Pteridines; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism
chemical_list: D011621:Pteridines; D001708:Biopterins; D006735:Horseradish Peroxidase; D010544:Peroxidases; D004093:Dihydropteridine Reductase; C003402:sapropterin