Open AccessPeroxidase catalysed aerobic degradation of 5,6,7,8‐tetrahydrobiopterin at physiological pH
Author(s) -
ARMAREGO Wilfred L. F.,
RANDLES David,
TAGUCHI Hiroyasu
Publication year - 1983
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1983.tb07666.x
Subject(s) - pterin , chemistry , enzyme kinetics , tetrahydrobiopterin , biopterin , stereochemistry , peroxidase , substrate (aquarium) , cofactor , enzyme , organic chemistry , active site , oceanography , geology
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, 1 H NMR and 13 C 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 K m of 16.3 μM and k cat of 22.5s −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 K m = 1.3μM and k cat = 22.8s −1 ; with ( 6S )‐7,8‐dihydro( 6H )biopterin K m = 13.5μM and k cat = 51.6s −1 ; and with (6 RS )‐7,8‐dihydro( 6H )neopterin k m = 19.2μM and k cat = 116s −1 .