Precursor to tetrahydrobiopterin, a required cofactor for a number of enzymes, including the biosynthesis of l-tyrosine; the inability to synthesize dihydrobiopterin can result in a form of malignant hyperphenylalaninemia.
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However, inflammation can also oxidate BH4 due to increasing production of ROS, which in turn forms dihydrobiopterin (BH2) and ultimately biopterin (B) [9].
BH4 concentrations, expressed as pmol/mg protein, were calculated by subtracting dihydrobiopterin (BH2) and oxidized biopterin from the total biopterins [30].
Furthermore, in course of catalytic function of the enzyme, B[H.sub.4] is converted to B[H.sub.3.sup.*] or dihydrobiopterin (B[H.sub.2]) which are no longer capable of reducing heme if they are not either reduced back to B[H.sub.4] or replenished by B[H.sub.4] [105,107].
Tetrahydrobiopterin (B[H.sub.4]) and dihydrobiopterin (B[H.sub.2]) were measured form cardiac homogenates, following the procedure described by Fukushima and Nixon [29], with modifications, by HPLC (Perkin Elmer series 200) separation and fluorescence detection at 350 nm (Shimadzu RF-20A).
Tetrahydrobiopterin (BH4) is one of the critical cellular non-enzymatic redox sensitive antioxidants and oxidation of BH4 to dihydrobiopterin (BH2) and other oxidized bio-pterin species causes endothelial nitric oxide synthases (eNOS) to produce higher superoxide levels instead of nitric oxide (NO), a phenomenon popularly termed as "eNOS uncoupling" resulting in increased oxidative stress (Crabtree et al., 2009).
Mancha de Llanos, "Determination of marker pteridins and biopterin reduced forms, tetrahydrobiopterin and dihydrobiopterin, in human urine, using a post-column photoinduced fluorescence liquid chromatographic derivatization method," Analytica Chimica Acta, vol.
Dihydrobiopterin reductase (DHPR) is a critically important enzyme in the BH4 cycle that is inhibited by Al, and calmodulin (CaM) is critically inhibited in the urea cycle.
For analysis of plasma levels of biopterin, dihydrobiopterin (BH2) and tetrahydrobiopterin (BH4), we used reversed phase HPLC with electrochemical detection and fluorescence detection.
eNOS: endothelial nitric oxide synthase; ROS: reactive oxygen species; NO: nitric oxide; [O.sub.2][??]: superoxide; BH4: tetrahydrobiopterin; BH2: dihydrobiopterin.
In the presence of 5-MTHF, the enzyme dihydrobiopterin reductase can reduce quininoid dihydrobiopterin to tetrahydrobiopterin (BH4) (12).
Cowley Jr., "NO synthase uncoupling in the kidney of Dahl S rats: role of dihydrobiopterin," Hypertension, vol.
Prenatal diagnosis of "dihydrobiopterin synthetase" deficiency, a variant form of phenylketonuria.