NADPH-Dependent Reactions

Typically, the P450-dependent metabolism of eicosanoids results in the hydroxylation of their 

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Glucuronate is reduced to L-gulonate in an NADPH-dependent reaction L-gulonate is the direct precursor of ascorbate in those animals capable of synthesizing this vitamin. In humans and other primates as well as guinea pigs, ascorbic acid cannot be synthesized because of the absence of L-g ulonolactone oxidase. L-Gulonate is metabolized ultimately to D-xylulose 5-phosphate, a constituent of the pentose phosphate pathway. 

Enoate reductase reduces a,/3-unsaturated carboxylate ions in an NADPH-dependent reaction to saturated carboxylated anions. Useful chiral synthons can be conveniently prepared by the asymmetric reduction of a triply substituted C—C bond by the action of enoate reductase, when the double bond is activated with strongly polarizing groups [22]. Enoate reductases are not commercially available as isolated enzymes therefore, microorganisms such as baker s yeast or Clostridium sp. containing enoate reductase are used to carry out the reduction reaction. 

Flavan-3,4-diols FIavan-3,4-diols, also known as leucoanthocyanidins, are not particularly prevalent in the plant kingdom, instead being themselves precursors of flavan-3-ols (catechins), anthocyanidins, and condensed tannins (proanthocyanidins) (see Fig. 5.4). Flavan-3,4-diols are synthesized from dihydroflavonol precursors by the enzyme dihydroflavonol 4-reductase (DFR), through an NADPH-dependent reaction (Anderson and Markham 2006). The substrate binding affinity of DFR is paramount in determining which types of downstream anthocyanins are synthesized, with many fruits and flowers unable to synthesize pelargonidin type anthocyanins, because their particular DFR enzymes cannot accept dihydrokaempferol as a substrate (Anderson and Markham 2006). 

The natural substrate for SR (EC 1.1.1.153) is the PTP that is converted in an NADPH-dependent reaction into BH4. However, the activity of SR is assayed by measuring the conversion of the artificial substrate sepipaterin to BH2 in the presence of NADPH in the dark. BH2 is then oxidized in the presence of iodine at pH 1.0 to biopterin, which is detected fluorimetrically at 350/440 nm upon HPLC separation. The assay is based, with some modifications, on the method by Ferre and Naylor [24]. 

Nitric oxide is synthesized from arginine in an NADPH-dependent reaction catalyzed by nitric oxide synthase (Fig. 22-31), a dimeric enzyme structurally related to NADPH cytochrome P-450 reductase (see Box 21-1). The reaction is a five-electron oxidation. Each subunit of the enzyme contains one bound molecule of each of four different cofactors FMN, FAD, tetrahydro-... 

BH4 biosynthesis proceeds in the de novo pathway in a Mg -, Zn -, and NADPH-dependent reaction from GTP through the two intermediates, 7,8-dihydroneopterin triphosphate (H2NTP, 7) and 6-pyruvoyl-5,6,7,8-tetrahydropterin (PTP, 42), which have been isolated although they are rather unstable. The three enzymes... 

The flavoprotein monooxygenase or MFMO enzyme is indicated to be present in sponges (Kurelec et al. 1985, 1987). Post-mitochondrial supernatants of G. cydonium, V. aerophoba, P. semitubulosa and T. aurantium readily activated (metabolized) 2-aminoanthracene (AA) to S. typhinurium TA98 mutagens in an NADPH-dependent reaction. T. aurantium also activated aminobiphenyl, 2-aminofluorene (AF) and 2-acetylaminofluorene (AAF). 

The last route is important for the formation of jasmonic acid. The action of allene oxide synthase on 13-hydroperoxy linolenic acid initially results in the generation of 12,13-epoxy-octadecatrienoic acid. This unstable epoxide is either chemically hydrolyzed to a- and y-ketols and racemic 12-oxo-phytodienoic acid or, in the presence of allene oxide cyclase (AOC), is further converted to enantiomeric pure 12-oxo-PDA [7]. The ring double bond of PDA is then reduced in a NADPH dependent reaction by 12-oxo-PDA reductase and after shortening of the side chain containing the carboxy group by 3 rounds of fi>-oxidation, the biosynthesis of jasmonic acid is completed. 

Liver microsomes from rabbits (or from rats and mice) effect oxidative sequential dealkylation of Pb(C2H5)4. The dealkylation is assumed to involve hydroxylation at the one position and alkene liberation [272]. In NADPH-dependent reactions, Pb(C2H5)4 is converted into [Pb(C2H5)3] by rabbit liver microsomes [343]. 

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