NADPH/NADP-system

The resulting thiol pair of the reduced thioredoxin is the reductant used for ribonucleotide reductase (Chapter 16). The standard redox potential E° of E. coli thioredoxin is -0.27 V, appropriately low for coupling to the NADPH / NADP+ system. 

Energy is coupled from catabolic sequences of energy-requiring activities of a cell by the ATP-ADP system. In a similar manner, reducing power is coupled by the NADPH-NADP+ system. 

Comparing this equation with the equation for the complete oxidation of palmitoyl-CoA (see table 18.1, equation 1), we find major differences in carriers and intermediates. The principal electron carrier in the anabolic pathway is the NADPH-NADP+ system in the catabolic pathway, /3 oxidation, the principal electron carriers are FAD-FADH2 and NAD+-NADH. The second striking difference between the two pathways is that malonyl-CoA is the principal substrate in the anabolic pathway but plays no role in the catabolic pathway. These differences reflect the fact that the two pathways do not share common enzymes. Indeed, in animal cells the reactions occur in separate cell compartments biosynthesis takes place in the cytosol, whereas catabolism occurs in the mitochondria. 

The isolated reductase (above) was used in assays to direct further fractionation experiments which culminated in the isolation of the physiological reducing system this turned out to be a previously unreci ized hydrogen transport system. This system, which connects ribonucleotide reductase to the NADPH-NADP+ system, was found to be a two-component system con.sisting of a small sulfhydryl protein, thioredoxin, and a flavoprotein, thioredoxin reductase. Thioredoxin is the reductant which specifically interacts with the ribonucleotide reductase. In the presence of catalytic amounts of thioredoxin, the thioredoxin reductase will link NADPH with the reduction of ribonucleoside diphosphates as follows ... 

The initial step of the oxidation of benzene to the epoxide has been widely studied (see [196, 217, 219, 220]) It is the reaction mediated by a mixed-function oxidase [196, 2i9] in the NADPH/NADP-system of the microsomes [220]. Cytochrome P-450, NADPH-reductase and phosphatide are important factors... 

Still another difference between biosynthesis of fatty acids and oxidation (in mammals) is that the former has an absolute requirement for NADPH (Fig. 17-12) while the latter requires NAD+ and flavo-proteins (Fig. 17-1). This fact, together with many other observations, has led to the generalization that biosynthetic reduction reactions usually require NADPH rather than NADH. Many measurements have shown that in the cytosol of eukaryotic cells the ratio [NADPH]/[NADP+] is high, whereas the ratio [NADH]/[NAD+] is low. Thus, the NAD+/NADH system is kept highly oxidized, in line with the role of NAD+ as a principal biochemical oxidant, while the NADP+/NADPH system is kept reduced. 

Protein concentrations are less than 0.1 mg/mL in two cases to avoid over-metabolism of the substrate. The buffer consists of potassium phosphate (50 mM, pH 7.4), MgCl2 (3 mM), EDTA (1 mM), and the NADPH-generating system [NADP (1 mM), glucose-6-phosphate (5 mM), and glucose-6-phosphate dehydrogenase (1 U/mL)]. The incubation time is five minutes for all assays. 

Aldehyde oxidase purified from maize coleoptiles is a multicomponent enzyme that contains a molybdenum cofactor, nonheme iron, and flavin adenine dinucleotide (FAD) as prosthetic groups.111 When substrate specificity of the aldehyde oxidase was tested, good activity was detected with IAAld, indole-3-aldehyde, and benzaldehyde among others. The addition of NADP and NADPH did not change the activity. In contrast, in maize endosperm, tryptophan-dependent IAA biosynthesis was dependent on an NADP/NADPH redox system, which may mean that the two tissues of maize are utilizing different pathways or different redox systems for IAA biosynthesis.112... 

Other less-studied defense enzymes NADPH oxidase "system 2O2 + NADPH + H+ = 2O2 + NADP+... 

If we accept that the redox state of nicotinamide nucleotides is a modulating system, we need to know how this works. An attractive idea involves the fact that the NADPH/NADP+ ratio is in equilibrium with the GSH/GSSG ratio which itself is of importance for the redox state and therefore the activity of functional proteins (Barron, 1951). 

NADPH/NADP+ and/or GSH/GSSG systems in insulin release (Ammon et al., 1979b Ammon and Verspohl, 1979). 

Additional information <2> (<2>, amplification of NADH without the influence of coexisting NAD", by phosphorylating NADH into NADPH and putting this NAPH through an NADP -NADPH cycling system [2]) [2]... 

Fig. 10. Hydrogen donor systems for ribonucleotide reduction. Enzyme reactions are I thioredoxin reductase (EC 1.6.4.5) II ribonucleotide reductase (EC 1.17.4) III glutathione reductase (EC 1.6.4.2). GSH, GSSG reduced and oxidized glutathione NADPH, NADP reduced and oxidized nicotinamide adenine dinucleotide phosphate coenzymes. The hydrogen transfer chain is continued in Fig. II...

A systematic study of the sulfoxidation by cyclohexanone monooxygenase from Acinetobacter using as the substrates many alkyl aryl sulfides, dialkyl sulfides and dialkyl disulfides has been carried out by Carrea, Colonna, and colleagues in the presence of NADP and a NADPH-regenerating system [143,144]. Chemical yields are good (at 0.8 mmol scale), with wide variations of ee s (from 0 to 98%) according to the structure of the sulfides. 

GSH can be regenerated from GSSG by the enzyme glutathione reductase (GR) linked to the NADPH/NADP+ (nicotinamide adenine dinucleotide phosphate) system. 

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