Glucose-6 phosphate dehydrogenase

As mentioned in Section 11.3.5 for the case where the rate determining step is sensitive to both isotopic species, elucidation of the intrinsic isotope effects is not possible using the equations given thus far (if neither the reverse nor the forward commitment is zero). Even then, however, it is possible to solve for the intrinsic iso- 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown  

Isotope effects Observed Kinetic Equilibrium Evaluated intrinsic 

Equation 11.74 allows for an explicit solution for all five unknowns. The intrinsic values obtained are listed in Table 11.2 together with the experimental ones. In addition to these intrinsic values of kinetic isotope effects to be used in further analysis of the transition state structure, the commitments were calculated as Cf = 0.8 0.3 and cr = 0.5 0.3. 

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Chemiluminescence and bioluminescence are also used in immunoassays to detect conventional enzyme labels (eg, alkaline phosphatase, P-galactosidase, glucose oxidase, glucose 6-phosphate dehydrogenase, horseradish peroxidase, microperoxidase, xanthine oxidase). The enhanced chemiluminescence assay for horseradish peroxidase (luminol-peroxide-4-iodophenol detection reagent) and various chemiluminescence adamantyl 1,2-dioxetane aryl phosphate substrates, eg, (11) and (15) for alkaline phosphatase labels are in routine use in immunoassay analyzers and in Western blotting kits (261—266). 

Two or more linked enzyme reactions can lead to a change in the concentration of NADH or NADPH that is equivalent to the concentration of the original analyte. The reference glucose measurement using hexokinase [9001-51-8] and glucose-6-phosphate dehydrogenase [9001-40-5] is an example ... 

In this biosensor a quartz radio crystal is functionalized with the enzyme glucose-6-phosphate dehydrogenase. As shown in Figure 3, a thin film of Pmssian blue [14038-43-8] C gN gFe, is then coated onto the crystal. 

Fig. 4. Schematic of a multisequence biosensor in which the target glucose is first converted to glucose-6-phosphate, G6P, in the test solution by hexokinase. G6P then reacts selectively with glucose-6-phosphate dehydrogenase immobilized on the quartz crystal surface. Electrons released in the reaction then chemically reduce the Pmssian blue film (see Fig. 3), forcing an uptake of potassium ions. The resulting mass increase is manifested as a...

FIGURE 23.27 The glucose-6-phosphate dehydrogenase reaction is the committed step in the pentose phosphate pathway. 

Favism is the haemolysis obseived after eating Vica fava. This reaction is observed in individuals with glucose-6-phosphate dehydrogenase deficiency. This common deficiency is also responsible for haemolysis in response to the antimalarial drug primaquine and others. 

An idiosyncratic reaction is a harmful, sometimes fatal reaction, that occurs in a small minority of individuals. The reaction may occur with low doses of drags. Genetic factors may be responsible, e.g. glucose-6-phosphate dehydrogenase deficiency, although the cause is often poorly understood. 

Glucose- 6-phosphate dehydrogenase Low or absent enzyme activity in about 10% of African populations. Hemolysis following intake of a number of drugs which have electrophilic reactive metabolites, but also, carriers of this enzyme deficiency have a partial protection from malaria. 

Prasada Rao KS, Ramana Rao KV. 1987. The possible role of glucose-6-phosphate dehydrogenase in the detoxification of methyl parathion. Toxicol Lett 39 211-214. 

The use of glutaric dialdehyde as a coupling agent bound the enzymes trypsin or glucose-6-phosphate dehydrogenase to the surface. A large part of the enzymic activity was retained (Fig. 4), and the activity was such that the particle-enzyme conjugate could be used in laboratory scale continuous-flow reactors. 

FIGURE 4 Activity of glucose-6-phosphate dehydrogenase as a function of time for ( ) the enzyme immobilized on the polyphosphazene/ alumina support and (o) in the presence of free enzyme and non-activated support. (From Ref. 23.)... 

Allcock, H. R., and Kwon, S., Covalent linkage of proteins to surface-modified poly(organophosphazenes) Immobilization of glucose-6-phosphate dehydrogenase and trypsin, Macromolecules. 19, 1502, 1986. 

Figure 7-10. Coupled enzyme assay for hexokinase activity. The production of glucose 6-phosphate by hexokinase is coupled to the oxidation of this product by glucose-6-phosphate dehydrogenase in the presence of added enzyme and NADP". When an excess of glucose-6-phosphate dehydrogenase is present, the rate of formation of NADPH, which can be measured at 340 nm, is governed by the rate of formation of glucose 6-phosphate by hexokinase.

Enzymes of the pentost Glucose-6-phosphate dehydrogenase phospha t e pathwa nI randlipogenesis Insulin ... 

Genetic deficiency of glucose-6-phosphate dehydrogenase, with consequent impairment of the generation of NADPH, is common in populations of Mediterranean and Afro-Caribbean origin. The defect is manifested as red cell hemolysis (hemolytic anemia) when susceptible individuals are subjected to oxidants, such as the an-timalarial primaquine, aspirin, or sulfonamides or when... 

Glucose-6-phosphate dehydrogenase (G6PD) deficiency (MIM 305900) A variety of mutations in the gene (X-linked) for G6PD, mostly single point mutations... 

The pentose phosphate pathway is operative in the RBC (it metabolizes about 5-10% of the total flux of glucose) and produces NADPH hemolytic anemia due to a deficiency of the activity of glucose-6-phosphate dehydrogenase is common. 

Luzzato L et al Glucose-6-phosphate dehydrogenase. In The Metabolic and Molecular Bases of Inherited Disease, 8th ed. Scriver CR et al (editors). McGraw-Hill, 2001. 

It is an important intracellular reductant, helping to maintain essential SH groups of enzymes in their reduced state. This role is discussed in Chapter 20, and its involvement in the hemolytic anemia caused by deficiency of glucose-6-phosphate dehydrogenase is discussed in Ghapters 20 and 52. 

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