Single enzymatic reactions reduction

In this section, we present the data on the enzymatic reactions of explosives relevant to the general cytotoxicity mechanisms of nitroaromatics (1) their single-electron enzymatic reduction to radicals accompanied by the formation of the reactive oxygen species (oxidative stress type of cytotoxicity) and (2) their two-electron reduction to nitroso and hydroxylamino metabolites causing the cytotoxicity by their covalent binding to proteins and DNA. 

The outstanding features of enzymatic reactions, e.g. high substrate specificity even in complex matrices, high reaction specificity, mild reaction conditions and reduction in waste product formation, are also of importance in the synthesis of single flavor compounds. Two additional factors have boosted the... 

Continuous enzymatic production of xylitol with simultaneous coenzyme regeneration in a charged membrane reactor was studied (210). An NADH dependent xylose reductase from C. tenuis catalyzed the reduction of xylose. This was coupled to enzymatic oxidation of glucose by glucose dehydrogenase from Bacillus cereus to make achievable an up to 10,000-fold regeneration of NADH per cycle of discontinuous conversion. Under suitoble conditions, 300 g/L of substrate could be converted in yields above 96% in one single batch reaction. 

As we have seen a stereoselective reaction is one in which there is a preponderance of one isomer irrespective of the stereochemistry of the reactant. The enzymatic reduction of pyruvic acid is stereoselective when the chiral molecules of the enzyme complexes with achiral pyruvic acid, they given a preponderance of one form of pyruvic acid-enzyme complex which then gives a single form of lactic acid. 

The kinetics of disappearance from the circulation of intravenously administered human insulin (Fig. 6.32) is nonlinear [145]. Within a few minutes after injection, it becomes localized in the liver, heart, and kidneys, where it is rapidly metabolized. Indeed, the hepatic extraction could be as high as 70% on a single passage, whereas kidneys could account for 10-40% degradation. Enzymatic reduction of the disulfide bridges appears to be the first step in the in vivo metabolism of insulin, although this reaction appears of limited significance under in vitro conditions. 

Figure 15-2 Absorption spectra of NAD+ and NADH. Spectra of NADP+ and NADPH are nearly the same as these. The difference in absorbance between oxidized and reduced forms at 340 nm is the basis for what is probably the single most often used spectral measurement in biochemistry. Reduction of NAD+ or NADP+ or oxidation of NADH or NADPH is measured by changes in absorbance at 340 nm in many methods of enzyme assay. If a pyridine nucleotide is not a reactant for the enzyme being studied, a coupled assay is often possible. For example, the rate of enzymatic formation of ATP in a process can be measured by adding to the reaction mixture the following enzymes and substrates hexokinase + glucose + glucose-6-phosphate dehydrogenase + NADP+. As ATP is formed, it phosphorylates glucose via the action of hexokinase. NADP+ then oxidizes the glucose 6-phosphate that is formed with production of NADPH, whose rate of appearance is monitored at 340 nm.

The radical nature of the oxygen molecule facilitates its reaction with various substrates to form radical species. Molecular oxygen can undergo a single electron reduction to form the superoxide (SO). SO can be converted in vivo to peroxides and various oxyradical species, including hydroxyl (OH), alkoxyl (RO), and peroxyl (ROO) radicals (Fig. 4.3). SO is usually disposed of in the body by enzymatic conversion to nonradical hydrogen peroxide (Fig. 4.4). 

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