Eliminations enzymatic

Fixation by formaldehyde virtually eliminates enzymatic activity in a process that is sometimes reversible (vide infra). Dehydration and embedding significantly reduces the reversibility of this process, suggesting that dehydration and embedding facilitate additional chemical reactions that are not observed in aqueous solution. 

Enzymes not only produce characteristic and desirable flavor (79) but also cause flavor deterioration (80,81) (see Enzyme Applications, Industrial). The latter enzyme types must be inactivated in order to stabilize and preserve a food. Freezing depresses enzymatic action. A more complete elimination of enzymatic action is accompHshed by pasteurization. 

Enzymatic Gravimetric Methods for TDF, SDF, and IDF. These methods use an a-amylase and protease to remove starch and reduce protein. They differ from each other in the conditions for gelatinization of starch. Elimination of detergent permits recovery of soluble fiber, which is not possible with the detergent methods. 

Saturation kinetics are also called zero-order kinetics or Michaelis-Menten kinetics. The Michaelis-Menten equation is mainly used to characterize the interactions of enzymes and substrates, but it is also widely applied to characterize the elimination of chemical compounds from the body. The substrate concentration that produces half-maximal velocity of an enzymatic reaction, termed value or Michaelis constant, can be determined experimentally by graphing r/, as a function of substrate concentration, [S]. 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

The reported molar masses of polyesters obtained by enzymatic catalysis are relatively low, generally below 8000, except for polymers recovered by precipitation.336 This procedure results in the elimination of a soluble fraction consisting of low-molar-mass linear and cyclic oligomers.336 An Mw as high as 46,400 has thus been reported for a poly(tetramethylene decanedioate) obtained... 

From the results of the urease activity test summarized in Figure 15, it is clear that the deposition procedure preserved to a certain extent the enzyme catalytic activity. Heating the sample before testing decreased the enzyme in the film by about 30% but did not eliminate it completely. The results of the activity test of two samples are summarized in Table 1 together with reference values for a spontaneous reaction without enzyme. It is necessary to underline that enzymatic activity on spherical supports was higher than the respective value in flat films, which could indicate that apparent catalytic efficiency was improved due to an increased area-to-volume ratio. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

The metabolic and pharmacokinetic profile of sucralose (this is a novel intense sweetener with a potency about 600 times that of sucrose) in human volunteers was studied by Roberts and coworkers [82]. Part of this study was realized using PLC in the following chromatographic system in which the stationary phase was silica gel and the mobile phase was ethyl acetate-methanol-water-concentrated ammonia (60 20 10 2, v/v). Separated substances were scraped off separately, suspended in methanol, and analyzed by filtration, scintillation counting, or enzymatic assay. It was shown that the characteristics of sucralose include poor absorption, rapid elimination, limited conjugative metabolism of the fraction absorbed, and lack of bio-accumulative potential. 

FIGURE 5.2 Activation mechanism of a tripartite prodrug through enzymatic cleavage and elimination of an azaquinone methide. (See the color version of this figure in Color Plates section.)... 

Under the experiment conditions, the enzymatic cleavage occurs within seconds. Therefore, the observed release time of the tryptophan is also the actual disappearance time of the intermediate forms after the enzymatic cleavage. This dramatic enhancement of tail-unit release with the elimination-based system (dendritic molecule 19) compared to the cyclization-based system (dendritic molecule 18) is best viewed by superimposition of the graphs (Fig. 5.13). 

Following the enzymatic cleavage, azaquinone methide was rapidly eliminated and decarboxylation occurred, leading to internal cyclization that released a urea derivative and phenol 35. The latter was disassembled as previously described to generate two equivalents of phenol 36, which was further fragmented to release the four reporter groups. 

SCHEME 10.2 Common pathways of QM formation in biological systems, (a) Stepwise two-electron oxidation by cytochrome P450 or a peroxidase, (b) Enzymatic oxidation of a catechol followed by spontaneous isomerization of the resulting n-quinone. (c) Enzymatic hydrolysis of a phosphate ester followed by base-catalyzed elimination of a leaving group from the benzylic position. 

FIGURE 5.2 Activation mechanism of a tripartite prodrug through enzymatic cleavage and elimination of an azaquinone methide. 

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