Oxidants, enzyme inactivation

Green tea processing is designed to achieve a dry product exhibiting the desirable twisted leaf appearance, but without flavanol oxidation. This is accomplished by carrying out rapid enzyme inactivation either with steam in a rotating cylinder as practiced in Japan or with dry heat as practiced in the People s Republic of China. The inactivated tea is cooled, rolled partially dried, rerolled, and then completely dried. 

Taking into account that heat treatment inactivates some oxidative enzymes and causes the rupture of some cellular structures, greater extractability of carotenoids is expected to occur in processed foods. Therefore, when mild temperatures are applied, it is very common to obtain higher carotenoid content in a processed food as compared to its fresh counter part. For example, total... 

In earlier studies the in vitro transition metal-catalyzed oxidation of proteins and the interaction of proteins with free radicals have been studied. In 1983, Levine [1] showed that the oxidative inactivation of enzymes and the oxidative modification of proteins resulted in the formation of protein carbonyl derivatives. These derivatives easily react with dinitrophenyl-hydrazine (DNPH) to form protein hydrazones, which were used for the detection of protein carbonyl content. Using this method and spin-trapping with PBN, it has been demonstrated [2,3] that protein oxidation and inactivation of glutamine synthetase (a key enzyme in the regulation of amino acid metabolism and the brain L-glutamate and y-aminobutyric acid levels) were sharply enhanced during ischemia- and reperfusion-induced injury in gerbil brain. 

For Aspergillus niger extracellular endo-D-galacturonanase, the role of histidine in the enzyme reaction was investigated by the method of photo-oxidative inactivation, catalyzed by Methylene Blue.140 The inactivation of the enzyme was paralleled by the decomposition of histidine. The similarity of pH profiles, as well as the values of the rate constants of enzyme inactivation (4.0 X 10-2 min-1) and of decomposition of histidine (3.9 X 10-2 min-1), indicate that one of the five histidine residues present in the molecule of the enzyme141 is essential for its activity. 

H. Cai, F. P. Guengerich, Acylation of Protein Lysines by Trichloroethylene Oxide , Chem. Res. Toxicol. 2000,13, 327 - 335 H. Cai, F. P. Guengerich, Reaction of Trichloroethylene and Trichloroethylene Oxide with Cytochrome P450 Enzymes Inactivation and Sites of Modification , Chem. Res. Toxicol. 2001, 14, 451 - 458. 

Figure 7.15 Inhibition of acetyl-CoA carboxylase by cyclic AMP dependent protein kinase and AMP dependent protein kinase the dual effect of glucagon. Phosphorylation of acetyl-CoA carboxylase by either or both enzymes inactivates the enzyme which leads to a decrease in concentration of malonyl-CoA, and hence an increase in activity of carnitine palmitoyltransferase-I and hence an increase in fatty acid oxidation. Insulin decreases the cyclic AMP concentration maintaining an active carboxylase and a high level of malonyl-CoA to inhibit fatty acid oxidation.

MAO oxidizes amine substrates also by a one-electron route via the cyclopropylamine radical cation 8 which undergoes ready ring opening to the iminium radical cation 9 [11]. Then capture by a flavin radical, may cause the enzyme inactivation [12]. This mechanism was established by labeling experiments, Eq.(4) [13]. 

Reduction of Ozonized Lysozyme. Methionine sulfoxide can revert to methionine with the generation of the lytic activity for photo-oxidized lysozyme (14). We tested whether the ozonized lysozyme could be reactivated by chemical reduction. The ozonized lysozyme was treated with 2-mercaptoethanol, dialysed, purified by passage through a column of Sephadex G-25 and lyoph-ilized. The product showed no increase in its lytic activity. This is not surprising because residues other than methionine are oxidized, but it may be concluded that the oxidation of methionine alone cannot account for enzyme inactivation. 

Hypertensin is soluble in alcohol, glacial acetic acid, phenol, and water, and insoluble in ether (61). Because it is inactivated by tyrosinase it probably contains a catechol or phenol group, and by amine oxidase, an amine group on an a-carbon atom (Figure 2). Hypertensin is inactivated by certain phenolic, catecholic, and amine oxidases, by pepsin, trypsin, chymotrypsin, and carboxypeptidase, and by hypertensinase found in plasma. The nature of hypertensinase is unknown, but it is probably not an oxidative enzyme. Because it is heat-labile, hypertensinase can be removed from blood and renin preparations by heating hypertensin itself is heat-stable. Lack of pure preparations of hypertensin has delayed its further chemical identification. 

Khan KK, He YQ, Domanski TL, et al. Midazolam oxidation by cytochrome P450 3 A4 and active-site mutants an evaluation of multiple binding sites and of the metabolic pathway that leads to enzyme inactivation. Mol Pharmacol 2002 61(3) 495-506. 

In order to preserve enzyme activity during the reaction process, special attention must be paid on the substrates that cause direct or indirect enzyme inactivation. Since peroxide is a strong peroxidase inhibitor, a low peroxide/enzyme ratio must be selected. When treating phenolic compounds, the polymeric products obtained from the action of peroxidases also cause enzyme inactivation [9]. If the enzyme is inactivated, not only is the reaction hindered but, sometimes, there is a direct oxidation of the substrate by the peroxide, which causes an enantioselective reduction in some synthetic reactions [10, 11]. In these cases, an appropriate enzyme concentration and usually an adequate enzyme addition strategy are considered [8],... 

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