Enzyme system, copper

In addition to all of the expected enzyme systems present in leaf tissue, fresh tea leaves contain a high level of polyphenol oxidase that catalyzes the oxidation of the catechins by atmospheric oxygen. Tea polyphenol oxidase exists as series of copper-containing (0.32%) isoenzymes. The major component has a molecular weight of about 144,000.54 The enzyme is concentrated in the leaf epidermis.55 Soil copper deficiency is sometimes responsible for inadequate oxidation during processing.56... 

Recent work in our laboratories has confirmed the existence of a similar pathway in the oxidation of vindoline in mammals (777). The availability of compounds such as 59 as analytical standards, along with published mass spectral and NMR spectral properties of this compound, served to facilitate identification of metabolites formed in mammalian liver microsome incubations. Two compounds are produced during incubations with mouse liver microsome preparations 17-deacetylvindoline, and the dihydrovindoline ether dimer 59. Both compounds were isolated and completely characterized by spectral comparison to authentic standards. This work emphasizes the prospective value of microbial and enzymatic transformation studies in predicting pathways of metabolism in mammalian systems. This work would also suggest the involvement of cytochrome P-450 enzyme system(s) in the oxidation process. Whether the first steps involve direct introduction of molecular oxygen at position 3 of vindoline or an initial abstraction of electrons, as in Scheme 15, remains unknown. The establishment of a metabolic pathway in mammals, identical to those found in Strep-tomycetes, with copper oxidases and peroxidases again confirms the prospective value of the microbial models of mammalian metabolism concept. 

Pyrazole also causes, both in vitro and in vivo, a reduction in the activity of the mixed function oxidase enzyme system (B-79MI10504), and has also been used to inhibit the copper-containing enzyme monoamine oxidase in vitro, as have several related heterocyclic compounds (80MI10507) such as indazole (78), isoxazole (75) and benzothiazole (79). 

Once the fungicides penetrate to the cell membrane or into the cytoplasm they may operate by devious means to disrupt vital functions. There is substantial evidence that the quinones immobilize the sultliydryl and imino prosthetic group of enzymes. The 8-hydroxyquinoline and dithiocarbamate compounds are active against copper and other metallic members of an enzyme system, presumably hy their ability to chelate metals. Heavy metals such as mercury alTect certain enzymes such as amylases and may serve as general protein precipitants. 

As with copper, zinc is an essential element for all plants and animals. It is necessary for the correct function of various enzyme systems. However, excessive intakes of zinc can have long-term effects as they can interfere with absorption of copper and iron in the diet, and may result in anaemia.39 JECFA has recommended a PTDI of 1 mg/kg bodyweight, equivalent to 60 mg/day for a 60 kg adult.38 In food, the major contributors to the diet are meat and its products, from which zinc is readily absorbed. Liver, with concentrations of around 62 mg/kg, contains the highest levels of any meat products, with other tissues having values of a half to a third of this figure. The second greatest source of zinc is cereals. Concentrations of zinc in whole cereal products are similar to those in meat. 

Nitrite reduction in assimilatory nitrate-reducing Neurospora crassa, Torulopsis nitratophila, Azotobacter vinelandii, and Azotobacter chro-ococcum appears to be catalyzed by enzyme systems which require flavin and metals. The enzyme from N. crassa has been partially purified, and its molecular weight has been estimated to be 300,000 (344, 346, 351, 367). The enzyme reduces both nitrite and hydroxylamine to ammonia and utilizes NADH or NADPH as electron donor. It is reported to be a FAD-dependent enzyme and to contain iron, copper, and active thiol (346, 367). Three moles of NADH are oxidized per mole of nitrite reduced to ammonia. It has been suggested that the reduction of nitrite occurs in three steps, each involving two electrons. Thus, hyponitrite and hydroxylamine have been proposed as successive intermediates in the re-... 

Role of Copper in the Enzyme System. It is now well recognized among workers in the ultratrace-metal field that copper is an essential nutrient for all forms of life, being the vital constituent of all living cells (32), The essentiality of copper is by virtue of its ability to catalyze biological oxidation whether the copper is in the protein-boimd or ionic form, although it is likely that the ionic copper is able to do this more eflBciently than the protein-bound copper (33),... 

Copper plays an important role in the constituents of many enzymes of the mammalian organism as well as in plants and anthropods. Several classes of oxidizing enzymes for copper have been described, including the cytochrome oxidases which are the terminal oxidases in the mitochondrial electron transport system, a key reaction in energy metabolism (34), and the amine oxidases (35) of which there are a number that contain copper (36,37,38), Lysyl oxidase (39) is probably the most important since it plays a major role in elastin and collagen synthesis... 

Copper has been used as a pesticide since the 1700s. Copper sulfate has herbicidal and fungicidal properties It inactivates critical enzyme systems in fungi, algae, and other plants. 

Similar to maneb, mancozeb also has chelating properties, allowing it to possibly interfere with a number of enzyme systems that contain metals, such as zinc, copper, and iron (e.g., dopamine )S-hydro-xylase). 

These findings have important implications for methane oxidation in natural samples. First, they suggest that the pMMO is the predominant enzyme system for methane oxidation in natural populations and thus provide more impetus for understanding this enzyme system. Second, the response of natural populations to changes in methane concentrations will most likely depend on a complex set of parameters, of which available copper concentration may be the key. It is now important to study how methanotrophs utilize copper and how they respond to changes in copper and methane concentrations and to copper speciation, in order to predict how natural populations will respond to environmental perturbations. 

Several studies have shown that the reactive binding species generated by 1,1-dimethylhydrazine metabolism may be free radical intermediates. Rat liver microsomes and rat hepatocytes are capable of metabolizing 1,1-dimethylhydrazine to form methyl radical intermediates (Albano et al. 1989 Tomasi et al. 1987). The formation of tliese radicals was inhibited by the addition of inhibitors of cytochrome P-450 (SKF 525A, metyrapone, and carbon monoxide) and inhibitors of the flavin-containing monooxygenase system (methimazole). The formation of free radicals could also be supported nonenzymatically by the presence of copper ion (Tomasi et al. 1987). These data indicate that at least two independent enzyme systems and one nonenzymatic pathway may be involved in the metabolism of... 

In biological systems, copper occurs as part of the active site of proteins or enzymes. The various ways in which copper is bound to these copper-binding centers are of special interest. 

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