Catecholates, 403 nicotinamide

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

A number of methyltransferases are able tc methylate small molecules (46,47). Thus, re. actions of methylation fulfill only two of the three criteria defined above, because the methyl group is small compared with the substrate. The main enzyme responsible for O-methylation is catechol 0-methyltransferas (EC 2.1.1.6 COMT), which is mainly cytosolic but also exists in membrane-bound form. Several enzymes catalyze reactions of xenobiotic N-methylation with different substrate specificities, e.g., nicotinamide iV-methyltrans-ferase (EC 2.1.1.1), histamine methyltrans-ferase (EC 2.1.1.8), phenylethanolamine N-methy 1 transferase (noradrenal ine A-meth-yltransferase EC 2.1.1.28), and nonspecific amine N-methyltransferase (arylamine N-methyltransferase, tryptamine JV-methyl-transferase EC 2.1.1.49) of which some isozymes have been characterized. Reactions of xenobiotic S-methylation are mediated by the membrane-bound thiol methyltransferase (EC 2.1.1.9) and the cytosolic thiopurine methyltransferase (EC 2.1.1.67) (3). 

A chlorophenoxyacetate-degrading Arthrobacter sp. contained an enzyme, which converted 2,4-dichlorophenol and other chlorophenols to catechols. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) and Oj were required for the reaction. The enzyme hydroaylating 2,4-dichlorophenol was partially separated from the catechol-degrading enzyme present in the bacterial extract. The compotmds formed from... 

Arene dioxygenases catalyze the first step in the metabolism of unactivated aromatic compounds, yielding cw-dihydrodiol of aromatics. As shown in eq. (22), the reaction requires nicotinamide adenine nucleotide (NADH) and molecular oxygen. Substrate specificity of enzymes is high, and products shown in Fig. 27 are produced by benzene-[351, 352], toluene- [353-356], naphthalene- [357-361], biphenyl- [362-364], benzoate-[365-370], and phthalate-dioxygenases [371-378]. In the cases of benzoate [379], 4-sulphobenzoate [380], and 6>-nitrotoluene [381], catechols are formed via unstable dihydroxylated intermediates as shown in eq. (23)... 

Electrodes modified by eiectrodeposited 3-methylPT can be used as chemical sensors for organic and especially biological interesting molecules. The modified surfaces catalyze the oxidation of several compounds (e.g. ferrocyanide, catechol, ascorbic acid, hydroquinone, dopamine, epinephrine, acetaminophene, p-aminophenol, and reduced nicotinamide adenine dinucleotide). The sensitivity of these polymer-coated electrodes is 4 to 10 times higher than pure electrodes of platinum. Binary and also ternary mixtures can also be analyzed these electrodes are also suitable as an amperometric detector for flow injection analysis [211, 212]. 

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