Methyl hydroxylases

Ullah AJH, R1 Murray, PK Bhattacharyya, GC Wagner, IC Gunsalus (1990) Protein components of a cytochrome P-450 linalool 8-methyl hydroxylase. J Biol Chem 265 1345-1351. 

The ability of these enzymes to contact directly the electrode is attributed to the peripheral location of the redox center. A detailed kinetic study [11] of the peroxidase-catalyzed reduction of H2O2 revealed that 42 % of the enzyme molecules are aligned on the electrode surface in a configuration where the redox heme site is accessible for direct electron transfer. Other enzymes possess two redox sites, and electron transfer proceeds vectorially from a peripheral site to an inner component. For example, /7-cresol methyl hydroxylase [PCMH, a flavin adenine dinucleotide (FAD)- and heme-containing redox enzyme] affects the direct oxidation of p-cresol to -hydroxybenzaldehyde [12[ ... 

The direct electronic communication of this series of redox enzymes allows their application as bioactive sensing interfaces p-cresol [12], methylamine [14] and fructose [ 15] detection have been reported in the presence ofp-cresol methyl hydroxylase, methylamine dehydrogenase and fructose dehydrogenase, respectively. 

The triazoles and related growth retardants are not general inhibitors of plant cytochrome P-450 monooxygenases [39]. Their action appears to be confined to methyl hydroxylases, with which they interact to differing extents depending on structure. Apart from e f-kaurene oxidase, there are several other such enzymes that are involved in important metabolic pathways (Fig. 5), the products of which may influence growth. These are potential sites of action and will be considered below. 

The metabolism of ABA to phaseic acid is initiated by hydroxylation at C-8 (Fig. 5). The enzyme catalyzing this reaction in Echinocystis lobata was found to be microsomal and to require O2 and NADPH [18]. Although it was not fully characterized, the enzyme has properties consistent with a monooxygenase which, as a methyl hydroxylase, is a potential site of action of the N-heterocyclics. There is evidence that this reaction may indeed be inhibited in some cases, leading to increased ABA levels [49] and therefore reduced transpiration [2]. Although there is little information on the structural requirements for inhibitors of ABA metabolism, it is of interest to note that inhibition of both this reaction and e/z kaurene oxidation would result in growth retardation. 

Methyldopa, through its metaboHte, CX-methyInorepinephrine formed in the brain, acts on the postsynaptic tt2-adrenoceptor in the central nervous system. It reduces the adrenergic outflow to the cardiovascular system, thereby decreasing arterial blood pressure. If the conversion of methyldopa to CX-methyl norepinephrine in the brain is prevented by a dopamine -hydroxylase inhibitor capable of penetrating into the brain, it loses its antihypertensive effects. 

Catecholamine biosynthesis begins with the uptake of the amino acid tyrosine into the sympathetic neuronal cytoplasm, and conversion to DOPA by tyrosine hydroxylase. This enzyme is highly localized to the adrenal medulla, sympathetic nerves, and central adrenergic and dopaminergic nerves. Tyrosine hydroxylase activity is subject to feedback inhibition by its products DOPA, NE, and DA, and is the rate-limiting step in catecholamine synthesis the enzyme can be blocked by the competitive inhibitor a-methyl-/)-tyrosine (31). 

Although /3-oxidation is universally important, there are some instances in which it cannot operate effectively. For example, branched-chain fatty acids with alkyl branches at odd-numbered carbons are not effective substrates for /3-oxidation. For such species, a-oxidation is a useful alternative. Consider phy-tol, a breakdown product of chlorophyll that occurs in the fat of ruminant animals such as sheep and cows and also in dairy products. Ruminants oxidize phytol to phytanic acid, and digestion of phytanic acid in dairy products is thus an important dietary consideration for humans. The methyl group at C-3 will block /3-oxidation, but, as shown in Figure 24.26, phytanic acid a-hydroxylase places an —OFI group at the a-carbon, and phytanic acid a-oxidase decar-boxylates it to yield pristanie add. The CoA ester of this metabolite can undergo /3-oxidation in the normal manner. The terminal product, isobutyryl-CoA, can be sent into the TCA cycle by conversion to succinyl-CoA. 

Therapeutic Function Tyrosine hydroxylase Inhibitor Chetnicel Name tt-Methyl-L-tyrosine Common Name Metirosine Structural Formula ... 

Trace Amines. Figure 1 The main routes of trace amine metabolism. The trace amines (3-phenylethylamine (PEA), p-tyramine (TYR), octopamine (OCT) and tryptamine (TRP), highlighted by white shading, are each generated from their respective precursor amino acids by decarboxylation. They are rapidly metabolized by monoamine oxidase (MAO) to the pharmacologically inactive carboxylic acids. To a limited extent trace amines are also A/-methylated to the corresponding secondary amines which are believed to be pharmacologically active. Abbreviations AADC, aromatic amino acid decarboxylase DBH, dopamine b-hydroxylase NMT, nonspecific A/-methyltransferase PNMT, phenylethanolamine A/-methyltransferase TH, tyrosine hydroxylase. 

Tyrosine is converted to dopa by the cytoplasmic enzyme tyrosine hydroxylase. This is the rate-limiting step 5 x 10 M) in DA synthesis, it requires molecular O2 and Fe + as well as tetrahydropterine (BH-4) cofactor and is substrate-specific. It can be inhibited by a-methyl-p-tyrosine, which depletes the brain of both DA and NA and it is particularly important for the maintenance of DA synthesis. Since the levels of tyrosine are above the for tyrosine hydroxylase the enzyme is normally saturated and so it is not possible to increase DA levels by giving tyrosine. 

While a number of drugs, e.g. a-methyl dopa, inhibit the enzyme they have little effect on the levels of brain DA and NA, compared with inhibition of tyrosine hydroxylase and they also affect the decarboxylation of other amino acids. Some compounds, e.g. a-methyl dopa hydrazine (carbidopa) and benserazide, which do not easily enter the CNS have a useful role when given in conjunction with levodopa in the treatment of Parkinsonism (see Chapter 15) since the dopa is then preserved peripherally and so more enters the brain. 

The enzyme /i-phenylethanolamine-A-methyl transferase, which is required to convert noradrenaline (NA) to adrenaline (Ad), is present in the CNS and there is histofluoro-metric evidence (positive staining with antibodies to that enzyme and to tyrosine hydroxylase and dopamine /i-hydroxylase as well) for adrenergic cell bodies in two groups (nuclei) alongside NA neurons of the locus coeruleus (EC) but ventral and lateral (Ci) and dorsal and medial (C2) to it. Projections go to the hypothalamus and in... 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

The alkane hydroxylase belongs to a family of nonheme iron oxygenases. There is some structural similarity between the nucleotide sequence of the integral-membrane alkane hydroxylase and the subunits of the monooxygenase encoded by xylA and xylM in the TOL plasmid that are involved in hydrox-ylation of the methyl groups in toluene and xylene in Pseudomonas putida PaWl (Suzuki et al. 1991). 

Smith CA, MR Flyman (2004) Oxidation of methyl tcrt-butyl ether by alkane hydroxylase in dicyclopro-pyUcetone-induced and n-octane-grown Pseudomonas putida Gpol. Appl Environ Microbiol 70 4544-4550. 

Champier, J., Claustrat, B., Besancon, R. el al. (1997). Evidence for tryptophan hydroxylase and hydroxy-indol-O-methyl-transferase mRNAs in human blood platelets. Life Sci 60, 2191-7. 

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