Adrenaline metabolites

It has been proposed that aziridines may be more widespread in biological systems than is generally realized [190]. Many drugs such as ephedrine (124 Scheme 11.18) and pronethalol (125) and endogenous metabolites such as adrenaline (126) contain a P-aminoalcohol moiety, which may act as a precursor to an aziridine metabolite that may explain the known carcinogenicity of some of these compounds such as pronethalol. 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

Disulfoton exposure altered catecholamine levels in animals, and this hormonal imbalance may be associated with elevated acetylcholine levels (Brzezinski 1969, 1972, 1973 Brzezinski and Ludwicki 1973 Brzezinski and Rusiecki 1970 Wysocka-Paruszewska 1970, 1971). In these studies, acute dosing with disulfoton caused increases in urinary and plasma noradrenaline and adrenaline levels, accompanied by decreases of adrenaline in the adrenal glands, in rats. In addition, the major urinary metabolite of catecholamine metabolism, 4-hydroxy-3-methoxymandelic acid (HMMA), was recovered in the urine from rats given acute doses of disulfoton (Wysocka-Paruszewska 1970,... 

Urine catecholamines may also serve as biomarkers of disulfoton exposure. No human data are available to support this, but limited animal data provide some evidence of this. Disulfoton exposure caused a 173% and 313% increase in urinary noradrenaline and adrenaline levels in female rats, respectively, within 72 hours of exposure (Brzezinski 1969). The major metabolite of catecholamine metabolism, HMMA, was also detected in the urine from rats given acute doses of disulfoton (Wysocka-Paruszewska 1971). Because organophosphates other than disulfoton can cause an accumulation of acetylcholine at nerve synapses, these chemical compounds may also cause a release of catecholamines from the adrenals and the nervous system. In addition, increased blood and urine catecholamines can be associated with overstimulation of the adrenal medulla and/or the sympathetic neurons by excitement/stress or sympathomimetic drugs, and other chemical compounds such as reserpine, carbon tetrachloride, carbon disulfide, DDT, and monoamine oxidase inhibitors (MAO) inhibitors (Brzezinski 1969). For these reasons, a change in catecholamine levels is not a specific indicator of disulfoton exposure. 

Increased levels of urinary catecholamines may also be associated with accumulation of acetylcholine that resulted from acetylcholinesterase inhibition by disulfoton. No human data were located to support this, but limited animal data provide some evidence. Disulfoton exposure caused a 173% and 313% increase in urinary noradrenaline and adrenaline levels in rats, respectively, within 72 hours (Brzezinski 1969). The major metabolite of catecholamine metabolism, HMMA, was also detected in the urine from rats given acute doses of disulfoton (Wysocka-Paruszewska 1971). 

Methyldopa is an false substrate for the dopamine-/ -hydroxylase resulting in a-methylnor-adrenaline. This metabolite is an a2-adrenoceptor agonist an induce, like clonidine, a centrally mediated reduction of sympathetic tonus. 

Tryptophan oxygenase is another iron-containing enzyme which, like catalase, is inhibited by pyrazole in vivo but not in vitro. Again, however, the metabolite 4-hydroxypyrazole is active in vitro and shows, in contrast to catalase, competitive inhibition with tryptophan for the enzyme (79MI10505). Adrenaline and other phenols are also inhibitors of this enzyme and in this case, therefore, the heterocyclic ring appears not to be essential for activity. 

An inca uscd production of cutcchuluinincs from a benign or malignuni tumour causes vasoconstriction. The increased catecholamine production may not be easy to confirm, as phaet)chromocytomas frequently secrete catecholamines episixlically. Urinary catecholamine metabolites (Fig.. 1) may not be elevated unless the patient has been symptomatic during the peritxl of urine collection. Plasma adrenaline and noradrenaline concentrations are usually increased but these measurements arc only available in a small number of centres. 

Spector and Willoughby - have pointed out that the vascular changes in the acute inflammatory reaction may be due to the destruction of local vasoconstrictor substances such as adrenaline. Evidence in favour of this mechanism includes the observation that increased capillary permeability after thermal injury is suppressed by iproniazid and other monoamine oxidase inhibitors. Such inhibitors are known to inhibit the conversion of adrenaline, noradrenaline, 5-hydroxytryptamine and other amines to inactive metabolites. The authors provide evidence that the action of the monoamine oxidase inhibitors on capillary permeability is dependent on their anti-enzymic activity and not on some other unrelated property. Nevertheless, the evidence remains indirect an attempt to detect pressor amines in the plasma of burned animals was unsuccessful. The potentiating effect of bretylium and the antagonistic action of an adrenolytic substance, dibenamine, on the action of iproniazid suggest that it is local depots of adrenaline rather than noradrenaline or 5-hydroxytryptamine which are involved. Independent support for this suggested role of catecholamines... 

The difficulty in obtaining satisfactory evidence may be due to the fact that thrombosis is a local circulatory problem which will require release of heparin locally for control. Such amounts will not be apparent in gross biochemical tests either as an increase in plasma concentration or a decrease in concentration in tissue. It is probable that heparin will be like other auto-pharmacological agents (e.g. adrenalin, steroids, insulin) in that the amount of heparin in the general circulation at any one time is only a secondary reflection of secretion levels. More important is the determination of rate of urinary excretion of metabolites and still more important the determination of rate of secretion by the glandular tissue (mast cells) itself. 

Axelrod has shown that, in man, circulating adrenaline and noradrenaline are metabolized predominantly by catechol 0-methyltransferase, approximately 70 per cent of an administered dose being inactivated by 0-methyla-tion and 20 per cent by deamination. Generally similar results were obtained in studies on other mammals. The principal urinary metabolite of the catechol amines is 3-methoxy-4-hydroxymandelic acid. 

It is generally employed in solution form either alone or in eombination with adrenaline (a vasodilator). It exhibits upto 95% ability to cause plasma protein binding. Beeause it exerts a minimal nerve motor block, hence it is specifically suitable for some surgieal operations. The drug is largely metabolized in the liver and its metabolites are chiefly exereted in the urine. 

This chapter describes two microbore LC-based methods for measurement of monoamine neurotransmitters in rat brain dialysates a method for the determination of the catecholamines (CA) noradrenaline (NAD), adrenaline (AD), and dopamine (DA) and a method for the determination of the indoleamine serotonin (5HT) and its major metabolite 5-hydroxyindoleacetic acid (SHIAA) (3). For all compounds, a limit of detection of 1 pmol or less (in an injected volume of 10 pL) can be achieved. Basal serotonin can be measured in microdialysates without including a reuptake inhibitor in the microdialysis perfusion fluid. Both of these systems are routinely used in the authors laboratory for the analysis of dialysates from striatum, hippocampus, and substantia nigra. Both methods are selective, robust, and can be automated. 

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