Metanephrines from catecholamines

The selective extraction of catecholamines has been the subject of some studies [250,269]. Recently B. Claude et al developed a method for the selective SPE of catecholamines and metanephrines from bovine serum using MIPs, and reported the recovery, RSD and linearity range of the method to be in the orderof64-98%, less than 6% and 0.30-0.90 pM respectively [250]. In another study M. Bouri et al reported the selective extraction of catecholamines from urine using a dopamine MIP coated magnetic molecularly imprinted polymer and capillary electrophoresis. They used methacrylic acid as the monomer, ethylene glycol dimethacrylate as the cross-Unker and coated the MIP on FCjO particles and used 2,2 - azobisisbutyronitrile as the initiator in methanol/water or acetonitrile/water solvents, and reported LOD of 0.04-0.06 pM, RSD of 0.7%-1.4% and a response range of 2.9%-5.5% [269]. 

B. Claude, P. Morin and L. Denoroy, Selective solid-phase extraction of catecholamines and metanephrines from serum using a new molecularly imprinted polymer, /. Liq. Chromatogr. RT, 37 (18) 2624-2638, 2014. 

The most well known of the naturally occurring phenethylamine derivatives (Table I) are the transmitters of the sympathetic nervous system, epinephrine, norepinephrine, and dopamine. All these compounds are 3,4-dioxygenated in the aromatic nucleus and are collectively known as the catecholamines. Norepinephrine is the transmitter of most sympathetic postganglionic fibers, dopamine is the predominant transmitter of the mammalian extrapyramidal system and of several mesocortical and mesolimbic neuronal pathways, and epinephrine is the major hormone of the adrenal medulla (363). The literature that has accumulated on the action of these compounds in higher animals is enormous. Metanephrine and normetanephrine are known from animals as deactivated metabolites of epinephrine and norepinephrine that result from the action of the enzyme catechol O-methyltransferase (364). 

Four catecholamines have been found in increased amounts in the urine after injury (W2), metanephrine, normetanephrine, iV-methyl-metanephrine, and 3-methoxytyramine. The JV-methylmetanephrine probably indicates an increased production of epinephrine (Cll). The increase in norepinephrine production is relatively greater than that of epinephrine, suggesting an active release of hormone from sympathetic nerve tissue as well as from the adrenal medulla. 

The basis for the high diagnostic efficacy of plasma free metanephrines is explained by several factors (1) plasma free metanephrines are produced by metabolism of catecholamines within pheochromocytomas, a process that occurs continuously and independently of variations in catecholamine release by tumors (2) normally only small amounts of metanephrines are produced in the body, and these are relatively unresponsive to sympathoadrenal activation compared with the parent amines and (3) VMA and the metanephrines commonly measured in urine are different metabofites from the free metanephrines measured in plasma, and are produced in different parts of the body by metabolic processes not directly related to the tumor itself." ... 

Additional markers of catecholamine overproduction have been employed to improve the biochemical detection of neuroblastomas. Free dopamine may be abnormal in urine from neuroblastoma patients with VMA and HVA excretion. Combined testing for VMA, HVA, and dopamine may therefore improve tumor detection, and in 1993 an international consensus report on neuroblastoma diagnosis added dopamine to the Hst of acceptable measurements to document the adrenergic nature of the tumor. Plasma measurements of dopamine and L-dopa, the amino acid precursor of dopamine, may also have clinical value and allow the alternate use of plasma. Measurement of methylated metabolites, especially normetanephrine, has also been explored. When urinary normetanephrine, metanephrine, methoxytyra-mine, dopamine, norepinephrine, VMA, and HVA were measured, clinical sensitivity for detection of neuroblastomas was 97% to 100% when results of normetanephrine testing were coupled either with VMA in the infants or with HVA in children greater than age 1. Even with an extended panel of catecholamines and metabolite measurements, a low incidence of nonsecreting tumors continues to be identified and should be considered in the interpretation of a negative test result. 

Catecholamines and metanephrines are prone to oxidative conversion to their corresponding quinones, 2 hence proper storage and preservation of 24-h urine samples after collection from patients is essential for accurate quantitation of these compounds. Various groups have reported stability studies of catecholamines in biological fluids. Boomsma et al. 

Fig. 4.4.12. Separation of a mixture of acidic and basic catecholamines on a LiChrosorb RP-8 column (0.28x25 cm). Eluent Water containing 0.02 M citrate (pH 2.5)/l% propanol/NaCIQ4 (0.08 M)/0.3% sodium dodecyl sulphate. Peaks DHMA, 3,4-dihydroxymandelic acid VMA, vanilmandelic acid HGA, 2,5-dihydroxyphenylacetic acid DOPAC, 3,4-dihydroxyphenylacetic acid 5-HIAA, 5-hydroxyindole-3-acetic acid HVA, homovanillylmandelic acid E, epinephrine NE, norepinephrine N-Syn, norsyneph-rine Syn, synephrine Dopa, 3,4-dihydroxyphenylalanine NM, normetanephrine MN, metanephrine Isopren, isoprenaline 3-H-Tyrm, dopamine Tyrm, tyramine 3-M-Tyrm, 3-methoxytyramine. Reprinted from Ref. 21 with permission.

Metanephrine and normetanephrine are deactivated O-methyl metabolites of epinephrine and norepinephrine, produced by the action of catechol O-methyl transferase. Both metanephrine and normetanephrine are produced at low concentrations from normal metabolism of catecholamines by the sympathetic nervous system, but their concentration can be greatly elevated by catecholamine-producing tumors, such as pheochromocytomas. Pheochromocytomas are rare malignancies that often present with symptoms of catecholamine excess, most prominently h5q)ertension. When used to identify pheochromocytoma, therefore. 

An interesting and rare potential adverse effect was reported in a 36-year-old pregnant woman who presented with recurrent occipital headaches (35 ). After only 4 doses of imipramine 25 mg, she became markedly hypertensive and suffered grand mal seizures. She was found to have raised urinary metanephrine levels and a phaeochro-mocytoma was surgically removed. This suggested that imipramine enhanced the vascular reactivity to catecholamines released from the phaeochromocytoma. 

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