Noradrenaline, determination oxidation

Noradrenaline (NAdr = norepinephrine, NE) is formed by the stereospecific oxidation of the fl-carbon of dopamine, which itself is formed by the decarboxylation of l-DOPA (L-3,4-dihydroxyphenylalanine). Epinephrine (Adr) is formed by the Af-methylation of norepinephrine. Thus, a compound that blocks the enzyme dopa decarboxylase (responsible for the decarboxylation of dopa) causes a decline in the level of catecholamines, such as dopamine and norepinephrine, and hypotensive activity is expected. The absolute stereostructures of norepinephrine and epinephrine were determined by the transformation of each of these alkaloids into R-mandelic acid [1]. 

One interesting redox reaction is the reversible interconversion of ferriin/ferroin complexes. The l,10-phenanthroline/iron(III) complex (ferriin) has been used to oxidize species of clinical and pharmaceutical interest such as analgesics and catecholamines, the reaction being monitored by measuring the initial rate of change of the absorbance of the ferroin formed at 510 nm. This type of reaction has also been used in simultaneous kinetic-based determinations, such as the resolution of mixtures of uric acid and ascorbic acid, paracetamol and oxyphenbutazone, and adrenaline and noradrenaline. 

The major enzymes in catecholamine catabolism are catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). COMT transfers a methyl group from S-adenosyhnethionine (SAM) (Chapter 47) to the oxygen at position 3 of the aromatic ring (Fig. 48.1). The pathway taken is a lottery depending on whether the noradrenaline and adrenaline are first of aU methylated (by COMT) or alternatively oxidatively deaminated (by MAO). If chance determines methylation has priority, then the methylated amines normetadrenaline and metadrenaline are formed prior to the MAO reaction and subsequent oxidation to HMMA (hydroxymethoxyman-... 

Finally noradrenaline and adrenaline have been determined ) after oxidation with iodate to iodonoradrenochrome (III) and iodoadrenochrome (IV). 

A radioisotope derivative procedure for the determination of adrenaline and noradrenaline, involving their oxidation to labelled 7-iodoaminochromes with high specific activity has recently been reported [74], The oxidation was carried out with radioactive potassium iodide solution, in the presence of persulphuric acid (Caro s acid) [74],... 

Methods said to be specific for the determination of cysteine and cystine, based on their reactions with an oxidation product of noradrenaline, possibly noradrenochrome, have been described [252, 253]. The precise nature of the reactions taking place however remains unclear. 

Another technique depends upon the fact that, on oxidation with manganese dioxide or potassium ferricyanide at pH 5 to 6, adrenaline and noradrenaline are converted to adrenochrome and noradrenochrome respectively. At pH 3 to 4 however only adrenaline is oxidised. Such a differential method would obviously be unsatisfactory for determination of a small amount of noradrenaline in the presence of a large excess of adrenaline. 

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