Adrenochrome, oxidation

In their original paper on adrenochrome, published in 1937, Green and Richter reported that the oxidation of adrenaline to adrenochrome is catalysed by (a) a cyanide-insensitive system present in heart and skeletal muscle and b) the cytochrome-indophenol oxidase system present in all tissues [22]. Since that time, there have been numerous reports of in vitro studies dealing with adrenaline to adrenochrome oxidations catalysed by mammalian body fluid and tissue preparations. 

On oxidation by potassium hexacyanoferrate(III) adrenaline is converted into adrenochrome which then condenses with ethylenediamine ... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

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). 

Phenylephrine (27) is a low-potency sympathomimetic amine used as a decongestant. Solutions become coloured due to an auto-oxidation accelerated by light. In a series of experiments, aqueous solutions of the hydrochloride were left under a UV lamp until a tan colour developed. HPLC analysis showed four main products of which one was identified as adrenaline (19). Even after prolonged irradiation, there was never more than 2% adrenaline in the solution. It was assumed that the catecholamine was removed as it formed by further reaction to adrenochrome and melanine, which accounted for the colour [34],... 

The oxidation of DOPA and adrenaline to dopachrome and adrenochrome, respectively, by a horse radish peroxidase-H202 system has been reported by Herzmann.29,30 The oxidation process was activated by trace quantities of caffeic acid, its esters, and related compounds.30 Ascorbic acid inhibited the oxidation of adrenaline by this enzyme in the initial stages of the reaction, but later had a stimulatory effect.30... 

There has been some controversy in the literature about the possibility of there being an enzyme system in mammalian body fluids or tissues capable of oxidizing adrenaline to adrenochrome. The claim of Payza and Hoffer that serum oxidizes adrenaline enzymically to adrenochrome42 has been queried by Geller et oZ.29... 

However, Van der Wender and Spoerlein have recently described the presence of an enzyme system in rat brain that is capable of oxidizing DOPA to melanitic pigments43 (an aminochrome, i.e. dopachrome, must be formed as an essential intermediate in this process) the same enzyme system apparently oxidizes adrenaline to adrenochrome.43 Kaliman has demonstrated the presence of an enzyme system in rabbit heart tissue which oxidizes adrenaline via the quinonoid pathway (presumably to adrenochrome).44 Heart, kidney, and brain tissues of white rats were also shown by Kaliman and Koshlyak to possess similar activity.45... 

Recently adrenochrome has been obtained (as its semicarbazone) by the air oxidation of synephrine [i.e. /9-hydroxy-/J-(4-hydroxy-phenyl)ethylmethylamine (5)] in the presence of potato tyrosinase.46 This overall reaction must involve an initial hydroxylation of synephrine (5) to give adrenaline (2), which is subsequently oxidized to adrenochrome (1). 

At neutral pH adrenaline was oxidized first to adrenochrome and then to melanin different products were produced at alkaline pH and melanin formation was low. With noradrenaline similar processes took place at neutral pH, but at a slower rate in this case the alkaline oxidation was characterized by increased melanin formation.61... 

The main procedure used for the preparation of adrenochrome (1) in the laboratory involves the oxidation of adrenaline by silver oxide in methanol. This method, which was first described by Veer in 194262,63 and subsequently used by many other workers (see refs. 2 and 3 for early references), is still the simplest procedure available for obtaining adrenochrome (1) and similar non-halogenated amino-... 

Adrenochrome methyl and ethyl ethers (8 and 9 respectively), first isolated by Hukki and Seppalainen as their semicarbazones,66 have now been obtained in crystalline form by oxidation of the corresponding catecholamine ethers with silver oxide in dry acetonitrile.65 A-Ethylnoradrenochrome (6) has also been prepared in crystalline form by the oxidation of N-ethylnoradrenaline in 90% methanol with the calculated quantity of iodic acid65 (cf. the preparation of adrenochrome by Macciotta67). Adrenochrome 08-acetate (3-acetoxy-epinochrome) (10) was obtained by the oxidation of acetyladrena-line [j3-acetoxy-j8-(3,4-dihydroxyphenyl)ethylmethylamine].68... 

The higher decomposition points were obtained when adrenochrome methyl and ethyl ethers were prepared by oxidation of the appropriate catecholamine in methanol with silver oxide. The BOlid aminochromes were then obtained as microcrystalline solids on addition of dry ether and cooling the resultant solution to — 80°. The slightly less pure products were obtained when the oxidation was carried out in acetonitrile. [R. A. Heacock and B. D. Scott, loc. cit. (footnote c )]. 

The characteristic transient yellow-green fluorescence exhibited by adrenaline solutions that were undergoing oxidation in the presence of alkali was first reported in 1918.182 This phenomenon was later shown to be general, and similar (but usually weaker) fluorescences were observed when other catecholamines were oxidized in alkaline solution.133 Many years were to elapse before the correct explanation of this phenomenon was forthcoming, i.e. that the fluorescent product derived from adrenaline was a rearrangement product of adrenochrome (the red oxidation product of adrenaline). 

No simple oxidation products of adrenochrome have been obtained in crystalline form. The evidence for and against the existence of oxoadrenochrome was summarized in an earlier review.3 Although the preparation of a substance described as 2-iodooxoadrenochrome (70) by the oxidation of adrenaline with iodic acid was reported,87,187 it was subsequently shown by an exhaustive consideration of its physical and chemical properties to be identical with 7-iodoadreno-chrome (12)109 (cf. ref. 70). 

The formation of relatively stable fluorescent products by the reaction of adrenaline with ethylenediamine (and certain other primary amines) in air, first reported in 1948 by Natelson et was adapted by Weil-Malherbe and Bone in 1952 for the assay of catecholamines.197 198 Since 1952 much work, largely of an empirical nature, has been carried out to improve the analytical procedure since often apparently minor variations of the reaction conditions have a significant effect on the fluorescence observed (see Section V, E, 4). Paper chromatographic examination of the reaction mixtures obtained from adrenaline and noradrenaline suggested that more than one product could be formed in each case.199-205 The main fluorescent product of the interaction of adrenochrome (1) (obtained by oxidation of adrenaline) and ethylenediamine in air has been obtained as a crystalline solid by Harley-Mason and Laird and shown to be 2,3-dihydro-3-hydroxy-l-methylpyrrolo[4,5-g]quinoxaline (94) (7% yield).206,207 This compound has two hydrogen atoms less than... 

The non-oxidative formation of melanin from adrenochrome in acid solution, reported by Harley-Mason,6 has recently been shown by Bu Lock to be controlled by a second-order reaction between adrenochrome and acid.107 It was suggested that the initial product (not isolated) was probably l-methylindole-5,6-quinone, which polymerized rapidly to melanin via dimers and oligomers.107 (The intermediate monomer quinone also gave a melanin-like copolymer with indole.107)... 

Although the two major routes for metabolism of adrenaline and noradrenaline are well established, and involve either methylation of the 3-hydroxyl group on the aromatic nucleus or oxidative deamination (cf. refs. 35, 93, 94, 229-231), the evidence to date does not warrant the complete rejection of a further possible metabolic pathway involving oxidation to an aminochrome (such as adrenochrome or noradrenochrome) in some instances. 

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

Because the product of these reactions, leucoadrenochrome, is more readily oxidized than adrenaline, a homogeneous electron transfer reaction (Eq. 21.12) involving leucoadrenochrome and unreacted adrenaline-quinone ensues. In addition to the formation of adrenochrome as the final product, adrenaline is regenerated by the solution redox reaction. The reader should be satisfied that the overall transformation of adrenaline to adrenochrome requires four electrons. 

Note that the anodic peak due to the oxidation of leucoadrenochrome to adrenochrome near 0 V is not seen until the second positive-going potential sweep is made. The voltage separation between the anodic and cathodic peaks for the oxidation of adrenaline (peak B, Fig. 21.4, bottom) and the reduction of adrenalinequinone (peak C) is large when compared to most chemically reversible redox couples. However, this behavior is typical of many quinone-hydroquinone systems on a carbon paste surface at intermediate values of pH. 

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