|Jmol-3D images||Image 1|
|Molar mass||165.23 g mol−1|
|Boiling point||173°C at 11 mm Hg; sublimes at 140-150°C|
|Solubility in water||high in: ethanol; ether; chloroform|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Hordenine, or N,N-dimethyltyramine, is an alkaloid of the phenethylamine class that occurs naturally in a variety of plants, taking its name from one of the commonest, barley (Hordeum species). Chemically, hordenine is the N-methyl derivative of N-methyltyramine, and the N,N-dimethyl derivative of the well-known biogenic amine tyramine, from which it is biosynthetically derived and with which it shares some pharmacological properties (see below). Currently, hordenine is widely sold as an ingredient of nutritional supplements, with the claims that it is a stimulant of the central nervous system, and has the ability to promote weight loss by enhancing metabolism. In experimental animals, given sufficiently large doses parenterally (i.e. by injection), hordenine does produce an increase in blood pressure, as well as other disturbances of the cardio-vascular, respiratory and nervous systems. These effects are generally not reproduced by oral administration of the drug in test animals, and there are virtually no scientific reports of the effects of hordenine in human beings. More detailed discussions of hordenine pharmacology and toxicology are given below.
The first report of the isolation from a natural source of the compound which is now known as hordenine was made by Arthur Heffter in 1894, who extracted this alkaloid from the cactus Anhalonium fissuratum (now reclassified as Ariocarpus fissuratum), naming it "anhalin". Twelve years later, E. Léger independently isolated an alkaloid which he named hordenine from germinated barley (Hordeum vulgare) seeds. Ernst Späth subsequently showed that these alkaloids were identical and proposed the correct molecular structure for this substance, for which the name "hordenine" was ultimately retained.
Hordenine is present in a fairly wide range of plants, notably amongst the cacti, but has also been detected in some algae and fungi. It occurs in grasses, and is found at significantly high concentrations in the seedlings of cereals such as barley (Hordeum vulgare) (~ 0.2%, or 2000 μg/g), proso millet (Panicum miliaceum) (~ 0.2%), and sorghum (Sorghum vulgare) (~0.1%). Reti, in his 1953 review of naturally-occurring phenethylamines, notes that the richest source of hordenine is the cactus Trichocereus candicans (now reclassified as Echinopsis candicans), which was found to contain 0.5-5% of the alkaloid.
Since barley, via its conversion to malt, is used extensively in the production of beer, beer and malt have been examined by several groups of investigators for the presence of hordenine. Citing a 1965 study by McFarlane, Poocharoen reported that beer contained ~ 12–24 mg/L, wort contained ~11–13 mg/L, and malt contained ~ 67 μg/g of hordenine. The hordenine content of various malts and malt fractions was extensively studied by Poocharoen himself, who also provided a good coverage of related literature up to 1983. This researcher found a mean concentration of hordenine in raw barley of ~ 0.7 μg/g; in green malts (i.e. barley that had been soaked in water for 2 days then germinated for 4 days), the mean concentration was ~ 21 μg/g, and in kilned malts (i.e. green malts that had been heated in a kiln for 1–2 days) the mean concentration was ~ 28 μg/g. When only green malt roots were examined, their mean content of hordenine was ~ 3363 μg/g, whereas the mean level in kilned malt roots was ~ 4066 μg/g.
It has been established that, in barley, hordenine levels reach a maximum within 5–11 days of germination, then slowly decrease until only traces remain after 1 month. Furthermore, hordenine is localized primarily in the roots. In comparing literature values for hordenine concentrations in "barley" or barley "malt", therefore, consideration should be made of the age and parts of the plant being analyzed: the figure of ~ 2000 μg/g cited in the review by Smith, for example, is consistent with Poocharoen's  figures for the hordenine levels in the roots of malted barley, but not in "whole" malt, where his figures of 21-28 μg/g are more consistent with McFarlane's figure of ~ 67 μg/g. On the other hand, there is a wide range of variability: a study by Lovett and co-workers of 43 different barley lines found concentrations of hordenine in roots ranging from 1-2625 μg/g fresh weight. These workers concluded that hordenine production was not under significant genetic control, but much more susceptible to environmental factors such as light duration.
It has been shown that hordenine is biosynthesized by the step-wise N-methylation of tyramine, which is first converted to N-methyltyramine, and which, in turn is methylated to hordenine. The first step in this sequence is accomplished by the enzyme tyramine N-methyltransferase (tyramine methylpherase), but it is uncertain if the same enzyme is responsible for the second methylation that actually produces hordenine.
Since the hordenine molecule contains both a basic (amine) and acidic (phenol) functional group, it is amphoteric.
The apparent (see original article for discussion) pKas for protonated hordenine are 9.78 (phenolic H) and 10.02 (ammonium H).
It should be noted that the "methyl hordenine HCl" which is listed as an ingredient on the labels of some "nutritional supplements" is in all likelihood simply hordenine hydrochloride, since the description of "methyl hordenine HCl" given by virtually all bulk suppliers of this substance corresponds to that for hordenine hydrochloride (or possibly just hordenine). There are five regioisomeric compounds that would correspond to the name "methyl hordenine HCl", if it were interpreted according to the rules of chemical nomenclature: α-methyl hordenine, β-methyl hordenine, 2-methyl hordenine, 3-methyl hordenine, and 4-O-methyl hordenine - each in the form of its HCl salt; N-methyl hordenine is better known as the natural product candicine, but is excluded from the possibilities because it is a quaternary ammonium salt that cannot be protonated and hence cannot form a HCl salt.
The first synthesis of hordenine is due to Barger: 2-phenylethyl alcohol was first converted to 2-phenylethyl chloride using PCl5; this chloride was reacted with dimethylamine to form N,N-dimethyl-phenylethylamine, which was then nitrated using HNO3; the N,N-dimethyl-4-nitro-phenethylamine was reduced to N,N-dimethyl-4-amino-phenethylamine with Sn/HCl; this amine was finally converted to hordenine by diazotization/hydrolysis using NaNO2/H2SO4/H2O.
A more efficient synthetic route was described by Chang and co-workers, who also provided references to earlier syntheses. This synthesis began with p-methoxy-phenylethyl alcohol, which was simultaneously O-demethylated and converted to the iodide by heating with HI; the resulting p-hydroxy-phenylethyl iodide was then heated with dimethylamine to give hordenine.
Radio-labelled hordenine has been prepared by the hydrogenation of a mixture of 2-[14C]-tyramine and 40% formaldehyde in the presence of 10% Pd-on-charcoal catalyst. The labelled C in the hordenine is thus the C which is β- to the N.
The first pharmacological study of hordenine to be recorded is that of Heffter, who was also the first to isolate it. Using the sulfate salt (see "Chemistry"), Heffter gave a subcutaneous dose of 0.3 g to a 2.8 kg cat (~ 107 mg/kg), and observed no effects besides violent vomiting; the cat behaved normally within 45 mins. He also took a dose of 100 mg orally himself, without experiencing any observable effect. However, the alkaloid was observed to produce a paralysis of the nervous system in frogs.
Working with Léger's (see "Occurrence") hordenine sulfate, Camus determined minimum lethal doses for the dog, rabbit, guinea pig and rat (see "Toxicology"). The associated symptoms of toxicity following parenteral doses were: excitation, vomiting, respiratory difficulties, convulsions, and paralysis, with death occurring as a result of respiratory arrest. In a subsequent paper, Camus reported that the i.v. administration of some hundreds of mg of hordenine sulfate to dogs or rabbits caused an increase in blood pressure and changes in the rhythm and force of contraction of the heart, noting also that the drug was not orally active.
The cardiovascular and other effects of hordenine were reviewed in detail by Reitschel, writing in 1937.
More modern studies were carried out by Frank and co-workers, who reported that i.v. administration of 2 mg/kg of hordenine to horses produced substantial respiratory distress, increased the rate of respiration by 250%, doubled the heart rate, and caused sweating without changes in basal body temperature or behavior. All effects disappeared within 30 mins. The same dose of hordenine given orally did not produce any of the effects seen after parenteral administration.
In a 1995 study, Hapke and Strathmann reported that in dogs and rats hordenine produced a positive inotropic effect on the heart (i.e. increased the strength of contraction), increased systolic and diastolic blood pressure, and increased the volume of peripheral blood flow. Movements of the gut were inhibited. Additional experiments on isolated tissue lead these investigators to conclude that hordenine was an indirectly acting adrenergic agent that produced its pharmacological effects by releasing stored norepinephrine (NE).
In a survey of the ability of a wide range of arylalkylamines and their derivatives to release norepinephrine (NE) from mouse heart, Daly and co-workers determined that 10 mg hordenine sulfate did not cause any significant NE release, in contrast to the same doses of the closely related compounds N-methyltyramine hydrochloride (which released 36% over control) and tyramine hydrochloride (which released 50% over control).
In contrast to tyramine, hordenine did not produce contraction of isolated rat vas deferens, but a 25 μM concentration of the drug did potentiate its response to submaximal doses of norepinephrine (NE), and inhibited its response to tyramine. However, the response to NE of isolated vas deferens taken from rats chronically pre-treated with guanethidine was not affected by hordenine. The investigators concluded that hordenine acted as an inhibitor of NE re-uptake in rat vas deferens.
Hordenine has been found to be a potent stimulant of gastrin release in the rat, being essentially equipotent with N-methyltyramine: 83 nM/kg of hordenine (corresponding to ~14 mg/kg of the free base) enhancing gastrin release by ~ 60%.
In a study of the effects of a large number of compounds on a rat trace amine receptor (rTAR1) expressed in HEK 293 cells, it was found that hordenine, at a concentration of 1 μM, had almost identical potency to that of the same concentration of β-phenethylamine in stimulating cAMP production through the rTAR1. The potency of tyramine in this receptor preparation was slightly higher than that of hordenine.
LD50 in mouse, by i.p. administration: 299 mg/kg. Other LD50 values given in the literature are: >100 mg/kg (mouse; i.p.), as HCl salt: 113.5 mg/kg (mouse; route of administration unspecified) Minimum lethal dose (as sulfate salt): 300 mg/kg (dog; i.v.); 2000 mg/kg (dog; p.o.); 250 mg/kg (rabbit; i.v.); 300 mg/kg (guinea pig; iv.); 2000 mg/kg (guinea pig; s.c.); ~ 1000 mg/kg (rat; s.c.).
From experiments aimed at identifying the toxin responsible for producing the locomotor disorder ("staggers") and rapidly lethal cardiac toxicosis ("sudden death") periodically observed in livestock feeding on the grass Phalaris aquatica, Australian researchers determined that the lowest doses of hordenine that would induce symptoms of "staggers" in sheep were 20 mg/kg i.v., and 800 mg/kg orally. However, the cardiac symptoms of "sudden death" could not be evinced by hordenine.
Although hordenine is capable of reacting with nitrosating agents (e.g. nitrite ion, NO2-) to form the carcinogen N-nitrosodimethylamine (NDMA), and was investigated as a possible precursor for the significant amounts of NDMA once found in beer, it was eventually established that the levels of hordenine present in malt were too low to account for the observed levels of NDMA.
The pharmacokinetics of hordenine have been studied in horses. After i.v. administration of the drug, the α-phase T1/2 was found to be ~3 mins., and the β-phase T1/2 was ~35 mins.
Hordenine has been found to act as a feeding deterrent to grasshoppers (Melanoplus bivittatus), and to caterpillars of Heliothis virescens and Heliothis subflexa; the estimated concentration of hordenine that reduced feeding duration to 50% of control was 0.4M for H. virescens and 0.08M for H. subflexa.
Hordenine has some plant growth-inhibiting properties: Liu and Lovett reported that, at a concentration of 50 ppm, it reduced the radicle length in seedlings of white mustard (Sinapis alba) by ~ 7%.; admixture with an equal amount of gramine markedly enhanced this inhibitory effect, in a synergistic manner.
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