Regioselective O-methylation of tetrahydropapaveroline and tetrahydroxyberbine in vivo in rat brain

An (R)-specific N-methyltransferase involved in human morphine biosynthesis

The biosynthesis of morphine, a stereochemically complex alkaloid, has been shown to occur in plants and animals. A search in the human genome for methyltransferases capable of catalyzing the N-methylation of benzylisoquinoline alkaloids, as biosynthetic precursors of morphine, yielded two enzymes, PNMT (EC 2.1.1.28) and NMT (EC 2.1.1.49). Introduction of an N-terminal poly-histidine tag enabled purification of both proteins by immobilized metal affinity chromatography. Recombinant PNMT and NMT were characterized for their catalytic activity towards four benzylisoquinolines: tetrahydropapaveroline (THP), 6-O-methyl-THP, 4'-O-methyl-THP and norreticuline. Human PNMT accepted none of the offered alkaloids and was only active with its established substrate, phenylethanolamine. The second enzyme, human NMT, converted all four benzylisoquinolines, however, with a strict preference for (R)-configured morphine precursors. Determination of kinetic parameters of NMT for the four (R)-configured benzylisoquinoline alkaloids by LC-MS/MS revealed (R)-norreticuline to be the best substrate with an even higher catalytic activity as compared to the previously reported natural substrate tryptamine. In addition, isolation of the morphine precursor salutaridine from urine of mice injected (i.p.) with (R)-THP provides new evidence that the initial steps of morphine biosynthesis in mammals occur stereochemically and sequentially differently than in plants and suggests an involvement of the herein characterized (R)-specific NMT.

How human neuroblastoma cells make morphine

Recently, our laboratory demonstrated that human neuroblastoma cells (SH-SY5Y) are capable of synthesizing morphine, the major active metabolite of opium poppy. Now our experiments are further substantiated by extending the biochemical studies to the entire morphine pathway in this human cell line. L-[1,2,3-13C3]- and [ring-2',5',6'-2H3]dopa showed high isotopic enrichment and incorporation in both the isoquinoline and the benzyl moiety of the endogenous morphine. [2,2-2H2]Dopamine, however, was exclusively incorporated only into the isoquinoline moiety. Neither the trioxygenated (R,S)-[1,3-13C2]norcoclaurine, the precursor of morphine in the poppy plant, nor (R)-[1,3,4-2H3]norlaudanosoline showed incorporation into endogenous morphine. However, (S)-[1,3,4-2H3]norlaudanosoline furnished a good isotopic enrichment and the loss of a single deuterium atom at the C-9 position of the morphine molecule, indicating that the change of configuration from (S)- to (R)-reticuline occurs via the intermediacy of 1,2-dehydroreticuline. Additional feeding experiments with potential morphinan precursors demonstrated substantial incorporation of [7-2H]salutaridinol, but not 7-[7-2H]episalutaridinol, and [7-2H,N-C2H3]oripavine, and [6-2H]codeine into morphine. Human morphine biosynthesis involves at least 19 chemical steps. For the most part, it is a reflection of the biosynthesis in opium poppy; however, there is a fundamental difference in the formation of the key intermediate (S)-reticuline: it proceeds via the tetraoxygenated initial isoquinoline alkaloid (S)-norlaudanosoline, whereas the plant morphine biosynthesis proceeds via the trioxygenated (S)-norcoclaurine. Following the plant biosynthetic pathway, (S)-reticuline undergoes a change of configuration at C-1 during its transformation to salutaridinol and thebaine. From thebaine, there is a bifurcate pathway leading to morphine proceeding via codeine or oripavine, in both plants and mammals.

Presence of reticuline in rat brain: a pathway for morphine biosynthesis

We demonstrate the presence of reticuline, an isoquinoline alkaloid that was purified and identified in the rat brain. This was achieved by high-performance liquid chromatography coupled with electrochemical detection. This material was finally identified by nano-electrospray ionization quadrupole time-of-flight tandem mass spectrometry. The expression of this tetrahydroisoquinoline alkaloid in rat brain is at 12.7+/-5.4 ng/g wet tissue. Furthermore, rat chow, rat small and large intestine and bacteria cultured from these tissues did not contain either morphine or reticuline, eliminating the possibility of contamination or an exogenous source of these compounds. This finding adds information which suggests that morphine biosynthesis may occur in rat neural tissues, and that its biosynthesis pathway may be similar to that reported in the poppy plant.

The role of opioid-dopamine interactions in the induction and maintenance of ethanol consumption

1. Alcohol is one of the most widely used recreational drugs, but also one of the most widely abused, causing vast economic, social and personal damage. 2. Several animal models are available to study the reinforcing mechanisms that are the basis of the abuse liability of ethanol. Innate differences in opioid or dopamine neurotransmission may enhance the abuse liability of ethanol, as indicated by animal and human studies. 3. Opioid antagonists have been shown to be effective, both experimentally and clinically, in decreasing ethanol consumption, presumably since ethanol induces the release of endogenous opioid peptides in vivo. However, ethanol may also stimulate the formation of opiate-like compounds, which could interact with opioid (or dopamine) receptors. Ethanol may cause changes in neurotransmission mediated via opioid receptors that determines whether alcohol abuse is more or less likely. 4. Ethanol appears to facilitate dopamine release by increasing opioidergic activity, disinhibiting dopaminergic neurons (by inhibition of GABAergic neurotransmission) via mu-opioid receptors in the ventral tegmental area (VTA) and delta-opioid receptors in the nucleus accumbens (NAcc). The effects of ethanol would be antagonised by presynaptic kappa-opioid receptors present on dopaminergic terminals in the NAcc. 5. Mesolimbic dopamine release induced by ethanol consumption seems to indicate ethanol-related stimuli are important, focussing attention on and enabling learning of the stimuli. However, studies indicate that there are redundant pathways, and neural pathways 'downstream' of the mesolimbic dopamine system, which also enable the reinforcing properties of ethanol to be mediated.

Determination of tetrahydropapaveroline in the urine of parkinsonian patients receiving L-dopa-carbidopa (Sinemet) therapy by high-performance liquid chromatography

Tetrahydropapaveroline (THP) concentrations were measured in the urine of Parkinsonian patients receiving L-dopa-carbidopa (Sinemet) therapy, using a method that employs a separation scheme that selectively isolates THP from urine and utilizes the Pictet-Spengler condensation of THP with formaldehyde combined with high-performance liquid chromatography for identification and determination. The mean (+/- S.D.) recoveries of THP from normal urine with 0.2 pmol/ml added and from Parkinsonian patients' urines with 0.5 pmol/ml added were 48.6 +/- 5.7 and 44.6 +/- 3.1%, respectively. Three Parkinsonian patients who were receiving either 250, 750 or 1000 mg of L-dopa (as Sinemet) daily had 24-h urinary THP excretion levels of 989, 1017 and 1600 pmol, respectively.

Tetrahydropapaveroline in brain regions of rats after acute ethanol administration

Tetrahydropapaveroline (THP), the condensation product of dopamine, and its aldehyde, dopaldehyde, have been detected in brain regions of rats after acute ethanol administration. THP levels were determined in eight brain regions of animals that received ethanol (3.0 g/kg) by intraperitoneal injection 100 or 120 minutes before decapitation. The levels of THP in two brain regions, i.e., the midbrain and striatum, were determined at time intervals ranging from 50 to 120 minutes after ethanol administration. THP was not found in brain regions of untreated animals. However, significant levels of THP were found in pooled midbrains (0.50 pmol/g tissue) and pooled hypothalami (0.20 pmol/g tissue) of animals that received ethanol 120 minutes before decapitation. Most brain regions had detectable levels of THP 100 minutes after the animals received ethanol and the striatum contained the highest concentration of the alkaloid. The concentration of THP in striata tissue of rats at 50, 70, 90, or 100 minutes after ethanol administration were 0.33, 0.38, 0.33, and 0.33 pmol/g tissue, respectively. These results demonstrate that THP can be detected in specific brain regions of the rat after acute ethanol administration.

Tetrahydropapaveroline and the blood-brain barrier in rats

The blood-brain barrier penetration of tetrahydropapaveroline (THP) was studied in male rats of the Sprague-Dawley strain. THP was not found in brains of untreated animals. However, THP was observed in the brains of animals that received THP.HBr by intraperitoneal (IP) injection 30 minutes before decapitation. Animals that received IP injections of 0.10, 1.0, 5.0, or 10 mg THP.HBr/kg exhibited brain levels of 3.1, 25, 95 or 126 pmoles THP per gram brain, respectively. Another group of rats received THP.HBr (5.0 mg/kg) IP 30 minutes before decapitation. The brain of each animal was dissected into nine regions and each region assayed for THP. All brain regions assayed had measurable levels of THP. The highest concentration of THP (132 pmoles/g) was observed in the olfactory lobes-frontal cortex while the lowest concentration of THP (27.3 pmol/g) was in the striatum. These results demonstrate that THP penetrates the blood-brain barrier in rats. They also suggest that if THP is formed in the periphery, it may penetrate the blood-brain barrier, and be localized in discrete brain regions.

Dopamine-derived alkaloids in alcoholism and in Parkinson's and Huntington's diseases

Tetrahydroisoquinoline (TIQ) alkaloids and 1-carboxy TIQ derivatives have been found in human fluids and/or tissues. The possible biosynthetic pathways of salsolinol (Sal), taken as an example of TIQs, are discussed, and the possibility that biosynthesis occurs through a stereospecific enzymatic reaction is considered. In this respect, it is reported that the R enantiomer of Sal predominates in urines of healthy volunteers, whereas the S enantiomer predominates in port wine and possibly in other beverages and foods, suggesting that Sal present in humans could have, at least partially, and endogenous enzymatic origin. TIQs and other dopamine-derived alkaloids are weak MAO inhibitors, the R enantiomer of Sal and salsolidine being more potent than the S form. The changes in monoamine oxidase activity and the nigrostriatal concentrations of dopamine and homovanillic acid in Parkinson's and Huntington's diseases and in alcoholism are reviewed. In these pathological situations, changes in the levels of dopamine-derived alkaloid levels may occur. The possibility that the modifications found might cause or contribute to changes in mental and/or neurophysiological states in these pathological situations is considered.

Metabolism of 6,7-dimethoxy 4-(4'-chlorobenzyl)isoquinoline. II. Role of liver catechol O-methyltransferase and glutathione

1. On i.v. administration to rats of 14C-6,7-dimethoxy 4-(4'-chlorobenzyl)isoquinoline (PV2) 23% dose of 14C was excreted in urine and 72% in faeces. The pattern of metabolites showed ten 14C-PV2 derivatives and unchanged PV2. Seven metabolites have been characterized by comparison with authentic compounds e.g. the ketone of PV2, the N-oxide PV2, the demethylated metabolites 6-hydroxy-PV2, 7-hydroxy-PV2 and 6,7-dihydroxy PV2, the benzyl ring-hydroxylated metabolites, 3'-hydroxy-PV2 and 6,7,3'-trihydroxy-PV2. Unchanged PV2 and its metabolites are excreted both free and conjugated. 2. Enzymic O-methylation of 6,7-dihydroxy-PV2 by liver catechol-O-methyl transferase (COMT) in vitro produced 6-hydroxy,7-methoxy-PV2. After blockade of COMT by pyrogallol in vivo, the excretion of 6,7-dihydroxy-PV2 was increased and the excretion of 6-hydroxy, 7-methoxy-PV2 decreased. 3. Hydroxylation of the benzyl ring of PV2 and its metabolites indicates the formation of an intermediate epoxide followed by glutathione conjugation. After glutathione depletion in vivo by diethyl maleate (DEM) liver covalent binding of 14C-PV2 metabolites was increased and biliary excretion of benzyl ring-hydroxylated PV2 metabolites decreased. Replacement of glutathione depletion by a cysteine derivative restored liver covalent binding and the excretion of PV2 metabolites to levels similar to those observed in control rats, indicating that glutathione conjugation may be an important metabolic pathway for the detoxication of PV2 and its metabolites in vivo.