2-propyl-1-aminopentane, its deamination by monoamine oxidase and semicarbazide-sensitive amine oxidase, conversion to valproic acid and behavioral effects

Semicarbazide-sensitive amine oxidase: role in the vasculature and vasodilation after in situ inhibition

1. The characteristics of semicarbazide-sensitive amine oxidase (SSAO) are reviewed and the unknown physiological or pathological role of this enzyme emphasized. 2. The various mechanisms of action proposed for the vasodilator drug hydralazine are considered. In particular, the inhibitory action on various enzymes, related or not to cardiovascular function, are discussed. 3. Studies linking inhibition of SSAO to hydralazine hypotension are reviewed and a general hypothesis relating both actions is presented. The hypothesis postulates that (a). vascular SSAO is involved in the regulation of vascular tone, and (b). hydralazine vasodilation is the consequence of vascular SSAO inhibition. 4. Evidence supporting these postulates is presented and vascular SSAO inhibition is proposed as a novel mechanism of vasodilation.

Monoamine oxidases and related amine oxidases as phase I enzymes in the metabolism of xenobiotics

To date most of the interest in oxidative metabolism of xenobiotics has been devoted to the role of the microsomal cytochrome P-450 system and to establish the basis for classifying and naming P450 enzymes. The contribution of amine oxidases to the metabolism of xenobiotics has been largely neglected, with the exception of the contribution of monoamine oxidases (MAOs) to the metabolism of exogenous tyramine and the studies of the "cheese effect" produced as the result of ingestion of large amounts of tyramine-containing foods under particular conditions. A review of the involvement of the mitochondrial MAOs in drug metabolism was published in 1988. Since that time, considerable additional evidence has appeared in the literature to support the contribution of MAOs to drug metabolism. In addition, the involvement of other amine oxidases in the metabolism of foreign compounds has been established. A second review on the contribution of amine oxidases to the metabolism of xenobiotics was therefore published in 1994. On an arbitrary basis, the heterogeneous class of amine oxidases can be divided into two types according to their prosthetic group: the flavineadenine dinucleotide (FAD)-dependent amine oxidases (Monoamine Oxidase and Polyamine Oxidase) and the amine oxidases not containing FAD (Semicarbazide-sensitive amine oxidases). In this overview, the contributions of these two types in xenobiotic metabolism are considered separately.

Substrate-specificity of mammalian tissue-bound semicarbazide-sensitive amine oxidase

Although the existence of a membrane-bound (probably plasmalemmal) semicarbazide-sensitive amine oxidase (SSAO) is well established in various mammalian tissues, and especially within vascular smooth muscle, its importance and the possible consequences of its metabolism of certain physiological and xenobiotic amines in vivo are under continuing investigation. In this respect, there are major species-related differences in substrate specificity determined in vitro, not only towards the synthetic amine benzylamine, but also towards some other aromatic amines (e.g. tyramine, tryptamine, 2-phenylethylamine, dopamine, histamine) which are possible endogenous substrates. Inhibition of SSAO can potentiate the pharmacological activity of some amines in isolated tissue (e.g. blood vessel) preparations from some species. Recent evidence has accumulated that SSAO may also be involved in metabolizing endogenous aliphatic amines such as methylamine and aminoacetone, focussing attention on the fact that the aldehyde products (formaldehyde and methylglyoxal, respectively) are potentially cytotoxic agents. Indeed, SSAO has been implicated in experimental models of cardiovascular toxicity involving conversion of the industrial aliphatic amine allylamine to acrolein. In summary, metabolism by SSAO may reduce the physiological/pharmacological effects of some amines, but the resulting metabolites (aldehydes, H2O2) may also have important actions.

Aliphatic propargylamines, a new series of potent selective, irreversible non-amphetamine-like MAO-B inhibitors. Their structures, function and pharmacological implications

1-Deprenyl, a selective irreversible MAO-B inhibitor, has been shown to prolong the onset of disability in Parkinson's patients and to improve cognitive behavior in Alzheimer's disease. It has been claimed that 1-deprenyl exhibits neuroprotective and neurorescue effects in several animal models. The precise mechanism of these effects is unknown. It is yet to be established whether or not the effects are unique to 1-deprenyl; a drug which possesses, in addition to inhibition of MAO-B activity, an amphetamine moiety. Based on the fact that several N-methylpropargylamine derivatives have been shown to be MAO inhibitors and that aliphatic amines are typical MAO-B substrates with a high affinity for the enzyme, we have synthesized a series of aliphatic propargylamines which have turned out to be highly potent, selective and irreversible MAO-B inhibitors, structurally unrelated to amphetamine. The potency of these inhibitors is related to their chain length and the substitution of a hydrogen on the terminal carbon of the aliphatic chain. MAO-I activity, as assessed in vitro, increased as the aliphatic carbon chain length increased; substitution of the hydrogen at the aliphatic chain terminal by hydroxyl, carboxyl or carboethoxyl groups or replacement of the methyl group on the nitrogen atom by an ethyl group considerably reduced their inhibitory activity. Stereospecific effects were observed with the R-(-)-enantiomer being 20-fold more active than the S-(+)-enantiomer. Inhibitors with relatively short carbon chain lengths (i.e. four to six carbons) were found to be more potent at inhibiting brain MAO-B activity in vivo especially after oral administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical, neuroprotective and neurorescue effects of aliphatic N-methylpropargylamines; new MAO-B inhibitors without amphetamine-like properties

A series of aliphatic N-methylpropargylamine MAO-B inhibitors have been synthesized and their structural and functional relationships have been investigated. 2-Hexyl-N-methylpropargylamine (2-HxMP), for example, has been found to be a highly potent, irreversible, selective, MAO-B inhibitor both in vitro and in vivo. The R-(-)-enantiomers are much more active than the S-(+)-enantiomers at inhibiting MAO-B activity. Some of these compounds protect mouse nigrostriatal dopamine neurons against the neurotoxin MPTP and the mouse hippocampal noradrenergic system against the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). They rescue hippocampal neurons after damage induced by ischemia and kainic acid treatment, as well as motoneurons in young mice following facial nerve axotomy. Such rescue effects are, interestingly, unrelated to inhibition of MAO-B activity. Some of the aliphatic propargylamines enhance the survival of neuroblastoma cells co-cultured with astrocytes following serum depletion. They stimulate the expression of AADC mRNA and inhibit GFAP mRNA expression. They do not possess amphetamine-like properties and exhibit no effect on noradrenaline or dopamine uptake nor do they increase hypertensive effects in the tyramine pressor test. Unlike R(-)-deprenyl, 2-HxMP does not potentiate dopamine toxicity in vitro. These new MAO-B inhibitors may possess significant chemotherapeutic implications for certain psychiatric and neurodegenerative disorders.

Deamination of aliphatic amines by type B monoamine oxidase and semicarbazide-sensitive amine oxidase; pharmacological implications

Straight and branched chain aliphatic monoamines, which are not normal tissue constituents, are deaminated selectively by type B monoamine oxidase (MAO-B). They exhibit a high affinity towards the active site of MAO-B and this made them very useful pharmacologically. An anticonvulsant prodrug, Milacemide [2-(N-pentyl)glycinamide] is deaminated by MAO-B and this facilitates a mechanism of delivering glycine into the CNS. We have found that 2-propyl-pentylamine (2-propyl-1-aminopentane) and N-(2-propylpentyl)glycinamide are also converted by MAO-B to valproic acid and glycine both in vitro and in vivo; these compounds, however, cause severe tremor. By attaching a propargylamine group the resultant series of aliphatic propargylamine derivatives have been shown to be very potent selective MAO-B inhibitors. They are chemically quite different from most other MAO-B inhibitors, since they do not possess any aromatic structures. The relatively short chain aliphatic propargylamines, i.e. N-2-pentyl-N-methylpropargylamine and N-2-hexyl-N-methylpropargylamine, are 4 to 5 times more potent and more selective than selegiline (1-deprenyl) with respect to the inhibition of MAO-B in brain following oral administration. Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the deamination of not only longer chain aliphatic amines but also short chain aliphatic amines including methylamine. Formaldehyde is produced from methylamine by SSAO. Increased methylamine deamination may cause cellular damage in some pathological conditions, such as uraemia and diabetes. We have observed that cultured human endothelial cells are damaged by methylamine in the presence of SSAO. Inhibition of the SSAO activity completely protects these cells from the methylamine-SSAO induced damage.

Simultaneous delivery of valproic acid and glycine to the brain. Deamination of 2-propylpentylglycinamide by monoamine oxidase B

2-Propylpentylglycinamide (2-PPG), a branched aliphatic amine derivative, was found to be readily deaminated by rat liver monoamine oxidase B in vitro and in vivo. The deamination leads to production of 2-propyl-1-pentaldehyde, which can be subsequently converted to valproic acid (VPA), and glycinamide, which is then subsequently converted to glycine. Absorption and biotransformation of a single ip dose of 2-PPG into blood as well as transfer of the drug and its metabolite into the brain were rapid processes. Although VPA (an anticonvulsant) and glycine (an inhibitory neurotransmitter) can be detected in the brain following administration of 2-PPG, its anticonvulsant action cannot be determined. 2-PPG at relatively low doses exhibited distinct tremor effects. Furthermore, 2-PPG appeared to potentiate the convulsant effect induced by pentylenetetrazol.