The deamination of n-pentylamine by monoamine oxidase and a semicarbazide-sensitive amine oxidase of rat heart

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.

The interactions of milacemide with monoamine oxidase

The interactions of the anticonvulsant drug milacemide (2-n-pentylaminoacetamide) with rat liver mitochondrial monoamine oxidases-A and -B have been studied. The compound acts as a substrate for the B-form of the enzyme, with an apparent Km value of 49 +/- 4.7 microM and a Vmax value of 1.1 +/- 0.2 nmol/min/mg. It is also a time-dependent irreversible inhibitor of that enzyme. Any activity of monoamine oxidase-A towards this substrate was too low to allow accurate determinations to be made by either luminometric determination of H2O2 formation or spectrophotometric coupling of aldehyde formation to NAD+ reduction in the presence of aldehyde dehydrogenase. Milacemide was a reversible competitive inhibitor towards monoamine oxidase-A. The inhibitor constant (Ki) was 115 +/- 35 microM indicating a higher affinity than that towards monoamine oxidase-B, which was also competitively inhibited in the absence of enzyme-inhibitor preincubation (Ki = 331 +/- 185 microM). Determination of the formation of H2O2 and the aldehyde product of the oxidative cleavage of milacemide by purified monoamine oxidase-B from ox liver indicated that cleavage resulted solely in the formation of pentanal and glycinamide. There was no evidence for alternative cleavage to pentylamine and oxamaldehyde.

Involvement of FAD-dependent polyamine oxidase in the metabolism of milacemide in the rat

1. It has previously been established that monoamine oxidase (MAO)-B participates in the metabolism of milacemide [2-(pentylamino)acetamide]. Furthermore, in rats, inhibition of FAD-dependent polyamine oxidase (PAO) was found to decrease the urinary excretion of two milacemide metabolites, termed UK1 and UK2. 2. Using gas chromatography-mass spectrometry, UK1 was identified as oxamic acid and UK2 as 2-hydroxyacetamide, confirming that PAO is involved in the metabolism of milacemide. 3. Thus, two FAD-dependent amine-oxidizing enzymes, MAO and PAO, contribute to the metabolism of milacemide. Milacemide appears to be the first non-polyamine xenobiotic in the metabolism of which PAO participates.

Oxidative deamination of aliphatic amines by rat aorta semicarbazide-sensitive amine oxidase

Rat aorta semicarbazide-sensitive amine oxidase (SSAO) exhibits very high affinity in the deamination of an homologous series of aliphatic amines of 1 to 18 straight chain carbon atoms. The Km value decreases substantially as the chain length of these amines increases. The Vmax values are higher for the short chain amines. Diamines are poor substrates for SSAO or are not acted upon by the enzyme. The substrate preference for SSAO differs from that for monoamine oxidase.

Does FAD-dependent polyamine oxidase contribute to the metabolism of milacemide?

Milacemide, a secondary amine derivative, was previously demonstrated to be a substrate of MAO-B and to be insensitive to the action of copper-dependent amine oxidases. In the present study, it was investigated whether the FAD-dependent secondary amine metabolizing enzyme polyamine oxidase (PAO), could participate in the metabolism of milacemide. For this purpose, the urinary metabolic pattern of oral 14C-milacemide was determined in rats with and without pretreatment with the irreversible PAO inhibitor MDL 72527 and, for comparison, after inhibition of MAO-B by l-deprenyl. While l-deprenyl was shown to significantly decrease the urinary excretion of glycinamide and of an unknown metabolite (UK1), pretreatment with MDL 72527 did not modify the elimination of milacemide as glycinamide but produced a decrease in UK1 equal to that induced by l-deprenyl. The percent of the dose of milacemide eliminated as unchanged drug was slightly but significantly increased after PAO inhibition, though considerably less than after l-deprenyl. These data suggest that milacemide might be a substrate of PAO. If confirmed, this result would constitute the first example of the involvement of the FAD-dependent PAO in drug metabolism.

Deamination of aliphatic amines of different chain lengths by rat liver monoamine oxidase A and B

Monoamines with from 1 to 18 straight chain carbon atoms have been analysed as rat liver monoamine oxidase substrates. Methylamine and ethylamine are clearly not substrates of monoamine oxidase (MAO). n-Propylamine, n-butylamine, n-dodecylamine and n-octadecylamine are relatively poor substrates, i.e. with high Km and low Vmax values for the enzyme. n-Pentylamine, n-hexylamine, n-heptylamine, n-octylamine, n-nonylamine and n-decylamine are all very good MAO substrates. All these aliphatic amines are found to be typical type B substrates according to the sensitivities of the enzyme towards the selective MAO-B inhibitor selegiline and the MAO-A inhibitor, clorgyline. The sensitivity towards selegiline with respect to these amines is even higher, i.e. Ki = 1 x 10(-9) M for butylamine, than that of the typical type B substrate beta-phenylethylamine (Ki = 1 x 10(-8) M). The sensitivity towards selegiline decreases slightly with increasing chain length of these aliphatic amines.

Inhibition of monoamine oxidase and semicarbazide-sensitive amine oxidase by mexiletine and related compounds

The in vitro inhibition by mexiletine and related compounds of the activity of rat brain, heart and lung monoamine oxidase-A (MAO-A), rat brain MAO-B, human platelet-poor plasma benzylamine oxidase and a clorgyline-resistant, semicarbazide-sensitive amine oxidase (SSAO) distinct from both MAO and benzylamine oxidase has been studied. The compounds were most active towards MAO-A and SSAO. IC50 values for mexiletine towards rat heart MAO-A and SSAO were 10 mumol/l and 320 mumol/l, respectively. Replacement of the para-hydrogen atom in the mexiletine aromatic ring by bromine increased potency towards both MAO-A and SSAO. Replacement of the ortho-methyl group in the mexiletine aromatic ring by hydrogen increased the potency towards SSAO alone. FLA 1042, with both these substitutions, was found to be a reversible mixed-type inhibitor of both MAO-A (Kislopei 1.4 mumol/l, Kiinti 24 mumol/l) and of SSAO (Kislopei 12 mumol/l, Kiinti 6 mumol/l).

Monoamine oxidase and semicarbazide-sensitive amine oxidase in spontaneously hypertensive and in normotensive control rats

The aim of the present work was to compare monoamine oxidase (MAO) and semicarbazide sensitive amine oxidase (SSAO) activity in several tissues from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Contribution of MAO-A, -B and SSAO to the metabolism of each substrate in each tissue was defined from experiments where the decrease of oxidative deamination of each substrate at a given concentration was measured as a function of increasing concentrations of a selective MAO-A, -B or SSAO inhibitor. In the heart, aorta and, to a lesser extent, the femoral arteries MAO-A activity was higher in SHR than in WKY. Similarly in the liver the enzyme activity was higher in SHR than in WKY but was due to the -B form of MAO. In all the other tissues studied (duodenum, brain, lungs, adrenals and kidneys) no difference in MAO-A, MAO-B or SSAO activity was found between SHR and WKY, except for the kidneys and brain, if the differences in the weights of these organs in SHR are taken into account.

The deamination of noradrenaline and 5-hydroxytryptamine by rat brain and heart monoamine oxidase and their inhibition by cimoxatone, toloxatone and MD 770222

In both rat brain and heart, noradrenaline and 5-hydroxytryptamine are metabolised predominantly by monoamine oxidase-A. The Km values for 14C-noradrenaline in the rat brain and heart are 290 microM and 300 microM, respectively, whereas for 14C-5-hydroxytryptamine the values are 180 microM and 140 microM, respectively. In the rat brain, mixed substrate experiments suggested that 14C-noradrenaline and 14C-5-hydroxytryptamine are metabolised at the same active centre. Both substrates are inhibited with similar Ki values by the reversible inhibitors cimoxatone, toloxatone and MD 770222.