Effects of aldosterone and related steroids on LPS-induced increased expression of inducible NOS in rat aortic smooth muscle cells

BACKGROUND AND PURPOSE

Expression of inducible NOS (iNOS) is important in certain inflammatory diseases. We determined if the hormone aldosterone, a mineralocorticoid receptor (MR) agonist, affects LPS activation of iNOS expression in rat aortic smooth muscle cells (RASMC).

EXPERIMENTAL APPROACH

Cultured RASMC were treated with LPS, with or without agonists/antagonists of steroid receptors. iNOS expression was determined by nitrite assays on culture medium removed from treated cells and by immunoblotting of cell protein extracts.

KEY RESULTS

LPS (1 µg·mL(-1) ) increased nitrite and iNOS protein above that in control (untreated) cells. These effects of LPS were reduced by aldosterone (0.1-10 µM). The MR antagonists, eplerenone (10 µM) and spironolactone (10 or 50 µM), did not inhibit these actions of 1 µM aldosterone, but the latter were prevented by 10 µM mifepristone, a glucocorticoid (GR) and progestogen receptor (PR) antagonist. Mifepristone also prevented the reduction of LPS-induced nitrite increase produced by 1 µM dexamethasone (GR agonist) and 10 µM progesterone (PR agonist). Spironolactone (10-50 µM) by itself decreased LPS-induced increases in nitrite and iNOS protein. Mifepristone (10 µM) partially reversed these effects of 10 µM spironolactone, but not those of 50 µM; the effects of 50 µM spironolactone were also unchanged when mifepristone was increased to 50 µM.

CONCLUSIONS AND IMPLICATIONS

This pharmacological profile suggests that aldosterone, and possibly 10 µM spironolactone, use mechanisms that are dependent on PR and/or GR, but not MR, to inhibit iNOS induction in RASMC. With 50 µM spironolactone, other inhibitory mechanisms requiring further investigation may become predominant.

Evidence for a clorgyline-resistant monoamine metabolizing activity in the rat heart

When benzylamine was used as substrate, a component of the total monoamine oxidase (MAO) activity in the rat heart was found to be resistant to inhibition by clorgyline. The proportion of the total activity represented by this component, decreased as the rat grew. It was also inhibited by both semicarbazide and isoniazid but not by potassium cyanide. Inhibitor studies with MAO in subcellular fractions showed that this component was more concentrated in the microsomal and soluble fractions. However, it could not be concluded that the activity was entirely a soluble enzyme. Determination of quasi-Michaelis constants (“Km”) for total benzylamine oxidizing activity revealed a high (“Km” of approximately 10−5M) and low (“Km” of approximately 5 times 10−4M) affinity component. The high affinity component was inhibited by semicarbazide and the low affinity component by clorgyline. In the presence of 10−3M clorgyline, the high affinity component showed substrate inhibition at higher substrate concentrations. The possibility is discussed that the clorgyline-resistant activity is due to an amine-oxidizing activity distinct from mitochondrial MAO.

Differential effects of some cell signalling inhibitors upon nitric oxide synthase expression and nuclear factor-kappaB activation induced by lipopolysaccharide in rat aortic smooth muscle cells

Treatment of rat aortic smooth muscle cells (RASMC) with 1 or 100 microg ml-1 lipopolysaccharide (LPS) for 20-24 h led to expression of the inducible form of nitric oxide synthase (iNOS) as detected by Western blotting for iNOS protein, and by determination of increased cellular nitrite formation. LPS-induced nitrite production was inhibited almost completely by concomitant treatment of cells with LPS and either (a) pyrrolidine dithiocarbamate (PDTC, 25 microm), an antioxidant inhibitor of NF-kappaB activation; (b) N-tosyl-L-phenylalanine chloromethyl ketone (TPCK, 20 and 40 microm), a proteasomal inhibitor which prevents NF-kappaB activation; (c) nordihydroguaiaretic acid (NDGA, 10 and 50 microm), a lipoxygenase inhibitor; or (d) apocynin (2, 3.5 and 5 m m), an inhibitor of NADPH oxidase. Gel-shift assays using nuclear protein extracts incubated with a 32P-labelled DNA binding probe for NF-kappaB detected two electrophoretically separable complexes containing NF-kappaB. A faster migrating complex obtained when using both LPS-treated and untreated cells appeared to represent a basal or constitutive NF-kappaB activity, whereas a slower band was found only after LPS-treatment. The latter band was abolished when using cells treated for 1 h with LPS in the presence of PDTC (25 microm) or TPCK (20 microm), but was not inhibited by NDGA (50 microm) or apocynin (3.5 m m). The basal band was unaffected by any of the cell signalling inhibitors. Densitometry of Western blots indicated that LPS-induced iNOS protein expression was inhibited to a similar extent (between 74 and 87%) by the latter concentrations of PDTC, TPCK, NDGA and apocynin. The ability of PDTC and TPCK to abolish LPS-specific NF-kappaB activation, while also producing considerable inhibition of iNOS protein expression and nitrite formation, suggests that induction of iNOS by LPS in RASMC involves NF-kappaB-dependent transcription. However, the failure of NDGA and apocynin to prevent NF-kappaB activation, at least during early stages (up to 1 h) of its nuclear accumulation, suggests that these agents may affect cell signalling pathways which regulate iNOS induction by another mechanism to be determined.

Properties and functions of tissue-bound semicarbazide-sensitive amine oxidases in isolated cell preparations and cell cultures

The demonstration of semicarbazide-sensitive amine oxidase (SSAO) activity in some freshly-dispersed cell preparations and in particular types of cells grown in culture, provides increasing opportunities for investigating the importance of SSAO in various aspects of cellular function. Assays of benzylamine and methylamine metabolism in homogenates of cultured cells have established clearly that SSAO is expressed in rat and pig vascular (aortic) smooth muscle cells, as well as in rat non-vascular (anococcygeus, trachea) smooth muscle, brown and white adipocytes. However, to date little or no SSAO activity has been detected in cultures of human vascular smooth muscle cells grown from blood vessels (e.g. umbilical artery) known to contain the enzyme, and the reason for this is not yet apparent. However, those cell cultures expressing SSAO are offering useful experimental models for studying biochemical and toxicological consequences upon cellular function which may result from the metabolism of various aromatic and aliphatic amines suggested to be possible physiological and xenobiotic substrates of the enzyme.

Studies on the behaviour of semicarbazide-sensitive amine oxidase in Sprague-Dawley rats treated with the monoamine oxidase inhibitor tranylcypromine

The possibility that increased levels of the activity of the semicarbazide-sensitive amine oxidase (SSAO) might, to some extent, compensate for the loss of monoamine oxidase (MAO) activity in the atypical form of Norrie Disease, was examined using the rat as a model. Long-term treatment with the MAO inhibitor tranylcypromine (1 mg/kg/day) resulted in sustained inhibition of MAO-A and MAO-B activities in liver and brain. After one week, the SSAO activity in heart had increased by 79% above the control levels. This increase was maintained for 3 weeks. Since such alterations might result from enzyme induction, the turnover of the enzyme was studied in cultured cells from rat aortic smooth muscle. The time-course of recovery of enzyme activity following irreversible inhibition by MDL 72145 corresponded to a half-life of approximately 6 days for this process.

Amplification of alpha 1D-adrenoceptor mediated contractions in rat aortic rings partially depolarised with KCl

Partial depolarisation of smooth muscle in endothelium-denuded rat aortic ring preparations, by increasing physiological buffer KC1 concentrations from 4.7 to 14.7 mM, produced a leftward shift of concentration response curves (CRCs) to the alpha 1-adrenoceptor agonist noradrenaline (NA), phenylephrine and methoxamine, without changing maximal responses, whereas maximal responses to clonidine (CLON), also an alpha 1-agonist in this tissue were considerably increased. Partial depolarisation did not alter responses to 10 nM NA or 100 nM CLON in Ca2+(-free) buffer, but significantly increased the contractions obtained on adding Ca2+ back in the presence of the agonists. The potentiation of NA (2.5 and 5 nm) contractions by partial depolarisation was prevented by the voltage-operated Ca2+ channel (VOCC) antagonist nifedipine (NIF, 1 microM). NIF did not significantly affect NA CRCs in 4.7 mM KCl, whereas responses in 14.7 mM KCl were significantly decreased, indicating VOCC recruitment by NA only in the latter condition. Initial depletion of intracellular Ca2+ stores with 1 microM thapsigargin (THAP) in Ca2+(-free) buffer did not alter NA CRCs subsequently obtained in normal Ca2+. However, after THAP-pretreatment, these NA responses (in both 4.7 and 14.7 mM KC1) were attenuated by NIF, indicating that VOCCs were activated by NA in THAP-treated tissues. SKF 96365 (SKF, 30 microM), which can block VOCC and non-VOCC routes of extracellular Ca2+ influx, inhibited NA responses in 4.7 mM and 14.7 mM KCl, possibly implying a role for both types of Ca2+ entry in contractions. However, the greater inhibitory effects of SKF in THAP-pretreated tissues, probably reflected the mobilisation of VOCCs by NA following THAP exposure, because SKF was shown separately to block VOCC-mediated contractions in tissues depolarised with 100 mM KCl alone. 10 microM niflumic acid, an inhibitor of Ca2+(-activated) Cl- channels, did not affect responses to NA in 4.7 mM or 14.7 mM KC1, suggesting that VOCC opening induced by NA in 14.7 mM KCl was not due to depolarisation produced by alpha 1-adrenoceptor induced Cl- efflux. CRCs for NA were unaffected by pretreatment of rings with 100 ng ml-1 pertussis toxin (PT), suggesting a lack of involvement of PT-sensitive G proteins in the contractions obtained either in 4.7 or 14.7 mM KCl. BMY 7378 (100 microM), a selective antagonist for alpha 1D-adrenoceptors, competitively inhibited NA contractions with apparent pKB values of 8.7 +/- 0.2 and 8.4 +/- 0.1 in 4.7 mM and 14.7 mM KCl, respectively. Pretreatment of rings with chloroethylclonidine (100 microM), an irreversible antagonist of alpha 1B-and alpha 1D-adrenoceptors, produced similar rightward shifts in CRCs to NA by 3.2 +/- 0.2 and 3.7 +/- 0.3 log concentration units in 4.7 mM and 14.7 mM KCl, respectively, without changing maximal responses. Inositol phosphate (IP) turnover produced by NA in aortic rings was not significantly different in 4.7 mM compared with 14.7 mM KCl. As a whole, these results suggest that partial depolarisation of the rat aorta with KCl enhances alpha 1-adrenoceptor mediated contractions predominantly via the alpha 1D-subtype, and by a mechanism to be identified which allows greater recruitment of VOCCs by NA. In addition, the ability of THAP-pretreatment also to enhance VOCC activation by NA suggests that Ca2+ release from, or prevention of its reuptake into, intracellular stores may contribute to those processes leading to VOCC opening.

[Effect of activity of semicarbazide-sensitive aminooxidases and cellular glutathione on the cytotoxic effect of allylamine, acrolein, and formaldehyde in human cultured endothelial cells]

The ability of allylamine (AA) administration to produce vascular lesions resembling atherosclerotic disease in animals, has been linked to metabolism of AA to the toxic aldehyde acrolein (ACR) by a semicarbazide-sensitive amine oxidase (SSAO) found in plasma and in vascular smooth muscle. Here, we have assessed the cytotoxicity of AA and ACR towards cultured human umbilical vein endothelial cells (HUVEC). After 20h treatment, 50 microM AA alone had no effect and 100 microM AA produced a modest reduction in cell viability. However, both concentrations produced considerable cell death when incubated with HUVEC in the presence of human umbilical artery homogenate as a source of human vascular SSAO activity. The cytotoxic actions of 50 microM AA were not altered by coincubation with 100 microM pargyline (an inhibitor of monoamine oxidase, MAO) but were completely prevented by 100 microM semicarbazide (SSAO inhibitor) and propargylamine (MAO/SSAO inhibitor). ACR at 50 and 100 microM was considerably cytotoxic, but had little effect at 5 and 10 microM. Since SSAO can also metabolize the biogenic aliphatic amine methylamine to formaldehyde (FA), effects of this aldehyde upon HUVEC were also studied. 50 microM FA did not significantly alter HUVEC viability whereas 200 microM FA produced a small but significant reduction in viability. However, 200 microM FA (but not 50 microM FA) was highly cytotoxic in HUVEC pretreated for 24h with the glutathione (GSH) synthesis inhibitor, D.L-buthionine sulphoximine (200 microM). These results suggest that endothelial integrity or function in blood vessels could be affected by these aliphatic aldehydes as environmental pollutants, dietary contaminants, and possible products of metabolic pathways, including those involving the action of SSAO upon biogenic and xenobiotic aliphatic amines. The availability of GSH-dependent mechanisms for metabolizing these aldehydes may have an important protective influence.

Mammalian plasma and tissue-bound semicarbazide-sensitive amine oxidases: biochemical, pharmacological and toxicological aspects

Mammalian plasma and tissues contain various soluble and membrane-bound enzymes which metabolize the synthetic amine benzylamine particularly well. The sensitivity of these enzymes to inhibition by semicarbazide and related compounds suggests that they contain a cofactor with a reactive carbonyl group, which has been proposed to be either pyridoxal phosphate, pyrroloquinoline quinone or (more recently) 6-hydroxydopa. It is not yet clear if all of these semicarbazide-sensitive amine oxidases (SSAOs) are copper-dependent enzymes. A variety of compounds have now been identified as relatively selective inhibitors to distinguish the SSAOs from other amine oxidases, in order to investigate the properties of SSAOs and their potential role in biogenic and xenobiotic amine metabolism in vivo. While plasma SSAO is soluble, most tissue SSAOs appear to be membrane-bound, probably plasmalemmal enzymes, which may be capable of metabolizing extracellular amines. Vascular (and non-vascular) smooth muscle cells have particularly high SSAO activity, although recently the enzyme has been found in other cell types (e.g. adipocytes, chondrocytes, odontoblasts) implying a functional importance not restricted solely to smooth muscle. The substrate specificity of plasma and tissue SSAOs shows considerable species-related variations. For example, while some endogenously-occurring aromatic amines such as tyramine and tryptamine are metabolized well by SSAO in homogenates of rat blood vessels, and also in vitro inhibition of SSAO can potentiate vasoconstrictor actions of these amines in rat vascular preparations, these amines are poor substrates for human SSAO, thus complicating attempts to generalize possible physiological roles for these enzymes. Vascular SSAO can metabolize the xenobiotic aliphatic amine, allylamine, to the cytotoxic aldehyde acrolein and this has been linked to the ability of allylamine administration to produce cardiovascular lesions in experimental animals, sometimes mimicking features of atherosclerotic disease. Recent studies showing that the endogenously-occurring aliphatic amines methylamine and aminoacetone are metabolized in vitro to formaldehyde and methylglyoxal, respectively, by SSAO in some animal (including human) tissues, suggest the possibility that toxicological consequences upon cellular function could result if such conversions occur in vivo.

Aminoacetone metabolism by semicarbazide-sensitive amine oxidase in rat aorta

High speed (105,000 g/60 min) membrane fractions from rat aorta homogenates metabolized the aliphatic amine aminoacetone (AA) to methylglyoxal (MG) with a Km of 19 +/- 3 microM, and Vmax of 510 +/- 169 nmol MG/hr/mg protein. This deaminating activity appears to be due to a semicarbazide-sensitive amine oxidase (SSAO), which is associated with smooth muscle cells in blood vessels of the rat and other species. AA was a competitive inhibitor (Ki of 28 +/- 6 microM) of the metabolism of benzylamine, a synthetic amine often used as an assay substrate for SSAO. AA is produced endogenously from mitochondrial metabolism of threonine and glycine, and thus could be a physiological substrate for SSAO, whereas the production of MG by SSAO could have cytotoxic implications for cellular function.

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.

Several aspects on the amine oxidation by semicarbazide-sensitive amine oxidase (SSAO) from bovine lung

The lung has been shown to be potentially important in the metabolism of amines. Since SSAO has been demonstrated to be active towards some volatile short-chain aliphatic amines in other tissues, the current study determined the specificity and kinetic constants for the metabolism by bovine lung SSAO, of several aliphatic and aromatic amines some of which have been suggested to be physiological substrates (e.g. methylamine, aminoacetone and beta-phenylethylamine), and others (e.g. benzylamine) which are non-physiological. In the case of benzylamine, an inhibition at high substrate concentration was observed. Kinetic assays ruled out the possibility that this inhibition was caused by products of the deamination of benzylamine, and consequently it is suggested that these results may indicate the presence of two binding sites for the interaction of benzylamine with SSAO.

Properties of mammalian tissue-bound semicarbazide-sensitive amine oxidase: possible clues to its physiological function?

Semicarbazide-sensitive amine oxidase (SSAO), occurs not only in vascular smooth muscle but also in other cell types (e.g. adipocytes, chondrocytes, odontoblasts), probably in the plasma membrane. Although certain aromatic biogenic amines (e.g. tryptamine, tyramine, beta-phenyl-ethylamine) may be endogenous substrates for SSAO in species such as the rat, the weak activity of SSAO in human tissues towards these amines makes this less likely in man. However SSAO in human and rat vascular homogenates readily converts the aliphatic biogenic amines methylamine and aminoacetone to formaldehyde and methylglyoxal, respectively. Also the xenobiotic aliphatic amine allylamine produces cardiovascular damage in experimental animals by a mechanism which involves its deamination by SSAO to acrolein. Further metabolism of these toxic aliphatic aldehydes may involve glutathione-dependent pathways. Thus, SSAO may be involved not only in the removal of physiologically-active endogenous/xenobiotic amines, but resulting metabolite (aldehyde/H2O2?) formation could also influence cellular function.

Metabolism of methylamine by semicarbazide-sensitive amine oxidase in white and brown adipose tissue of the rat

The metabolism of [14C]methylamine (MA) by amine oxidase activity in rat white and brown adipose tissue homogenates, and in mature adipocytes from these tissues has been studied. Oxidation of MA was completely inhibited by 0.1-1 mM semicarbazide, without being affected by the monoamine oxidase (MAO) inhibitor, pargyline (1 mM), indicating that MA is metabolized by semicarbazide-sensitive amine oxidase (SSAO) and not by MAO. The mean Km for MA deamination in all of these sources was around 250-300 microM. SSAO activity towards MA was also demonstrated in white and brown pre-adipocytes, transformed to the adipose phenotype by treatment in culture for 7 days with lipogenic agents. These results are similar to previous findings that SSAO in vascular smooth muscle is able to metabolize aliphatic amines such as MA, and furthermore suggest that SSAO may play a role in adipose tissue function and/or maturation.

The metabolism of aminoacetone to methylglyoxal by semicarbazide-sensitive amine oxidase in human umbilical artery

The aliphatic amine aminoacetone has been described previously as a product of mitochondrial metabolism of threonine and glycine. Here, aminoacetone is shown to be deaminated to methylglyoxal by supernatants obtained by low speed centrifugation (600 g/10 min) of human umbilical artery homogenates, and also by membrane fractions isolated by high speed centrifugation (105,000 g/60 min) of these supernatants. Metabolism of 100 microM aminoacetone was completely inhibited by 1 mM propargylamine and MDL 72145, drugs which are capable of inhibiting the membrane-bound semicarbazide-sensitive amine oxidase (SSAO) activity found in vascular smooth muscle cells, whereas 1 mM pargyline and deprenyl which are inhibitors of monoamine oxidase, were without inhibitory effect. Estimated kinetic constants (at pH 7.8) for aminoacetone metabolism were Km = 92 microM; Vmax = 270 nmol/hr/mg protein. In addition, aminoacetone was a competitive inhibitor (Ki = 83 microM and 128 microM in low speed supernatants and high speed membrane fractions, respectively) of [14C]benzylamine metabolism by SSAO in this tissue. Aminoacetone would appear to be an endogenously occurring amine with a Km for metabolism by SSAO far lower than other aliphatic and aromatic biogenic amines examined previously as potential physiological substrates for the human vascular enzyme and possible implications of this are discussed.

Methylamine metabolism to formaldehyde by vascular semicarbazide-sensitive amine oxidase

The capacity of the vascular enzyme, semicarbazide-sensitive amine oxidase (SSAO), to metabolize methylamine to the potentially toxic product, formaldehyde, was tested using rat aortic homogenates and purified porcine aortic SSAO. Formaldehyde production in incubations of enzyme source with methylamine (1 mM) was detected by high performance liquid chromatography and product was confirmed by desorption chemical ionization mass spectrometry (DCI-MS). Inhibitor studies using the specific SSAO inhibitor semicarbazide and the monoamine oxidase inhibitor pargyline indicate that SSAO is responsible for metabolism of methylamine to formaldehyde. These results suggest the possibility that elevated methylamine found in several pathologic states (such as uremia and diabetes mellitus), or generated from exogenous sources, could result in overproduction of formaldehyde in tissues with high SSAO activity, especially blood vessels.

Further studies on the metabolism of methylamine by semicarbazide-sensitive amine oxidase activities in human plasma, umbilical artery and rat aorta

An ion exchange radiochemical assay has been developed to study the deamination of [14C]methylamine (MA) in homogenates of rat aorta and human umbilical artery, as well as in samples of human plasma. MA metabolism was found to be inhibited almost completely by 1 mM semicarbazide, but virtually unaffected by 0.1 mM clorgyline, suggesting that MA is a substrate for the semicarbazide-sensitive amino oxidase (SSAO) activities which also metabolize benzylamine (BZ) in these sources. Mean Km values for MA metabolism by aorta, umbilical artery and plasma were 182, 832 and 516 microM, respectively, with corresponding Vmax values in aorta and umbilical artery of 100 and 590 nmol (mg prot.)-1 h-1, and in plasma of 48 nmol (mL serum)-1 h-1. Kinetic constants determined for [14C]BZ metabolism in plasma (by an organic solvent extraction assay) and in umbilical artery (by the ion exchange assay) yielded mean Km values of 225 microM (plasma), 222 microM (umbilical artery), and Vmax values of 28 nmol (mL serum)-1 h-1 (plasma) and 377 nmol (mg prot.)-1 h-1 (umbilical artery). The deamination of [14C]MA was inhibited competitively by unlabelled BZ, with Ki values in umbilical artery and plasma of 220 and 172 microM, respectively. Also, metabolite formation from mixtures of [14C]BZ (200 microM) and [14C]MA (800 microM) was extremely close to that predicted for a single enzyme capable of metabolizing two alternative substrates in a competitive fashion.(ABSTRACT TRUNCATED AT 250 WORDS)

Semicarbazide-sensitive amine oxidase activity in rat aortic cultured smooth muscle cells

Metabolism of 5 microM benzylamine (BZ) by rat aortic cultured smooth muscle cells (SMC) is inhibited almost completely by 10(-3) M semicarbazide and 10(-6) M propargylamine, but is little affected by 10(-4) M and 10(-3) M pargyline and clorgyline, indicating BZ metabolism predominantly by the semicarbazide-sensitive amine oxidase (SSAO) previously characterized in rat aortic homogenates. Km values of 7-9 microM for BZ metabolism by SSAO in SMC cultures, indicate similar, if not identical properties, to the enzyme in the parent blood vessel.

The enhanced daily excretion of urinary methylamine in rats treated with semicarbazide or hydralazine may be related to the inhibition of semicarbazide-sensitive amine oxidase activities

The effects of amine oxidase inhibitors upon the daily urinary excretion of monomethylamine (MMA), dimethylamine (DMA), trimethylamine (TMA) and ammonia in the rat have been examined. Administration of hydralazine (5 mg kg-1) or semicarbazide (100 mg kg-1), drugs which irreversibly inhibit semicarbazide-sensitive amine oxidases (SSAO) but not monoamine oxidase (MAO), enhanced MMA excretion by around three- to six-fold above pretreatment levels, whereas no effect of pargyline (25 mg kg-1), a selective irreversible inhibitor of MAO was found. No apparent changes in DMA or TMA excretion in response to drug-treatment were observed. Ammonia excretion also was generally unchanged except for an apparent marked increase (approximately four-fold) over the 24 h following semicarbazide, a result which might be explained if ammonia is a degradation product of semicarbazide metabolism in the rat. With recent evidence that MMA is a substrate in-vitro for SSAO activities, results here may indicate that SSAO or related enzymes are involved in endogenous MMA turnover.

Properties of a semicarbazide-sensitive amine oxidase in human umbilical artery

The metabolism of some aromatic amines by amine oxidase activities in human umbilical artery homogenates has been studied. The inhibitory effects of clorgyline showed that 5-hydroxytryptamine (5-HT) and tryptamine, 1 mM, were predominantly substrates for monoamine oxidase (MAO) type A, whereas MAO-A and B were both involved in the metabolism of beta-phenylethylamine (PEA), 100 microM, and tyramine, 1 mM. About 20-30% of tyramine and PEA metabolism was resistant to 1 mM clorgyline, but sensitive to inhibition by semicarbazide, 1 mM, indicating the presence of a semicarbazide-sensitive amine oxidase (SSAO). Benzylamine, 1 mM, appeared to be metabolized exclusively by SSAO with a Km (161 microM) at pH 7.8 similar to that found for SSAO in other human tissues. Tyramine and PEA were relatively poor substrates for SSAO, with very high apparent Km values of 17.6 and 13.3 mM, respectively, when determined in the presence of clorgyline, 10(-3) M, added to inhibit any metabolism of those amines by MAO activities. However, kinetic studies with benzylamine indicated that clorgyline, 10(-3) M, also appears to inhibit SSAO competitively such that the true Km values for tyramine and PEA may be about 60% of those apparent values given above. No evidence for the metabolism of 5-HT or tryptamine by SSAO was obtained. The aliphatic amine methylamine was recently shown to be a specific substrate for SSAO in umbilical artery homogenates. We have used benzylamine and methylamine as SSAO substrates in histochemical studies to localize SSAO in tissue sections.(ABSTRACT TRUNCATED AT 250 WORDS)

Deamination of methylamine by semicarbazide-sensitive amine oxidase in human umbilical artery and rat aorta

The deamination of methylamine (MA) by amine oxidase enzymes has been studied and compared with that of benzylamine (BZ) in homogenates of rat aorta and human umbilical artery by means of a radiochemical assay to estimate the radiolabelled deaminated metabolites produced, and also a spectrophotometric assay to measure H2O2 formation during the metabolism of these substrates. The effects of various inhibitors used in these assays suggest that a semicarbazide-sensitive amine oxidase (SSAO) is predominantly if not wholly responsible for the deamination of both MA and BZ in these tissues. MA was found to have a relatively higher apparent Km (102 microM in aorta; 779 microM in umbilical artery) than BZ (6.8 microM in aorta; 207 microM in umbilical artery) for metabolism by SSAO in these tissues. However, these large differences between species in the apparent Km values for each amine indicate that the biochemical properties of SSAO in human and rat vasculature are not identical. SSAO in human umbilical artery was particularly active towards MA, with a Vmax which was approximately 70% greater than that for BZ as substrate, whereas in rat aorta the Vmax for MA was around 60% of that for BZ. MA is known to occur endogenously in man and other species, and the possibility that it may be a physiological substrate in vivo for SSAO is discussed.

Inhibition of rat aorta semicarbazide-sensitive amine oxidase by 2-phenyl-3-haloallylamines and related compounds

The inhibition of semicarbazide-sensitive amine oxidase (SSAO) in rat aorta homogenates by some 2-phenyl-3-haloallylamines has been studied. Derivatives containing a fluorine atom were approximately three times more potent than the corresponding 3-chloroallylamines. These halogen-containing compounds were irreversible inhibitors of SSAO after preincubation with aorta homogenates; kinetic evidence for an initial competitive, reversible interaction (Ki around 0.4-0.6 microM) was found with two compounds (MDL 72145 and 72274). A similar Ki (approx. 0.7 microM) was obtained with 2-phenylallylamine (MDL 72200). However, this compound which lacks a halogen atom was a reversible inhibitor, even after preincubation. The use of a spectrophotometric assay to measure H2O2 production from amine metabolism demonstrated that MDL 72200 was a substrate (Km = 1.4 microM) for SSAO, with a Vmax approximately five times smaller than that of benzylamine (Km = 8.1 microM). Of particular interest in this study is the finding that (E)-2-phenyl-3-chloroallylamine (MDL 72274) is highly selective as an inhibitor of SSAO, compared with MAO-A or B activities, and may be a useful compound for investigating the importance of SSAO in animal tissues.

Comparative studies with the enantiomers of the glycol metabolite of propranolol and their effects on the cardiac beta-adrenoceptor

The two enantiomers ((R)- and (S)-) of propranolol glycol, a metabolite of propranolol, have been synthesized, and their effects upon the beta-adrenoceptor studied by two methods. The ability of these compounds to antagonize the inotropic actions of isoprenaline was examined on spontaneously beating rat atrial preparations. Also, the effects of these enantiomers upon the binding of [3H]dihydroalprenolol to beta-receptors in rat cardiac ventricular membranes was studied. Experiments with the atria indicated that the (S)-glycol was a reversible competitive antagonist of isoprenaline with a potency approximately one thousand times lower than that of (+/-)-propranolol. In contrast, the (R)-glycol appeared to act as an irreversible antagonist, producing complex dose-response curves. The effects of these compounds to cause displacement of alprenolol binding were consistent with the organ bath data. The interaction of the (S)-glycol with the beta-receptor binding site was reversible (Ki of 27.6 +/- 4.2 microM) but less potent than that of (+/-)-propranolol (Ki of 0.99 +/- 0.07 nM). On the other hand, pretreatment of ventricular membranes with the (R)-glycol, followed by extensive washing techniques, resulted in alprenolol binding which did not regain control values, providing further evidence for an irreversible effect upon the beta-receptor. The possible significance of these pharmacological actions of the two enantiomers is discussed in terms of the in vivo metabolic pathways for propranolol.

Monoamine oxidase activities in dissociated cell fractions from rat skeletal muscle

Collagenase was used to dissociate rat skeletal muscle (gastrocnemius) into its constituent cells, from which a myocyte fraction enriched in striated muscle cells, and a non-myocyte fraction containing cells of connective tissue and vascular origin, were prepared. The activities of amine oxidase enzymes and alkaline phosphatase (AP) were then assayed in these fractions, and also in homogenates prepared from corresponding samples of non-dissociated tissue. The specific activities of the semicarbazide-sensitive amine oxidase (SSAO) and AP were considerably higher (30 to 35-fold) in non-myocyte than in myocyte fractions. AP is generally considered to be present predominantly in vascular cells of skeletal muscle, with little, if any, in skeletal muscle cells themselves. Thus, the results obtained may indicate a similar localization for SSAO activity. Support for this came from histochemical studies, which showed staining for SSAO primarily over the walls of larger blood vessels in the muscle. Unlike SSAO and AP, were marked differences in MAO-A activity between myocyte and non-myocyte fractions were not found, suggesting that MAO-A is more probably a constituent of cells within both fractions.

Vascular smooth muscle cells: a major source of the semicarbazide-sensitive amine oxidase of the rat aorta

Several methods have been used to study the distribution of the semicarbazide-sensitive amine oxidase (SSAO) within the wall of the rat aorta. After separation of the smooth muscle-containing layers of the tunica media from the connective tissue of the tunica adventitia, much higher specific enzyme activity (measured with 1 microM benzylamine) was found in homogenates of the media than of adventitia. Similar results were obtained for MAO-A (with 1 mM 5-HT as substrate). SSAO activity was also considerably higher in homogenates of cells (predominantly smooth muscle) isolated from medial tissue by enzymatic dissociation with collagenase and elastase compared with homogenates of cells (mostly of connective tissue origin) from the adventitia. Histochemical staining resulting from SSAO activity (with benzylamine as substrate) occurred predominantly and intensely over the tunica media in rat aortic sections, although some occasional staining of adventitial sites was also observed. Staining was prevented by the SSAO inhibitors hydroxylamine (1 microM) and semicarbazide (1 mM), but not by the MAO inhibitor, clorgyline (1 mM). These results indicate that SSAO is associated predominantly, although not exclusively, with the smooth muscle cells in the rat aorta. Our findings that beta-aminopropionitrile (BAPN) is a reversible, competitive inhibitor (Ki around 2 X 10(-4)M) of SSAO, in contrast to the irreversible inhibition of the connective tissue lysyl oxidase by BAPN reported by others, provides further evidence that these enzymes are not identical.

An allylamine derivative (MDL 72145) with potent irreversible inhibitory actions on rat aorta semicarbazide-sensitive amine oxidase

(E)-2-(3,4-dimethoxyphenyl)-3-fluoroallylamine (MDL 72145) was found to be an extremely potent inhibitor of the semicarbazide-sensitive amine oxidase (SSAO) in rat aorta homogenates. Considerable inhibition, which was not reversed by dialysis, could be produced under appropriate in-vitro conditions at drug concentrations around 10 nM. The pseudo first order kinetics for time-dependent inhibition by MDL 72145 (10-100 nM) were found to be consistent with a bimolecular reaction between enzyme and inhibitor with a rate constant for this step of 2 X 10(6) min-1 M-1. A similar rate of inhibition under an oxygen atmosphere to that obtained under nitrogen was produced upon incubation of enzyme with inhibitor, suggesting that oxidation of the inhibitor to an active metabolite was not required for its activity. Incubation of homogenates for very short periods (1 min) with inhibitor (0.05-0.5 microM) and benzylamine (1-10 microM) as substrate indicated non-competitive kinetics for the early interaction of enzyme with the drug. Benzylamine (50 microM), but not pyridoxal phosphate (100 microM), was able to protect SSAO from inhibition by 10 nM MDL 72145. However, pyridoxal phosphate alone appeared to produce some irreversible inhibition of the enzyme. Dialysis against buffer containing 50 microM or 1 mM benzylamine was unable to reactivate SSAO inhibited by 10 nM MDL 72145. It is concluded that MDL 72145 irreversibly inhibits SSAO by acting at, or near, the substrate binding site, but the exact nature of the complex formed remains to be identified.

Monoamine oxidase activities of dissociated cell fractions from rat ventricular muscle

Cell fractions enriched in cardiac muscle cells (myocytes), on the one hand, and in non-myocytes, on the other, were prepared by dissociation of rat ventricular tissue with collagenase. Amine oxidase activities in homogenates of these cell fractions and also in homogenates of the corresponding undissociated ventricular tissue were compared. In addition, the activity of alkaline phosphatase (AP), an enzyme found predominantly associated in the heart with non-myocytes, particularly capillary endothelial cells, was also measured. No significant difference in the activity of MAO-A (assayed with 1 mM 5-hydroxytryptamine) was found between myocyte and non-myocyte fractions. In contrast, the activities of alkaline phosphatase (AP) and also the semicarbazide-sensitive amine oxidase (SSAO), assayed with 1 microM benzylamine (BZ), were both significantly higher in non-myocytes, by several-fold, than in myocyte fractions. Studies of the inhibition by clorgyline of 1 mM BZ metabolism confirmed that both MAO-A and MAO-B can also contribute to BZ oxidation in the rat heart. These experiments indicated different ratios of MAO-A: MAO-B in the various cell fractions. The ratios of the percentage contributions of MAO-A and MAO-B, respectively, to the total metabolism of 1 mM BZ were 78:20 (myocytes), 43:52 (non-myocytes) and 57:32 (undissociated tissue). These results suggest that MAO-B, in addition to AP and SSAO, may be associated preferentially with non-myocyte constituents of the rat heart. Although cardiac myocytes appear to contain predominantly MAO-A, this enzyme form is also localized, with high activity, to the non-myocyte fraction. However, since the non-myocyte fraction is heterogeneous in its cell content, containing vascular components of the coronary microcirculation, as well as other cells of connective tissue origin, the exact cellular localization of the enzyme activities within this fraction has not yet been defined.

Inhibitory actions of hydralazine upon monoamine oxidizing enzymes in the rat

The inhibition by hydralazine of the clorgyline-resistant amine oxidase (CRAO) and monoamine oxidase (MAO) activities in various rat tissues has been studied. Hydralazine was a potent, time-dependent inhibitor of rat heart CRAO activity in vitro. The inhibition was not reversed by dialysis for 18 hr at 4 degrees, and only partially reversed by dialysis at 37 degrees. Dialysis at 4 degrees in the presence of pyridoxal phosphate (10(-4) M) also did not reverse the inhibition. Ex vivo inhibition of CRAO was found in heart and aorta homogenates in a dose-dependent manner after administration of hydralazine (1-40 mg/kg i.p.) to rats. In contrast, MAO-A activity was unaffected or, in some cases, significantly increased in these tissue homogenates from drug-treated animals. However, in vitro inhibition by hydralazine of both MAO-A and B activities of rat liver mitochondrial fractions was found, and these effects were fully reversible by dialysis for 18 hr at 4 degrees. Inhibition of MAO-A was competitive (Ki of 2.5 X 10(-6) M), while inhibition of MAO-B showed complex mixed non-competitive kinetics. These results indicate that hydralazine possesses different inhibitory properties towards the various amine oxidases in rat tissues, and these actions are discussed in relation to the clinical use of the drug as an anti-hypertensive agent.

In vitro and in vivo inhibition by benserazide of clorgyline-resistant amine oxidases in rat cardiovascular tissues

Bernserazide (D,L-serine 2-[2,3,4-trihydroxybenzyl]-hydrazide) as been shown to inhibit the clorgyline-resistant amine oxidase (CRAO) activities which metabolize benzylamine in homogenates of rat aorta, heart and brown adipose tissue. In vitro studies showed a concentration- and time-dependent inhibition of CRAO in heart and aorta which was reversed by dialysis for 18hr. At high concentrations (10(-4)-10(-3)M) benserazide appeared to increase enzyme activity towards and occasionally above control value. These increases became more prominent after long periods of preincubation (especially in the presence of saturating benzylamine concentrations) and remained after dialysis of those homogenates preincubated with benserazide. The administration of benserazide for one or seven days in daily doses of 5-150 mg/kg also inhibited CRAO activity in vivo in a dose-dependent manner, with greater inhibition after seven days treatment. Reversal of inhibition, by dialysis of tissue homogenates from benserazide-treated rats, was much slower than was found with homogenates incubated in vitro with the drug. After benserazide administration to rats, MAO-A activity towards 5-hydroxytryptamine was generally not inhibited, and in fact was significantly increased in some cases. The administration of L-DOPA (250 mg/kg) together with benserazide (40 mg/kg) resulted in a similar degree of CRAO inhibition in aorta and heart to that seen after benserazide alone. These findings are discussed with regard to the use of these drugs in the therapy of Parkinson's Disease, although the paucity of information about the physiological function of CRAO makes the significance of its inhibition by benserazide unclear.

The effect of DSP-4 (N-[2-chloroethyl]-N-ethyl-2-bromobenzylamine) on monoamine oxidase activities in tissues of the rat

The in vitro inhibition of monoamine oxidase (MAO) in rat liver, and of the clorgyline-resistant amine oxidase (CRAO) in rat heart and aorta, by DSP-4 (N-[2-chloroethyl]-N-ethyl-2-bromobenzylamine) has been studied. Inhibition of each enzyme activity was independent of prolonged preincubation, was reversed by dialysis, and also Ackermann-Potter plots were consistent with reversible inhibition. Simple linear competitive inhibition of MAO-A and MAO-B was observed, with Ki values of 6 x 10(-6) and 8 x 10(-5) M, respectively. CRAO was inhibited in a mixed, non-competitive manner (Ki of 3.2 x 10(-5) and 7.8 x 10(-6) M in heart and aorta, respectively) which conformed to a kinetic model in which the binding of DSP-4 to CRAO increased the affinity of substrate binding, but prevented product formation. The possible significance of these results for the in vivo actions of the drug is discussed.

Some factors influencing the metabolism of benzylamine by type A and B monoamine oxidase in rat heart and liver

The ability of MAO-A and MAO-B to metabolize benzylamine in vitro has been investigated in mitochondrial preparations from rat liver and heart. Although under normal circumstances benzylamine appeared to be metabolized exclusively by MAO-B in the rat liver, a contribution by both MAO-A and a clorgyline-resistant enzyme component was revealed when the MAO-B activity was much reduced by pretreatment of the mitochondria with appropriate concentrations of deprenyl. These three enzyme activities also contributed to benzylamine deamination in rat heart mitochondria. However, binding studies with [3H]pargyline, which provided an estimate of the respective concentrations of MAO-A and MAO-B active centres in heart mitochondria, indicated a ratio between MAO-A and MAO-B, markedly different from that shown by plots of inhibition of benzylamine metabolism by various concentrations of clorgyline. The interpretation of these clorgyline plots is discussed in terms of the kinetic constants of both MAO-A and MAO-B, and the relative amounts of each enzyme. It is proposed that although the turnover rate constant for benzylamine metabolism by MAO-A is much smaller than that shown by MAO-B, in those tissues containing a large ratio of MAO-A:MAO-B content, the metabolism of benzylamine by MAO-A can be detected.

The influence of benserazide on changes in monoamine oxidase activity in some rat tissues following treatment with L-DOPA

1. This study was designed to see whether or not increases in monoamine oxidase (MAO) specific activity that follow chronic treatment of rats with L-dihydroxyphenylalanine (L-DOPA) could be modified by benserazide (Ro 4-4602), an inhibitor of L-aromatic amino acid decarboxylase, and to compare the properties of the increased MAO activity with those of control animals. 2. Male wistar rats were treated with L-DOPA (250 mg/kg) and benserazide (40 mg/kg) either alone or in combination for 10 days. 3. The activity of MAO in homogenates of heart, kidney, liver and brain was measured with 5-hydroxytryptamine (5-HT) and benzylamine (BZ). 4. The significant increases in MAO specific activity seen in heart and kidney following L-DOPA treatment could be reduced or prevented by benserazide. 5. Use of the selective MAO inhibitor clorgyline showed that the increases in MAO specific activity, when measured with either 5-HT or BZ were due to an increase in the number of active centres of MAO-A, in the rat heart. 6. There was a significant increase in the Vmax of the enzyme reaction with 5-HT, in the rat heart and kidney homogenates. 7. It is concluded that L-DOPA increases the specific activity of MAO-A in rat heart and kidney as a result of its decarboxylation.

Selective influences of age and thyroid hormones on type A monoamine oxidase of the rat heart

The specific actiivty of rat heart MAO, towards both tyramine and benzylamine as substrates, was found to increase with the age of the animal, and also after administration of (-)-thyroxine to young male rats. Conversely, enzyme activity was decreased in animals made hypothyroid by including 2-thiouracil in their diet. However, with both age and altered thyroid status, relatively greater changes in the deamination of tyramine rather than in that of benzylamine, were obtained. Clorgyline and deprenyl, used as inhibitors of rat heart MAO, indicated that tyramine is metabolized solely by MAO-A, whereas benzylamine is a substrate for both MAO-A and -B, and also a clorgyline- and deprenyl-resistant enzymic activity. The proportional contribution of MAO-A, -B and the clorgyline-resistant enzyme towards the total benzylamine deamination in the rat heart was found to vary with the age and with altered thyroid status of the animal in such a way that selective changes in the activity of MAO-A appear to be largely responsible for the overall changes in the specific activity of rat heart MAO which occur in response to these developmental factors.

Evidence for a clorgyline-resistant monoamine metabolizing activity in the rat heart

When benzylamine was used as substrate, a component of the total monoamine oxidase (MAO) activity in the rat heart was found to be resistant to inhibition by clorgyline. The proportion of the total activity represented by this component, decreased as the rat grew. It was also inhibited by both semicarbazide and isoniazid but not by potassium cyanide. Inhibitor studies with MAO in subcellular fractions showed that this component was more concentrated in the microsomal and soluble fractions. However, it could not be concluded that the activity was entirely a soluble enzyme. Determination of quasi-Michaelis constants ("Km") for total benzylamine oxidizing activity revealed a high ("Km" of approximately 10(-5)M) and low ("Km" of approximately 5 X 10(-4)M) affinity component. The high affinity component was inhibited by semicarbazide and the low affinity component by clorgyline. In the presence of 10(-3)M clorgyline, the high affinity component showed substrate inhibition at higher substrate concentrations. The possibility is discussed that the clorgyline-resistant activity is due to an amine-oxidizing activity distinct from mitochondrial MAO.

The effects of thyroid hormones on monoamine oxidase in the rat heart

The administration of thyroxine to young male rats produced an increase in the specific activity of their cardiac monoamine oxidase (MAO). A reduction in the circulating concentrations of thyroid hormones, brought about by 2-thiouracil, led to a decrease. The relative change in activity produced was greater with tyramine than with benzylamine as substrate. By following the time-course of the return of enzyme activity, with tyramine as substrate, after a single injection of pargyline in vivo, it was concluded that both excess and lack of thyroid hormones cause their effects on MAO activity by changing the rate of synthesis of the enzyme and not its degradation rate constant. The degradation rate constant did change with the age of the animal. The MAO activity, which increased towards tyramine as substrate in hyperthyroid rat hearts, behaved in the same way as that of controls to heat treatment, irreversible inhibition by pargyline or by clorgyline and also in Km determinations. The pattern for benzylamine oxidation was similar, except for the effect of the inhibitor clorgyline which shifted the plateau region of the double sigmoid inhibition curve significantly using enzyme from hyperthyroid rat hearts. The plateau region was also shown to be affected by the age of the animal. The possibility is discussed that the increased cardiac MAO activity produced by thyroid hormones and by the growth of the animal is mediated by that form of the enzyme primarily responsible for the oxidation of tyramine. Mixed substrate experiments suggested that tyramine oxidation could be inhibited competitively by benzylamine.