Subcellular location of semicarbazide-sensitive amine oxidase in rat aorta

Comparison of Inhibitor and Substrate Selectivity between Rodent and Human Vascular Adhesion Protein-1

Vascular adhesion protein-1 (VAP-1) is an ectoenzyme that functions as a copper-containing amine oxidase and is involved in leukocyte adhesion at sites of inflammation. Inhibition of VAP-1 oxidative deamination has become an attractive target for anti-inflammatory therapy with demonstrated efficacy in rodent models of inflammation. A previous comparison of purified recombinant VAP-1 from mouse, rat, monkey, and human gene sequences predicted that rodent VAP-1 would have higher affinity for smaller hydrophilic substrates/inhibitors because of its narrower and more hydrophilic active site channel. An optimized in vitro oxidative deamination fluorescence assay with benzylamine (BA) was used to compare inhibition of five known inhibitors in recombinant mouse, rat, and human VAP-1. Human VAP-1 was more sensitive compared to rat or mouse VAP-1 (lowest IC₅₀ concentration) to semicarbazide but was least sensitive to hydralazine and LJP-1207. Hydralazine had a lower IC₅₀ in rats compared to humans, although not significant. However, the IC₅₀ of hydralazine was significantly higher in the rat compared to mouse VAP-1. The larger hydrophobic compounds from Astellas (compound 35c) and Boehringer Ingelheim (PXS-4728A) were hypothesized to have higher binding affinity for human VAP-1 compared to rodent VAP-1 since the channel in human VAP-1 is larger and more hydrophobic than that in rodent VAP-1. Although the sensitivity of these two inhibitors was the lowest in the mouse enzyme, we found no significant differences between mouse, rat, and human VAP-1. Michaelis-Menten kinetics of the small primary amines phenylethylamine and tyramine were also compared to the common marker substrate BA demonstrating that BA had the highest affinity among the substrates. Rat VAP-1 had the highest affinity for all three substrates and mouse VAP-1 had intermediate affinity for BA and phenylethylamine, but tyramine was not a substrate for mouse VAP-1 under these assay conditions. These results suggest that comparing oxidative deamination in mouse and rat VAP-1 may be important if using these species for preclinical efficacy models.

Oxidative Deamination of Emixustat by Human Vascular Adhesion Protein-1/Semicarbazide-Sensitive Amine Oxidase

Emixustat potently inhibits the visual cycle isomerase retinal pigment epithelium protein 65 (RPE65) to reduce the accumulation of toxic bisretinoid by-products that lead to various retinopathies. Orally administered emixustat is cleared rapidly from the plasma, with little excreted unchanged. The hydroxypropylamine moiety that is critical in emixustat's inhibition of RPE65 is oxidatively deaminated to three major carboxylic acid metabolites that appear rapidly in plasma. These metabolites greatly exceed the plasma concentrations of emixustat and demonstrate formation-rate-limited metabolite kinetics. This study investigated in vitro deamination of emixustat in human vascular membrane fractions, plasma, and recombinant human vascular adhesion protein-1 (VAP-1), demonstrating single-enzyme kinetics for the formation of a stable aldehyde intermediate (ACU-5201) in all in vitro systems. The in vitro systems used herein established sequential formation of the major metabolites with addition of assay components for aldehyde dehydrogenase and cytochrome P450. Reaction phenotyping experiments using selective chemical inhibitors and recombinant enzymes of monoamine oxidase, VAP-1, and lysyl oxidase showed that only VAP-1 deaminated emixustat. In individually derived human vascular membranes from umbilical cord and aorta, rates of emixustat deamination were highly correlated to VAP-1 marker substrate activity (benzylamine) and VAP-1 levels measured by enzyme-linked immunosorbent assay. In donor-matched plasma samples, soluble VAP-1 activity and levels were lower than in aorta membranes. A variety of potential comedications did not strongly inhibit emixustat deamination in vitro.

Semicarbazide-sensitive Amine Oxidase from the Smooth Muscles of Dog Aorta and Trachea: Activation by the MAO-A Inhibitor Clorgyline

Semicarbazide-sensitive amine oxidase (SSAO) has been identified in the dog trachea and aorta smooth muscles. The dog SSAO is blocked by hydrazine inhibitors. SSAOs from several different vascular smooth muscle sources, such as the rat and bovine aorta, and human umbilical artery, as well as the bovine plasma, are insensitive to the MAO-A inhibitor clorgyline; the dog SSAO on the other hand is significantly activated by clorgyline. Two methods, i.e. radioenzymatic and fluoroemetric methods, have been applied to substantiate this clorgyline-induced activation. The activation was detected with respect to the deamination of different substrates, such as benzylamine, β-phenylethylamine and longer carbon chain aliphatic amines, but not with respect to methylamine. The clorgyline effect is reversible, non-competitive and time-independent; it depends on electrostatic and hydrophobic interactions between clorgyline and hydrophobic regions of the dog SSAO enzyme.

Modifications of arterial phenotype in response to amine oxidase inhibition by semicarbazide

Semicarbazide-sensitive amine oxidase (SSAO)-deficient mice present no alteration in elastin cross-linking processes and carotid mechanical properties. In contrast, previous studies have shown that SSAO inhibitors induced marked anomalies in arterial structure and function. The aim of the present study was to examine the effect of semicarbazide (SCZ), an efficient SSAO inhibitor, on the arterial phenotype of the carotid artery in relation to modulation of SSAO and lysyl oxidase activities in growing rats. We first show that after 6 weeks of SCZ treatment (100 mg/kg per day), SSAO activity was reduced by 90%, whereas lysyl oxidase activity was only partially inhibited (<60%) in carotid artery, compared with controls. There was significant growth inhibition and no difference in mean arterial pressure but an increase in pulse pressure with a smaller arterial diameter in SCZ-treated rats. SCZ decreased aortic insoluble elastin without a change in total collagen. In addition, extracellular proteins other than insoluble elastin and collagen were increased in SCZ-treated rats. All of the elastic lamellae presented globular masses along their periphery, and focal disorganization was observed in the ascending aorta. Carotid artery mechanical strength was lower in SCZ-treated rats, and the elastic modulus-wall stress curve was shifted leftward compared with controls, indicating increased stiffness. Thus, SCZ modifies arterial geometry and mechanical properties, alters elastic fiber structure, and reduces the content of cross-linked elastin. Because these abnormalities are essentially absent in SSAO-deficient mice, our results suggest that lysyl oxidase inhibition is responsible for the major part of the vascular phenotype of SCZ-treated rats.

Diabetes and semicarbazide-sensitive amine oxidase (SSAO) activity: A review

The enzyme of semicarbazide-sensitive amine oxidase (SSAO) activity has been reported to be elevated in blood from diabetic patients. SSAO are widely distributed in plasma membranes of various tissues and blood plasma. SSAO-mediated production of toxic aldehydes has been proposed to be related to pathophysiological conditions. Cytotoxic metabolites by SSAO may cause endothelial injury and subsequently induce atherosclerosis. The precise physiological functions of SSAO could play an important role in the control of energy balance in adipose tissue. It is possible that the increased SSAO activity in diabetes may be a result of up-regulation due to increase of SSAO substrates, such as methylamine or aminoacetone. SSAO could play an important role in the regulation of adipocyte homeostasis. Inhibition of SSAO could be of therapeutic value for treatment of diabetic patient.

The serotonin transporter is present and functional in peripheral arterial smooth muscle

We tested the hypothesis that the 5-HT transporter (5-HTT) is present and functional in peripheral arterial smooth muscle. In aorta and mesenteric resistance arteries, real time RT-PCR and western analyses indicated the presence of 5-HTT mRNA and a 74 kDa 5-HTT protein. Immunohistochemistry localized the transporter to smooth muscle and endothelial cells. 5-HT and the metabolite 5-hydroxyindole acetic acid (5-HIAA) were detected in aorta, carotid, and superior mesenteric arteries using HPLC; the MAOA inhibitor pargyline significantly increased (over 400%) arterial 5-HT concentration. 5-HT was taken up by arteries in a time-dependent manner and uptake was independent of the endothelium, sympathetic nerves, and norepinephrine transporter. 5-HT-induced contraction of normal aorta was potentiated by the 5-HTT inhibitor fluvoxamine. A change in arterial 5-HTT function occurs in deoxycorticosterone (DOCA)-salt hypertension as the potency and threshold of 5-HT in contracting aorta from the DOCA-salt rat was increased by fluoxetine and fluvoxamine (1 micromol/L; DOCA fluvoxamine -log EC50 [mol/L] = 6.85 +/- 0.08, DOCA-control = 6.44 +/- 0.08); expression of transporter was significantly increased in aorta of DOCA salt rats (145% Sham). These studies show for the first time the presence of the 5-HTT in peripheral arterial smooth muscle and raise the question as to the function of the 5-HTT in regulating peripheral effects of 5-HT.

Vasoactive effects of methylamine in isolated human blood vessels: role of semicarbazide-sensitive amine oxidase, formaldehyde, and hydrogen peroxide

It is hypothesized that methylamine (MA) and semicarbazide-sensitive amine oxidase (SSAO) activity are involved in the cardiovascular complications in human diabetics. To test this, we 1) determined the acute vasoactive effects of MA (1-1,000 micromol/l) in uncontracted and norepinephrine (NE; 1 micromol/l)-precontracted human blood vessels used for coronary artery bypass grafts [left internal mammary artery (LIMA), radial artery (RA), and right saphenous vein (RSV)]; 2) tested whether MA effects in LIMA and RSV were dependent on SSAO activity using the SSAO inhibitor semicarbazide (1 mmol/l, 15 min); 3) determined the effects of MA metabolites formaldehyde and hydrogen peroxide in LIMA and RSV; 4) tested whether the MA response was nitric oxide, prostaglandin, or hyperpolarization dependent; 5) measured the LIMA and RSV cGMP levels after MA exposure; and 6) quantified SSAO activity in LIMA, RA, and RSV. In NE-precontracted vessels, MA stimulated a biphasic response in RA and RSV (rapid contraction followed by prolonged relaxation) and dominant relaxation in LIMA (mean +/- SE, %relaxation: 55.4 +/- 3.9, n = 30). The MA-induced relaxation in LIMA was repeatable, nontoxic, and age independent. Semicarbazide significantly blocked MA-induced relaxation (%inhibition: 82.5 +/- 4.8, n = 7) and SSAO activity (%inhibition: 98.1 +/- 1.3, n = 26) in LIMA. Formaldehyde (%relaxation: 37.3 +/- 18.6, n = 3) and H(2)O(2) (%relaxation: 55.6 +/- 9.0, n = 9) at 1 mmol/l relaxed NE-precontracted LIMA comparable with MA. MA-induced relaxation in LIMA was nitric oxide, prostaglandin, and possibly cGMP independent and blocked by hyperpolarization. We conclude that vascular SSAO activity may convert endogenous amines, like MA, to vasoactive metabolites.

Semicarbazide-sensitive amine oxidase substrates potentiate hydralazine hypotension: possible role of hydrogen peroxide

The relation between inhibition of semicarbazide-sensitive amine oxidase (SSAO) and vasodilation by hydralazine (HYD) was evaluated in chloralose/urethane-anesthetized rats pretreated with various substrates of the enzyme and subsequently administered a threshold hypotensive dose of the vasodilator. The SSAO substrates benzylamine, phenethylamine, and methylamine potentiate the hypotensive response to HYD. Methylamine, which was studied in greater detail because of its status as a possible endogenous SSAO substrate, does not influence the response to the reference vasodilator pinacidil; it does enhance HYD relaxation in aortic rings obtained from pretreated rats. Experiments designed to identify the product of SSAO activity responsible for potentiation by methylamine suggest involvement of hydrogen peroxide (H2O2), as evidenced by the findings that such potentiation is abolished by additional pretreatment with the H2O2-metabolizing enzyme catalase, and that the plasma concentration of H2O2 is increased by methylamine and decreased by HYD. These results are interpreted as a substantiation of the relation between the known SSAO inhibitory effect of HYD and its vasodilator activity. Pretreatment with the SSAO substrates would increase production of H2O2 in vascular smooth muscle and thus magnify the influence of this vasoconstrictor agent on vascular tone. In these conditions, the decrease in H2O2 production and hence in vascular tone caused by SSAO inhibition by HYD would also be magnified. It is speculated that inhibition of vascular SSAO could represent a novel mechanism of vasodilation.

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.

Physiological and pathological implications of semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the deamination of primary amines. Such deamination has been shown capable of regulating glucose transport in adipose cells. It has been independently discovered that the primary structure of vascular adhesion protein-1 (VAP-1) is identical to SSAO. VAP-1 regulates leukocyte migration and is related to inflammation. Increased serum SSAO activities have been found in patients with diabetic mellitus, vascular disorders and Alzheimer's disease. The SSAO-catalyzed deamination of endogenous substrates, that is, methylamine and aminoacetone, led to production of toxic formaldehyde and methylglyoxal, hydrogen peroxide and ammonia, respectively. These highly reactive aldehydes have been shown to initiate protein cross-linkage, exacerbate advanced glycation of proteins and cause endothelial injury. Hydrogen peroxide contributes to oxidative stress. 14C-methylamine is converted to 14C-formaldehyde, which then forms labeled long-lasting protein adduct in rodents. Chronic methylamine treatment increased the excretion of malondialdehyde and microalbuminuria, and enhanced the formation of fatty streaks in C57BL/6 mice fed with an atherogenic diet. Treatment with selective SSAO inhibitor reduces atherogenesis in KKAy diabetic mice fed with high-cholesterol diet. Aminoguanidine, which blocks advanced glycation and reduces nephropathy in animals, is in fact more potent at inhibiting SSAO than its effect on glycation. It suggests that SSAO is involved in vascular disorders under certain pathological conditions. Although SSAO has been known for several decades, its physiological and pathological implications are just beginning to be recognized.

Elevated plasma semicarbazide-sensitive amine oxidase (SSAO) activity in Type 2 diabetes mellitus complicated by retinopathy

AIMS

To measure plasma semicarbazide-sensitive amine oxidase (SSAO) activities and detect retinopathy in Type 2 diabetes mellitus (DM).

METHODS

Cross-sectional, population-based study of 65 diabetes patients (61 diagnosed from the age of 30 years) with or without retinopathy as determined by fundus photography in primary care. HbA1c was analysed by ion exchange chromatography on a Mono S for HbA1c column. SSAO activities were assayed radiometrically and formaldehyde-albumin adducts by ELISA in plasma samples from patients and 136 healthy controls.

RESULTS

Subjects with diabetes had higher plasma SSAO activity, measured as nmol benzylamine x mlplasma(-11) x h(-1)(mean 20.6), than controls (mean 14.3), P<0.0001; 95% confidence interval (CI) for difference 4.9-7.7. SSAO activity was higher in patients with retinopathy (mean 23.2) than in those without (mean 18.9), P=0.012; 95% CI for difference 1.0-7.5, and related to the HbA1c value. No statistically significant relationship between diabetes duration and SSAO activity was found. With HbA1c values and insulin treatment entered into a multiple logistic regression model, SSAO activity no longer predicted retinopathy, P increasing from 0.025 to 0.17. SSAO activity and the presence of any retinopathy were unrelated to titres of antibodies against formaldehyde-treated human serum albumin.

CONCLUSIONS

SSAO activity, earlier found to be elevated in Type 1 DM, is also elevated in Type 2 DM. The SSAO family of enzymes may be involved in the development of diabetic retinopathy, possibly by catalysing the formation of toxic metabolites. A potent and specific inhibitor of human SSAO might help prevent retinopathy in Type 1 and Type 2 DM.

Molecular cloning of a major mRNA species in murine 3T3 adipocyte lineage. differentiation-dependent expression, regulation, and identification as semicarbazide-sensitive amine oxidase

In an effort to identify novel mRNAs modulated during the course of adipose conversion, we have used a simplified differential display technique and have isolated a cDNA encoding an amine oxidase tremendously expressed in the adipocyte, the semicarbazide-sensitive amine oxidase (SSAO). The predicted amino acid sequence (765 amino acids) is likely to be the homologue of the human placental amine oxidase and of the partially known sequence of the rat adipocyte membrane amine oxidase. SSAO mRNAs are present in several tissues, but strikingly, the highest levels of gene expression are found in adipose tissue and aorta. Enzyme transcript levels are barely detectable in preadipocytes but are induced several hundred-fold during the adipocyte differentiation of 3T3-L1 or 3T3-F442A cells and of rat precursor primary cultures. These changes in transcript levels parallel a sharp increase in SSAO enzyme activity. The biochemical properties of the SSAO present in 3T3-L1 or 3T3-F442A adipocytes closely resemble the features of the SSAO activity previously described in white and brown adipose tissues. Interestingly, SSAO mRNA levels and enzyme activity drop in response to effectors of the cAMP pathway and to the cytokine tumor necrosis factor-alpha, indicating that two major signaling molecules of adipose tissue development and metabolism can control SSAO function. Moreover, the expression of SSAO transcripts and activity are clearly down-regulated in white adipose tissue from obese Zücker rats. Because of its known stimulatory effect on glucose transport, its biochemical properties and its pattern of expression and regulation, SSAO could play an important role in the regulation of adipocyte homeostasis.

Developmental vasculotoxicity associated with inhibition of semicarbazide-sensitive amine oxidase

The endogenous substrate(s) and physiological function(s) of semicarbazide-sensitive amine oxidase (SSAO), a group of enzymes exhibiting highest activity in vascular smooth muscle cells of the mammalian aortic wall, remain undetermined. This study examines the pathophysiological effects in the thoracic aortic wall resulting from specific in vivo SSAO inhibition. Weanling Sprague-Dawley rats were treated acutely or chronically with either semicarbazide hydrochloride or the allylamine derivatives MDL-72274 or MDL-72145 (Marion Merrell Dow Research Institute, Cincinnati, OH). Treatment with these compounds produced acute (6 and 24 h) and chronic (21 day) lowering of SSAO activity in aorta and lung with little effect on the activity of the vital matrix-forming enzyme, lysyl oxidase, in aortas of chronically treated animals. Chronic SSAO inhibition produced lesions consisting of striking disorganization of elastin architecture within the aortic media accompanied by degenerative medial changes and metaplastic changes in vascular smooth muscle cells. No significant difference in the total weight of dry, lipid-extracted aortic elastin and collagen components were observed between chronically SSAO inhibited and control animals. However, the amount of mature elastin was lowered and mature collagen was raised in the aortas of animals treated chronically with semicarbazide. Descending thoracic aortic rings isolated from chronically SSAO-inhibited animals had larger cross-sectional diameters (i.e., exhibited dilation) when compared to corresponding rings from control animals. This study demonstrates that developmental toxicity, characterized by striking vascular lesions and dilated thoracic aortas, can result from specific in vivo SSAO inhibition, suggesting a role for SSAO in connective tissue matrix development and maintenance, and specifically in the development of normal elastin.

Semicarbazide-sensitive amine oxidase (SSAO) from human and bovine cerebrovascular tissues: biochemical and immunohistological characterization

Semicarbazide-sensitive amine oxidase (SSAO) is widely distributed in almost all tissues, especially in vascularized ones. However, its presence in brain microvessels is still controversial. We have investigated the presence of SSAO in human and bovine brain microvessels by biochemical and immunohistological techniques, and we have compared it with SSAO present in meninges from the same species. SSAO metabolizes benzylamine and methylamine in all tissues tested and possibly dopamine and octopamine as well, as shown in competition studies. Kynuramine inhibited the metabolism of benzylamine by SSAO with high affinity in a non-competitive manner. Western-blot analysis rendered a positive staining of a 100 kDa band, in tissues from both species. These results were confirmed by immunohistological studies: the tunica media and intima of the meninges from both species were positively stained, and so was the endothelial layer of microvessels. SSAO was absent in brain parenchyma. These results definitively confirm the presence of SSAO in human and bovine cerebrovascular tissues and they demonstrate for the first time, the presence of this amine oxidase in endothelial cells from microvessels, through biochemical and immunological approaches.

Endogenous formaldehyde as a potential factor of vulnerability of atherosclerosis: involvement of semicarbazide-sensitive amine oxidase-mediated methylamine turnover

The mouse is known to be highly resistant to atherosclerosis. However, some inbred mouse strains are vulnerable to atherosclerosis when they are fed a high-cholesterol, high-fat diet. Increased deamination of methylamine (MA) and the subsequent production of formaldehyde has been recently shown to be a potential risk factor of atherosclerosis. In the present study semicarbazide-sensitive amine oxidase (SSAO)-mediated MA turnover in C57BL/6 mouse, a strain very susceptible to atherosclerosis, has been assessed in comparison to a moderate, i.e. BALB/c, and resistant, i.e. CD1, mouse strains. Kidney and aorta SSAO activities were found to be significantly increased in C57BL/6 in comparison to BALB/c and CD1 mice. A significant increase of urinary MA and formaldehyde were detected in C57BL/6. [14C]MA following intravenous injection would be quickly metabolized by SSAO. The labeled formaldehyde product would cross link with proteins. C57BL/6 exhibits significantly higher labeled protein adducts than BALB/c and CD1 in response to [14C]MA. The results indicated that mice vulnerable to atherosclerosis possess an increased SSAO-mediated MA turnover. The increase of production of formaldehyde, possibly other aldehydes, may induce endothelial injury or be chronically involved in protein cross-linking and subsequent angiopathy.

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.

Deamination of methylamine and angiopathy; toxicity of formaldehyde, oxidative stress and relevance to protein glycoxidation in diabetes

Semicarbazide-sensitive amine oxidase (SSAO) is located in the vascular smooth muscles, retina, kidney and the cartilage tissues, and it circulates in the blood. The enzyme activity has been found to be significantly increased in blood and tissues in diabetic patients and animals. Methylamine and aminoacetone are endogenous substrates for SSAO. The deaminated products are formaldehyde and methylglyoxal respectively, as well as H2O2 and ammonia, which are all potentially cytotoxic. Formaldehyde and methylglyoxal are cytotoxic towards endothelial cells. Excessive SSAO-mediated deamination may directly initiate endothelial injury and plaque formation, increase oxidative stress, which can potentiate oxidative glycation, and/or LDL oxidation and damage vascular systems. Formaldehyde is also capable of exacerbating advanced glycation, and thus increase the complexity of protein cross-linking. Uncontrolled SSAO-mediated deamination may be involved in the acceleration of the clinical complications in diabetes.

Evidence of a coupled mechanism between monoamine oxidase and peroxidase in the metabolism of tyramine by rat intestinal mitochondria

The relationship between monoamine oxidase (EC 1.4.3.4; MAO) and peroxidase (EC 1.11.1.7; POD) in the metabolism of tyramine was investigated using the crude mitochondrial fraction of rat intestine. When tyramine was incubated with mitochondria, the formation of the peroxidase-catalysed oxidation product, 2,2'-dihydroxy-5,5'-bis(ethylamino)diphenyl (dityramine), identified by mass spectrometric analysis, was monitored spectrophotometrically. After an initial lag time, the formation rate of dityramine was linear up to 2 hr, amounting to 17 nmol x hr(-1) x mg protein(-1). A similar value was found for the oxidative deamination of tyramine catalysed by intestinal MAO. Either 10(-3) M clorgyline or 10(-3) M NaCN suppressed this reaction by completely inhibiting MAO or POD, respectively. In the former case, however, addition of H2O2 to the incubation mixture promptly started the reaction. Selective inhibition of MAO-A and MAO-B was achieved with 3 x 10(-7) M clorgyline and 3 x 10(-7) M deprenyl, respectively, and the formation rate of dityramine decreased in a corresponding manner. Preincubation with histamine or spermidine reduced the lag time without affecting the steady-state reaction rate. Higher levels of dityramine were also detected in vivo in rat intestine after oral administration of tyramine. These results indicate that the peroxidase-dependent metabolism of tyramine in the gut may be driven by H2O2 produced by MAO activities and that MAO-A is mainly responsible for this process, as well as for the oxidative deamination of tyramine.

Aminoguanidine inhibits semicarbazide-sensitive amine oxidase activity: implications for advanced glycation and diabetic complications

Aminoguanidine, a nucleophilic hydrazine, has been shown to be capable of blocking the formation of advanced glycation end products. It reduces the development of atherosclerotic plaques and prevents experimental diabetic nephropathy. We have found that aminoguanidine is also quite potent at inhibiting semicarbazide-sensitive amine oxidase (SSAO) both in vitro and in vivo. The inhibition is irreversible. This enzyme catalyses the deamination of methylamine and aminoacetone, which leads to the production of cytotoxic formaldehyde and methylglyoxal, respectively. Serum SSAO activity was reported to be increased in diabetic patients and positively correlated with the amount of plasma glycated haemoglobin. Increased SSAO has also been demonstrated in diabetic animal models. Urinary excretion of methylamine is substantially increased in the rats following acute or chronic treatment with aminoguanidine. Urinary methylamine levels were substantially increased in streptozotocin (STZ)-induced diabetic rats following administration of aminoguanidine. The non-hydrazine SSAO inhibitor (E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride (MDL-72974A) has been shown to reduce urinary excretion of lactate dehydrogenase (an indicator of nephropathy) in STZ-induced diabetic rats. Formaldehyde not only induces protein crosslinking, but also enhances the advanced glycation of proteins in vitro. The results support the hypothesis that increased SSAO-mediated deamination may be involved in structural modification of proteins and contribute to advanced glycation in diabetes. The clinical implications for the use of aminoguanidine to prevent glycoxidation have been discussed.

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.

Formaldehyde produced endogenously via deamination of methylamine. A potential risk factor for initiation of endothelial injury

Methylamine can be converted by semicarbazide-sensitive amine oxidase (SSAO) to formaldehyde and hydrogen peroxide, which have been proven to be toxic towards cultured endothelial cells. We investigated whether or not these deaminated products from methylamine can exert potentially hazardous toxic effects in vivo. Long lasting residual radioactivity in different tissues was detected following administration of [14C]-methylamine in the mouse. Approximately 10% of the total administered radioactivity could even be detected 5 days after injection of [14C]-methylamine. Eighty percent of the formation of irreversible adducts can be blocked by a highly selective SSAO inhibitor, (E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride (MDL-72974A). The residual radioactivity was primarily associated with the insoluble tissue components and the soluble macromolecules. Radioactively labelled macromolecules were fragmented following enzymatic proteolysis. Results suggest that the formaldehyde derived from methylamine interacts with proteins in vivo. In the streptozotocin-induced diabetic mice, both SSAO activity and the formation of residual radioactivity were found to be significantly increased in the kidney. Chronic administration of methylamine enhances blood prorenin level, which strongly suggests that uncontrolled deamination of methylamine may be a risk factor for initiation of endothelial injury, and subsequent genesis of atherosclerosis.

Further studies on the ex-vivo effects of procarbazine and monomethylhydrazine on rat semicarbazide-sensitive amine oxidase and monoamine oxidase activities

Following administration of the anticancer agent, procarbazine, or one of its metabolites, monomethylhydrazine, to rats, activities of monoamine oxidases A and B (MAO A and MAO B) and of semicarbazide-sensitive amine oxidase (SSAO) were measured ex-vivo. Both compounds were found to be potent inhibitors of SSAO in tissue homogenates, exhibiting ID50 values in most tissues of approximately 8 mg kg-1 (procarbazine) and 0.08 mg kg-1 (monomethylhydrazine). Concurrent dose-dependent inhibition of MAO activities did not occur. However, in liver, potentiation of MAO B activity, to 140% of that in controls, was apparent following monomethyl-hydrazine and this effect was independent of the drug dose. Both compounds produced a dose-dependent potentiation of MAO A in brown adipose tissue, the elevation being more pronounced following monomethylhydrazine, with activity rising to 350% of that in control homogenates. In a parallel in-vitro study, monomethylhydrazine was without effect on MAO A in brown adipose tissue homogenates. By perfusing the SSAO substrate, benzylamine, through the isolated mesenteric arterial bed of the rat, it was found that pretreatment of animals with procarbazine or monomethylhydrazine reduced metabolism of this amine by a similar degree as had been determined ex-vivo in blood vessel homogenates. The results presented suggest that these compounds would be suitable for use as selective inhibitors in pharmacological examinations of SSAO function in isolated tissues and organs.

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.

Semicarbazide-sensitive amine oxidases: some biochemical properties and general considerations

Semicarbazide-sensitive amine oxidases with a high affinity for benzylamine (Bz.SSAO) (E.C.1.4.3.6) have been biochemically described in many mammalian tissues (adipose tissue, lung, heart, blood vessels). The enzymic activity appears to be expressed by mesenchymal cells (fibroblasts, adipocytes, smooth muscles). Although the physiological role of this enzymic activity is still unclear, some possible physiological substrates such as histamine are discussed. Some enzymes of this class (SSAO) have been purified. They share many similarities, among which are that they contain copper and a carbonyl active site. The nature of the organic cofactor of these enzymes is discussed and data are presented which have identified pyridoxal in pig kidney diamine oxidase and in pig plasma benzylamine oxidase by gas chromatography-mass spectrometry.

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.

Some aspects of the pathophysiology of semicarbazide-sensitive amine oxidase enzymes

The widespread distribution of enzymes classed as semicarbazide-sensitive amine oxidases (SSAO enzymes) throughout a very wide range of eukaryotic as well as prokaryotic organisms encourages the aspirations of those who wish to demonstrate physiological, pathological or pharmacological importance. Such enzymes are found in several tissues of mammals, both freely soluble, as in blood plasma, and membrane-bound, for example, in smooth muscle and adipose tissue. While they are capable of deaminating many amines with the production of an aldehyde and hydrogen peroxide, doubt still surrounds the identity of the most important endogenous substrates for these enzymes. At present, methylamine and aminoacetone appear to head the list of candidates. The possibility that SSAO enzymes can convert amine substrates to highly toxic metabolites is illustrated by the production of acrolein from the xenobiotic amine, allylamine and formaldehyde and methylglyoxal from methylamine and aminoacetone, respectively. Activities of SSAO enzymes may be influenced by physiological changes, such as pregnancy or pathologically by disease states, including diabetes, tumours and burns. Increased deamination of aminoacetone by tissue and plasma SSAO enzymes as a result of its increased production from L-threonine in conditions such as exhaustion, starvation and diabetes mellitus may be harmful. Such dangers could be mitigated either physiologically by a compensatory reduction in SSAO activity or pharmacologically by treatment with inhibitors of SSAO.

The role of semicarbazide-sensitive amine oxidase with a high affinity for benzylamine (Bz. SSAO) in the catabolism of histamine in the mesenteric arterial bed of the rat

In the mesenteric arterial bed of the rat the semicarbazide-sensitive amine oxidase with a high affinity for benzylamine (Bz. SSAO) (E.C. 1.4.3.6) is also able to oxidize histamine. In the perfused mesenteric arterial bed of the rat the complete inhibition of the Bz. SSAO obtained with a specific inhibitor, B24, almost completely reduces the efflux of imidazole acetic acid and increases the relaxing effect of histamine. Bz. SSAO appears to be the only enzyme present in these blood vessels able to catabolize histamine.

Characterization of human serum and umbilical artery semicarbazide-sensitive amine oxidase (SSAO). Species heterogeneity and stereoisomeric specificity

Semicarbazide-sensitive amine oxidases (SSAOs) are located in cardiovascular smooth muscle, cartilage and brown adipose tissues of different species, including human. The enzyme is also present in blood, and its activity appears to be altered under certain pathological conditions. SSAOs from both human umbilical arteries and serum were partially purified, and some of their biochemical properties were investigated. Both human artery and blood SSAO exhibited very similar substrate preference, lack of stereospecificity catalyzing the deamination of pro-R and pro-S benzylamine-deuterated enantiomers, and were very sensitive towards (E)-2-(4-fluorophenethyl)-3-fluoroallylamine (MDL-72974A). It was concluded that circulating serum SSAO is identical to the SSAO from vascular tissues. Human SSAO exhibited distinctly different properties in comparison to bovine and rat SSAOs.

Comparative localization of inositol 1,4,5-trisphosphate and ryanodine receptors in intestinal smooth muscle: an analytical subfractionation study

[3H]Ins(1,4,5)P3- and [3H]ryanodine-binding sites were characterized in membrane fractions from guinea-pig intestinal smooth muscle (longitudinal layer) and their subcellular localization was investigated by analytical cell-fractionation techniques. Fractions collected at low centrifugal fields (N and M fractions) contained predominantly low-affinity [3H]Ins(1,4,5)P3-binding sites (KD 80 nM), whereas microsomal (P) fractions contained only high-affinity binding sites (KD 5 nM). Total sedimentable high-affinity binding sites of [3H]Ins(1,4,5)P3 were 9-10-fold more numerous than those of [3H]ryanodine. Both high-affinity binding sites were purified in microsomal fractions, and their sub-microsomal distribution patterns after isopycnic density-gradient centrifugation were similar to those of presumed endoplasmic reticulum (ER) constituents, indicating that Ins(1,4,5)P3 and ryanodine receptors were localized primarily in ER and probably associated with rough as well as smooth ER. However, the stoichiometric ratio of Ins(1,4,5)P3 to ryanodine receptors was distinctly higher in high-density RNA-rich subfractions than in low-density RNA-poor subfractions, suggesting that Ins(1,4,5)P3 receptors were somewhat concentrated in the ribosome-coated portions of ER. The low overall stoichiometric ratio of ryanodine to Ins(1,4,5)P3 receptors in intestinal smooth muscle (1:9-10) might explain, at least partly, the existence of a Ca(2+)-storage compartment devoid of ryanodine-sensitive Ca2+ channels, but equipped with Ins(1,4,5)P3-sensitive channels, in saponin-permeabilized smooth-muscle cells [Iino, Kobayashi and Endo (1988) Biochem. Biophys. Res. Commun. 152, 417-422].

Semicarbazide-sensitive amine oxidase and monoamine oxidase in rat brain microvessels, meninges, retina and eye sclera

Monoamine oxidase-A and -B (MAO-A and MAO-B) and semicarbazide-sensitive amine oxidase (SSAO) activities were assessed in several rat micro-vascular tissues and eyes using selective substrates and inhibitors. In rat brain microvessels both MAO-A and MAO-B activities are relatively high and the levels of the two types of MAO's are comparable. Retina possesses a similar ratio of MAO-A and B but the activities are much lower. Eye sclera and meninges exhibit mainly MAO-A and MAO-B, respectively. Aorta is the only tissue where SSAO is the predominant amine oxidase. Relatively low, but significant amounts of SSAO were also detected in brain microvessels, meninges, retina and eye sclera. Methylamine was observed to be deaminated by SSAO from different tissues. The physiological and toxicological implications of amine oxidases in these tissues are discussed.

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.

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.

Radioligand and functional estimates of the interaction of the 1,4-dihydropyridines, isradipine and lacidipine, with calcium channels in smooth muscle

1. The present experiments were undertaken in order to characterize further the apparently irreversible inhibition of the contraction of depolarized rat aorta caused by lacidipine, a 1,4-dihydropyridine calcium antagonist. 2. We studied the effect of lacidipine on contraction evoked by 100 mM KCl solution in rat aorta, treated by N omega-nitro-L-arginine (0.1 mM), an inhibitor of nitric oxide (NO) synthesis. We compared the effect of prolonged depolarization on lacidipine and (+)-isradipine inhibition and the reversal of this inhibition after washout in the absence of dihydropyridines. Assuming that the onset of lacidipine-evoked inhibition was a pseudo-first order association kinetics, we estimated the dissociation rate constant (k-1 = 0.031 min-1), the association rate constant (k1 = 2.70 x 10(8) M-1 min-1) and the dissociation constant (KD = k-1/k1 = 115 pM) which was close to the IC50 value in steady-state conditions (160 pM). 3. The inhibitory effects of lacidipine and (+)-isradipine on rat aorta contraction were reversibly enhanced after preincubation with the drug in a 40 mM KCl-solution. Washout with drug-free 40 mM K(+)-depolarizing solution reversed inhibition in the (+)-isradipine-treated preparations, but not in the lacidipine-treated ones. 4. Radioligand binding studies were performed with [3H]-lacidipine and [3H]-isradipine in microsomes from rat aorta and rat ileum. Both ligands bound to a homogeneous population of binding sites (for[3H]-lacidipine: KD = 23 +/- 2.6 pM, Bmax = 380 +/- 21 fmol mg-1 protein in membranes from aorta; KD =23 +/- 3.1 pM, Bmax = 790 +/- 60 fmol mg-1 protein in membranes from ileum; for [3H]-isradipine:KD = 140 +/- 46 pM, Bmax = 350 +/- 64 fmol mg-1 protein in membrane from aorta; KD = 68 +/- 14 pM,Bmax = 760 +/- 75 fmol mg-1 protein in membranes from ileum). After isotopic dilution, [3H]-lacidipine and [3H]-isradipine dissociated according to a monoexponential kinetics. In membranes from ileum, the calculated dissociation rate constants (kappa_ 1) were 0.0257 min-1 and 0.0595 min-1, for [3H]-lacidipine and[3H]-isradipine, respectively.5. The non specific binding of [3H]-lacidipine and [3H]-isradipine, was measured in intact rat aorta preparations incubated under the conditions of the functional experiments, in the presence of nifedipine(1 microM). After incubation with [3H]-lacidipine 77.6 +/- 1.9 pM for 2 h the concentration of drug in the tissue was 15.15 +/- 1.18 fmol mg-1 w.wt. and still amounted to 7.24 +/- 0.61 fmol mg-1 w.wt. after 3.5 h washout in drug-free solution. After incubation with [3H]-isradipine 47.2 +/- 0.4 pM for 2 h it was 2.26 +/-0.07 fmol mg-1 w.wt. and was undetectable after 3.5 h washout in a drug-free solution.6. It is concluded that lacidipine interacts reversibly with dihydropyridine binding sites and that the apparent irreversible inhibition of contraction in depolarized preparations could be related to a nonspecific binding in a tissue compartment different from the plasma membrane.

Stereochemical course of tyramine oxidation by semicarbazide-sensitive amine oxidase

Two semicarbazide-sensitive amine oxidases (SSAO's) from bovine and porcine aortic tissue were partially purified and characterized, and the stereochemical course of amine oxidation was evaluated. The porcine and bovine SSAO's were membrane bound glycoproteins, with Km values for benzylamine of 8 and 16 microM, respectively. The reactivity of SSAO with semicarbazide and phenylhydrazine suggests that the cofactor is a carbonyl type molecule. The stereochemical course of the bovine and porcine aortic semicarbazide-sensitive amine oxidase reaction was investigated using chiral tyramines, deuterated at C-1 and C-2, and 1H-NMR spectroscopy to establish the loss or retention of deuterium in product p-hydroxyphenethyl alcohols. The preferred mode of tyramine oxidation was found to occur with the loss of pro-S proton at C-1, coupled with solvent exchange into C-2, a pattern which has not been observed for any copper amine oxidase examined to date. The solvent exchange reaction also occurred stereospecifically, with loss from and reprotonation to the pro-R position, suggesting that these two processes occur from the same face of the enamine double bond.

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.

Effect of benzylamine and its metabolites on the responses of the isolated perfused mesenteric arterial bed of the rat

1. Semicarbazide-sensitive amine oxidase (SSAO) is an enzyme activity which can be found in the plasma membrane of rat vascular smooth muscle cells. We have investigated the possibility that the products of deamination by this enzyme, namely ammonia, hydrogen peroxide and the aldehyde, may be important in the modulation of the responses of vascular smooth muscle to extracellular stimuli. 2. The isolated perfused mesenteric arterial bed of the rat was used and dose-pressure response curves (DRC) to bolus injections of adrenaline (Ad) or ATP were plotted by non-linear curve fitting. The relaxant effects of carbachol (CCh), which releases endothelium dependent relaxing factor (ERDF), were studied by co-administering CCh with Ad. The effects of including the preferred SSAO substrate, benzylamine (BZ; 25 microM), in the perfusion fluid throughout the experiment and of inhibition of SSAO by treatment of rats with (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine (MDL 72145; 1 mg kg-1) 1 h before dissection, have been studied. 3. Neither BZ nor SSAO inhibition affected the DRC to ATP. BZ shifted Ad responses to the left, inhibition of SSAO increased this shift indicating that the amine, but not its metabolites, were responsible for the potentiation of the responses to Ad. DRC to CCh showed a shift to the left and a significant decrease in the Hill slope with BZ, indicative of a potentiation of low doses of CCh more than high doses. Inhibition of SSAO prevented this change and so the metabolites of BZ deamination appeared to be involved in the potentiation. 4. Ammonia generated by SSAO may contribute to the production of EDRF or hydrogen peroxide may sensitize guanylate cyclase to stimulation by EDRF and so explain these findings.

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

2-Propyl-1-aminopentane (2-PAPN), a branched aliphatic amine, was found to be readily deaminated by monoamine oxidase B in the liver of the rat and semicarbazide-sensitive amine oxidase in the aorta of the rat. The deaminated product, 2-propyl-1-pentaldehyde, could be subsequently converted to valproic acid in the presence of aldehyde dehydrogenase and beta-NAD cofactor in vitro as well as in vivo. Valproic acid was identified after derivatization with 4-bromomethyl-6,7-dimethoxycoumarin, followed by HPLC-fluorometric assessment. Absorption and biotransformation of a single intraperitoneal dose of 2-PAPN resulted in the rapid appearance of the drug and its metabolite in the blood and in the brain. The formation of valproic acid from 2-PAPN in vivo, however, was insufficient to facilitate anticonvulsant action. In fact, 2-PAPN itself, at relatively small doses, exhibited distinct tremor effects. Such tremor effects could be prevented by valproic acid. However, 2-PAPN was also found to potentiate the convulsant effect induced by mercaptopropionic acid (MPA) and, in addition, the 2-PAPN-induced tremor could be potentiated by MPA in mice.

Postnatal maturation of excitation-contraction coupling in rat ventricle in relation to the subcellular localization and surface density of 1,4-dihydropyridine and ryanodine receptors

To better understand excitation-contraction coupling in cardiac muscle, we investigated the main Ca2+ channels involved in that process in adult and neonatal rat ventricle. Voltage-dependent (L-type) Ca2+ channels and sarcoplasmic reticulum Ca2+ release channels were labeled by means of [3H] (+)-PN200-110 and [3H]ryanodine, respectively. The number of [3H]ryanodine binding sites (per gram tissue) increased more than that of [3H] (+)-PN200-110 binding sites over the postnatal period (2.1-fold versus 1.35-fold, respectively). After equilibration of microsomal fractions in density gradient, ryanodine receptors were characterized by a heavy distribution pattern that did not change appreciably between days 1 and 30 after birth. In neonatal tissue, 1,4-dihydropyridine receptors were found mainly in low-density subfractions, together with other sarcolemmal constituents, whereas in adult tissue, they were recovered predominantly in high-density subfractions, together with ryanodine receptors. Thus, after birth, and in parallel with the development of T tubules, there was a progressive concentration of L-type Ca2+ channels in junctional structures of high equilibrium density, where they were situated close to the Ca2+ release channels of the sarcoplasmic reticulum. In adult ventricle, L-type channels were, on an average, threefold more abundant in T tubules than in external sarcolemma. In parallel mechanical studies, we found that the inhibitory action of ryanodine on systolic contraction was much more pronounced in adult than in neonatal right ventricle, and that, conversely, neonatal tissue was more sensitive that adult tissue to inhibitors of L-type channels. We conclude that, in view of the presumed mechanism of Ca2+ release from the sarcoplasmic reticulum, that is, Ca(2+)-induced Ca2+ release, the predominant localization in adult rat ventricle of the major Ca2+ entry pathway in the vicinity of the Ca2+ release pathway is of great functional significance. Furthermore, owing to the relative stoichiometry of Ca2+ entry and Ca2+ release channels in junctional structures (about 1:9), a physical link between these channels is not likely to be involved in the modulation of Ca2+ release from the sarcoplasmic reticulum in cardiac muscle.

Distribution of alpha 1 and alpha 2 (Na+,K+)-ATPase isoforms between the junctional (t-tubular) and non-junctional sarcolemmal domains of rat ventricle

The alpha 1 and alpha 2 (Na+,K+)-ATPase isoforms in microsomal fractions from adult rat ventricle could not be separated by density gradient centrifugation. Both isoforms were mainly recovered in low-density subfractions and their distribution pattern was superimposable to those of other typical plasma membrane constituents (5'-nucleotidase, muscarinic receptors) but differed from that of 1,4-dihydropyridine receptors, which were mainly associated with high-density subfractions. Thus, both (Na+,K+)-ATPase isoforms were present essentially in the non-junctional sarcolemmal domain, i.e. at the cell surface, while 1,4-dihydropyridine receptors (voltage-dependent calcium channels) seemed much more concentrated in the junctional domain, which is predominantly of t-tubular origin. Therefore, the high inotropic efficacy of low ouabain concentrations in rat ventricle cannot be explained on the basis of a preferential localization of the high-affinity receptors (alpha 2 isoform) in the vicinity of junctional structures. The difference in inotropic efficacy between high and low ouabain concentrations might be related to differences in stimulus response coupling associated with alpha 1 and alpha 2 isoforms, as suggested by the greater sensitivity of the effect of low concentrations to ethylisopropylamiloride, an inhibitor of Na(+)-H+ exchange.

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.

Metabolism of amines in the isolated perfused mesenteric arterial bed of the rat

1. Semicarbazide-sensitive amine oxidase (SSAO) activity has been demonstrated in the isolated mesenteric arterial bed of the rat in vitro by studying the metabolism of benzylamine (Bz) and tyramine (Tyr) added to the perfusing fluid. 2. Pretreatment of rats with (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine (MDL72145), a potent inhibitor of SSAO in rat mesenteric blood vessels, reduced the amount of metabolites, following the addition of Bz (25 microM) or Tyr (100 microM) to the perfusing fluid, by 83% and 52% respectively. Inactivation of monoamine oxidase type A (MAO-A) by the addition of clorgyline (10 microM) to the perfusing fluid, had little effect on the appearance of metabolites from Tyr. 3. The presence of 3 microM cocaine in the perfusing fluid increased the amount of metabolites produced from Tyr. 4. The metabolites of Tyr appearing in the perfusion fluid from control preparations were 85% p-hydroxyphenylacetic and the remainder consisted of a mixture of p-hydroxyphenylacetaldehyde and, possible, p-hydroxyphenylethanol. 5. The metabolism of Tyr by homogenates of the rat mesenteric vascular bed was carried out by SSAO (60%) and MAO-A (40%) with very little contribution from MAO-B. Homogenates from rats pretreated with MDL 72145 showed metabolism of Tyr by MAO-A only. 6. These data indicate that SSAO is capable of metabolizing amines present in the fluid perfusing blood vessels to metabolites that are readily released. Histochemical evidence has shown that whereas MAO-A is present in the mitochondria of smooth muscle cells and nerve endings, SSAO is located in the plasma membrane of the smooth muscle cells. This subcellular distribution may explain the differences found between metabolites released from intact vessels and the metabolism seen in homogenates. The identity of the Tyr metabolizing activity in intact vessels that is resistant to both MDL 72145 and clorgyline remains to be determined.

The influence of amine metabolizing enzymes on the pharmacology of tyramine in the isolated perfused mesenteric arterial bed of the rat

1. The pressor response to the infusion of tyramine (Tyr) into the isolated perfused mesenteric arterial bed of the rat has been studied at both a low and a high dose (0.2 and 2.0 mumol) and the effect of monoamine oxidase-A (MAO-A) and semicarbazide-sensitive amine oxidase (SSAO) inhibition was examined. Very little MAO-B activity is found in homogenates of this tissue when Tyr is used as substrate. 2. Inhibition of SSAO by treating rats with 1 mg kg-1 (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroally lamine (MDL 72145) 1 h before dissection, had no significant effect on the maximum pressure attained or the area under the curve (AUC) of the response to both low and high doses of Tyr. Inhibition of MAO-A, by inclusion of 10 microM clorgyline in the perfusing fluid, resulted in no significant potentiation at both low or high doses of Tyr. The inhibition of both these enzymes together substantially increased the AUC of the pressor response. 3. Cocaine (3 microM) significantly potentiated the responses to adrenaline (Ad). At this dose, cocaine significantly reduced the peak height and the AUC of the responses to both doses of Tyr. 4. Inhibition of extraneuronal uptake mechanisms with corticosterone (29 microM) did not potentiate the response to Ad and did not significantly alter the response to Tyr (low dose). 5. The effects of MDL 72145 and clorgyline on the directly acting amine, Ad, were studied. MDL 72145 caused a small but significant increase in the EC50 and in the maximum response to Ad, whilst clorgyline (10 microM) increased the EC50 value slightly and decreased the maximum response.