Regulation of semicarbazide-sensitive amine oxidase expression by tumor necrosis factor-alpha in adipocytes: functional consequences on glucose transport

Multiple Direct Effects of the Dietary Protoalkaloid N-Methyltyramine in Human Adipocytes

Dietary amines have been the subject of a novel interest in nutrition since the discovery of trace amine-associated receptors (TAARs), especially TAAR-1, which recognizes tyramine, phenethylamine, tryptamine, octopamine, N-methyltyramine (NMT), synephrine, amphetamine and related derivatives. Alongside the psychostimulant properties of TAAR-1 ligands, it is their ephedrine-like action on weight loss that drives their current consumption via dietary supplements advertised for ‘fat-burning’ properties. Among these trace amines, tyramine has recently been described, at high doses, to exhibit an antilipolytic action and activation of glucose transport in human adipocytes, i.e., effects that are facilitating lipid storage rather than mobilization. Because of its close structural similarity to tyramine, NMT actions on human adipocytes therefore must to be reevaluated. To this aim, we studied the lipolytic and antilipolytic properties of NMT together with its interplay with insulin stimulation of glucose transport along with amine oxidase activities in adipose cells obtained from women undergoing abdominal surgery. NMT activated 2-deoxyglucose uptake when incubated with freshly isolated adipocytes at 0.01–1 mM, reaching one-third of the maximal stimulation by insulin. However, when combined with insulin, NMT limited by half the action of the lipogenic hormone on glucose transport. The NMT-induced stimulation of hexose uptake was sensitive to inhibitors of monoamine oxidases (MAO) and of semicarbazide-sensitive amine oxidase (SSAO), as was the case for tyramine and benzylamine. All three amines inhibited isoprenaline-induced lipolysis to a greater extent than insulin, while they were poorly lipolytic on their own. All three amines—but not isoprenaline—interacted with MAO or SSAO. Due to these multiple effects on human adipocytes, NMT cannot be considered as a direct lipolytic agent, potentially able to improve lipid mobilization and fat oxidation in consumers of NMT-containing dietary supplements.

Sick fat: the good and the bad of old and new circulating markers of adipose tissue inflammation

Adipose tissue (AT) is one of the largest endocrine organs contributing to metabolic homeostasis. The functional pleiotropism of AT depends on its ability to secrete a large number of hormones, cytokines, extracellular matrix proteins and growth factors, all influencing many local and systemic physiological and pathophysiological processes. In condition of chronic positive energy balance, adipocyte expansion, hypoxia, apoptosis and stress all lead to AT inflammation and dysfunction, and it has been demonstrated that this sick fat is a main risk factor for many metabolic disorders, such as type 2 diabetes mellitus, fatty liver, cardiovascular disease and cancer. AT dysfunction is tightly associated with aberrant secretion of bioactive peptides, the adipocytokines, and their blood concentrations often reflect the expression in the AT. Despite the existence of an association between AT dysfunction and systemic pro-inflammatory state, most of the circulating molecules detectable in obese and dysmetabolic individuals do not identify specifically the condition of sick fat. Based on this premise, this review provides a concise overview of "classic" and novel promising adipocytokines associated with AT inflammation and discusses possible critical approaches to their interpretation in clinical practice.

Vascular Adhesion Protein-1: A Cell Surface Amine Oxidase in Translation

Significance: Vascular adhesion protein-1 (VAP-1) is an ectoenzyme that oxidates primary amines in a reaction producing also hydrogen peroxide. VAP-1 on the blood vessel endothelium regulates leukocyte extravasation from the blood into tissues under physiological and pathological conditions. Recent Advances: Inhibition of VAP-1 by neutralizing antibodies and by several novel small-molecule enzyme inhibitors interferes with leukocyte trafficking and alleviates inflammation in many experimental models. Targeting of VAP-1 also shows beneficial effects in several other diseases, such as ischemia/reperfusion, fibrosis, and cancer. Moreover, soluble VAP-1 levels may serve as a new prognostic biomarker in selected diseases. Critical Issues: Understanding the contribution of the enzyme activity-independent and enzyme activity-dependent functions, which often appear to be mediated by the hydrogen peroxide production, in the VAP-1 biology will be crucial. Similarly, there is a pressing need to understand which of the VAP-1 functions are regulated through the modulation of leukocyte trafficking, and what is the role of VAP-1 synthesized in adipose and smooth muscle cells. Future Directions: The specificity and selectivity of new VAP-1 inhibitors, and their value in animal models under therapeutic settings need to be addressed. Results from several programs studying the therapeutic potential of VAP-1 inhibition, which now are in clinical trials, will reveal the relevance of this amine oxidase in humans.

Hypoxia inhibits semicarbazide-sensitive amine oxidase activity in adipocytes

Semicarbazide-sensitive amine oxidase (SSAO), an enzyme highly expressed on adipocyte plasma membranes, converts primary amines into aldehydes, ammonium and hydrogen peroxide, and is likely involved in endothelial damage during the course of diabetes and obesity. We investigated whether in vitro, adipocyte SSAO was modulated under hypoxic conditions that is present in adipose tissue from obese or intensive care unit. Physical or pharmacological hypoxia decreased SSAO activity in murine adipocytes and human adipose tissue explants, while enzyme expression was preserved. This effect was time-, dose-dependent and reversible. This down-regulation was confirmed in vivo in subcutaneous adipose tissue from a rat model of hypoxia. Hypoxia-induced suppression in SSAO activity was independent of the HIF-1-α pathway or of oxidative stress, but was partially antagonized by medium acidification. Hypoxia-induced down-regulation of SSAO activity could represent an adaptive mechanism to lower toxic molecules production, and may thus protect from tissue injury during these harmful conditions.

Influence of prolonged fasting on monoamine oxidase and semicarbazide-sensitive amine oxidase activities in rat white adipose tissue

Monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) activities are very high in white adipose tissue (WAT). SSAO, also known as Vascular Adhesion Protein-1 in vessels, is present at the surface of fat cells and independent approaches have evidenced its impressive increase during adipogenesis. However, the factors that might regulate the expression SSAO and MAO in adipose tissue are still poorly defined. Here, we report the influence of fasting on MAO and SSAO activities in adipose depots. A decrease of MAO activity occurred after three days of starvation in the intra-abdominal adipose tissue (INWAT) of male Wistar rats, regardless of their initial adiposity or fat loss. The reduced fat stores of seven-week old rats, loosing 59 % of INWAT mass during fasting, contained only one half of the MAO activity found in fed control. The same reduction of MAO was observed after prolonged fasting in older rats which lose only 26% of their INWAT during the same starvation duration, leading to a fat mass comparable to that of younger fed control rats. It was therefore the endocrine and metabolic changes occurring during fasting that were responsible for the reduced MAO activity and not the amount of INWAT. Surprisingly, SSAO activity remained unchanged during starvation. In subcutaneous WAT, the changes in MAO and SSAO activities exhibited the same tendencies than those found in INWAT. Taken together, these data show that both MAO and SSAO activities increase in INWAT with age-dependent fattening, and indicate that only MAO diminishes during fasting.

Antidepressant phenelzine alters differentiation of cultured human and mouse preadipocytes

Change in body weight is a frequent side effect of antidepressants and is considered to be mediated by central effects on food intake and energy expenditure. The antidepressant phenelzine (Nardil) potently inhibits both monoamine oxidase and semicarbazide-sensitive amine oxidase activities, two enzymes that are highly expressed in adipose tissue, raising the possibility that it could directly alter adipocyte biology. Treatment with this compound is rather associated with weight gain. The aim of this work was to examine the effects of phenelzine on differentiation and metabolism of cultured human and mouse preadipocytes and to characterize the mechanisms involved in these effects. In all preadipocyte models, phenelzine induced a time- and dose-dependent reduction in differentiation and triglyceride accumulation. Modulation of lipolysis or glucose transport was not involved in phenelzine action. This effect was supported by the reduced expression in the key adipogenic transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein-alpha, which was observed only at the highest drug concentrations (30-100 microM). The PPAR-gamma agonists thiazolidinediones did not reverse phenelzine effects. By contrast, the reduction in both cell triglycerides and sterol regulatory element-binding protein-1c (SREBP-1c) was detectable at lower phenelzine concentrations (1-10 microM). Phenelzine effect on triglyceride content was prevented by providing free fatty acids to the cells and was partially reversed by overexpression of a dominant-positive form of SREBP-1c, showing the privileged targeting of the lipogenic pathway. When considered together, these findings demonstrate that an antidepressant directly and potently inhibits adipocyte lipid storage and differentiation, which could contribute to psychotropic drug side effects on energy homeostasis.

Kinetics and spectroscopic evidence that the Cu(I)-semiquinone intermediate reduces molecular oxygen in the oxidative half-reaction of Arthrobacter globiformis amine oxidase

The role of copper during the reoxidation of substrate-reduced amine oxidases by O(2) has not yet been definitively established. Both outer-sphere and inner-sphere pathways for the reduction of O(2) to H(2)O(2) have been proposed. A key step in the inner-sphere mechanism is the reaction of O(2) directly with the Cu(I) center of a Cu(I)-semiquinone intermediate. To thoroughly examine this possibility, we have measured the spectral changes associated with single-turnover reoxidation by O(2) of substrate-reduced Arthrobacter globiformis amine oxidase (AGAO) under a wide range of conditions. We have previously demonstrated that the internal electron-transfer reaction [Cu(II)-TPQ(AMQ) --> Cu(I)-TPQ(SQ)] (where TPQ(AMQ) is the aminoquinol form of reduced TPQ and TPQ(SQ) is the semiquinone form) occurs at a rate that could permit the reaction of O(2) with both species to be observed on the stopped-flow time scale [Shepard, E. M., and Dooley, D. M. (2006) J. Biol. Inorg. Chem. 11, 1039-1048]. The transient absorption spectra observed for the reaction of O(2) with substrate-reduced AGAO provide compelling support for the reaction of the Cu(I)-TPQ(SQ) form. Further, global analysis of the kinetics and the transient absorption spectra are fully consistent with an inner-sphere reaction of the Cu(I)-semiquinone intermediate with O(2) and are inconsistent with an outer-sphere mechanism for the reaction of the reduced enzyme with O(2).

Modelling the roles of MAO and SSAO in glucose transport

Amine oxidase substrates such as benzylamine and methylamine have been shown to stimulate glucose uptake by increasing the recruitment of the glucose transporter GLUT4 from vesicles within the cell to the cell surface. Inhibition of this effect by the presence of semicarbazide and catalase led to the suggestion that the process is mediated by the H(2)O(2) produced in the oxidation of these amines. Tyramine, which is a substrate for both MAO and SSAO, can also stimulate this process and in that case both MAO and SSAO inhibitors attenuate the effect. Benzylamine does not occur physiologically and tyramine is normally present in only very low amounts. We have suggested that adrenaline, which also stimulates glucose metabolism through adrenoceptors, may act as the physiological substrate for GLUT4 recruitment. It is a substrate for MAO but not SSAO. However, oxidation of adrenaline by MAO releases both H(2)O(2) and methylamine for further oxidation by SSAO. In order to gain a fuller understanding of this process we have performed simulation studies that may be used to assess the contributions of the amine oxidases to the process under a variety of conditions. The results are consistent with the experimentally observed behaviour. This approach not only helps to establish the feasibility of this process but also allows behaviour prediction and the identification of further experimental approaches.

Decreased serum activity of semicarbazide-sensitive amine oxidase (SSAO) in patients treated with second generation antipsychotics: a link to impaired glucose metabolism?

OBJECTIVE

Although the treatment of schizophrenia with many second generation antipsychotics is known to be associated with metabolic changes, such as hyperglycemia or hypercholesterolemia, the underlying mechanisms of these adverse reactions remain unclear. In light of the recent focus on the involvement of semicarbazide-sensitive amine oxidase (SSAO) in glucose metabolism, we investigated SSAO serum activity in schizophrenic patients treated with antipsychotics with the objective of determining a possible link between SSAO and impaired glucose metabolism.

METHODS

Blood samples were drawn from 44 schizophrenic patients (24 receiving second generation antipsychotics known to disturb glucose metabolism) on day 1 and day 5 of inpatient treatment. Forty-one healthy adults with no medical condition known to influence SSAO served as controls.

RESULTS

Of the 44 schizophrenic patients, the 24 treated with second generation antipsychotics known to disturb glucose metabolism showed significantly lower SSAO serum activity [day 1 (mean +/- SD): 477 +/- 105 mU/L; day 5: 438 +/- 86 mU/L] than the 20 patients treated with other antipsychotics not known to influence glucose metabolism (day 1: 513 +/- 124 mU/L, p = 0.359; day 5: 542 +/- 204 mU/L, p = 0.021) only after 5 days of treatment and compared to healthy controls (526 +/- 142 mU/L, p = 0.021). No differences were observed between schizophrenic patients treated with first generation antipsychotics and the controls.

CONCLUSION

We found decreased SSAO serum activity exclusively in schizophrenic patients treated with second generation antipsychotics known to disturb glucose metabolism. In terms of the role of SSAO in glucose metabolism, the observed decrease in SSAO serum activity may be linked to metabolic changes that are known to occur in schizophrenic patients being treated with many second generation antipsychotics.

Intramolecular electron transfer rate between active-site copper and TPQ in Arthrobacter globiformis amine oxidase

Copper amine oxidases catalyze the oxidative deamination of primary amines operating through a ping-pong bi bi mechanism, divided into reductive and oxidative half-reactions. Considerable debate still exists regarding the role of copper in the oxidative half-reaction, where O2 is reduced to H2O2. Substrate-reduced amine oxidases display an equilibrium between a Cu(II) aminoquinol and a Cu(I) semiquinone, with the magnitude of the equilibrium constant being dependent upon the enzyme source. The initial electron transfer to dioxygen has been proposed to occur from either the reduced Cu(I) center or the reduced aminoquinol cofactor. In order for Cu(I) to be involved, it must be shown that the rate of electron transfer (kET) between the aminoquinol and Cu(II) is sufficiently rapid to place the Cu(I) semiquinone moiety on the mechanistic pathway. To further explore this issue, we measured the intramolecular electron transfer rate for the Cu(II) aminoquinol left arrow over right arrow Cu(I) semiquinone equilibrium in Arthrobacter globiformis amine oxidase (AGAO) by temperature-jump relaxation techniques. The results presented herein establish that kET is greater than the rate of catalysis (kcat) for the preferred amine substrate beta-phenylethylamine at three pH values, thereby permitting the Cu(I) semiquinone to be a viable catalytic intermediate during enzymatic reoxidation in this enzyme. The data show that kET is approximately equivalent at pH 6.2 and 7.2, being 2.5 times kcat for these pH values. At pH 8.2, however, kET decreases, becoming comparable to kcat. Potential reasons for the decreased kET at basic pH are presented. The implications of these results in light of a previously published study measuring reoxidation rates of substrate-reduced AGAO are also addressed.

Short- and long-term insulin-like effects of monoamine oxidases and semicarbazide-sensitive amine oxidase substrates in cultured adipocytes

Semicarbazide-sensitive amine oxidase (SSAO) is known to increase during in vitro adipogenesis and to be one of the most highly expressed membrane proteins of white adipocytes. Although less well documented, mitochondrial monoamine oxidases (MAOs) are also present in adipocytes and share with SSAO the capacity to generate hydrogen peroxide. This work therefore aimed to compare several biologic effects of MAO and SSAO substrates in 3T3-F442A adipocytes. In differentiated cells, tyramine oxidation was predominantly MAO dependent, whereas benzylamine oxidation was SSAO dependent. Both amines partially mimicked insulin actions, including stimulation of Akt phosphorylation and glucose uptake. In addition, tyramine and benzylamine impaired tumor necrosis factor alpha-dependent nitric oxide formation in a pargyline- and semicarbazide-sensitive manner, respectively. Various biogenic amines were tested in competition for tyramine or benzylamine oxidation and classified as MAO-preferring (methoxytyramine, tryptamine) or SSAO-preferring substrates (methylamine, octopamine). Short-term incubation with 1 mmol/L of all amines except histamine stimulated glucose uptake up to 20% to 50% of maximal insulin activation. One-week treatment with either MAO or SSAO substrates alone allowed postconfluent cells to differentiate into adipocytes, reproducing 60% of insulin-promoted lipid accumulation. All amines also exerted a slight improvement in the adipogenic action of insulin. Therefore, like SSAO, substrate activation of MAO can interact with adipocyte metabolism by mimicking diverse effects of insulin in addition to preventing tumor necrosis factor alpha-dependent responses.

A comparative study of the binding and inhibition of four copper-containing amine oxidases by azide: implications for the role of copper during the oxidative half-reaction

Copper amine oxidases (CuAOs) catalyze the oxidative deamination of primary amines operating through a ping-pong bi-bi mechanism. In this work, azide (an exogenous monodentate ligand) was used to probe the role of copper during the oxidative half-reaction of CuAO catalysis. The effects of azide on both the reductive and oxidative half-reactions of pea seedling amine oxidase (PSAO), the recombinant human kidney diamine oxidase (rhDAO), Arthrobacter globiformis amine oxidase (AGAO), and Pichia pastoris amine oxidase (PPLO) have been examined. For the reductive half-reaction, defined as the oxidation of amine substrate to an aldehyde, azide was discovered to exhibit either noncompetitive or competitive inhibition with respect to the amine, depending on the enzyme source. With regard to the oxidative half-reaction, defined as the reoxidation of the enzyme via reduction of O(2) to H(2)O(2), azide has been determined to exhibit competitive inhibition with respect to O(2) in PSAO with a calculated K(i) value that is in excellent agreement with the experimentally determined K(d) value for the Cu(II)-N(3)(-) complex. Azide was found to exhibit mixed-type/partially competitive inhibition with respect to substrate O(2) in rhDAO, with an apparent K(i) that is similar to the K(d) value for the Cu(II)-N(3)(-) complex. The competitive inhibition for PSAO and the partially competitive inhibition for rhDAO are consistent with O(2) interacting directly with copper during enzymatic reoxidation. For the enzymes AGAO and PPLO, pure noncompetitive and mixed-type/partially competitive inhibition is observed. K(i) values for reductive and oxidative half-reactions are equivalent and are lower than measured K(d) values for the Cu(II)-N(3)(-) complexes in oxidized and substrate-reduced forms of these enzymes. Given these observations, it appears that substantial inhibition of the reductive half-reaction occurs at the concentrations of azide used for the oxidative half-reaction experiments, thereby complicating kinetic interpretation. At this time, the data do not permit us to distinguish between two possibilities: (1) inhibition by azide with respect to O(2) is intrinsically competitive in CuAOs, but this effect cannot always be deconvolved experimentally from the effects of azide on the reductive half-reaction; or (2) CuAOs differ in some steps of their reoxidation mechanisms.

Amine oxidase substrates for impaired glucose tolerance correction

Amine oxidases are widely distributed from microorganisms to vertebrates and produce hydrogen peroxide plus aldehyde when catabolizing endogenous or xenobiotic amines. Novel roles have been attributed to several members of the amine oxidase families, which cannot be anymore considered as simple amine scavengers. Semicarbazide-sensitive amine oxidase (SSAO) is abundantly expressed in mammalian endothelial, smooth muscle, and fat cells, and plays a role in lymphocyte adhesion to vascular wall, arterial fiber elastic maturation, and glucose transport, respectively. This latter role was studied in detail and the perspectives of insulin-like actions of amine oxidase substrates are discussed in the present review. Independent studies have demonstrated that SSAO substrates and monoamine oxidase substrates mimic diverse insulin effects in adipocytes: glucose transport activation, lipogenesis stimulation and lipolysis inhibition. These substrates also stimulate in vitro adipogenesis. Acute in vivo administration of amine oxidase substrates improves glucose tolerance in rats, mice and rabbits, while chronic treatments with benzylamine plus vanadate exert an antihyperglycaemic effect in diabetic rats. Dietary supplementations with methylamine, benzylamine or tyramine have been proven to influence metabolic control in rodents by increasing glucose tolerance or decreasing lipid mobilisation, without noticeable changes in the plasma markers of lipid peroxidation or protein glycation, despite adverse effects on vasculature. Thus, the ingested amines are not totally metabolized at the intestinal level and can act on adipose and vascular tissues. In regard with this influence on metabolic control, more attention must be paid to the composition or supplementation in amines in foods and nutraceutics.

Activity and expression of semicarbazide-sensitive benzylamine oxidase in a rodent model of diabetes: interactive effects with methylamine and alpha-aminoguanidine

Previous data indicate that methylamine injection in fasted healthy mice produced a hypophagic effect dependent of neuronal K(v)1.6 channels expression and increased by alpha-aminoguanidine, an inhibitor of semicarbazide-sensitive benzylamine oxidase enzymes mainly involved in amine degradation. In the present work we have investigated: 1) the level of expression and activity of the semicarbazide-sensitive benzylamine oxidase; 2) the effect of methylamine alone and in the presence of alpha-aminoguanidine on food intake of genetic obese and type II diabetes mice (the db/db mice). Db/db mice showed higher levels of semicarbazide-sensitive benzylamine oxidase activities in adipose tissue and in plasma than their lean counterpart (db/db(+) mice). Methylamine (30-75 microg, i.c.v.) showed similar hypophagic effects in obese and lean mice consistently with the levels of neuronal K(v)1.6 found in both animal strains. Alpha-aminoguandine (50 mg/kg, i.p.) increased methylamine (i.c.v.) hypophagia in both obese and lean mice and only in obese mice when methylamine was given i.p. These results suggest a crucial role of semicarbazide-sensitive benzylamine oxidase activity in controlling methylamine hypophagia in hyperphagic diabetic mice.

Differential inhibition of six copper amine oxidases by a family of 4-(aryloxy)-2-butynamines: evidence for a new mode of inactivation

A series of compounds derived from a previously identified substrate analogue of copper amine oxidases (CuAOs) (Shepard et al. (2002) Eur. J. Biochem. 269, 3645-3658) has been screened against six different CuAOs with a view to designing potent and selective inhibitors. The substrate analogues investigated were 4-(1-naphthyloxy)-2-butyn-1-amine, 4-(2-methylphenoxy)-2-butyn-1-amine, 4-(3-methylphenoxy)-2-butyn-1-amine, 4-(4-methylphenoxy)-2-butyn-1-amine, and 4-phenoxy-2-butyn-1-amine. These compounds were screened against equine plasma amine oxidase (EPAO), Pisum sativum amine oxidase (PSAO), Pichia pastoris lysyl oxidase (PPLO), bovine plasma amine oxidase (BPAO), human kidney diamine oxidase (KDAO), and Arthrobacter globiformis amine oxidase (AGAO) to examine the effect of different substituent groups on potency. Despite the similar structures of the 4-aryloxy analogues evaluated, striking differences in potency were observed. In addition, crystal structures of AGAO derivitized with 4-(2-naphthyloxy)-2-butyn-1-amine and 4-(4-methylphenoxy)-2-butyn-1-amine were obtained at a resolution of 1.7 A. The structures reveal a novel and unprecedented reaction mechanism involving covalent attachment of the alpha,beta-unsaturated aldehyde turnover product to the amino group of the reduced 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor. Collectively, the structural and inhibition results support the feasibility of designing selective mechanism-based inhibitors of copper amine oxidases.

Cyanide as a copper and quinone-directed inhibitor of amine oxidases from pea seedlings ( Pisum sativum) and Arthrobacter globiformis: evidence for both copper coordination and cyanohydrin derivatization of the quinone cofactor

The interactions of cyanide with two copper-containing amine oxidases (CuAOs) from pea seedlings (PSAO) and the soil bacterium Arthrobacter globiformis (AGAO) have been investigated by spectroscopic and kinetic techniques. Previously, we rationalized the effects of azide and cyanide for several CuAOs in terms of copper coordination by these exogenous ligands and their effects on the internal redox equilibrium TPQ(amr)-Cu(II) right harpoon over left harpoon TPQ(sq)-Cu(I). The mechanism of cyanide inhibition was proposed to occur through complexation to Cu(I), thereby directly competing with O(2) for reoxidation of TPQ. Although cyanide readily and reversibly reacts with quinones, no direct spectroscopic evidence for cyanohydrin derivatization of TPQ has been previously documented for CuAOs. This work describes the first direct spectroscopic evidence, using both model and enzyme systems, for cyanohydrin derivatization of TPQ. K(d) values for Cu(II)-CN(-) and Cu(I)-CN(-), as well as the K(i) for cyanide inhibition versus substrate amine, are reported for PSAO and AGAO. In spite of cyanohydrin derivatization of the TPQ cofactor in these enzymes, the uncompetitive inhibition of amine oxidation is determined to arise almost exclusively through CN(-) complexation of Cu(I).

Semicarbazide-sensitive amine oxidases: enzymes with quite a lot to do

The semicarbazide-sensitive amine oxidases (SSAO) (EC 1.4.3.6) were believed to be detoxifying enzymes, primarily involved in the oxidative deamination of endogenous amines, such as methylamine and aminoacetone, together with some xenobiotic amines. However, it appears that the reaction products may have important signalling functions in the regulation of cell development and glucose homeostasis. Furthermore, enzyme, from some sources, behaves as a cellular adhesion protein under inflammatory and it may also be involved in lipid transport. This review considers what is known about the activities and potential functions of this hardworking protein.

Benzylamine exhibits insulin-like effects on glucose disposal, glucose transport, and fat cell lipolysis in rabbits and diabetic mice

Benzylamine, a substrate of semicarbazide-sensitive amine oxidase (SSAO), stimulates glucose transport in rat adipocytes and improves glucose disposal in diabetic rats only in the presence of vanadate. These effects have been described to result from a synergism between the hydrogen peroxide formed during amine oxidation and vanadate, via the generation of pervanadate, a powerful insulin mimicker. However, it has also been reported that benzylamine alone can stimulate glucose uptake and inhibit lipolysis in human fat cells. In this work, we therefore investigated whether benzylamine on its own was able to induce both in vivo and in vitro insulin-like responses in animal models other than rat. In rabbits, the i.v. infusion of 7 micromol/kg benzylamine before a glucose tolerance test resulted in a net reduction of the hyperglycemic response without a change in insulin secretion. Benzylamine also improved glucose tolerance and reduced lipid mobilization in hyperglycemic/obese mice. In vitro, 0.1 mM benzylamine stimulated glucose transport and inhibited lipolysis in mouse and rabbit adipocytes. These effects were blocked by previous treatments with semicarbazide, a SSAO inhibitor. Levels of benzylamine oxidation were more elevated in mouse than in rabbit adipose tissues, whereas the reverse was observed for skeletal muscles. Finally, benzylamine was unable to stimulate insulin secretion by isolated pancreatic islets from both species and SSAO activity was hardly detectable in pancreas. Together, our results bring evidence that benzylamine on its own can improve glucose tolerance in rabbit and mouse, likely by stimulating glucose uptake via amine oxidase activation in insulin-sensitive tissues.