Modulation of bovine serum amine oxidase activity by hydrogen peroxide

Zinc-α2-glycoprotein as an inhibitor of amine oxidase copper-containing 3

Zinc-α2-glycoprotein (ZAG) is a major plasma protein whose levels increase in chronic energy-demanding diseases and thus serves as an important clinical biomarker in the diagnosis and prognosis of the development of cachexia. Current knowledge suggests that ZAG mediates progressive weight loss through β-adrenergic signalling in adipocytes, resulting in the activation of lipolysis and fat mobilization. Here, through cross-linking experiments, amine oxidase copper-containing 3 (AOC3) is identified as a novel ZAG binding partner. AOC3-also known as vascular adhesion protein 1 (VAP-1) and semicarbazide sensitive amine oxidase (SSAO)-deaminates primary amines, thereby generating the corresponding aldehyde, H2O2 and NH3. It is an ectoenzyme largely expressed by adipocytes and induced in endothelial cells during inflammation. Extravasation of immune cells depends on amine oxidase activity and AOC3-derived H2O2 has an insulinogenic effect. The observations described here suggest that ZAG acts as an allosteric inhibitor of AOC3 and interferes with the associated pro-inflammatory and anti-lipolytic functions. Thus, inhibition of the deamination of lipolytic hormone octopamine by AOC3 represents a novel mechanism by which ZAG might stimulate lipolysis. Furthermore, experiments involving overexpression of recombinant ZAG reveal that its glycosylation is co-regulated by oxygen availability and that the pattern of glycosylation affects its inhibitory potential. The newly identified protein interaction between AOC3 and ZAG highlights a previously unknown functional relationship, which may be relevant to inflammation, energy metabolism and the development of cachexia.

Substrate specificity of copper-containing plant amine oxidases

The steady-state kinetic parameters of the amine oxidases purified from Lathyrus cicera (LCAO) and Pisum sativum (PSAO) seedling were measured on a series of common substrates, previously tested on bovine serum amine oxidase (BSAO). LCAO, as PSAO, was substantially more reactive than BSAO with aliphatic diamines and histamine. The k(cat) and k(cat)/Km for putrescine were four and six order of magnitude higher, respectively. Differences were smaller with some aromatic monoamines. The plot of k(cat) versus hydrogen ions concentration produced bell-shaped curves, the maximum of which was substrate dependent, shifting from neutral pH with putrescine to alkaline pH with phenylethylamine and benzylamine. The latter substrates made the site more hydrophobic and increased the pK(a) of both enzyme-substrate and enzyme-product adducts. The plot of k(cat)/Km versus hydrogen ion concentration produced approximately parallel bell-shaped curves. Similar pK(a) couples were obtained from the latter curves, in agreement with the assignment as free enzyme and free substrate pK(a). The limited pH dependence of kinetic parameters suggests a predominance of hydrophobic interactions.

Polyamines as a defense mechanism against lipoperoxidation in Trypanosoma cruzi

The polyamines, spermine and spermidine--organic polycations that are absolutely required for eukaryotic cell growth--are shown here to function in Trypanosoma cruzi epimastigotes, as protectors of membrane lipoperoxidation by reactive oxygen species generated either by H2O2/Fe2+ or nifurtimox. In vitro, spermine and spermidine inhibited lipoperoxidation in a dose dependent manner. Spermine was more efficient than spermidine in its inhibitory effect. Lipid peroxidation induced by H2O2 showed an IC50 of 0.55 mM for spermine and 0.9 mM for spermidine while an IC50 of 0.8 mM for spermine and 1.5 mM for spermidine were observed when lipoperoxidation was elicited by nifurtimox. Likewise in vivo, both exogenously added spermine and spermidine or endogenous increase of spermine levels induced by phorbol ester, protected against lipoperoxidation and decreased citotoxicity provoked by nifurtimox. Putrescine and cadaverine, also present in T. cruzi had no effect at all. None of the polyamines had any effect neither on the scavenging of superoxide anion nor on the regulation of antioxidant enzymes such as superoxide dismutase and peroxidases involved in H2O2 detoxification. Here we point out that spermine, by acting as a protector of membrane lipoperoxidation might contribute to survival of T. cruzi continuously exposed to oxidative stress.

Amine oxidases in apoptosis and cancer

Amine oxidases, the major enzymes of biogenic amines metabolism, are considered to be biological regulators, especially for cell growth and differentiation. A primary involvement of amine oxidases in cancer growth inhibition and progression, especially by means of aldehydes, H(2)O(2) and other reactive oxygen species, the amine oxidase-mediated products of biogenic amines oxidation, has been demonstrated. Amine oxidases are involved in cancer growth inhibition because of the higher content in tumour cells of biogenic amines in comparison to normal cells. The cytotoxic effect can be explained by a damage to cell membranes and/or nuclei or, indirectly, through modulation of membrane permeability transition and therefore apoptosis. The oxidation products of biogenic amines appears to be also carcinogenic, while acrolein, produced from the oxidation of spermine and spermidine, should be a key compound both carcinogenic and cytotoxic. The cancer inhibition/promotion effect of amine oxidases could be explained by taking into consideration the full pattern of the enzyme content of the cell. The balance of amine oxidases and antioxidant enzymes appear to be a crucial point for cancer inhibition or progression. A long lasting imbalance of these enzymes appears to be carcinogenic, while, for a short time, amine oxidases are cytotoxic for cancer cells.

Inactivation of copper-containing amine oxidases by turnover products

For bovine serum amine oxidase, two different mechanisms of substrate-induced inactivation have been proposed. One consists of a slow oxidation by H2O2 of a conserved residue in the reduced enzyme after the fast turnover phase [Pietrangeli, P., Nocera, S., Fattibene, P., Wang, X.T., Mondovì, B. & Morpurgo, L. (2000) Biochem. Biophys. Res. Commun.267, 174-178] and the other of the oxidation by H2O2 of the dihydrobenzoxazole in equilibrium with the product Schiff base, during the catalytic cycle [Lee, Y., Shepard, E., Smith, J., Dooley, D.M. & Sayre, L.M. (2001) Biochemistry40, 822-829]. To discriminate between the two mechanisms, the inactivation was studied using Lathyrus cicera (red vetchling) amine oxidase. This, in contrast to bovine serum amine oxidase, formed the Cu+-semiquinolamine radical with a characteristic UV-vis spectrum when oxygen was exhausted by an excess of any tested amine in a closed cuvette. The inactivation, lasting about 90 min, was simultaneous with the radical decay and with the formation of a broad band (shoulder) at 350 nm. No inactivation occurred when a thousand-fold excess of amine was rapidly oxidized in an L. cicera amine oxidase solution stirred in open air. Thus, the inactivation is a slow reaction of the reduced enzyme with H2O2, following the turnover phase. Catalase protected L. cicera amine oxidase from inactivation. This effect was substrate-dependent, varying from full protection (benzylamine) to no protection (putrescine). In the absence of H2O2, a specific inactivating reaction, without formation of the 350 nm band, was induced by some aldehydes, notably putrescine. Some mechanisms of inactivation are proposed.

Some new functions of amine oxidases

Two contrasting topics are examined in this account: the protective actions of amine oxidases (AOs) resulting from the elimination and/or modulation of the levels of polyamines and some biogenic amines, such as histamine, in anaphylactic shock and the cell damaging effect of AOs catabolic products. Other functions of the plasma copper-containing amine oxidase are considered; namely the modification of some proteins by oxidation of their free amino groups, the auto-regulation of the catalytic activity of AOs, the protective effect against free radicals, and the regulation of K(+)-channels.

Is the catalytic mechanism of bacteria, plant, and mammal copper-TPQ amine oxidases identical?

This short review is mostly concerned with the work carried out in Rome on the copper amine oxidase from bovine serum (BSAO). The first target was the copper oxidation state and its relationship with the organic cofactor. It was found that copper is not reduced on reaction with amines under anaerobic conditions or along the catalytic cycle and that it is not within bonding distance of the quinone cofactor. The copper stability in the oxidised state was supported by BSAO ability to oxidise benzylhydrazine, a slow substrate, in the presence of N,N-diethyldithiocarbamate (DDC) and by the substantial catalytic activity of Co(2+)-substituted BSAO. Parallel work established that only one subunit of the dimeric enzyme readily binds reagents of the carbonyl group. Flexible hydrazides with a long aromatic tail were found to be highly specific inhibitors, suggesting the presence of an extended hydrophobic region at the catalytic site. A study by stopped-flow transient spectroscopy and steady state kinetics led to the formulation of a simplified, yet complete and consistent, catalytic mechanism for BSAO that was compared with that available for lentil seedling amine oxidase (LSAO). As in other copper amine oxidases, BSAO is inactivated by H(2)O(2) produced in the catalytic reaction, while the cofactor is stabilised in its reduced state. A conserved tyrosine hydrogen-bonded to the cofactor might be oxidised.

The inhibition of semicarbazide-sensitive amine oxidase by aminohexoses

Semicarbazide-sensitive amine oxidase (EC 1.4.3.6; amine:oxygen oxidoreductase (deaminating) (copper-containing); SSAO) is a multifunctional protein. It acts under inflammatory conditions as a vascular-adhesion protein (VAP-1), mediating the adhesion of lymphocytes to vascular endothelial cells. The relationships, if any, between this adhesion function and the enzymatic functions (amine-substrate specificity and catalysis) of SSAO have not yet been defined. Since cell surface amino sugars and their derivatives are known to be involved in cell-to-cell recognition, we have investigated their possible effects on the enzyme activity of SSAO. The aminohexoses galactosamine, glucosamine and mannosamine were not oxidatively deaminated by SSAO. However, their presence during the assay of benzylamine oxidation resulted in a time-dependent inhibition. This inhibition was shown to follow saturation kinetics with respect to hexosamine concentration. Although time-dependent, the inhibition of SSAO activity was found to be reversible by dilution. In contrast, there is no such inhibition when the N-acetylamino sugar derivatives or the parent sugars (galactose, glucose and mannose) replaced the amino sugars in the reaction mixture. These results suggest that the interactions between SSAO and aminohexoses are specific and, therefore, that the cell-adhesion functions and amine-recognition functions of VAP-1/SSAO may be interlinked.