Semicarbazide-sensitive amine oxidase: role in the vasculature and vasodilation after in situ inhibition

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.

Association between semicarbazide-sensitive amine oxidase, a regulator of the glucose transporter, and elastic lamellae thinning during experimental cerebral aneurysm development

Object

Amine oxidases play a key role in the polymerization and cross-linking of the collagens and elastic lamellae of the arterial wall. The loss of elastic lamellae integrity is one of the first steps in the genesis of a cerebral aneurysm. The authors investigated the relation between semicarbazide-sensitive amine oxidase (SSAO) and the organization of the cerebral arterial wall during aneurysm development.

Methods

Intracranial aneurysms were induced in rats via unilateral carotid artery ligation and renovascular hypertension. This modified Hashimoto model was used to create elevated blood pressure associated with shear stress in cerebral arteries. The authors immunohistologically investigated some markers of the extracellular matrix (Types I, III, and IV collagen and elastin), vascular smooth muscle cell differentiation (smooth muscle myosin heavy chain [sm-MHC], α–smooth muscle actin, and desmin), and amine oxidases (SSAO and lysyl oxidase [LOX]) in the cerebral arterial wall in control and treated rats 1, 2, 3, 4, and 6 months after the surgical procedure.

Results

The authors found severe disorganization and thinning of the elastic lamellae and a dramatic reduction in SSAO activity and immunostaining during cerebral aneurysm development. In contrast, LOX markers were slightly increased. Elastic lamellae thinning was highly correlated with decreases in SSAO (r = 0.76, p < 0.0001). There was also a correlation between sm-MHC and SSAO levels.

Conclusions

The data suggested that cerebral hemodynamic modifications induce decreases in SSAO activity resulting in cell dedifferentiation and inducing dysregulation of glucose transport.

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.

Hypotensive effect of hydroxylamine, an endogenous nitric oxide donor and SSAO inhibitor

The endogenous compound hydroxylamine relaxes vascular smooth muscle in vitro, apparently through conversion to the vasodilator factor nitric oxide, but its effect on blood pressure has not been characterized. We found that in the anesthetized rat the amine elicits dose-related hypotension when administered by continuous iv infusion. In experiments designed to explore the mechanism of this effect, hydroxylamine was compared with the nitric oxide donor nitroprusside and the direct-acting vasodilator hydralazine, using pretreatments known to modify diverse mechanisms of vasodilation. Hydroxylamine hypotension was enhanced by the SSAO inhibitor isoniazid and the SSAO substrate methylamine, a pattern shared by hydralazine. Responses were blocked by the guanylate cyclase inhibitor methylene blue and were increased by the nitric oxide synthase inhibitor L-NAME, a pattern shared by nitroprusside. It was concluded that hydroxylamine exerts hypotension partly through conversion to nitric oxide and partly by a "hydralazine-like" mechanism involving SSAO inhibition.

2-bromoethylamine, a suicide inhibitor of semicarbazide-sensitive amine oxidase, increases hydralazine hypotension in rats

Previous work has shown that inhibitors of the predominantly vascular enzyme semicarbazide-sensitive amine oxidase (SSAO) potentiate the hypotensive response to hydralazine, itself a SSAO inhibitor, in anesthetized rats. The present study was carried out to determine whether the recently described suicide SSAO inhibitor 2-bromoethylamine shares this effect. Hypotensive responses to hydralazine, 0.1 mg/kg IV, were obtained in chloralose-urethane-anesthetized rats, either unpretreated or receiving bromoethylamine at different doses and pretreatment intervals. Parallel experiments were run with semicarbazide, the prototypical hydrazine SSAO inhibitor. Both inhibitors potentiated hydralazine hypotension, bromoethylamine having a longer latency and a shorter duration of action than semicarbazide. High doses of bromoethylamine did not produce potentiation, a phenomenon attributed to SSAO inactivation by excess substrate and decreased formation by the enzyme of the inhibitor product. Experiments with combined administration of both inhibitors were also carried out. When semicarbazide was administered before bromoethylamine, potentiaton was prevented, apparently by a mechanism similar to the above; when it was given after the amine, potentiation was increased. This was attributed to enzyme inhibition by interaction with 2 different active sites. The charactertistics of hydralazine potentiation by bromoethylamine were considered compatible with the mechanism of SSAO inhibition by the amine.