Potent inhibition of the inducible isoform of nitric oxide synthase by aminoethylisoselenourea and related compounds

The generation of nitric oxide (NO) by nitric oxide synthase (NOS) can be inhibited by certain guanidines and S-alkylisothioureas. In particular, aminoethylisothiourea (AE-TU) shows selectivity towards the inducible isoform (iNOS) over the endothelial isoform (ecNOS). Here we report on the effects of the selenium analog of AE-TU, aminoethylisoselenourea (AE-SeU), and its homologue, aminopropylisoselenourea (AP-SeU), on the activities of iNOS and ecNOS. AE-SeU and AP-SeU inhibited the conversion of L-arginine to L-citrulline in homogenates of lung taken from endotoxin-treated rats (a model of iNOS activity) with potencies (EC₅₀ = 1.1, and 0.1 μM, respectively) greater than that of N(G)-methyl-L-arginine (L-NMA) (22 μM). In contrast, AE-SeU and AP-SeU were weaker than or similar to L-NMA at inhibiting ecNOS activity in homogenized bovine endothelial cells (EC₅₀ values = 104, 15, and 16 μM, respectively). AE-SeU and AP-SeU potently inhibited nitrite formation by immunostimulated J774 macrophages (a model of iNOS activity) with EC₅₀ values of 10 and 4 μM respectively. The corresponding EC₅₀ value for L-NMA was 160 μM. The inhibition was dose-dependently reduced by increasing concentrations of L-arginine in the medium. In vivo, AE-SeU had only modest effects on blood pressure when given as a bolus to anesthetized rats, suggesting only a small effect on ecNOS in vivo, whereas AP-SeU had potent presser effects similar to those of L-NMA. We found that both AE-SeU and AP-SeU were unstable in aqueous solution at pH values above 6. Their disappearance from solution was accompanied by the appearance of a reductive species, probably free selenol. These findings suggest that AE-SeU and AP-SeU exert their inhibitory effects through intramolecular rearrangement to yield selenoethylguanidine and seleno-propylguanidine. Thus, selenoalkylguanidines are novel inhibitors of iNOS.