Microsomal metabolism of 2-(methylseleno) benzanilide

Synthesis and Characterization of a New Five and Six Membered Selenoheterocyclic Compounds Homologues of Ebselen

The discovery of the antioxidant activity of selenoenzyme glutathione peroxidase (GPx) has attracted growing attention in the biochemistry of selenium. Among molecules which mimic the structure of the active site of the enzyme, N-phenyl-1,2-benzisoselenazolin-3-one 1, Ebselen, exhibited useful anti-inflammatory properties. It has been extensively investigated and has undergone clinical trials as an anti-inflammatory agent. Unfortunately, Ebselen exhibits relatively poor catalytic activity, prompting attempts to design more efficacious GPx mimetics that would retain his low toxicity while manifesting improved catalytic properties. In this context, novel 1,2-benzoselenazine and 1,2-benzoselenazols, which are five and six membered homologues of Ebselen were synthesized and characterized. One structure has been proven by single crystal X-ray crystallography.

Metabolites of sodium selenite and methylated selenium compounds administered at cancer chemoprevention levels in the rat

1. The metabolism of orally-administered sodium selenite and five methylated selenium compounds was investigated in the female rat at dosages equivalent to those used in other studies for prevention of mammary cancer. Dimethyl selenide (DMSe) exhaled within 24 h following dosing was measured, along with inorganic and monomethylated (MMSe) forms of selenium plus trimethylselenonium ion (TMSe+) in urine. 2. MMSe was the dominant metabolite of selenite given at low levels (0.1 ppm in the diet), but excretion of DMSe and TMSe+ increased sharply when selenite dosage was increased to the chemopreventive range of 3 ppm dietary Se. When similar chemopreventive levels of mono-, di-, or trimethylated compounds were administered, the total quantity of methylated metabolites was greater than for selenite and the metabolite profile reflected the expected point of entry into the intermediary metabolism pathway; the major metabolites were MMSe from Se-methylselenocysteine, DMSe from selenobetaine methyl ester, and TMSe+ from selenobetaine. However, the profile of metabolites provided clear evidence that the methylated selenium compounds underwent demethylation, as shown by the excretion of inorganic and MMSe. Selenium administered as dimethyl selenoxide was almost completely excreted and about 90% of the dose was recovered as DMSe, indicating that reduction was the major pathway. For TMSe+, about 10% of the dose was excreted as DMSe and 84% as TMSe+. 3. A low, non-toxic level of sodium arsenite (5 ppm As in the diet) that is known to modify differentially the anticarcinogenic activity of selenite and methylated selenium compounds did not modify the excretion of the methylated selenium metabolites. 4. It is concluded that high anticarcinogenic activity is associated with extensive excretion of methylated Se excretory metabolites, but high output of such metabolites per se does not necessarily lead to anticarcinogenic activity. The whole animal has extensive capabilities for interconverting forms of selenium, and retains significant amounts in tissues, complicating the interpretation of Se metabolism and anticarcinogenic action. Further research is needed on the forms of selenium present in tissues.

Ebselen, a selenoorganic compound as glutathione peroxidase mimic

The selenoorganic compound ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)-one, exhibits activity as an enzyme mimic. The reaction catalyzed is that of a glutathione (GSH) peroxidase (i.e., the reduction of a hydroperoxide at the expense of thiol). The specificity for substrates ranges from hydrogen peroxide and smaller organic hydroperoxides to membrane-bound phospholipid and cholesterol hydroperoxides. In addition to glutathione, the thiol reductant cosubstrate can be dithioerythritol, N-acetylcysteine or dihydrolipoate, or other suitable thiol compounds. Ebselen also has properties such as free radical and singlet oxygen quenching. Model experiments in vitro with liposomes, microsomes, isolated cells, and organs show that the protection against oxidative challenge afforded by ebselen can be explained largely by the activity as GSH peroxidase mimic. Whether this also explains the known preliminary results in clinical settings is yet open. The metabolism and disposition of ebselen is presented in this review. The main point is that the selenium is not bioavailable, explaining the extremely low toxicity observed in animal studies. The occurrence of natural GPx mimics, ovothiol and related compounds, is briefly mentioned.

Deactivation of singlet molecular oxygen by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur-containing homologs

The bimolecular rate constants (k) of quenching of molecular singlet oxygen 1O2 (1 delta g) by organo-selenium compounds exhibiting glutathione peroxidase activity and by sulfur analogs have been determined by time resolved phosphorescence detection of 1O2 in CD3OD and C6D6, with no solvent effect. The rate constants of quenching by the Se-containing compounds were found to be approximately one order of magnitude higher than those of the S-containing homologs. A linear correlation was observed between log k and the Hammett constant omega ortho with p = -0.89, the rate constant being higher for molecules with an electron-donating substituent and lower for those with an electron-withdrawing substituent. This observation is consistent with the involvement of a charge transfer complex in the deactivation of singlet oxygen.