The hepatotoxicity of thioacetamide (TA) has been known since 1948. In rats, single doses cause centrilobular necrosis accompanied by increases in plasma transaminases and bilirubin. To elicit these effects TA requires oxidative bioactivation leading first to its S-oxide (TASO) and then to its chemically reactive S,S-dioxide (TASO2) which ultimately modifies amine-lipids and proteins. To generate a suite of liver proteins adducted by TA metabolites for proteomic analysis, and to reduce the need for both animals and labeled compounds, we treated isolated hepatocytes directly with TA. Surprisingly, TA was not toxic at concentrations up to 50 mM for 40 hr. On the other hand, TASO was highly toxic to isolated hepatocytes as indicated by LDH release, cellular morphology and vital staining with Hoechst 33342/propidium iodide. TASO toxicity was partially blocked by the CYP2E1 inhibitors diallyl sulfide and 4-methylpyrazole, and was strongly inhibited by TA. Significantly, we found that hepatocytes produce TA from TASO relatively efficiently by back-reduction. The covalent binding of [14C]-TASO is inhibited by unlabeled TA which acts as a “cold-trap” for [14C]-TA and prevents its re-oxidation to [14C]-TASO. This in turn increases the net consumption of [14C]-TASO despite the fact that its oxidation to TASO2 is inhibited. The potent inhibition of TASO oxidation by TA, coupled with the back-reduction of TASO and its futile redox cycling with TA may help explain phenomena previously interpreted as “saturation toxicokinetics” in the in vivo metabolism and toxicity of TA and TASO. The improved understanding of the metabolism and covalent binding of TA and TASO facilitates the use of hepatocytes to prepare protein adducts for target protein identification.
A series of trifluoromethyl, benzothiazolyl or thiazolyl ketone-containing peptidic compounds as SARS-CoV 3CL protease inhibitors were developed and their potency was evaluated by in vitro protease inhibitory assays. Three candidates had encouraging results for the development of new anti-SARS compounds.
Thioacetamide (TA) has long been known as a hepatotoxicant whose bioactivation requires S-oxidation to thioacetamide S-oxide (TASO) and then to the very reactive S,S-dioxide (TASO2). The latter can tautomerize to form acylating species capable of covalently modifying cellular nucleophiles including phosphatidylethanolamine (PE) lipids and protein lysine side chains. Isolated hepatocytes efficiently oxidize TA to TASO but experience little covalent binding or cytotoxicity because TA is a very potent inhibitor of the oxidation of TASO to TASO2. On the other hand hepatocytes treated with TASO show extensive covalent binding to both lipids and proteins accompanied by extensive cytotoxicity. In this work, we treated rat hepatocytes with [14C]-TASO and submitted the mitochondrial, microsomal and cytosolic fractions to 2DGE which revealed a total of 321 radioactive protein spots. To facilitate the identification of target proteins and adducted peptides we also treated cells with a mixture of TASO/[13C2D3]-TASO. Using a combination of 1DGE- and 2DGE-based proteomic approaches, we identified 187 modified peptides (174 acetylated, 50 acetimidoylated and 37 in both forms) from a total of 88 non-redundant target proteins. Among the latter, 57 are also known targets of at least one other hepatotoxin. The formation of both amide- and amidine-type adducts to protein lysine side chains is in contrast to the exclusive formation of amidine-type adducts with PE phospholipids. Thiobenzamide (TB) undergoes the same two-step oxidative bioactivation as TA, and it also gives rise to both amide and amidine adducts on protein lysine side chains but only amidine adducts to PE lipids. Despite their similarity in functional group chemical reactivity, only 38 of 62 known TB target proteins are found among the 88 known targets of TASO. The potential roles of protein modification by TASO in triggering cytotoxicity are discussed in terms of enzyme inhibition, protein folding and chaperone function, and the emerging role of protein acetylation in intracellular signaling and the regulation of biochemical pathways.
A highly efficient green protocol for palladium acetate-catalysed ligand-free Suzuki–Miyaura cross-coupling reactions in neat ‘water extract of banana (WEB)’ was developed. Suzuki–Miyaura reaction proceeds in WEB at very short reaction times under ‘ligand/external base/external promoters/organic medium’ free conditions.
Chitosan stabilised copper-iron oxide nanocomposite material is found to be a very efficient recyclable/reusable heterogeneous catalyst for azide-alkyne cycloaddition reaction under mild condition. The as-synthesized nanocomposite material is magnetically recoverable and reusable at least up to four times without significant loss in their catalytic activity. The catalytic system is found to be highly useful for the regioselective synthesis of a wide range of 1,2,3-triazoles from various azides and alkynes.
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