Ammonium trichloro(dioxoethylene-O,O‘)tellurate (AS101) is an organotellurium(IV) compound that exhibits immunomodulation activity. In light of the unique Te(IV)−thiol chemistry, it was tested as a selective cysteine protease inhibitor. Although no inhibitory activity of serine-, metallo-, or aspartic proteases was observed, AS101 exhibited time- and concentration-dependent inactivation of cysteine proteases. The kinetic parameters of inactivation of papain were K i = 3.5 ± 2.0 μM and k i = (5.1 ± 0.4) × 10-2 min-1. The enzymatic activity could be recovered by treatment with thiols, indicating that the inactivation involves oxidation of the active-site thiol to a disulfide bond (Enz−S−S−R) or to a species containing a Te−S bond such as Enz−S−Te−S−R. Gel permeation chromatography established that the R group is a small molecule and excludes the possibility of dimerization of the enzyme itself. It was further established that some other Te(IV) derivatives could also inactivate cysteine proteases, while Te(VI) derivatives did not exhibit any such inhibitory activity. In order to understand the chemistry underlying the cysteine protease inactivation by AS101 and other organotellurium(IV) compounds, their interaction with the model compound cysteine was studied. While the Te(VI) derivatives did not interact with cysteine, all of the Te(IV) compounds interacted with 4 equiv of cysteine. The kinetics of this interaction is first order in Te and second order in thiol, yielding a third-order rate constant of ∼106 M-2 s-1, as determined for the interaction between AS101 with cysteine. The interactions between Te derivatives and cysteine in DMSO were followed by 125Te and 13C NMR. While Te(VI) compounds did not undergo any changes upon interaction with cysteine, on the basis of their 125Te NMR, the Te(IV) derivatives interacted with 4 equiv of cysteine, yielding new stable Te(IV) compounds. These compounds were tentatively designated as Te(cysteine)4 or its high-valence complex with other components in the reaction mixture. These results expand our understanding of tellurium chemistry and correlate well with its biological activity. Such knowledge can be applied for the development of novel biologically active tellurium compounds.

In Parkinson's disease (PD) dopaminergic neurons in the substantia nigra (SN) become dysfunctional and many ultimately die. We report that the tellurium immunomodulating compound ammonium trichloro(dioxoethylene-O,O'-)tellurate (AS101) protects dopaminergic neurons and improves motor function in animal models of PD. It is effective when administered systemically or by direct infusion into the brain. Multifunctional activities of AS101 were identified in this study. These were mainly due to the peculiar Tellur(IV)-thiol chemistry of the compound, which enabled the compound to interact with cysteine residues on both inflammatory and apoptotic caspases, resulting in their inactivation. Conversely, its interaction with a key cysteine residue on p21(ras), led to its activation, an obligatory activity for AS101-induced neuronal differentiation. Furthermore, AS101 inhibited IL-10, resulting in up-regulation of GDNF in the SN. This was associated with activation of the neuroprotective kinases Akt and mitogen-activated protein kinases, and up-regulation of the antiapoptotic protein Bcl-2. Inhibition of caspase-1 and caspase-3 activities were associated with decreased neuronal death and inhibition of IL-1beta. We suggest that, because multiple mechanisms are involved in the dysfunction and death of neurons in PD, use of a multifunctional compound, exerting antiapoptotic, anti-inflammatory, and neurotrophic-inducing capabilities may be potentially efficacious for the treatment of PD.

The organotellurium compound, trichloro(dioxoethylene-O,O') tellurate (AS101) has been shown previously to exert diverse biologic activities both in vitro and in vivo. This compound was recently found to react with thiols and to catalyze their oxidation. This property of AS101 raises the possibility that it may serve as a cysteine protease inhibitor. In the present study, using a substrate-specific enzymatic assay, we show that treatment of caspase-1 (interleukin-1beta [IL-1beta] converting enzyme [ICE]) with AS101 inhibits its enzymatic activity in a dose-dependent manner. Moreover, the results show that AS101 treatment causes a significant reduction in the active form of IL-18 and IL-1beta in peripheral blood mononuclear cells (PBMC) and in human HaCat keratinocytes. We further demonstrate that the inhibitory effect of AS101 does not involve nitric oxide (NO) or interferon-gamma (IFN-gamma), two possible regulators of IL-18 production, and does not occur at the mRNA level, suggesting a posttranscriptional mechanism of action. More importantly, AS101 downregulates IL-18 and IL-1beta serum levels in a mouse model of lipopolysaccharide (LPS)-induced sepsis, resulting in increased survival. Recent studies emphasize the pathophysiologic role of IL-18 and IL-1beta in a variety of inflammatory diseases. Thus, their blockage by the nontoxic compound, AS101, currently used in clinical studies, may provide clinical advantage in the treatment of these diseases.

The pKa of the catalytic His57 N(epsilon)H in the tetrahedral complex (TC) of chymotrypsin with trifluoromethyl ketone inhibitors is 4-5 units higher relative to the free enzyme (FE). Such stable TC's, formed with transition state (TS) analog inhibitors, are topologically similar to the catalytic TS. Thus, analysis of this pKa shift may shed light on the role of water solvation in the general base catalysis by histidine. We applied our QM/SCRF(VS) approach to study this shift. The method enables explicit quantum mechanical DFT calculations of large molecular clusters that simulate chemical reactions at the active site (AS) of water solvated enzymes. We derived an analytical expression for the pKa dependence on the degree of water exposure of the ionizable group, and on the total charge in the enzyme AS, Q(A) and Q(B), when the target ionizable functional group (His57 in this study) is in the acidic (A) and basic (B) forms, respectively. Q2(B) > Q2(A) both in the FE and in the TC of chymotrypsin. Therefore, water solvation decreases the relative stability of the protonated histidine in both. Ligand binding reduces the degree of water solvation of the imidazole ring, and consequently elevates the histidine pKa. Thus, the binding of the ligand plays a triggering role that switches on the cascade of catalytic reactions in serine proteases.

Peptidyl epoxides were designed as selective pseudo-mechanism-based inactivators of cysteine proteases. Both threo- and erythro-peptidyl epoxides were synthesized and tested as potential inactivators of serine proteases (chymotrypsin, subtilisin, and elastase) and of cysteine proteases (papain, cathepsin B, and clostripain). Four tripeptidyl epoxides (Cbz-Gly-Leu-Phe-epoxide, Cbz-Ala-Ala-Phe-epoxide, Cbz-Gly-Leu-Ala-epoxide, and Cbz-Ala-Ala-Ala-epoxide), bearing amino acid sequences similar to those of good substrates or known inhibitors of the serine proteases, were tested in this study. Neither the threo- nor the erythro-peptidyl epoxides exhibited any inhibitory activity toward the serine proteases, even at high concentration and long incubation time. Nor did the threo-peptidyl epoxides inhibit the cysteine proteases. On the other hand, the erythro-peptidyl epoxides were time- and concentration-dependent inactivators of the cysteine proteases. Furthermore, stereoselectivity toward the natural l-amino acid at the P1 position was also exhibited upon inhibition of papain. In order to demonstrate selectivity within the cysteine protease family, two other erythro-peptidyl epoxides (Cbz-Phe-Ala-epoxide and Cbz-Phe-O-Bn-Thr-epoxide) were synthesized and tested as inhibitors of the three cysteine proteases. These new peptidyl epoxides exhibited selective inactivation of cysteine proteases, with second-order rate constants (k i/K i) ranging over 4 orders of magnitude (0.04−330 M-1 s-1). Thus, this new family of highly selective cysteine protease inhibitors offers mechanistic implications and may have useful applications.