Discovery of Heteroaromatic Sulfones As a New Class of Biologically Compatible Thiol-Selective Reagents.

The selective reaction of chemical reagents with reduced protein thiols is critical to biological research. This reaction is utilized to prevent cross-linking of cysteine-containing peptides in common proteomics workflows and is applied widely in discovery and targeted redox investigations of the mechanisms underlying physiological and pathological processes. However, known and commonly used thiol blocking reagents like iodoacetamide, N-ethylmaleimide, and others were found to cross-react with oxidized protein sulfenic acids (-SOH) introducing significant errors in studies employing these reagents. We have investigated and are reporting here a new heteroaromatic alkylsulfone, 4-(5-methanesulfonyl-[1,2,3,4]tetrazol-1-yl)-phenol (MSTP), as a selective and highly reactive -SH blocking reagent compatible with biological applications.

Channel-lining residues of the AMPA receptor M2 segment: structural environment of the Q/R site and identification of the selectivity filter.

In AMPA receptor channels, a single amino acid residue (Q/R site) of the M2 segment controls permeation of calcium ions, single-channel conductance, blockade by intracellular polyamines, and permeation of anions. The structural environment of the Q/R site and its positioning with regard to a narrow constriction were probed with the accessibility of substituted cysteines to positively and negatively charged methanethiosulfonate reagents, applied from the extracellular and cytoplasmic sides of the channel. The accessibility patterns confirm that the M2 segment forms a pore loop with the Q/R site positioned at the tip of the loop (position 0) facing the extracellular vestibule. Cytoplasmically accessible residues on the N- and C-terminal sides of position 0 form the ascending alpha-helical (-8 to -1) and descending random coil (+1 to +6) components of the loop, respectively. Substitution of a glycine residue at position +2 with alanine strongly decreased the permeability of organic cations, indicating that position +2 contributes to the narrow constriction. The anionic 2-sulfonatoethyl-methanethiosufonate reacted with a cysteine at position 0 only from the external side and with cysteines at positions +1 to +4 only from the cytoplasmic side. These results suggest that charge selectivity occurs external to the constriction (+2) and possibly involves interactions of ions with the negative electrostatic potential created by the dipole of the alpha-helix formed by the ascending limb of the loop.

Reactivity of sulfhydryl groups in the brush-border membranes of chick duodena is increased by 1,25-dihydroxycholecalciferol.

The total amount and reactivity of SH-groups were determined in isolated duodenal brush-border membranes from rachitic chicks given 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3) before isolation of the membranes. 1,25(OH)2D3 treatment significantly increased the total amount of SH groups (9.7 +/- 2.3 vs. 23.9 +/- 2.1, P less than 0.001, n = 6) in brush-border membranes solubilized in 1% sodium dodecyl sulfate. The rate of reaction (reactivity) of membrane-bound SH-groups, determined with the fluorescent thiol reagent, N-(7-dimethylamino-4-methylcoumarin-3-yl)maleimide (DACM), was also significantly enhanced by the intravenous injection of various doses (0.005-0.10 microgram) of 1,25(OH)2D3 into vitamin D-deficient chicks. An increase in reactivity occurred as early as 10 min after dosing of the chicks with 1,25(OH)2D3. Fluorescence scanning of the membrane proteins labeled with DACM and separated by polyacrylamide gel electrophoresis revealed three major peaks of fluorescence and a generally higher degree of fluorescent labeling of these and many other proteins in the membranes isolated from the 1,25(OH)2D3-treated chicks. The physiological significance of the 1,25(OH)2D3-mediated increase in the reactivity and total amount of membrane-bound SH-groups in terms of vitamin-D-dependent epithelial transport and epithelial membrane properties needs to be further explored.

An attempt to explain interindividual variability in 24-h urinary excretion of inorganic arsenic metabolites by C57 BL/6J mice.

Forty C57 BL/6J mice, injected subcutaneously with 0.5 mg/kg arsenic as sodium arsenite, were examined for 24-h urinary excretion of total arsenic metabolites, creatinine and S-adenosylmethionine (SAM) and for 24-h faecal excretion of arsenic and levels of arsenic in the blood, liver, kidneys, lung, skin, spleen and bone at 24-h post-dose. Total urinary arsenic metabolites were calculated by summing up the inorganic (Asi), monomethylated (MMA) and dimethylated (DMA) derivatives directly measured by selective arsine generation-atomic absorption spectrometry (AG-AAS) or were measured by AG-AAS following complete mineralization. Both sets of results showed interindividual differences varying by as much as 7-fold and correlated with the 24-h urinary excretion of both SAM (r = 0.84 and r = 0.86, respectively) and creatinine (r = 0.82 and r = 0.87, respectively). There was interindividual variability of about a 30-fold range in 24-h faecal excretion of arsenic which correlated inversely with 24-h urinary excretion of arsenic metabolites (r = -0.69) and 24-h urinary excretion of both creatinine (r = -0.70) and SAM (r = -0.67). Body tissue levels of arsenic were low and not related to 24-h urinary excretion of arsenic metabolites, SAM and creatinine. Taken together, the results indicate that differences in the profile of urinary arsenic excretion and in the retention of arsenic in a particular organ do not contribute to interindividual variability in 24-h urinary excretion of arsenic metabolites by C57 BL/6J mice, but that variability in faecal excretion does, at least in part. It is speculated that there is most likely a predominant contribution from a diffuse tissue retention of arsenic or from a third route of arsenic elimination, i.e. respiratory, to this phenomenon in view of the small faecal contribution.

5,5'-Dithiobis-(2-nitrobenzoic acid) as a probe for a non-essential cysteine residue at the medium chain acyl-coenzyme A dehydrogenase binding site of the human 'electron transferring flavoprotein' (ETF).

Human 'electron transferring flavoprotein' (ETF) was inactivated by the thiol-specific reagent 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). The kinetic profile showed the reaction followed pseudo-first-order kinetics during the initial phase of inactivation. Monitoring the release of 5-thio-2-nitrobenzoate (TNB) showed that modification of 1 cysteine residue was responsible for the loss of activity. The inactivation of ETF by DTNB could be reversed upon incubation with thiol-containing reagents. The loss of activity was prevented by the inclusion of medium chain acyl-CoA dehydrogenase (MCAD) and octanoyl-CoA. Cyanolysis of the DTNB modified-ETF with KCN led to the release of TNB accompanied presumably by the formation of the thio-cyano enzyme and with almost full recovery of activity. Conservation studies and the lack of 100% inactivation, however, suggested that this cysteine residue is not essential for the interaction with MCAD.

Synthesis and characterization of thiobutyltriphenylphosphonium bromide, a novel thiol reagent targeted to the mitochondrial matrix.

Mitochondria are continually exposed to oxidative stress due to superoxide formation by the respiratory chain which increases in pathological situations such as ischemia reperfusion and neurodegeneration. During oxidative stress there are a number of changes in mitochondrial low-molecular-weight and protein thiols. In particular, the mitochondrial glutathione pool becomes oxidized and forms mixed disulfides with protein thiols. To investigate changes in the redox state and conjugation of mitochondrial glutathione, and other mitochondrial thiols, we designed and characterized a thiol probe specifically targeted to the mitochondrial matrix. This molecule, thiobutyltriphenylphosphonium bromide, contains a thiol group linked to a lipophilic triphenylphosphonium cation which causes it to accumulate in the negatively charged mitochondrial matrix. Using [14C]thiobutyltriphenylphosphonium bromide we confirmed that it was selectively accumulated by isolated mitochondria. In the mitochondrial matrix the thiol group equilibrated with endogenous thiols and during oxidative stress became disulfide-bonded to protein and nonprotein thiols. Therefore, this novel thiol probe can be used to label protein thiol groups and to investigate changes in conjugation and redox state of mitochondrial thiols during oxidative stress.