Chemical modification of thiol group(s) in protein: application to the study of anti-microtubular drugs binding

Ferroptosis is involved in cisplatin sensitivity of the S3 segment of immortalized proximal tubule cells

Cisplatin (CDDP) is administered as an anticancer drug across a broad spectrum of cancer treatments, but it causes severe renal damage. Several studies have attempted to elucidate the cause of CDDP-induced renal injury, but the detailed mechanism remains unclear. We previously found that S3 cells are more sensitive to CDDP than S1 and S2 cells by using immortalized cells derived from S1, S2, and S3 segments of proximal tubules. In this study, we investigated the potential contribution of reactive oxygen species (ROS) to the sensitivity of S3 cells to CDDP. The results showed that S3 cells have high sensitivity to CDDP, paraquat (PQ) and three ROS substances. To examine the mechanisms underlying the sensitivity to ROS in S3 cells, we compared the cellular responses of CDDP- and PQ-exposed S3 cells. The results indicated that the levels of intracellular ROS and lipid peroxides were increased in S3 cells after CDDP and PQ exposure. The intracellular levels of antioxidant proteins such as thioredoxin, thioredoxin reductase 1 and glutathione peroxidase 4 were also increased by exposure to PQ, but these proteins were decreased by CDDP exposure in S3 cells. Furthermore, the levels of intracellular free Fe2+ were increased by CDDP exposure only in S3 cells but not S1 or S2 cells, and cytotoxicity by exposure to CDDP in S3 cells was suppressed by ferroptosis inhibitors. These results suggested that the induction of ferroptosis due to the ROS production through attenuation of the antioxidant system and elevated free Fe2+ is partly responsible for the sensitivity of S3 cells to CDDP.

Biochemical and structural insight into the chemical resistance and cofactor specificity of the formate dehydrogenase from Starkeya novella

Formate dehydrogenases (Fdhs) mediate the oxidation of formate to carbon dioxide and concomitant reduction of nicotinamide adenine dinucleotide (NAD+ ). The low cost of the substrate formate and importance of the product NADH as a cellular source of reducing power make this reaction attractive for biotechnological applications. However, the majority of Fdhs are sensitive to inactivation by thiol-modifying reagents. In this study, we report a chemically resistant Fdh (FdhSNO ) from the soil bacterium Starkeya novella strictly specific for NAD+ . We present its recombinant overproduction, purification and biochemical characterization. The mechanistic basis of chemical resistance was found to be a valine in position 255 (rather than a cysteine as in other Fdhs) preventing the inactivation by thiol-modifying compounds. To further improve the usefulness of FdhSNO as for generating reducing power, we rationally engineered the protein to reduce the coenzyme nicotinamide adenine dinucleotide phosphate (NADP+ ) with better catalytic efficiency than NAD+ . The single mutation D221Q enabled the reduction of NADP+ with a catalytic efficiency kCAT /KM of 0.4 s-1 ·mm-1 at 200 mm formate, while a quadruple mutant (A198G/D221Q/H379K/S380V) resulted in a fivefold increase in catalytic efficiency for NADP+ compared with the single mutant. We determined the cofactor-bound structure of the quadruple mutant to gain mechanistic evidence behind the improved specificity for NADP+ . Our efforts to unravel the key residues for the chemical resistance and cofactor specificity of FdhSNO may lead to wider use of this enzymatic group in a more sustainable (bio)manufacture of value-added chemicals, as for instance the biosynthesis of chiral compounds.

Peptide nucleic acid probe for protein affinity purification based on biotin-streptavidin interaction and peptide nucleic acid strand hybridization

We describe a new method for protein affinity purification that capitalizes on the high affinity of streptavidin for biotin but does not require dissociation of the biotin-streptavidin complex for protein retrieval. Conventional reagents place both the selectively reacting group (the "warhead") and the biotin on the same molecule. We place the warhead and the biotin on separate molecules, each linked to a short strand of peptide nucleic acid (PNA), synthetic polymers that use the same bases as DNA but attached to a backbone that is resistant to attack by proteases and nucleases. As in DNA, PNA strands with complementary base sequences hybridize. In conditions that favor PNA duplex formation, the warhead strand (carrying the tagged protein) and the biotin strand form a complex that is held onto immobilized streptavidin. As in DNA, the PNA duplex dissociates at moderately elevated temperature; therefore, retrieval of the tagged protein is accomplished by a brief exposure to heat. Using iodoacetate as the warhead, 8-base PNA strands, biotin, and streptavidin-coated magnetic beads, we demonstrate retrieval of the cysteine protease papain. We were also able to use our iodoacetyl-PNA:PNA-biotin probe for retrieval and identification of a thiol reductase and a glutathione transferase from soybean seedling cotyledons.

Ellman's reagent in promoting crystallization and structure determination of Anabaena CcbP

Obtaining crystals presented a bottleneck in the structural study of Anabaena cyanobacterial Ca2+-binding protein (CcbP). In this report, the promoting effect of Ellman's reagent [5,5'-dithiobis(2-nitrobenzoic acid); DTNB] on the crystallization of CcbP is described. CcbP contains one free cysteine. A quick and simple oxidation reaction with DTNB blocked the free cysteine in purified CcbP and generated a homogenous monomeric protein for crystallization. The crystal structure of DTNB-modified CcbP was determined by the single-wavelength anomalous diffraction method. Structure analysis indicated that DTNB modification facilitated crystallization of CcbP by inducing polar interactions in the crystal lattice. DTNB-mediated cysteine modification was demonstrated to have little effect on the overall structure and the Ca2+ binding of CcbP. Thus, DTNB modification may provide a simple and general approach for protein modification to improve the success of crystallization screening.

Identification of a novel non-AT1, non-AT2 angiotensin binding site in the rat brain

Efforts to protect radiolabeled angiotensins from metabolism during receptor binding assays date back more than 30 years. However, this continues to be a problem. This study focused on the effects of a protease inhibitor, p-chloromercuribenzoate (PCMB), on the binding of (125)I-Ang II to rat brain membranes. Addition of PCMB to the incubation medium revealed a high affinity binding site for (125)I-Ang II in brain membranes (K(d)=1-4 nM) with a greater amount of binding than revealed in previous studies of brain Ang II receptors. Further characterization of this binding, revealed it to be insensitive to inhibition by losartan (an AT(1) receptor antagonist) and PD123319 (an AT(2) receptor antagonist). This non-AT1, non-AT2 binding site was not present in liver or adrenal membranes. It was activated by a limited range of concentrations of PCMB, with maximal activation at 0.3-1 mM. This binding site was equally abundant in cerebral cortex (a brain region with few Ang II receptors) and the hypothalamus (a brain region with abundant Ang II receptors). The binding site was also present in mouse brain, but not mouse liver. The binding site shows high affinity for Ang I, Ang II and Ang III (K(i) approximately 40-100 nM), but lesser affinity for smaller angiotensin fragments and other neuropeptides. This binding site shares some characteristics with the liver cytosolic Ang II binding proteins, later identified as endopeptidases EC 3.4.24.15 and/or EC 3.4.24.16. However, some unique characteristics of this non-AT1, non-AT2 binding site suggest that it may be a novel angiotensin binding substance.

Neurotoxicity of organomercurial compounds

Mercury is a ubiquitous contaminant, and a range of chemical species is generated by human activity and natural environmental change. Elemental mercury and its inorganic and organic compounds have different toxic properties, but all them are considered hazardous in human exposure. In an equimolecular exposure basis, organomercurials with a short aliphatic chain are the most harmful compounds and they may cause irreversible damage to the nervous system. Methylmercury (CH(3)Hg(+)) is the most studied following the neurotoxic outbreaks identified as Minamata disease and the Iraq poisoning. The first description of the CNS pathology dates from 1954. Since then, the clinical neurology, the neuropathology and the mechanisms of neurotoxicity of organomercurials have been widely studied. The high thiol reactivity of CH(3)Hg(+), as well as all mercury compounds, has been suggested to be the basis of their harmful biological effects. However, there is clear selectivity of CH(3)Hg(+) for specific cell types and brain structures, which is not yet fully understood. The main mechanisms involved are inhibition of protein synthesis, microtubule disruption, increase of intracellular Ca(2+) with disturbance of neurotransmitter function, oxidative stress and triggering of excitotoxicity mechanisms. The effects are more damaging during CNS development, leading to alterations of the structure and functionality of the nervous system. The major source of CH(3)Hg(+) exposure is the consumption of fish and, therefore, its intake is practically unavoidable. The present concern is on the study of the effects of low level exposure to CH(3)Hg(+) on human neurodevelopment, with a view to establishing a safe daily intake. Recommendations are 0.4 micro g/kg body weight/day by the WHO and US FDA and, recently, 0.1 micro g/kg body weight/day by the US EPA. Unfortunately, these levels are easily attained with few meals of fish per week, depending on the source of the fish and its position in the food chain.

Redox regulation of cytosolic glycerol-3-phosphate dehydrogenase: Cys(102) is the target of the redox control and essential for the catalytic activity

Cytosolic glycerol-3-phosphate dehydrogenase (cG3PDH) occupies the branch point between the glycolytic pathway and triglyceride biosynthesis. However, the regulatory mechanism of the cG3PDH activity has remained obscure. Here we report that cG3PDH is efficiently inhibited by modification of the thiol group through a redox mechanism. In this study, we found that sodium selenite and nitric oxide (NO) donors such as S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine inhibited cG3PDH activity, and that similar effects could be achieved with selenium metabolites such as selenocysteine and selenomethionine. Furthermore, we found that reducing agents, such as dithiothreitol and beta-mercaptoethanol, restored the cG3PDH activity suppressed by selenite and NO both in vitro and in cultured cells. Buthionine sulfoximine depleted levels of both reduced glutathione and the oxidized form but had no effect on the suppression of cG3PDH activity by selenite in cultured cells. Moreover, thiol-reactive agents, such as N-ethylmaleimide and o-iodosobenzoic acid, blocked the enzyme activity of cG3PDH through the modification of redox-sensitive cysteine residues in cG3PDH. The inhibitor of NO synthase, L-N(G)-nitro-arginine, restored the cG3PDH activity inhibited by NO in cultured cells, whereas the inhibitor of guanylyl cyclase, 1H-[1,2,4] oxadiazole[4,3-alpha] quinoxalin-1-one (ODQ), has no effect. NO directly inhibits cG3PDH activity not via a cGMP-dependent mechanism. Finally, using site-directed mutagenesis, we found that Cys(102) of cG3PDH was sensitive to both selenite and NO. From the results, we suggest that cG3PDH is a target of cellular redox regulation.