Photocatalytic solar hydrogen production from water on a 100-m2 scale.

The unprecedented impact of human activity on Earth's climate and the ongoing increase in global energy demand have made the development of carbon-neutral energy sources ever more important. Hydrogen is an attractive and versatile energy carrier (and important and widely used chemical) obtainable from water through photocatalysis using sunlight, and through electrolysis driven by solar or wind energy1,2. The most efficient solar hydrogen production schemes, which couple solar cells to electrolysis systems, reach solar-to-hydrogen (STH) energy conversion efficiencies of 30% at a laboratory scale3. Photocatalytic water splitting reaches notably lower conversion efficiencies of only around 1%, but the system design is much simpler and cheaper and more amenable to scale-up1,2-provided the moist, stoichiometric hydrogen and oxygen product mixture can be handled safely in a field environment and the hydrogen recovered. Extending our earlier demonstration of a 1-m2 panel reactor system based on a modified, aluminium-doped strontium titanate particulate photocatalyst4, we here report safe operation of a 100-m2 array of panel reactors over several months with autonomous recovery of hydrogen from the moist gas product mixture using a commercial polyimide membrane5. The system, optimized for safety and durability, and remaining undamaged on intentional ignition of recovered hydrogen, reaches a maximum STH of 0.76%. While the hydrogen production is inefficient and energy negative overall, our findings demonstrate that safe, large-scale photocatalytic water splitting, and gas collection and separation are possible. To make the technology economically viable and practically useful, essential next steps are reactor and process optimization to substantially reduce costs and improve STH efficiency, photocatalyst stability and gas separation efficiency.

Photocatalytic water splitting with a quantum efficiency of almost unity.

Overall water splitting, evolving hydrogen and oxygen in a 2:1 stoichiometric ratio,  using particulate photocatalysts is a potential means of achieving scalable and economically viable solar hydrogen production. To obtain high solar energy conversion efficiency, the quantum efficiency of the photocatalytic reaction must be increased over a wide range of wavelengths and semiconductors with narrow bandgaps need to be designed. However, the quantum efficiency associated with overall water splitting using existing photocatalysts is typically lower than ten per cent1,2. Thus, whether a particulate photocatalyst can enable a quantum efficiency of 100 per cent for the greatly endergonic water-splitting reaction remains an open question. Here we demonstrate overall water splitting at an external quantum efficiency of up to 96 per cent at wavelengths between 350 and 360 nanometres, which is equivalent to an internal quantum efficiency of almost unity, using a modified aluminium-doped strontium titanate (SrTiO3:Al) photocatalyst3,4. By selectively photodepositing the cocatalysts Rh/Cr2O3 (ref. 5) and CoOOH (refs. 3,6) for the hydrogen and oxygen evolution reactions, respectively, on different crystal facets of the semiconductor particles using anisotropic charge transport, the hydrogen and oxygen evolution reactions could be promoted separately. This enabled multiple consecutive forward charge transfers without backward charge transfer, reaching the upper limit of quantum efficiency for overall water splitting. Our work demonstrates the feasibility of overall water splitting free from charge recombination losses and introduces an ideal cocatalyst/photocatalyst structure for efficient water splitting.

Carbon Nanotubes as a Solid-State Electron Mediator for Visible-Light-Driven Z-Scheme Overall Water Splitting.

So-called Z-scheme systems, which typically comprise an H2 evolution photocatalyst (HEP), an O2 evolution photocatalyst (OEP), and an electron mediator, represent a promising approach to solar hydrogen production via photocatalytic overall water splitting (OWS). The electron mediator transferring photogenerated charges between the HEP and OEP governs the performance of such systems. However, existing electron mediators suffer from low stability, corrosiveness to the photocatalysts, and parasitic light absorption. In the present work, carbon nanotubes (CNTs) were shown to function as an effective solid-state electron mediator in a Z-scheme OWS system. Based on the high stability and good charge transfer characteristics of CNTs, this system exhibited superior OWS performance compared with other systems using more common electron mediators. The as-constructed system evolved stoichiometric amounts of H2 and O2 at near-ambient pressure with a solar-to-hydrogen energy conversion efficiency of 0.15%. The OWS reaction was also promoted in the case that this CNT-based Z-scheme system was immobilized on a substrate. Hence, CNTs are a viable electron mediator material for large-scale Z-scheme OWS systems.

Overall Water Splitting by a SrTaO2N-Based Photocatalyst Decorated with an Ir-Promoted Ru-Based Cocatalyst.

The development of narrow-bandgap photocatalysts for one-step-excitation overall water splitting (OWS) remains a critical challenge in the field of solar hydrogen production. SrTaO2N is a photocatalytic material having a band structure suitable for OWS under visible light (λ ≤ 600 nm). However, the presence of defects in the oxynitride and the lack of cocatalysts to promote simultaneous hydrogen and oxygen evolution make it challenging to realize OWS using this material. The present work demonstrates a SrTaO2N-based particulate photocatalyst for OWS. This photocatalyst, which was composed of single crystals, was obtained by nitriding SrCl2 and Ta2O5 together with NaOH, with the latter added to control the formation of defects. The subsequent loading of bimetallic RuIrOx nanoparticles accelerated charge separation and allowed the SrTaO2N photocatalyst to exhibit superior OWS activity. This research presenting the strategies of controlling the oxygen sources and promoting the cocatalyst function is expected to expand the range of potential OWS-active oxynitride photocatalysts and permit the design of efficient cocatalysts for photocatalytic OWS.

A perspective on two pathways of photocatalytic water splitting and their practical application systems.

Photocatalytic water splitting has been widely studied as a means of converting solar energy into hydrogen as an ideal energy carrier in the future. Systems for photocatalytic water splitting can be divided into one-step excitation and two-step excitation processes. The former uses a single photocatalyst while the latter uses a pair of photocatalysts to separately generate hydrogen and oxygen. Significant progress has been made in each type of photocatalytic water splitting system in recent years, although improving the solar-to-hydrogen energy conversion efficiency and constructing practical technologies remain important tasks. This perspective summarizes recent advances in the field of photocatalytic overall water splitting, with a focus on the design of photocatalysts, co-catalysts and reaction systems. The associated challenges and potential approaches to practical solar hydrogen production via photocatalytic water splitting are also presented.

Cystathionine γ-Lyase Self-Inactivates by Polysulfidation during Cystine Metabolism.

Cystathionine γ-lyase (CSE) is an enzyme responsible for the biosynthesis of cysteine from cystathionine in the final step of the transsulfuration pathway. It also has ß-lyase activity toward cystine, generating cysteine persulfide (Cys-SSH). The chemical reactivity of Cys-SSH is thought to be involved in the catalytic activity of particular proteins via protein polysulfidation, the formation of -S-(S)n-H on their reactive cysteine residues. The Cys136/171 residues of CSE have been proposed to be redox-sensitive residues. Herein, we investigated whether CSE polysulfidation occurs at Cys136/171 during cystine metabolism. Transfection of wild-type CSE into COS-7 cells resulted in increased intracellular Cys-SSH production, which was significantly increased when Cys136Val or Cys136/171Val CSE mutants were transfected, instead of the wild-type enzyme. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that CSE polysulfidation occurs at Cys136 during cystine metabolism. In vitro incubation of CSE with CSE-enzymatically synthesized Cys-SSH resulted in the inhibition of Cys-SSH production. In contrast, the mutant CSEs (Cys136Val and Cys136/171Val) proved resistant to inhibition. The Cys-SSH-producing CSE activity of Cys136/171Val CSE was higher than that of the wild-type enzyme. Meanwhile, the cysteine-producing CSE activity of this mutant was equivalent to that of the wild-type enzyme. It is assumed that Cys-SSH-producing CSE activity could be auto-inactivated via the polysulfidation of the enzyme during cystine metabolism. Thus, the polysulfidation of CSE at the Cys136 residue may be an integral feature of cystine metabolism, which functions to down-regulate Cys-SSH synthesis by the enzyme.

Criteria for Efficient Photocatalytic Water Splitting Revealed by Studying Carrier Dynamics in a Model Al-doped SrTiO3 Photocatalyst.

Overall water splitting (OWS) using semiconductor photocatalysts is a promising method for solar fuel production. Achieving a high quantum efficiency is one of the most important prerequisites for photocatalysts to realize high solar-to-fuel efficiency. In a recent study (Nature 2020, 58, 411-414), a quantum efficiency of almost 100 % has been achieved in an aluminum-doped strontium titanate (SrTiO3 : Al) photocatalyst. Herein, using the SrTiO3 : Al as a model photocatalyst, we reveal the criteria for efficient photocatalytic water splitting by investigating the carrier dynamics through a comprehensive photoluminescence study. It is found that the Al doping suppresses the generation of Ti3+ recombination centers in SrTiO3 , the surface band bending facilitates charge separation, and the in situ photo-deposited Rh/Cr2 O3 and CoOOH co-catalysts render efficient charge extraction. By suppressing photocarrier recombination and establishing a facile charge separation and extraction mechanism, high quantum efficiency can be achieved even on photocatalysts with a very short (sub-ns) intrinsic photocarrier lifetime, challenging the belief that a long carrier lifetime is a fundamental requirement. Our findings could provide guidance on the design of OWS photocatalysts toward more efficient solar-to-fuel conversion.

Flux-Assisted Synthesis of Y2 Ti2 O5 S2 for Photocatalytic Hydrogen and Oxygen Evolution Reactions.

Photocatalytic water splitting is an ideal means of producing hydrogen in a sustainable manner, and developing highly efficient photocatalysts is a vital aspect of realizing this process. The photocatalyst Y2 Ti2 O5 S2 (YTOS) is capable of absorbing at wavelengths up to 650 nm and exhibits outstanding thermal and chemical durability compared with other oxysulfides. However, the photocatalytic performance of YTOS synthesized using the conventional solid-state reaction (SSR) process is limited owing to the large particle sizes and structural defects associated with this synthetic method. Herein, we report the synthesis of YTOS particles by a flux-assisted technique. The enhanced mass transfer efficiency in the flux significantly reduced the preparation time compared with the SSR method. In addition, the resulting YTOS showed improved photocatalytic H2 and O2 evolution activity when loaded with Rh and Co3 O4 co-catalysts, respectively. These improvements are attributed to the reduced particle size and enhanced crystallinity of the material as well as the slower decay of photogenerated carriers on a nanosecond to sub-microsecond time range. Further optimization of this flux-assisted method together with suitable surface modification is expected to produce high-quality YTOS crystals with superior photocatalytic activity.

An Oxysulfide Photocatalyst Evolving Hydrogen with an Apparent Quantum Efficiency of 30 % under Visible Light.

Photocatalytic water splitting is a simple means of converting solar energy into storable hydrogen energy. Narrow-band gap oxysulfide photocatalysts have attracted much attention in this regard owing to the significant visible-light absorption and relatively high stability of these compounds. However, existing materials suffer from low efficiencies due to difficulties in synthesizing these oxysulfides with suitable degrees of crystallinity and particle sizes, and in constructing effective reaction sites. The present work demonstrates the production of a Gd2 Ti2 O5 S2 (λ<650 nm) photocatalyst capable of efficiently driving photocatalytic reactions. Single-crystalline, plate-like Gd2 Ti2 O5 S2 particles with atomically ordered surfaces were synthesized by flux and chemical etching methods. Ultrafine Pt-IrO2 cocatalyst particles that promoted hydrogen (H2 ) and oxygen (O2 ) evolution reactions were subsequently loaded on the Gd2 Ti2 O5 S2 while ensuring an intimate contact by employing a microwave-heating technique. The optimized Gd2 Ti2 O5 S2 was found to evolve H2 from an aqueous methanol solution with a remarkable apparent quantum efficiency of 30 % at 420 nm. This material was also stable during O2 evolution in the presence of a sacrificial reagent. The results presented herein demonstrates a highly efficient narrow-band gap oxysulfide photocatalyst with potential applications in practical solar hydrogen production.

Surface Modifications of (ZnSe)0.5(CuGa2.5Se4.25)0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting.

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution.

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta3N5 (Ta3N5:Mg+Zr) nanoparticles by a brief NH3 nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta3N5 under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Methods in sulfide and persulfide research.

Sulfides and persulfides/polysulfides (R-Sn-R', n > 2; R-Sn-H, n > 1) are endogenously produced metabolites that are abundant in mammalian and human cells and tissues. The most typical persulfides that are widely distributed among different organisms include various reactive persulfides-low-molecular-weight thiol compounds such as cysteine hydropersulfide, glutathione hydropersulfide, and glutathione trisulfide as well as protein-bound thiols. These species are generally more redox-active than are other simple thiols and disulfides. Although hydrogen sulfide (H2S) has been suggested for years to be a small signaling molecule, it is intimately linked biochemically to persulfides and may actually be more relevant as a marker of functionally active persulfides. Reactive persulfides can act as powerful antioxidants and redox signaling species and are involved in energy metabolism. Recent evidence revealed that cysteinyl-tRNA synthetases (CARSs) act as the principal cysteine persulfide synthases in mammals and contribute significantly to endogenous persulfide/polysulfide production, in addition to being associated with a battery of enzymes including cystathionine ß-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase, which have been described as H2S-producing enzymes. The reactive sulfur metabolites including persulfides/polysulfides derived from CARS2, a mitochondrial isoform of CARS, also mediate not only mitochondrial biogenesis and bioenergetics but also anti-inflammatory and immunomodulatory functions. The physiological roles of persulfides, their biosynthetic pathways, and their pathophysiology in various diseases are not fully understood, however. Developing basic and high precision techniques and methods for the detection, characterization, and quantitation of sulfides and persulfides is therefore of great importance so as to thoroughly understand and clarify the exact functions and roles of these species in cells and in vivo.

Oxysulfide photocatalyst for visible-light-driven overall water splitting.

Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the corresponding oxides1,2. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen3. Here, we demonstrate the activation and stabilization of Y2Ti2O5S2, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y2Ti2O5S2 during a 20 h reaction. The discovery of the overall water splitting capabilities of Y2Ti2O5S2 extends the range of promising materials for solar hydrogen production.

Oxidative Stress Orchestrates MAPK and Nitric-Oxide Synthase Signal.

Reactive oxygen species (ROS) are not only harmful to cell survival but also essential to cell signaling through cysteine-based redox switches. In fact, ROS triggers the potential activation of mitogen-activated protein kinases (MAPKs). The 90 kDa ribosomal S6 kinase 1 (RSK1), one of the downstream mediators of the MAPK pathway, is implicated in various cellular processes through phosphorylating different substrates. As such, RSK1 associates with and phosphorylates neuronal nitric oxide (NO) synthase (nNOS) at Ser847, leading to a decrease in NO generation. In addition, the RSK1 activity is sensitive to inhibition by reversible cysteine-based redox modification of its Cys223 during oxidative stress. Aside from oxidative stress, nitrosative stress also contributes to cysteine-based redox modification. Thus, the protein kinases such as Ca2+/calmodulin (CaM)-dependent protein kinase I (CaMKI) and II (CaMKII) that phosphorylate nNOS could be potentially regulated by cysteine-based redox modification. In this review, we focus on the role of post-translational modifications in regulating nNOS and nNOS-phosphorylating protein kinases and communication among themselves.

Coordination between Calcium/Calmodulin-Dependent Protein Kinase II and Neuronal Nitric Oxide Synthase in Neurons.

Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is highly abundant in the brain and exhibits broad substrate specificity, thereby it is thought to participate in the regulation of neuronal death and survival. Nitric oxide (NO), produced by neuronal NO synthase (nNOS), is an important neurotransmitter and plays a role in neuronal activity including learning and memory processes. However, high levels of NO can contribute to excitotoxicity following a stroke and neurodegenerative disease. Aside from NO, nNOS also generates superoxide which is involved in both cell injury and signaling. CaMKII is known to activate and translocate from the cytoplasm to the post-synaptic density in response to neuronal activation where nNOS is predominantly located. Phosphorylation of nNOS at Ser847 by CaMKII decreases NO generation and increases superoxide generation. Conversely, NO-induced S-nitrosylation of CaMKII at Cys6 is a prominent determinant of the CaMKII inhibition in ATP competitive fashion. Thus, the "cross-talk" between CaMKII and NO/superoxide may represent important signal transduction pathways in brain. In this review, we introduce the molecular mechanism of and pathophysiological role of mutual regulation between CaMKII and nNOS in neurons.

Reactive sulfur species impair Ca2+/calmodulin-dependent protein kinase II via polysulfidation.

We previously reported that Ca2+/calmodulin-dependent protein kinase II (CaMKII) is inhibited by S-nitrosylation of Cys6 in cells. Herein, we show that polysulfidation of CaMKII at Cys6 limits its enzyme activity following reactive sulfur species (RSS) stimulus. In vitro incubation of CaMKII with the RSS donor, Na2S4, induced the inhibition of the enzyme via its polysulfidation. Treatment with dithiothreitol reversed the polysulfidation and the subsequent inhibition. The inhibition of CaMKII by Na2S4 is competitive with ATP but not with the peptide substrate Syntide-2. In transfected cells expressing CaMKII, the enzyme activity decreased upon treatment with Na2S4, whereas cells expressing mutant CaMKII (C6A) were resistant to this treatment. In addition, the endogenous CaMKII was inhibited by treatment with Na2S4 in RAW264.7 murine macrophage cells. These results suggest a novel regulation of CaMKII by RSS via its Cys6 polysulfidation in cells.

The active-site cysteine residue of Ca2+/calmodulin-dependent protein kinase I is protected from irreversible modification via generation of polysulfidation.

Ca2+/calmodulin (CaM)-dependent protein kinase (CaMK) I is activated by the phosphorylation of a crucial activation loop Thr177 by upstream kinases, CaMK kinase (CaMKK), and regulates axonal or dendritic extension and branching. Reactive sulfur species (RSS) modulate protein functions via polysulfidation of the reactive Cys residues. Here, we report that the activity of CaMKI was reversibly inhibited via its polysulfidation at Cys179 by RSS. In vitro incubation of CaMKI with the exogenous RSS donor Na2S3 resulted in a dose-dependent inhibition of the phosphorylation at Thr177 by CaMKK and inactivation of the enzymatic activity. Dithiothreitol (DTT), a small molecule reducing reagent, rescued these inhibitions. Conversely, mutated CaMKI (C179V) was resistant to the Na2S3-induced inactivation. In transfected cells expressing CaMKI, ionomycin-induced CaMKI activity was decreased upon treatment with Na2S4, whereas cells expressing mutant CaMKI (C179V) proved resistant to this treatment. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that CaMKI was a target for polysulfidation in cells. Furthermore, the polysulfidation of CaMKI protected Cys179 from its irreversible modification, known as protein succination. Thus, we propose that CaMKI was reversibly inhibited via polysulfidation of Cys179 by RSS, thereby protecting it from irreversible modification.

Core-Shell-Structured LaTaON2 Transformed from LaKNaTaO5 Plates for Enhanced Photocatalytic H2 Evolution.

LaTaON2 is a photocatalyst with intense visible light absorption up to 650 nm, but exhibits low H2 evolution activity owing to uncontrolled facets and high defect densities. In this work, core-shell-structured plate-like LaKNaTaO5 /LaTaON2 was synthesized by nitriding a layered perovskite-type LaKNaTaO5 . The volatilization of K and Na species during the nitridation promoted the rapid transformation of LaKNaTaO5 into LaTaON2 along [010] direction with the plate-like shape retained. This yielded high-quality LaTaON2 shells exposing (010) facets on the lattice-matched LaKNaTaO5 cores. After loading with a Rh co-catalyst, LaKNaTaO5 /LaTaON2 showed photocatalytic H2 evolution activity four times greater than that obtained from conventional irregular-shaped LaTaON2 powders and utilized visible light up to 620 nm. This work provides a novel strategy yielding oxynitrides with well-defined facets and low defect densities by selecting lattice-matched oxide precursors containing volatile components.

Involvement of nitric oxide/reactive oxygen species signaling via 8-nitro-cGMP formation in 1-methyl-4-phenylpyridinium ion-induced neurotoxicity in PC12 cells and rat cerebellar granule neurons.

To investigate the role of nitric oxide (NO)/reactive oxygen species (ROS) redox signaling in Parkinson's disease-like neurotoxicity, we used 1-methyl-4-phenylpyridinium (MPP+) treatment (a model of Parkinson's disease). We show that MPP+-induced neurotoxicity was dependent on ROS from neuronal NO synthase (nNOS) in nNOS-expressing PC12 cells (NPC12 cells) and rat cerebellar granule neurons (CGNs). Following MPP+ treatment, we found production of 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP), a second messenger in the NO/ROS redox signaling pathway, in NPC12 cells and rat CGNs, that subsequently induced S-guanylation and activation of H-Ras. Additionally, following MPP+ treatment, extracellular signal-related kinase (ERK) phosphorylation was enhanced. Treatment with a mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) inhibitor attenuated MPP+-induced ERK phosphorylation and neurotoxicity. In conclusion, we demonstrate for the first time that NO/ROS redox signaling via 8-nitro-cGMP is involved in MPP+-induced neurotoxicity and that 8-nitro-cGMP activates H-Ras/ERK signaling. Our results indicate a novel mechanism underlying MPP+-induced neurotoxicity, and therefore contribute novel insights to the mechanisms underlying Parkinson's disease.

Reactive sulfur species inactivate Ca2+/calmodulin-dependent protein kinase IV via S-polysulfidation of its active-site cysteine residue.

Reactive sulfur species (RSS) modulate protein functions via S-polysulfidation of reactive Cys residues. Here, we report that Ca2+/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) was reversibly inactivated by RSS via polysulfidation of the active-site Cys residue. CaMKIV is phosphorylated at Thr196 by its upstream CaMK kinase (CaMKK), resulting in the induction of its full activity. In vitro incubation of CaMKIV with the exogenous RSS donors Na2S n (n = 2-4) resulted in dose-dependent inhibition of the CaMKK-induced phospho-Thr196 and consequent inactivation of the enzyme activity. Conversely, mutated CaMKIV (C198V) was refractory to the Na2S n -induced enzyme inhibition. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that Cys198 in CaMKIV represents a target for S-polysulfidation. Furthermore, phosho-Thr196 and CaMKIV activity were inhibited by incubation with cysteine hydropersulfide, a newly identified RSS that is generated from cystine by cystathionine-γ-lyase. In transfected cells expressing CaMKIV, ionomycin-induced CaMKIV phosphorylation at Thr196 was decreased upon treatment with either Na2S4 or the endoplasmic reticulum (ER) stress inducer thapsigargin, whereas cells expressing mutant CaMKIV (C198V) were resistant to this treatment. In addition, the ionomycin-induced phospho-Thr196 of endogenous CaMKIV was also inhibited by treatment either with Na2S4 or thapsigargin in Jurkat T lymphocytes. Taken together, these data define a novel signaling function for intracellular RSS in inhibiting CaMKIV activity via S-polysulfidation of its Cys198 during the response to ER stress.