Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

On the Question of Uncatalyzed CO Insertion into a Hydrazone Double Bond: A Comparative Study Using Different CO Sources and Substrates

Because of endogenous signaling roles of carbon monoxide (CO) and its demonstrated pharmacological effects, there has been extensive interests in developing fluorescent CO probes. Palladium-mediated CO insertion has been successfully used for such applications. However, recent years have seen many publications of using uncatalyzed CO insertion into a hydrazone double bond as a way to sense CO. Such chemistry has no precedents otherwise. Further, the rigor of the CO-sensing work was largely based on using ruthenium-carbonyl complexes such as CORM-3 as CO surrogates, which have been reported to have extensive chemical reactivity and to release largely CO2 instead of CO unless in the presence of a strong nucleophile such as dithionite. For all of these, it is important to reassess the feasibility of such a CO-insertion reaction. By studying two of the reported "CO probes" using CO gas, this study finds no evidence of CO insertion into a hydrazone double bond. Further, the chemical reaction between CO gas and a series of eight hydrazone compounds was conducted, leading to the same conclusion. Such findings are consistent with the state-of-the-art knowledge of carbonylation chemistry and do not support uncatalyzed CO insertion as a mechanism for developing fluorescent CO probes.

Xanthene-Based Dyes for Photoacoustic Imaging and their Use as Analyte-Responsive Probes

Developing imaging tools that can report on the presence of disease-relevant analytes in multicellular organisms can provide insight into fundamental disease mechanisms as well as provide diagnostic tools for the clinic. Photoacoustic imaging (PAI) is a light-in, sound-out imaging technique that allows for high resolution, deep-tissue imaging with applications in pre-clinical and point-of-care settings. The continued development of near-infrared (NIR) absorbing small-molecule dyes promises to improve the capabilities of this emerging imaging modality. For example, new dye scaffolds bearing chemoselective functionalities are enabling the detection and quantification of disease-relevant analytes through activity-based sensing (ABS) approaches. Recently described strategies to engineer NIR absorbing xanthenes have enabled development of analyte-responsive PAI probes using this classic dye scaffold. Herein, we present current strategies for red-shifting the spectral properties of xanthenes via bridging heteroatom or auxochrome modifications. Additionally, we explore how these strategies, coupled with chemoselective spiroring-opening approaches, have been employed to create ABS probes for in vivo detection of hypochlorous acid, nitric oxide, copper (II), human NAD(P)H: quinone oxidoreductase isozyme 1, and carbon monoxide. Given the versatility of the xanthene scaffold, we anticipate continued growth and development of analyte-responsive PAI imaging probes based on this dye class.

Metal-based carbon monoxide releasing molecules with promising cytotoxic properties

Carbon monoxide, the "silent killer" gas, is increasingly recognised as an important signalling molecule in human physiology, which has beneficial biological properties. A particular way of achieving controlled CO administration is based on the use of biocompatible molecules that only release CO when triggered by internal or external factors. These approaches include the development of pharmacologically effective prodrugs known as CO releasing molecules (CORMs), which can supply biological systems with CO in well-regulated doses. An overview of transition metal-based CORMs with cytotoxic properties is here reported. The mechanisms at the basis of the biological activities of these molecules and their potential therapeutical applications with respect to their stability and CO releasing properties have been discussed. The activation of metal-based CORMs is determined by the type of metal and by the nature and features of the auxiliary ligands, which affect the metal core electronic density and therefore the prodrug resistance towards oxidation and CO release ability. A major role in regulating the cytotoxic properties of these CORMs is played by CO and/or CO-depleted species. However, several mysteries concerning the cytotoxicity of CORMs remain as intriguing questions for scientists.

Recent advances in the application of gasotransmitters in spinal cord injury

Spinal Cord Injury (SCI) is a condition characterized by complete or incomplete motor and sensory impairment, as well as dysfunction of the autonomic nervous system, caused by factors such as trauma, tumors, or inflammation. Current treatment methods primarily include traditional approaches like spinal canal decompression and internal fixation surgery, steroid pulse therapy, as well as newer techniques such as stem cell transplantation and brain-spinal cord interfaces. However, the above methods have limited efficacy in promoting axonal and neuronal regeneration. The challenge in medical research today lies in promoting spinal cord neuron regeneration and regulating the disrupted microenvironment of the spinal cord. Studies have shown that gas molecular therapy is increasingly used in medical research, with gasotransmitters such as hydrogen sulfide, nitric oxide, carbon monoxide, oxygen, and hydrogen exhibiting neuroprotective effects in central nervous system diseases. The gas molecular protect against neuronal death and reshape the microenvironment of spinal cord injuries by regulating oxidative, inflammatory and apoptotic processes. At present, gas therapy mainly relies on inhalation for systemic administration, which cannot effectively enrich and release gas in the spinal cord injury area, making it difficult to achieve the expected effects. With the rapid development of nanotechnology, the use of nanocarriers to achieve targeted enrichment and precise control release of gas at Sites of injury has become one of the emerging research directions in SCI. It has shown promising therapeutic effects in preclinical studies and is expected to bring new hope and opportunities for the treatment of SCI. In this review, we will briefly outline the therapeutic effects and research progress of gasotransmitters and nanogas in the treatment of SCI.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Structure-Photoreactivity Relationship Study of Substituted 3-Hydroxyflavones and 3-Hydroxyflavothiones for Improving Carbon Monoxide Photorelease

Carbon monoxide (CO) is notorious for its toxic effects but is also recognized as a gasotransmitter with considerable therapeutic potential. Due to the inherent challenges in its delivery, the utilization of organic CO photoreleasing molecules (photoCORMs) represents an interesting alternative to CO administration characterized by high spatial and temporal precision of release. This paper focused on the design, synthesis, and photophysical and photochemical studies of 20 3-hydroxyflavone (flavonol) and 3-hydroxyflavothione derivatives as photoCORMs. Newly synthesized compounds bearing various electron-donating and electron-withdrawing groups show bathochromically shifted absorption maxima and considerably enhanced CO release yields compared to the parent unsubstituted flavonol, exceeding 0.8 equiv of released CO in derivatives exhibiting excited states with a charge-transfer character. Until now, such outcomes have been limited to flavonol derivatives possessing a π-extended aromatic system. In addition, thione analogs of flavonols, 3-hydroxyflavothiones, show substantial bathochromic shifts of their absorption maxima and enhanced photosensitivity but provide lower yields of CO formation. Our study elucidates in detail the mechanism of CO photorelease from flavonols and flavothiones, utilizing steady-state and time-resolved spectroscopies and photoproduct analyses, with a particular emphasis on unraveling the structure-photoreactivity relationship and understanding competing side processes.

Recent progress on anti-nociceptive effects of carbon monoxide releasing molecule-2 (CORM-2)

The role of carbon monoxide (CO) has evolved albeit controversial disputes on its toxicity. This biological gasotransmitter participates in the endogenous regulation of neurotransmitters and neuropeptides released in the nervous system. Exogenous CO gas inhalation at a lower concentration has been the subject of investigations, which have revealed its biological homeostatic mechanisms and protective effects against many pathological conditions. This therapeutic procedure of CO is, however, limited due to its immediate release, which favours haemoglobin at a high affinity with the subsequent generation of toxic carboxyhaemoglobin in tissues. In order to address this problem, carbon monoxide releasing molecule-2 (CORM-2) or also known as tricarbonyldichlororuthenium II dimer is developed to liberate a controlled amount of CO in the biological systems. In this review, we examine several potential mechanisms exerted by this therapeutic compound to produce the anti-nociceptive effect that has been demonstrated in previous studies. This review could shed light on the role of CORM-2 to reduce pain, especially in cases of chronic and neuropathic pain.

Carbon monoxide affects early cardiac development in an avian model

INTRODUCTION

Carbon monoxide (CO) is a toxic gas that can be lethal in large doses and may also cause physiological damage in lower doses. Epidemiological studies suggest that CO in lower doses over time may impact on embryo development, in particular cardiac development, however other studies have not observed this association.

METHODS

Here, we exposed chick embryos in ovo to CO at three different concentrations (3, 9, 18 ppm) plus air control (4 protocols in total) for the first 9 days of development, at which point we assessed egg and embryo weight, ankle length, developmental stage, heart weight, ventricular wall thickness, ventricular-septal thickness and atrial wall thickness.

RESULTS

We found that heart weight was reduced for the low and moderate exposures compared to air, that atrial wall and ventricular wall thickness was increased for the moderate and high exposures compared to air and that ventricular septal thickness was increased for low, moderate and high exposures compared to air. Ventricular wall thickness was also significantly positively correlated with absolute CO exposures across all protocols.

CONCLUSIONS

This intervention study thus suggests that CO even at very low levels may have a significant impact on cardiac development.

Advancing piezoelectric 2D nanomaterials for applications in drug delivery systems and therapeutic approaches

Precision drug delivery and multimodal synergistic therapy are crucial in treating diverse ailments, such as cancer, tissue damage, and degenerative diseases. Electrodes that emit electric pulses have proven effective in enhancing molecule release and permeability in drug delivery systems. Moreover, the physiological electrical microenvironment plays a vital role in regulating biological functions and triggering action potentials in neural and muscular tissues. Due to their unique noncentrosymmetric structures, many 2D materials exhibit outstanding piezoelectric performance, generating positive and negative charges under mechanical forces. This ability facilitates precise drug targeting and ensures high stimulus responsiveness, thereby controlling cellular destinies. Additionally, the abundant active sites within piezoelectric 2D materials facilitate efficient catalysis through piezochemical coupling, offering multimodal synergistic therapeutic strategies. However, the full potential of piezoelectric 2D nanomaterials in drug delivery system design remains underexplored due to research gaps. In this context, the current applications of piezoelectric 2D materials in disease management are summarized in this review, and the development of drug delivery systems influenced by these materials is forecast.

Multimodal Carbon Monoxide Photorelease from Flavonoids

Photooxygenation of flavonoids leads to the release of carbon monoxide (CO). Our structure-photoreactivity study, employing several structurally different flavonoids, including their 13C-labeled analogs, revealed that CO can be produced via two completely orthogonal pathways, depending on their hydroxy group substitution pattern and the reaction conditions. While photooxygenation of the enol 3-OH group has previously been established as the CO liberation channel, we show that the catechol-type hydroxy groups of ring B can predominantly participate in photodecarbonylation.

A long-wavelength mitochondria-targeted CO fluorescent probe for living cells and zebrafish imaging

Carbon monoxide (CO) not only causes damage to life and health as an environmental pollutant, but also undertakes many physiological functions in organisms. In particular, developing means that can be used for the determination of CO in organelles will provide insight into the vital role it plays. Studies have shown that mitochondrial respiration is closely related to CO concentrations, so it is critical to develop tools for CO detection in mitochondria. Here, we use a rhodamine derivative that can target mitochondria as fluorophores to construct a mitochondrial-labeled CO fluorescence probe (Rh-CO) with high sensitivity (detection limit: 9.4 nM), excellent water-solubility, and long emission (λem = 630 nm). Prominently, the probe has outstanding mitochondria-targeting capabilities. Moreover, we used transient glucose deprivation (TGD) and heme to stimulate endogenous CO production in living cells and zebrafish, respectively, and the probe exhibited excellent imaging capabilities. All in all, we expect this probe to contribute to a deeper understanding of the role played by CO in mitochondria.

A Class of Activatable NIR-II Photoacoustic Dyes for High-Contrast Bioimaging

Photoacoustic (PA) imaging is emerging as one of the important non-invasive imaging techniques in biomedical research. Small molecule- second near-infrared window (NIR-II) PA dyes combined with imaging data can provide comprehensive and in-depth in vivo physiological and pathological information. However, the NIR-II PA dyes usually exhibit "always-on" properties due to the lack of a readily optically tunable group, which hinders the further applications in vivo. Herein, a novel class of dyes GX have been designed and synthesized as an activatable NIR-II PA platform, in which the absorption/emission wavelength of GX-5 extends up to 1082/1360 nm. Importantly, the GX dyes have a strong tissue penetration depth and high-resolution for the mouse vasculature structures in NIR-II PA 3D imaging and high signal-to-noise ratio in NIR-II fluorescence (FL) imaging. Furthermore, to demonstrate the applicability of GX dyes, the first NIR-II PA probe GX-5-CO activated by carbon monoxide (CO) was engineered and employed to reveal the enhancement of the CO levels in the hypertensive mice by high-contrast NIR-II PA and FL imaging. We expect that many derivatives of GX dyes will be developed to afford versatile NIR-II PA platforms for designing a wide variety activatable NIR-II PA probes as biomedical tools.

Transcription Factor EB: A Promising Therapeutic Target for Ischemic Stroke

Transcription factor EB (TFEB) is an important endogenous defensive protein that responds to ischemic stimuli. Acute ischemic stroke is a growing concern due to its high morbidity and mortality. Most survivors suffer from disabilities such as numbness or weakness in an arm or leg, facial droop, difficulty speaking or understanding speech, confusion, impaired balance or coordination, or loss of vision. Although TFEB plays a neuroprotective role, its potential effect on ischemic stroke remains unclear. This article describes the basic structure, regulation of transcriptional activity, and biological roles of TFEB relevant to ischemic stroke. Additionally, we explore the effects of TFEB on the various pathological processes underlying ischemic stroke and current therapeutic approaches. The information compiled here may inform clinical and basic studies on TFEB, which may be an effective therapeutic drug target for ischemic stroke.

Strategies for gaseous neuromodulator release in chemical neuroscience: Experimental approaches and translational validation

Gasotransmitters are a group of short-lived gaseous signaling molecules displaying diverse biological functions depending upon their localized concentration. Nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO) are three important examples of endogenously produced gasotransmitters that play a crucial role in human neurophysiology and pathogenesis. Alterations in their optimal physiological concentrations can lead to various severe pathophysiological consequences, including neurological disorders. Exogenous administration of gasotransmitters has emerged as a prominent therapeutic approach for treating such neurological diseases. However, their gaseous nature and short half-life limit their therapeutic delivery. Therefore, developing synthetic gasotransmitter-releasing strategies having control over the release and duration of these gaseous molecules has become imperative. However, the complex chemistry of synthesis and the challenges of specific quantified delivery of these gases, make their therapeutic application a challenging task. This review article provides a focused overview of emerging strategies for delivering gasotransmitters in a controlled and sustained manner to re-establish neurophysiological homeostasis.

KAN0438757, a novel PFKFB3 inhibitor, prevent the progression of severe acute pancreatitis via the Nrf2/HO-1 pathway in infiltrated macrophage

Acute pancreatitis (AP) is a non-infectious pancreatic enzyme-induced disorder, a life-threatening inflammatory condition that can cause multi-organ dysfunction, characterized by high morbidity and mortality. Several therapies have been employed to target this disorder; however, few happen to be effectively employable even in the early phase. PFKFB3(6-phosphofructo-2-kinase/fructose-2,6-biphosphatase-3) is a critical regulator of glycolysis and is upregulated under inflammatory, mitogenic, and hypoxia conditions. Essential information on the targeting of the inflammatory pathway will present the termination of the disorder and recovery. Herein we investigated the protective function of KAN0438757, a potent inhibitor of PFKFB3, and its mechanism of impeding AP induced in mice. KAN0438757 was confirmed to activate the Nrf2/HO-1 inflammatory signaling pathways in response to caerulein induced acute pancreatitis (CAE-AP) and fatty acid ethyl ester induced severe acute pancreatitis (FAEE-SAP). Additionally, KAN0438757 alleviated the inflammatory process in infiltrated macrophage via the Nrf2/HO-1 inflammatory signaling pathway and demonstrated a significant effect on the growth of mice with induced AP. And more importantly, KAN0438757 displayed negligible toxicity in vivo. Taken together our data suggest KAN0438757 directly suppresses the inflammatory role of PFKFB3 and induces a protective role via the Nrf2/HO-1 pathway, which could prove as an excellent therapeutic platform for SAP amelioration.

Carbon Monoxide: A Second Biomarker to Couple with Viscosity for the Construction of "Dual-Locked" Near-Infrared Fluorescent Probes for Accurately Diagnosing Non-Alcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) can progress to cirrhosis and liver cancer if left untreated. Therefore, it is of great importance to develop useful tools for the noninvasive and accurate diagnosis of NAFLD. Increased microenvironmental viscosity was considered as a biomarker of NAFLD, but the occurrence of increased viscosity in other liver diseases highly reduces the diagnosis accuracy of NAFLD by a single detection of viscosity. Hence, it is very necessary to seek a second biomarker of NAFLD. It has been innovatively proposed that the overexpressed heme oxygenase-1 enzyme in NAFLD would produce abnormally high concentrations of CO in hepatocytes and that CO could serve as a potential biomarker. In this work, we screened nine lactam Changsha dyes (HCO-1-HCO-9) with delicate structures to obtain near-infrared (NIR), metal-free, and "dual-locked" fluorescent probes for the simultaneous detection of CO and viscosity. Changsha dyes with a 2-pyridinyl hydrazone substituent could sense CO, and the 5-position substituents on the 2-pyridinyl moiety had a great electron effect on the reaction rate. The double bond in these dyes served as the sensing group for viscosity. Probe HCO-9 was utilized for precise diagnosis of NAFLD by simultaneous detection of CO and viscosity. Upon reacting with CO in a high-viscosity microenvironment, strong fluorescence at 745 nm of probe HCO-9 was turned on with NIR excitation at 700 nm. Probe HCO-9 was proven to be an effective tool for imaging CO and viscosity. Due to the advantages of NIR absorption and low toxicity, probe HCO-9 was successfully applied to image NAFLD in a mouse model.

Simultaneous Light-Triggered Release of Nitric Oxide and Carbon Monoxide from a Lipid-Coated Upconversion Nanosystem Inhibits Colon Tumor Growth

Gas therapy has gained noteworthy attention in biomedical research, with the rise of gas-releasing molecules enhancing their therapeutic potential, especially when integrated into nano-based drug delivery systems. Herein, we present a lipid-coated gas delivery system to simultaneously shuttle two gas-releasing molecules carrying nitric oxide (NO) and carbon monoxide (CO), respectively. Upconversion nanoparticles (UCNPs) are designed to generate photons at 360 nm upon 808 nm of near-infrared (NIR) irradiation. These in situ-generated UV photons trigger simultaneous NO and CO release from S-nitrosoglutathione (GSNO) and the CO-releasing molecule (CORM), respectively, which are coloaded into lipid-coated UCNP/GSNO/CORM/FA nanoparticles (LUGCF). LUGCF with a GSNO/CORM mass ratio of 2:1 is determined to be optimal in terms of synergistically instigating apoptosis in HCT116 and CT26 colon cancer cells, where both NO/CO are released and subsequent production of ROS are detected. This CO/NO combination nanoplatform exhibits a very effective inhibition of colon tumor growth in vivo at relatively low doses upon a mild 808 nm irradiation. Overall, we effectively integrated two therapeutic gas-releasing molecules in one NIR-responsive nanosystem, presenting a promising therapeutic strategy for future biomedical applications in dual-gas cancer therapy.

Heme Oxygenase-1 and Its Role in Colorectal Cancer

Heme oxygenase-1 (HO-1) is an enzyme located at the endoplasmic reticulum, which is responsible for the degradation of cellular heme into ferrous iron, carbon monoxide and biliverdin-IXa. In addition to this main function, the enzyme is involved in many other homeostatic, toxic and cancer-related mechanisms. In this review, we first summarize the importance of HO-1 in physiology and pathophysiology with a focus on the digestive system. We then detail its structure and function, followed by a section on the regulatory mechanisms that control HO-1 expression and activity. Moreover, HO-2 as important further HO isoform is discussed, highlighting the similarities and differences with regard to HO-1. Subsequently, we describe the direct and indirect cytoprotective functions of HO-1 and its breakdown products carbon monoxide and biliverdin-IXa, but also highlight possible pro-inflammatory effects. Finally, we address the role of HO-1 in cancer with a particular focus on colorectal cancer. Here, relevant pathways and mechanisms are presented, through which HO-1 impacts tumor induction and tumor progression. These include oxidative stress and DNA damage, ferroptosis, cell cycle progression and apoptosis as well as migration, proliferation, and epithelial-mesenchymal transition.

Carbon monoxide intoxication with a CO-Hb of 30% while smoking waterpipe: a case report

Carbon monoxide (CO) poisoning is a significant public health issue and a considerable economic burden in developed countries. While the majority of non-fire-related CO poisonings are attributed to gas heating, there are several other less recognized sources that should be considered in the initial differential diagnosis.The patient in this case was a 21-year-old who experienced a brief episode of loss of consciousness and was subsequently admitted to the Emergency department. Upon evaluation, the patient was diagnosed with CO poisoning, which necessitated hyperbaric oxygen therapy to mitigate the effects of this toxic exposure.Despite exhibiting harmful symptoms initially, the patient stated in a phone interview two and a half years post-incident that they have not experienced any enduring effects such as cardiac arrhythmia or concentration deficits. While their understanding of the risks associated with waterpipe smoking has increased, it has not influenced any major changes in their waterpipe smoking habits.

Carbon Monoxide-Loaded Red Blood Cell Prevents the Onset of Cisplatin-Induced Acute Kidney Injury

Cisplatin-induced acute kidney injury (AKI) is an important factor that limits the clinical use of this drug for the treatment of malignancies. Oxidative stress and inflammation are considered to be the main causes of not only cisplatin-induced death of cancer cells but also cisplatin-induced AKI. Therefore, developing agents that exert antioxidant and anti-inflammatory effects without weakening the anti-tumor effects of cisplatin is highly desirable. Carbon monoxide (CO) has recently attracted interest due to its antioxidant, anti-inflammatory, and anti-tumor properties. Herein, we report that CO-loaded red blood cell (CO-RBC) exerts renoprotective effects on cisplatin-induced AKI. Cisplatin treatment was found to reduce cell viability in proximal tubular cells via oxidative stress and inflammation. Cisplatin-induced cytotoxicity, however, was suppressed by the CO-RBC treatment. The intraperitoneal administration of cisplatin caused an elevation in the blood urea nitrogen and serum creatinine levels. The administration of CO-RBC significantly suppressed these elevations. Furthermore, the administration of CO-RBC also reduced the deterioration of renal histology and tubular cell injury through its antioxidant and anti-inflammatory effects in cisplatin-induced AKI mice. Thus, our data suggest that CO-RBC has the potential to substantially prevent the onset of cisplatin-induced AKI, which, in turn, may improve the usefulness of cisplatin-based chemotherapy.

The Triple Crown: NO, CO, and H2S in cancer cell biology

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are three endogenously produced gases with important functions in the vasculature, immune defense, and inflammation. It is increasingly apparent that, far from working in isolation, these three exert many effects by modulating each other's activity. Each gas is produced by three enzymes, which have some tissue specificities and can also be non-enzymatically produced by redox reactions of various substrates. Both NO and CO share similar properties, such as activating soluble guanylate cyclase (sGC) to increase cyclic guanosine monophosphate (cGMP) levels. At the same time, H2S both inhibits phosphodiesterase 5A (PDE5A), an enzyme that metabolizes sGC and exerts redox regulation on sGC. The role of NO, CO, and H2S in the setting of cancer has been quite perplexing, as there is evidence for both tumor-promoting and pro-inflammatory effects and anti-tumor and anti-inflammatory activities. Each gasotransmitter has been found to have dual effects on different aspects of cancer biology, including cancer cell proliferation and apoptosis, invasion and metastasis, angiogenesis, and immunomodulation. These seemingly contradictory actions may relate to each gas having a dual effect dependent on its local flux. In this review, we discuss the major roles of NO, CO, and H2S in the context of cancer, with an effort to highlight the dual nature of each gas in different events occurring during cancer progression.

BODIPY Based OFF-ON Fluorescent Probe for Endogenous Carbon Monoxide Imaging in Living Cells

Carbon monoxide (CO) is one of the signaling molecules that are ubiquitous in humans, which involves in the regulation of human physiology and pathology. In this work, the probe PEC was designed and synthesized based on BODIPY fluorophore that can selectively detect CO through reducing the nitro group to amino group, resulting in a "turn-on" fluorescence response with a simultaneous increase in the concentration of CO. The response is selective over a variety of relevant reactive free radicals, ions, and amino acid species. PEC has the advantages of good stability, good water solubility, and obvious changes in fluorescence signals. In addition, PEC can be used to detect and track endogenous CO in living cells.

The importance of and difficulties involved in creating molecular probes for a carbon monoxide gasotransmitter

As one of the triumvirate of recognized gasotransmitter molecules, namely NO, H2S, and CO, the physiological effects of CO and its potential as a biomarker have been widely investigated, garnering particular attention due to its reported hypotensive, anti-inflammatory, and cytoprotective properties, making it a promising therapeutic agent. However, the development of CO molecular probes has remained relatively stagnant in comparison with the fluorescent probes for NO and H2S, owing to its inert molecular state under physiological conditions. In this review, starting from elucidating the definition and significance of CO as a gasotransmitter, the imperative for the advancement of CO probes, especially fluorescent probes, is expounded. Subsequently, the current state of development of CO probe methodologies is comprehensively reviewed, with an overview of the challenges and prospects in this burgeoning field of research.

The emergence and advancement of Tsuji-Trost reaction triggered carbon monoxide recognition and bioimaging

Considering that carbon monoxide is both a vital gasotransmitter and an obnoxious gas, tremendous efforts have been dedicated toward its recognition through various methods. However, the fluorescent light-up approach through the exploration of optical markers remains one of the most convenient methods owing to its several advantages. Amongst the different approaches towards the development of CO responsive optically active molecular markers, the Tsuji-Trost reaction-based CO recognition strategy has remained one of the most significant areas of interest across researchers working in this field. However, there have been no attempts to exclusively summarize the commendable work done in this area yet. The current review, therefore, attempts to summarize the developments of various optical probes following this reaction strategy until the year 2022. This review provides detailed mechanistic insights into the Tsuji-Trost mediated CO detection strategy. Besides, discussions on the strategic development and employment of probes based on various allyl derivatives - allyl carbamate/carbonate/ethers - will provide a thorough understanding of the detection method. The significant advancements of the Tsuji-Trost reaction as an interesting strategy that is accepted and extensively explored for monitoring CO in various media including air, aqueous solutions and living systems have been elaborately discussed. Various potential applications and utilization of these developed fluorogenic probes for tracing CO in different living systems have been examined systematically. Moreover, monitoring of exogenous/endogenous CO levels, modulation of intracellular CO concentration under various induced conditions and bioimaging of CO in in vivo models have also been detailed here. Briefly, this review summarizes the current prospects of this detection method and the future directions in related fields.

Clinical Applications for Gasotransmitters in the Cardiovascular System: Are We There Yet?

Ischemia is the underlying mechanism in a wide variety of acute and persistent pathologies. As such, understanding the fine intracellular events occurring during (and after) the restriction of blood supply is pivotal to improving the outcomes in clinical settings. Among others, gaseous signaling molecules constitutively produced by mammalian cells (gasotransmitters) have been shown to be of potential interest for clinical treatment of ischemia/reperfusion injury. Nitric oxide (NO and its sibling, HNO), hydrogen sulfide (H2S), and carbon monoxide (CO) have long been proven to be cytoprotective in basic science experiments, and they are now awaiting confirmation with clinical trials. The aim of this work is to review the literature and the clinical trials database to address the state of development of potential therapeutic applications for NO, H2S, and CO and the clinical scenarios where they are more promising.

Direct hydrogen selenide (H2Se) release from activatable selenocarbamates

Hydrogen selenide (H₂Se) is a possible bioregulator, potential gasotransmitter, and important precursor in biological organoselenium compound synthesis. Early tools for H₂Se research have benefitted from available mechanistic understanding of analogous small molecules developed for detecting or delivering H₂S. A now common approach for H₂S delivery is the use of small molecule thiocarbamates that can be engineered to release COS, which is quickly converted to H₂S by carbonic anhydrase. To expand our understanding of the chemical underpinnings that enable H₂Se delivery, we investigated whether selenocarbamates undergo similar chemistry to release carbonyl selenide (COSe). Using both light- and hydrolysis-activated systems, we demonstrate that unlike their lighter thiocarbamate congeners, selenocarbamates release H₂Se directly with concomitant isocyanate formation rather than by the intermediate release of COSe. This reaction mechanism for direct H₂Se release is further supported by computational investigations that identify a ΔΔG^‡ ∼ 25 kcal mol^-1 between the H₂Se and COSe release pathways in the absence of protic solvent. This work highlights fundamentally new approaches for H₂Se release from small molecules and advances the understanding of reactivity differences between reactive sulfur and selenium species.

Use of Enzymatically Activated Carbon Monoxide Donors for Sensitizing Drug-Resistant Tumor Cells

The application of gaseous signaling molecules like NO, H₂S or CO to overcome the multidrug resistance in cancer treatment has proven to be a viable therapeutic strategy. The development of CO-releasing molecules (CORMs) in a controlled manner and in targeted tissues remains a challenge in medicinal chemistry. In this paper, we describe the design, synthesis and chemical and enzymatic stability of a novel non-metal CORM (1) able to release intracellularly CO and, simultaneously, facilitate fluorescent degradation of products under the action of esterase. The toxicity of 1 against different human cancer cell lines and their drug-resistant counterparts, as well as the putative mechanism of toxicity were investigated. The drug-resistant cancer cell lines efficiently absorbed 1 and 1 was able to restore their sensitivity vs. chemotherapeutic drugs by causing a CO-dependent mitochondrial oxidative stress that culminated in mitochondrial-dependent apoptosis. These results demonstrate the importance of CORMs in cases where conventional chemotherapy fails and thus open the horizons towards new combinatorial strategies to overcome multidrug resistance.

Heme Oxygenase-1 and Its Metabolites Carbon Monoxide and Biliverdin, but Not Iron, Exert Antiviral Activity against Porcine Circovirus Type 3

Porcine circovirus type 3 (PCV3) is a newly discovered pathogen that causes porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, multisystemic inflammation, and reproductive failure. Heme oxygenase-1 (HO-1), a stress-inducible enzyme, exerts protective functions by converting heme into carbon monoxide (CO), biliverdin (BV), and iron. However, the effects of HO-1 and its metabolites on PCV3 replication remain unknown. In this study, experiments involving specific inhibitors, lentivirus transduction, and small interfering RNA (siRNA) transfection revealed that active PCV3 infection reduced HO-1 expression and that the expression of HO-1 negatively regulated virus replication in cultured cells, depending on its enzymatic activity. Subsequently, the effects of the HO-1 metabolites (CO, BV, and iron) on PCV3 infection were investigated. The CO inducers (cobalt protoporphyrin IX [CoPP] or tricarbonyl dichloro ruthenium [II] dimer [CORM-2]) mediate PCV3 inhibition by generating CO, and this inhibition is reversed by hemoglobin (Hb; a CO scavenger). The inhibition of PCV3 replication by BV depended on BV-mediated reactive oxygen species (ROS) reduction, as N-acetyl-l-cysteine affected PCV3 replication while reducing ROS production. The reduction product of BV, bilirubin (BR), specifically promoted nitric oxide (NO) generation and further activated the cyclic GMP/protein kinase G (cGMP/PKG) pathway to attenuate PCV3 infection. Both the iron provided by FeCl3 and the iron chelated by deferoxamine (DFO) with CoPP treatment failed to affect PCV3 replication. Our data demonstrate that the HO-1-CO-cGMP/PKG, HO-1-BV-ROS, and HO-1-BV-BR-NO-cGMP/PKG pathways contribute crucially to the inhibition of PCV3 replication. These results provide important insights regarding preventing and controlling PCV3 infection. IMPORTANCE The regulation of host protein expression by virus infection is the key to facilitating self-replication. As an important emerging pathogen of swine, clarification of the interaction between PCV3 infection and the host enables us to understand the viral life cycle and pathogenesis better. Heme oxygenase-1 (HO-1) and its metabolites carbon monoxide (CO), biliverdin (BV), and iron have been demonstrated to involve a wealth of viral replications. Here, we, for the first time, demonstrated that HO-1 expression decreases in PCV3-infected cells and negatively regulates PCV3 replication and that the HO-1 metabolic products CO and BV inhibit PCV3 replication by the CO- or BV/BR/NO-dependent cGMP/PKG pathway or BV-mediated ROS reduction, but the iron (the third metabolic product) does not. Specifically, PCV3 infection maintains normal proliferation by downregulating HO-1 expression. These findings clarify the mechanism by which HO-1 modulates PCV3 replication in cells and provide important targets for preventing and controlling PCV3 infection.

Sensing a CO-Releasing Molecule (CORM) Does Not Equate to Sensing CO: The Case of DPHP and CORM-3

Carbon monoxide (CO) is an endogenous signaling molecule with demonstrated pharmacological effects. For studying CO biology, there is a need for sensitive and selective fluorescent probes for CO as research tools. In developing such probes, CO gas and/or commercially available metal-carbonyl-based "CO-releasing molecules" (CORMs) have been used as CO sources. However, new findings are steadily emerging that some of these commonly used CORMs do not release CO reliably in buffers commonly used for studying such CO probes and have very pronounced chemical reactivities of their own, which could lead to the erroneous identification of "CO probes" that merely detect the CORM used, not CO. This is especially true when the CO-sensing mechanism relies on chemistry that is not firmly established otherwise. Cu2+ can quench the fluorescence of an imine-based fluorophore, DPHP, presumably through complexation. The Cu2+-quenched fluorescence was restored through the addition of CORM-3, a Ru-based CORM. This approach was reported as a new "strategy for detecting carbon monoxide" with the proposed mechanism being dependent on CO reduction of Cu2+ to Cu1+ under near-physiological conditions ( Anal. Chem. 2022, 94, 11298-11306). The study only used CORM-3 as the source of CO. CORM-3 has been reported to have very pronounced redox reactivity and is known not to release CO in an aqueous solution unless in the presence of a strong nucleophile. To assess whether the fluorescent response of the DPHP-Cu(II) cocktail to CORM-3 was truly through detecting CO, we report experiments using both pure CO and CORM-3. We confirm the reported DPHP-Cu(II) response to CORM-3 but not pure CO gas. Further, we did not observe the stated selectivity of DPHP for CO over sulfide species. Along this line, we also found that a reducing agent such as ascorbate was able to induce the same fluorescent turn-on as CORM-3 did. As such, the DPHP-Cu(II) system is not a CO probe and cannot be used to study CO biology. Corollary to this finding, it is critical that future work in developing CO probes uses more than a chemically reactive "CO donor" as the CO source. Especially important will be to confirm the ability of the "CO probe" to detect CO using pure CO gas or another source of CO.

Diabetic Nephropathy and Gaseous Modulators

Diabetic nephropathy (DN) remains the leading cause of vascular morbidity and mortality in diabetes patients. Despite the progress in understanding the diabetic disease process and advanced management of nephropathy, a number of patients still progress to end-stage renal disease (ESRD). The underlying mechanism still needs to be clarified. Gaseous signaling molecules, so-called gasotransmitters, such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), have been shown to play an essential role in the development, progression, and ramification of DN depending on their availability and physiological actions. Although the studies on gasotransmitter regulations of DN are still emerging, the evidence revealed an aberrant level of gasotransmitters in patients with diabetes. In studies, different gasotransmitter donors have been implicated in ameliorating diabetic renal dysfunction. In this perspective, we summarized an overview of the recent advances in the physiological relevance of the gaseous molecules and their multifaceted interaction with other potential factors, such as extracellular matrix (ECM), in the severity modulation of DN. Moreover, the perspective of the present review highlights the possible therapeutic interventions of gasotransmitters in ameliorating this dreaded disease.

Unraveling prevalence of homoacetogenesis and methanogenesis pathways due to inhibitors addition

Three inhibitors targeting different microorganisms, both from Archaea and Bacteria domains, were evaluated for their effect on CO₂ biomethanation: sodium ionophore III (ETH2120), carbon monoxide (CO), and sodium 2-bromoethanesulfonate (BES). This study examines how these compounds affect the anaerobic digestion microbiome in a biogas upgrading process. While archaea were observed in all experiments, methane was produced only when adding ETH2120 or CO, not when adding BES, suggesting archaea were in an inactivated state. Methane was produced mainly via methylotrophic methanogenesis from methylamines. Acetate was produced at all conditions, but a slight reduction on acetate production (along with an enhancement on CH₄ production) was observed when applying 20 kPa of CO. Effects on CO₂ biomethanation were difficult to observe since the inoculum used was from a real biogas upgrading reactor, being this a complex environmental sample. Nevertheless, it must be mentioned that all compounds had effects on the microbial community composition.

A mitochondria-targeted fluorescent probe for imaging of endogenous carbon monoxide in living cells

Carbon monoxide (CO), a vital gasotransmitter, plays critical functions in many physiological processes. Mitochondrial CO is closely related to mitochondrial respiration, thus the detection and imaging of mitochondrial CO in living cells is very important and has attracted much attention recently. In this paper, we developed a hemicyanine-based off-on fluorescent probe, CO-H1, which was used for monitoring endogenous mitochondrial CO levels in living cells. After reacted with CO in the presence of PdCl₂, the fluorescence of CO-H1 was enhanced notably, accompanied by a significant red shift of absorption. CO-H1 exhibits low cytotoxicity, high sensitivity (detection limit of 0.048 μM), and good selectivity for CO. When incubated with living cells, probe CO-H1 mainly entered the mitochondria. CO-H1 was successfully applied to imaging the exogenous/endogenous mitochondrial CO in living cells, suggesting its potential application for further studying the biological functions of mitochondrial CO in living cells.

Platelets and Cardioprotection: The Role of Nitric Oxide and Carbon Oxide

Nitric oxide (NO) and carbon monoxide (CO) represent a pair of biologically active gases with an increasingly well-defined range of effects on circulating platelets. These gases interact with platelets and cells in the vessels and heart and exert fundamentally similar biological effects, albeit through different mechanisms and with some peculiarity. Within the cardiovascular system, for example, the gases are predominantly vasodilators and exert antiaggregatory effects, and are protective against damage in myocardial ischemia-reperfusion injury. Indeed, NO is an important vasodilator acting on vascular smooth muscle and is able to inhibit platelet activation. NO reacts with superoxide anion (O₂(^-•)) to form peroxynitrite (ONOO(^-)), a nitrosating agent capable of inducing oxidative/nitrative signaling and stress both at cardiovascular, platelet, and plasma levels. CO reduces platelet reactivity, therefore it is an anticoagulant, but it also has some cardioprotective and procoagulant properties. This review article summarizes current knowledge on the platelets and roles of gas mediators (NO, and CO) in cardioprotection. In particular, we aim to examine the link and interactions between platelets, NO, and CO and cardioprotective pathways.

The Role of Gasotransmitter-Dependent Signaling Mechanisms in Apoptotic Cell Death in Cardiovascular, Rheumatic, Kidney, and Neurodegenerative Diseases and Mental Disorders

Cardiovascular, rheumatic, kidney, and neurodegenerative diseases and mental disorders are a common cause of deterioration in the quality of life up to severe disability and death worldwide. Many pathological conditions, including this group of diseases, are based on increased cell death through apoptosis. It is known that this process is associated with signaling pathways controlled by a group of gaseous signaling molecules called gasotransmitters. They are unique messengers that can control the process of apoptosis at different stages of its implementation. However, their role in the regulation of apoptotic signaling in these pathological conditions is often controversial and not completely clear. This review analyzes the role of nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H₂S), and sulfur dioxide (SO₂) in apoptotic cell death in cardiovascular, rheumatic, kidney, and neurodegenerative diseases. The signaling processes involved in apoptosis in schizophrenia, bipolar, depressive, and anxiety disorders are also considered. The role of gasotransmitters in apoptosis in these diseases is largely determined by cell specificity and concentration. NO has the greatest dualism; scales are more prone to apoptosis. At the same time, CO, H₂S, and SO₂ are more involved in cytoprotective processes.

Lighting up trace carbon monoxide and residual palladium species by a low cytotoxic mitochondria targetable red fluorescent probe: Its large scaled applications

High levels of residual palladium can lead to serious negative health effects. Carbon monoxide (CO) is a significant gasotransmitter in transporting intermolecular and intramolecular signals to balance several physiological processes. Therefore, there is a need for rapid detection of CO and palladium residue. To address these issues, we have designed a novel light-up fluorescent probe for the detection of Pd and CO. It can not only detect Pd and CO selectively with a remarkable chromogenic and red fluorescent response over other metal ions allowing detection with naked eyes but also discriminate Pd⁰ and Pd²⁺/Pd⁴⁺ species. The detection reaction is confirmed by HPLC analysis. The probe demonstrates biocompatibility and mitochondrial target ability for potential biological applications. The practical applications based on drug residue and soil analysis, and smartphone have been successfully performed. Bioimaging of the concentration change of Pd and CO in HeLa cells using the probe is successfully applied. Therefore, the present approach can provide early diagnosis of Pd and CO with low detection limit, low cytotoxicity, high selectivity, and sensitivity.

Toward "CO in a Pill": Silica-Immobilized Organic CO Prodrugs for Studying the Feasibility of Systemic Delivery of CO via In Situ Gastrointestinal CO Release

Carbon monoxide (CO), an endogenous signaling molecule, is known to exert a range of pharmacological effects, including anti-inflammation, organ protection, and antimetastasis in various animal models. We have previously shown the ability of organic prodrugs to deliver CO systemically through oral administration. As part of our efforts for the further development of these prodrugs, we are interested in minimizing the potential negative impact of the "carrier" portion of the prodrug. Along this line, we have previously published our work on using benign "carriers" and physically trapping the "carrier" portion in the gastrointestinal (GI) tract. We herein report our feasibility studies on using immobilized organic CO prodrugs for oral CO delivery while minimizing systemic exposure to the prodrug and the "carrier portion." In doing so, we immobilize a CO prodrug to silica microparticles, which are generally recognized as safe by the US FDA and known to provide large surface areas for loading and water accessibility. The latter point is essential for the hydrophobicity-driven activation of the CO prodrug. Amidation-based conjugation with silica is shown to provide 0.2 mmol/g loading degree, effective prodrug activation in buffer with comparable kinetics as the parent prodrug, and stable tethering to prevent detachment. One representative silica conjugate, SICO-101, is shown to exhibit anti-inflammation activity in LPS-challenged RAW264.7 cells and to deliver CO systemically in mice through oral administration and GI CO release. We envision this strategy as a general approach for oral CO delivery to treat systemic and GI-specific inflammatory conditions.

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent probes for biomarker detection: design, mechanism, and application

Biomarkers are essential in biology, physiology, and pharmacology; thus, their detection is of extensive importance. Fluorescent probes provide effective tools for detecting biomarkers exactly. Excited state intramolecular proton transfer (ESIPT), one of the significant photophysical processes that possesses specific photoisomerization between Keto and Enol forms, can effectively avoid annoying interference from the background with a large Stokes shift. Hence, ESIPT is an excellent choice for biomarker monitoring. Based on the ESIPT process, abundant probes were designed and synthesized using three major design methods. In this review, we conclude probes for 14 kinds of biomarkers based on ESIPT explored in the past five years, summarize these general design methods, and highlight their application for biomarker detection in vitro or in vivo.

Light activated simultaneous release and recognition of biological signaling molecule carbon monoxide (CO)

The therapeutic action of carbon monoxide (CO) is very well known and has been studied on various types of tissues and animals. However, real-time spatial and temporal tracking and release of CO is still a challenging task. This paper reported an amphiphilic CO sensing probe NP and phospholipid 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) based nanoscale vesicular sensing system Ves-NP consisting of NP. The liposomal sensing system (Ves-NP) showed good selectivity and sensitivity for CO without any interference from other relevant biological analytes. Detection of CO is monitored by fluorescence OFF-ON signal. Ves-NP displayed LOD of 5.94 µM for CO detection with a response time of 5 min. Further, in a novel attempt, Ves-NP is co-embedded with the amphiphilic CO-releasing molecule 1-Mn(CO)₃ to make an analyte replacement probe Ves-NP-CO. Having a both CO releasing and sensing moiety at the surface of the same liposomal system Ves-NP-CO play a dual role. Ves-NP-CO is used for the simultaneous release and recognition of CO that can be controlled by light. Thus, in this novel approach, for the first time we have attached both the release and recognition units of CO in the vesicular surface, both release and recognition simultaneously monitored by the change in fluorescent OFF-ON signal.

De Novo Construction of Fluorophores via CO Insertion-Initiated Lactamization: A Chemical Strategy toward Highly Sensitive and Highly Selective Turn-On Fluorescent Probes for Carbon Monoxide

Extensive studies in the last few decades have led to the establishment of CO as an endogenous signaling molecule and subsequently to the exploration of CO's therapeutic roles. In the current state, there is a critical conundrum in CO-related research: the extensive knowledge of CO's biological effects and yet an insufficient understanding of the quantitative correlations between the CO concentration and biological responses of various natures. This conundrum is partially due to the difficulty in examining precise concentration-response relationships of a gaseous molecule. Another reason is the need for appropriate tools for the sensitive detection and concentration determination of CO in the biological system. We herein report a new chemical approach to the design of fluorescent CO probes through de novo construction of fluorophores by a CO insertion-initiated lactamization reaction, which allows for ultra-low background and exclusivity in CO detection. Two series of CO detection probes have been designed and synthesized using this strategy. Using these probes, we have extensively demonstrated their utility in quantifying CO in blood, tissue, and cell culture and in cellular imaging of CO from exogenous and endogenous sources. The probes described will enable many biology and chemistry labs to study CO's functions in a concentration-dependent fashion with very high sensitivity and selectivity. The chemical and design principles described will also be applicable in designing fluorescent probes for other small molecules.

Strategies toward Metal-Free Carbon Monoxide Prodrugs: An Update

Carbon monoxide is an important gasotransmitter in mammals, with pleiotropic therapeutic potential against a wide range of human diseases. However, clinical translation of CO is severely hampered by the lack of a reliable CO delivery form. The development of metal-free CO prodrugs is the key to resolving such delivery issues. Over the past three years, some new exciting progress has been made in this field. In this review, we highlight these advances and discuss related issues.

Improving carbon monoxide tolerance of Cupriavidus necator H16 through adaptive laboratory evolution

Background: The toxic gas carbon monoxide (CO) is abundantly present in synthesis gas (syngas) and certain industrial waste gases that can serve as feedstocks for the biological production of industrially significant chemicals and fuels. For efficient bacterial growth to occur, and to increase productivity and titres, a high resistance to the gas is required. The aerobic bacterium Cupriavidus necator H16 can grow on CO₂ + H₂, although it cannot utilise CO as a source of carbon and energy. This study aimed to increase its CO resistance through adaptive laboratory evolution. Results: To increase the tolerance of C. necator to CO, the organism was continually subcultured in the presence of CO both heterotrophically and autotrophically. Ten individual cultures were evolved heterotrophically with fructose in this manner and eventually displayed a clear growth advantage over the wild type strain. Next-generation sequencing revealed several mutations, including a single point mutation upstream of a cytochrome bd ubiquinol oxidase operon (cydA2B2), which was present in all evolved isolates. When a subset of these mutations was engineered into the parental H16 strain, only the cydA2B2 upstream mutation enabled faster growth in the presence of CO. Expression analysis, mutation, overexpression and complementation suggested that cydA2B2 transcription is upregulated in the evolved isolates, resulting in increased CO tolerance under heterotrophic but not autotrophic conditions. However, through subculturing on a syngas-like mixture with increasing CO concentrations, C. necator could also be evolved to tolerate high CO concentrations under autotrophic conditions. A mutation in the gene for the soluble [NiFe]-hydrogenase subunit hoxH was identified in the evolved isolates. When the resulting amino acid change was engineered into the parental strain, autotrophic CO resistance was conferred. A strain constitutively expressing cydA2B2 and the mutated hoxH gene exhibited high CO tolerance under both heterotrophic and autotrophic conditions. Conclusion: C. necator was evolved to tolerate high concentrations of CO, a phenomenon which was dependent on the terminal respiratory cytochrome bd ubiquinol oxidase when grown heterotrophically and the soluble [NiFe]-hydrogenase when grown autotrophically. A strain exhibiting high tolerance under both conditions was created and presents a promising chassis for syngas-based bioproduction processes.

Reassessing CORM-A1: redox chemistry and idiosyncratic CO-releasing characteristics of the widely used carbon monoxide donor

Redox activity and unreliable carbon monoxide production of CO donor, CORM-A1, presents new complications in its use for studying CO biology.

Carboxyhemoglobin as a diagnostic and prognostic biomarker of hemolytic anemias in dogs

BACKGROUND

Endogenous production of carbon monoxide during hemoglobin metabolism leads to the formation of carboxyhemoglobin. Carboxyhemoglobin concentration is abnormally high in humans with hemolytic anemia (HA).

HYPOTHESIS

Measurement of carboxyhemoglobin concentration can discriminate HA from other forms of anemia.

ANIMALS

Twenty-seven dogs with HA (immune-mediated HA, n = 22; microangiopathic HA, n = 5), 27 dogs with non-HA (kidney disease, n = 14; immune-mediated thrombocytopenia, [n = 6]; miscellaneous, n = 7) and 24 nonanemic control dogs.

METHODS

Prospective cohort study. Carboxyhemoglobin quantification, a CBC and biochemistry profile were performed upon admission, and survival to hospital discharge and at 30 days were the measured outcomes. Groups were compared by the Mann-Whitney and Kruskal-Wallis tests. Receiver-operator characteristic (ROC) analyses were used to examine the predictive utility of carboxyhemoglobin for the diagnosis of HA in anemic dogs.

RESULTS

Carboxyhemoglobin (median [interquartile range]) differed between dogs with HA (7.7% [2.5%]) and non-HA (3.6% [1.05]; P < .001) and dogs with HA and nonanemic dogs (3.5% [0.65%]; P < .001). No difference was detected between nonHA and nonanemic dogs. The area under the ROC curve for carboxyhemoglobin as predictor of HA in anemic dogs was 0.997 (95% CI, 0.99-1.00). Three optimal cut-off points were identified, including 5.05%, 4.55% and 4.85%, with corresponding sensitivity/specificity of 92.6%/100%, 100%/92.6% and 96.3%/96.3%, respectively. Neither carboxyhemoglobin nor any of the CBC or chemistry analytes were associated with survival.

CONCLUSIONS AND CLINICAL IMPORTANCE

Carboxyhemoglobin proved an excellent predictor of HA in dogs and might constitute a useful, ancillary tool for diagnosing and monitoring hemolytic anemias.

Common xanthene fluorescent dyes are visible-light activatable CO-releasing molecules

Fluorescein, eosin Y, and rose bengal are dyes used in clinical medicine and considered (photo-)chemically stable. Upon extensive irradiation with visible light in aqueous solutions, we found that these compounds release carbon monoxide (CO) - a bioactive gasotransmitter - in 40-100% yields along with the production of low-mass secondary photoproducts, such as phthalic and formic acids, in a multistep degradation process. Such photochemistry should be considered in applications of these dyes, and they could also be utilized as visible-light activatable CO-releasing molecules (photoCORMs) with biological implications.

Deep-Depth Imaging of Hepatic Ischemia/Reperfusion Injury Using a Carbon Monoxide-Activated Upconversion Luminescence Nanosystem

Exploring a chemical imaging tool for visualizing the endogenous CO biosignaling molecule is of great importance in understanding the pathophysiological functions of CO in complex biological systems. Most of the existing CO fluorescent probes show excitation and emission in the region of ultraviolet and visible light, which are not suitable for application in in vivo deep-depth imaging of CO. Herein, a new near-infrared (NIR) to NIR upconversion luminescence (UCL) nanosystem for in vivo visualization of CO was developed, which possesses the merits of high selectivity and sensitivity, a deep tissue penetration depth, and a high signal-to-noise ratio. In this design, upon interaction with CO, the maxima absorption peak of the nanosystem showed a significant blue shift from 795 nm to 621 nm and triggered a remarkable turn-on NIR UCL signal due to the luminescence resonance energy transfer process. Leveraging this nanosystem, we achieved an NIR UCL visualization of the generation of CO biosignals caused by hypoxic, acute inflammation, or ischemic injury in living cells, zebrafish, and mice. Moreover, the protective effect of CO in zebrafish models of oxygen and glucose deprivation/reperfusion (OGD/R) and mice models of lipopolysaccharide-induced oxidative stress (LOS) and hepatic ischemia/reperfusion (HI/R) was also further verified. Therefore, this work discloses that the nanosystem not only serves as a promising nanoplatform to study biological signaling pathways of CO in pathophysiological events, but may also provide a powerful tool for HI/R injury diagnosis.

Phytomelatonin and gasotransmitters: a crucial combination for plant physiological functions

Melatonin, a molecule that was first identified in animal tissues, has been confirmed to be involved as a potential phytohormone in a variety of plant physiological responses. It is considered primarily as an antioxidant with important actions in controlling reactive oxygen and reactive nitrogen species. In addition to its role in regulating plant growth and development, phytomelatonin is involved in protection against abiotic and biotic stresses. The 'gasotransmitter'-that is, a gaseous signaling molecule-is a new concept that has been advanced in the past two decades, with functions in animal and plant physiological regulation. Gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide, methane, and, more recently identified, hydrogen gas are critical and indispensable in a wide range of biological processes. This review investigates the interrelationship between phytomelatonin and the above-mentioned gasotransmitters from the perspective of biosynthetic origin and functions. Moreover, the potential future research directions for phytomelatonin and gasotransmitters interactions are discussed.

Photoactive carbon monoxide-releasing coordination polymer particles

We report the synthesis of photoactive carbon monoxide-releasing coordination polymer particles through the assembly of Mn(I) carbonyl complexes with bis(imidazole) ligands. The use of Mn(I) carbonyl complexes as metallic nodes in the coordination network avoids the potential for aggregation-induced self-quenching, favouring their use in the solid state.