Novel GPIb-independent platelet aggregation induced by botrocetin: implications for diagnosis and antithrombotic therapy.

BACKGROUND: Snake venom botrocetin facilitates von Willebrand factor (VWF) binding to platelet GPIbα and has been widely used for the diagnosis of von Willebrand disease and GPIb-related disorders. Botrocetin is also commonly employed for the development/characterization of antithrombotics targeting the GPIb-VWF axis. OBJECTIVES: To explore the alternative receptor(s)/mechanisms that participate in botrocetin-induced platelet aggregation. METHODS: The effects of botrocetin on platelet aggregation were examined using platelets from wild-type, VWF- and fibrinogen-deficient, GPIbα-deficient, IL4Rα/GPIbα-transgenic, ITGA2B and ITGB3-deficient mice, and Bernard-Soulier syndrome and healthy human samples. Platelet-fibrinogen and platelet-VWF interaction were measured using flow cytometry. GPIbα-VWF binding was evaluated utilizing enzyme-linked immunosorbent assay. Botrocetin-αIIbß3 and botrocetin-GPIbα interactions were measured using enzyme-linked immunosorbent assay and fluorescence anisotropy assays. Heparinized whole blood from healthy donors was examined for thrombus formation and growth in a perfusion chamber. RESULTS: Botrocetin could induce aggregation of platelets from a Bernard-Soulier syndrome patient and GPIbα-deficient mice as well as platelets lacking the N-terminal extracellular domain of GPIbα. Botrocetin could interact with αIIbß3 and facilitated αIIbß3-VWF interaction independent of GPIb. Botrocetin competitively bound to the ligand-binding domain of activated rather than resting αIIbß3. Although botrocetin-induced platelet aggregation requires VWF, strikingly, in the absence of VWF, botrocetin blocked fibrinogen and other ligand binding to αIIbß3 and inhibited platelet aggregation and thrombus formation. Consistently, recombinant botrocetin defective in VWF binding inhibited αIIbß3- and GPIb-mediated platelet aggregation, spreading, and thrombus formation. CONCLUSION: Our study provides insights into avoiding the misdiagnosis of GPIb-related disorders and developing botrocetin mutants as potential new antithrombotics that may simultaneously target both αIIbß3 and GPIbα.

Salvianolic acid B inhibits thrombosis and directly blocks the thrombin catalytic site.

Background: Salvianolic acid B (SAB) is a major component of Salvia miltiorrhiza root (Danshen), widely used in East/Southeast Asia for centuries to treat cardiovascular diseases. Danshen depside salt, 85% of which is made up of SAB, is approved in China to treat chronic angina. Although clinical observations suggest that Danshen extracts inhibited arterial and venous thrombosis, the exact mechanism has not been adequately elucidated. Objective: To delineate the antithrombotic mechanisms of SAB. Methods: We applied platelet aggregation and coagulation assays, perfusion chambers, and intravital microscopy models. The inhibition kinetics and binding affinity of SAB to thrombin are measured by thrombin enzymatic assays, intrinsic fluorescence spectrophotometry, and isothermal titration calorimetry. We used molecular in silico docking models to predict the interactions of SAB with thrombin. Results: SAB dose-dependently inhibited platelet activation and aggregation induced by thrombin. SAB also reduced platelet aggregation induced by adenosine diphosphate and collagen. SAB attenuated blood coagulation by modifying fibrin network structures and significantly decreased thrombus formation in mouse cremaster arterioles and perfusion chambers. The direct SAB-thrombin interaction was confirmed by enzymatic assays, intrinsic fluorescence spectrophotometry, and isothermal titration calorimetry. Interestingly, SAB shares key structural similarities with the trisubstituted benzimidazole class of thrombin inhibitors, such as dabigatran. Molecular docking models predicted the binding of SAB to the thrombin active site. Conclusion: Our data established SAB as the first herb-derived direct thrombin catalytic site inhibitor, suppressing thrombosis through both thrombin-dependent and thrombin-independent pathways. Purified SAB may be a cost-effective agent for treating arterial and deep vein thrombosis.

Amodiaquine Nonspecifically Binds Double Stranded and Three-Way Junction DNA Structures.

The 4-aminoquinoline class of compounds includes the important antimalarial compounds amodiaquine and chloroquine. Despite their medicinal importance, the mode of action of these compounds is poorly understood. In a previous study we observed these compounds, as well as quinine and mefloquine, tightly bind the DNA cocaine-binding aptamer. Here, we further explore the range of nucleic acid structures bound by these compounds. To gauge a wide range of binding affinities, we used isothermal titration calorimetry to explore high affinity binding (nM to tens of µM) and NMR spectroscopy to assay weak binding biding in the hundreds of micromolar range. We find that amodiaquine tightly binds all double stranded DNA structures explored. Mefloquine binds double stranded DNA duplex molecules tightly and weakly associates with a three-way junction DNA construct. Quinine and chloroquine only weakly bind duplex DNA but do not tightly bind any of the DNA constructs explored. A simulation of the free energy of binding of these ligands to the Dickerson-Drew dodecamer resulted in an excellent agreement between the simulated and experimental free energy. These results provide new insight into the DNA binding of clinically important antimalarial compounds and may play a role in future development of new antimalarials.

SARS-CoV-2 RBD and Its Variants Can Induce Platelet Activation and Clearance: Implications for Antibody Therapy and Vaccinations against COVID-19.

The COVID-19 pandemic caused by SARS-CoV-2 virus is an ongoing global health burden. Severe cases of COVID-19 and the rare cases of COVID-19 vaccine-induced-thrombotic-thrombocytopenia (VITT) are both associated with thrombosis and thrombocytopenia; however, the underlying mechanisms remain inadequately understood. Both infection and vaccination utilize the spike protein receptor-binding domain (RBD) of SARS-CoV-2. We found that intravenous injection of recombinant RBD caused significant platelet clearance in mice. Further investigation revealed the RBD could bind platelets, cause platelet activation, and potentiate platelet aggregation, which was exacerbated in the Delta and Kappa variants. The RBD-platelet interaction was partially dependent on the ß3 integrin as binding was significantly reduced in ß3-/- mice. Furthermore, RBD binding to human and mouse platelets was significantly reduced with related αIIbß3 antagonists and mutation of the RGD (arginine-glycine-aspartate) integrin binding motif to RGE (arginine-glycine-glutamate). We developed anti-RBD polyclonal and several monoclonal antibodies (mAbs) and identified 4F2 and 4H12 for their potent dual inhibition of RBD-induced platelet activation, aggregation, and clearance in vivo, and SARS-CoV-2 infection and replication in Vero E6 cells. Our data show that the RBD can bind platelets partially though αIIbß3 and induce platelet activation and clearance, which may contribute to thrombosis and thrombocytopenia observed in COVID-19 and VITT. Our newly developed mAbs 4F2 and 4H12 have potential not only for diagnosis of SARS-CoV-2 virus antigen but also importantly for therapy against COVID-19.

Redox Reporter - Ligand Competition to Support Signaling in the Cocaine-Binding Electrochemical Aptamer-Based Biosensor.

Electrochemical aptamer-based (E-AB) biosensors have demonstrated capabilities in monitoring molecules directly in undiluted complex matrices and in the body with the hopes of addressing personalized medicine challenges. This sensing platform relies on an electrode-bound, redox-reporter-modified aptamer. The electrochemical signal is thought to originate from the aptamer undergoing a binding-induced conformational change capable of moving the redox reporter closer to the electrode surface. While this is the generally accepted mechanism, it is notable that there is limited evidence demonstrating conformational change or distance-dependent change in electron transfer rates in E-AB sensors. In response, we investigate here the signal transduction of the well-studied cocaine-binding aptamer with different analytical methods and found that this sensor relies on a redox-reporter - ligand competition mechanism rather than a ligand-induced structure formation mechanism. Our results show that the covalently bound redox reporter, methylene blue, binds at or near the ligand binding site on the aptamer resulting in a folded conformation of the cocaine-binding aptamer. Addition of ligand then competes with the redox reporter for binding, altering its electron transfer rate. While we show this for the cocaine-binding aptamer, given the prevalence of methylene blue in E-AB sensors, a similar competition-based may occur in other systems.

Dual roles of fucoidan-GPIbα interaction in thrombosis and hemostasis: implications for drug development targeting GPIbα.

BACKGROUND: Platelet GPIbα-von Willebrand factor (VWF) interaction initiates platelet adhesion, activation, and thrombus growth, especially under high shear conditions. Therefore, the GPIb-VWF axis has been suggested as a promising target against arterial thrombosis. The polysaccharide fucoidan has been reported to have opposing prothrombotic and antithrombotic effects; however, its binding mechanism with platelets has not been adequately studied. OBJECTIVE: The objective of this study was to explore the mechanism of fucoidan and its hydrolyzed products in thrombosis and hemostasis. METHODS: Natural fucoidan was hydrolyzed by using hydrochloric acid and was characterized by using size-exclusion chromatography, UV-visible spectroscopy, and fluorometry techniques. The effects of natural and hydrolyzed fucoidan on platelet aggregation were examined by using platelets from wild-type, VWF and fibrinogen-deficient, GPIbα-deficient, and IL4Rα/GPIbα-transgenic and αIIb-deficient mice and from human beings. Platelet activation markers (P-selectin expression, PAC-1, and fibrinogen binding) and platelet-VWF A1 interaction were measured by using flow cytometry. GPIbα-VWF A1 interaction was evaluated by using enzyme-linked immunosorbent assay. GPIb-IX-induced signal transduction was detected by using western blot. Heparinized whole blood from healthy donors was used to test thrombus formation and growth in a perfusion chamber. RESULTS: We found that GPIbα is critical for fucoidan-induced platelet activation. Fucoidan interacted with the extracellular domain of GPIbα and blocked its interaction with VWF but itself could lead to GPIbα-mediated signal transduction and, subsequently, αIIbß3 activation and platelet aggregation. Conversely, low-molecular weight fucoidan inhibited GPIb-VWF-mediated platelet aggregation, spreading, and thrombus growth at high shear. CONCLUSION: Fucoidan-GPIbα interaction may have unique therapeutic potential against bleeding disorders in its high-molecular weight state and protection against arterial thrombosis by blocking GPIb-VWF interaction after fucoidan is hydrolyzed.

DNA binding by the antimalarial compound artemisinin.

Artemisinin (ART) is a vital medicinal compound that is used alone or as part of a combination therapy against malaria. ART is thought to function by attaching to heme covalently and alkylating a range of proteins. Using a combination of biophysical methods, we demonstrate that ART is bound by three-way junction and duplex containing DNA molecules. Binding of ART by DNA is first shown for the cocaine-binding DNA aptamer and extensively studied using this DNA molecule. Isothermal titration calorimetry methods show that the binding of ART is both entropically and enthalpically driven at physiological NaCl concentration. Native mass spectrometry methods confirm DNA binding and show that a non-covalent complex is formed. Nuclear magnetic resonance spectroscopy shows that ART binds at the three-way junction of the cocaine-binding aptamer, and that binding results in the folding of the structure-switching variant of this aptamer. This structure-switching ability was exploited using the photochrome aptamer switch assay to demonstrate that ART can be detected using this biosensing assay. This study is the first to demonstrate the DNA binding ability of ART and should lay the foundation for further work to study implications of DNA binding for the antimalarial activity of ART.

Weak Binding of Levamisole by the Cocaine-Binding Aptamer Does Not Interfere with an Aptamer-Based Detection Assay.

Levamisole is a common and harmful adulterant of street samples of cocaine and can cause electrochemical tests for cocaine to give false negative results. To see if levamisole would interfere with aptamer-based bioassays, we analyzed the binding of levamisole to the cocaine-binding DNA aptamer. At low aptamer concentrations (0.5 to 20 µM) using isothermal titration calorimetry methods and thermal stability measurements, no binding of levamisole to the cocaine-binding aptamer was observed. At higher levamisole concentrations (500 µM), weak binding to the cocaine-binding aptamer was detected using nuclear magnetic resonance (NMR) spectroscopy chemical shift perturbations. NMR-detected titrations show that levamisole binding is competitive with cocaine binding, indicating that both ligands share a common binding site. Finally, we show that the presence of levamisole does not interfere with the photochrome aptamer switch binding assay for cocaine. We conclude that assays using low concentrations of cocaine, and consequently low concentration of levamisole as an adulterant, should be unaffected by the weak binding of levamisole.

Visible Fluorescent Light-up Probe for DNA Three-Way Junctions Provides Host-Guest Biosensing Applications.

DNA three-way junctions (3WJs) consist of a Y-shaped hydrophobic branch point connecting three double-stranded stems and are viewed as druggable targets for cancer treatment. They are also important building blocks for the construction of DNA nanostructures and serve as recognition elements for DNA aptasensors for a wide variety of diagnostic applications. However, visible fluorescent light-up probes for specific staining of DNA 3WJs are currently lacking. Herein, we report that a merocyanine containing the N-methylbenzothiazolium (Btz) acceptor vinyl linked to a 2-fluorophenolic (FPhO) donor (FPhOBtz) serves as a universal fluorescent turn-on dye for DNA 3WJs. Our evidence is based on a multifaceted approach to define the specificity and affinity of FPhOBtz for 3WJ DNA aptamers; the cocaine binding aptamer MN4, the cholic acid binding aptamer (CABA), and four steroid aptamers (DOGS.1, DISS.1, BES.1, DCAS.1). FPhOBtz exhibits impressive turn-on (up to 730-fold) fluorescence at 580 nm upon aptamer binding with low micromolar affinity. Direct FPhOBtz displacement from the 3WJ binding domain through competitive alkaloid and steroid binding provides immediate fluorescent read out for host-guest detection strategies in human blood serum in the low micromolar regime. Our results present the first visible light-up fluorescent probe for DNA 3WJ detection strategies.

Thermodynamic analysis of cooperative ligand binding by the ATP-binding DNA aptamer indicates a population-shift binding mechanism.

The ATP-binding DNA aptamer is often used as a model system for developing new aptamer-based biosensor methods. This aptamer follows a structure-switching binding mechanism and is unusual in that it binds two copies of its ligand. We have used isothermal titration calorimetry methods to study the binding of ATP, ADP, AMP and adenosine to the ATP-binding aptamer. Using both individual and global fitting methods, we show that this aptamer follows a positive cooperative binding mechanism. We have determined the binding affinity and thermodynamics for both ligand-binding sites. By separating the ligand-binding sites by an additional four base pairs, we engineered a variant of this aptamer that binds two adenosine ligands in an independent manner. Together with NMR and thermal stability experiments, these data indicate that the ATP-binding DNA aptamer follows a population-shift binding mechanism that is the source of the positive binding cooperativity by the aptamer.

Analysis of the role played by ligand-induced folding of the cocaine-binding aptamer in the photochrome aptamer switch assay.

The Photochrome Aptamer Switch Assay (PHASA) relies on ligand binding by an aptamer to alter the local environment of a stilbene compound covalently attached to the 5' end of the aptamer. We used the PHASA with both structure switching and non-structure switching versions of the cocaine-binding aptamer. We show that the largest change in fluorescence intensity and the lowest concentration limit of detection (CLooD) is obtained using the structure-switching cocaine-binding aptamer. Fluorescence anisotropy measurements were used to quantify the affinity of the conjugated aptamer to cocaine. We also used thermal melt analysis and Nuclear Magnetic Resonance (NMR) spectroscopy to show that the addition of the stilbene to the aptamer increases the melt temperature of the cocaine-bound structure-switching aptamer by (6.4 ± 0.3) °C compared to the unconjugated aptamer while the free form of the structure-switching aptamer-stilbene conjugate remains unfolded.

The Bispidinone Derivative 3,7-Bis-[2-(S)-amino-3-(1H-indol-3-yl)-propionyl]-1,5-diphenyl-3,7-diazabicyclo[3.3.1]nonan-9-one Dihydrochloride Induces an Apoptosis-Mediated Cytotoxic Effect on Pancreatic Cancer Cells In Vitro.

Pancreatic cancer (PC) is a complex, heterogeneous disease with a dismal prognosis. Current therapies have failed to improve survival outcomes, urging the need for discovery of novel targeted treatments. Bispidinone derivatives have yet to be investigated as cytotoxic agents against PC cells. The cytotoxic effect of four bispidinone derivatives (BisP1: 1,5-diphenyl-3,7-bis(2-hydroxyethyl)-3,7-diazabicyclo[3.3.1]nonan-9-one; BisP2: 3,7-bis-(2-(S)-amino-4-methylsulfanylbutyryl)-1,5-diphenyl-3,7-diazabicyclo[3.3.1]nonan-9-one dihydrochloride; BisP3: [2-{7-[2-(S)-tert-butoxycarbonylamino-3-(1H-indol-3-yl)-propionyl]-9-oxo-1,5-diphenyl-3,7-diazabicyclo[3.3.1]non-3-yl}-1-(S)-(1H-indol-3-ylmethyl)-2-oxoethyl]-carbamic acid tertbutyl ester; BisP4: 3,7-bis-[2-(S)-amino-3-(1H-indol-3-yl)-propionyl]-1,5-diphenyl-3,7-diazabicyclo[3.3.1]nonan-9-one dihydrochloride) was assessed against PC cell lines (MiaPaca-2, CFPAC-1 and BxPC-3). Cell viability was assessed using a Cell Counting Kit-8 (CCK-8) colorimetric assay, while apoptotic cell death was confirmed using fluorescence microscopy and flow cytometry. Initial viability screening revealed significant cytotoxic activity from BisP4 treatment (1 µM⁻100 µM) on all three cell lines, with IC50 values for MiaPaca-2, BxPC-3, and CFPAC-1 16.9 µM, 23.7 µM, and 36.3 µM, respectively. Cytotoxic treatment time-response (4 h, 24 h, and 48 h) revealed a 24 h treatment time was sufficient to produce a cytotoxic effect on all cell lines. Light microscopy evaluation (DAPI staining) of BisP4 treated MiaPaca-2 PC cells revealed dose-dependent characteristic apoptotic morphological changes. In addition, flow cytometry confirmed BisP4 induced apoptotic cell death induction of activated caspase-3/-7. The bispidinone derivative BisP4 induced an apoptosis-mediated cytotoxic effect on MiaPaca-2 cell lines and significant cytotoxicity on CFPAC-1 and BxPC-3 cell lines. Further investigations into the precise cellular mechanisms of action of this class of compounds are necessary for potential development into pre-clinical trials.

A proof of concept application of aptachain: ligand-induced self-assembly of a DNA aptamer.

A challenge for the use of aptamers as biosensors is how to signal the occurrence of their ligand binding event into a signal that can be exploited in a detection scheme. Here, we present the concept of "aptachain" formation, where an aptamer is split into two overlapping or staggered strands and assembles into an extended oligomer upon ligand binding. This assembly of aptamers can then be used as a way to detect ligand binding by the aptamer. As an example of this concept, we employed the cocaine-binding aptamer as a model system, used its ability to tightly bind quinine and demonstrated its capability in a gold nanoparticle-based biosensing application. We used isothermal titration calorimetry to demonstrate that, when split into two overlapping DNA strands, the aptamer remains functional. Size-exclusion chromatography showed that the quinine-bound oligos form a larger assembly of aptamer units than in the absence of ligand. Finally, we used the oligomer forming ability of the aptachain oligos in a biosensor application for quinine that brings gold nanoparticles closer together resulting in a shift in their plasmonic resonance to a longer wavelength and an observed colour shift. We propose that splitting aptamers into overlapping strands that form oligomers in the presence of a ligand, aptachain formation, will be generally applicable to aptamers and prove useful in a variety of biotechnology applications.

Development of a thermal-stable structure-switching cocaine-binding aptamer.

We have developed a new cocaine-binding aptamer variant that has a significantly higher melt temperature when bound to a ligand than the currently used sequence. Retained in this new construct is the ligand-induced structure-switching binding mechanism that is important in biosensing applications of the cocaine-binding aptamer. Isothermal titration calorimetry methods show that the binding affinity of this new sequence is slightly tighter than the existing cocaine-binding aptamer. The improved thermal performance, a Tm increase of 4 °C for the cocaine-bound aptamer and 9 °C for the quinine-bound aptamer, was achieved by optimizing the DNA sequence in stem 2 of the aptamer to have the highest stability based on the nearest neighbor thermodynamic parameters and confirmed by UV and fluorescence spectroscopy. The sequences in stem 1 and stem 3 were unchanged in order to retain the structure switching and ligand binding functions. The more favorable thermal stability characteristics of the OR3 aptamer should make it a useful construct for sensing applications employing the cocaine-binding aptamer system.

Optimizing Stem Length To Improve Ligand Selectivity in a Structure-Switching Cocaine-Binding Aptamer.

Understanding how aptamer structure and function are related is crucial in the design and development of aptamer-based biosensors. We have analyzed a series of cocaine-binding aptamers with different lengths of their stem 1 in order to understand the role that this stem plays in the ligand-induced structure-switching binding mechanism utilized in many of the sensor applications of this aptamer. In the cocaine-binding aptamer, the length of stem 1 controls whether the structure-switching binding mechanism for this aptamer occurs or not. We varied the length of stem 1 from being one to seven base pairs long and found that the structural transition from unfolded to folded in the unbound aptamer is when the aptamer elongates from 3 to 4 base pairs in stem 1. We then used this knowledge to achieve new binding selectivity of this aptamer for quinine over cocaine by using an aptamer with a stem 1 two base pairs long. This selectivity is achieved by means of the greater affinity quinine has for the aptamer compared with cocaine. Quinine provides enough free energy to both fold and bind the 2-base pair-long aptamer while cocaine does not. This tuning of binding selectivity of an aptamer by reducing its stability is likely a general mechanism that could be used to tune aptamer specificity for tighter binding ligands.

Subcutaneous versus intravenous insulin therapy for glucose control in non-diabetic trauma patients. A randomized controlled trial.

WHAT IS KNOWN AND OBJECTIVE: Hyperglycaemia in trauma patients admitted to the intensive care unit (ICU) is associated with increased morbidity and mortality. Our pilot study is a prospective randomized controlled trial comparing the impact of two glucose control regimens on outcomes in non-diabetic trauma patients admitted with hyperglycaemia to the ICU. METHODS: Trauma patients with blood glucose levels (BGLs) ≥7·8 mm within the first 48 h of the hospital admission were randomized to receive intermittent SQ or continuous IV insulin to maintain BGLs between 4·4 and 6·1 mm. We excluded diabetics on the basis of history, or a glycosylated haemoglobin ≥6% on admission. We compared the effect of SQ vs. IV insulin therapy on the ICU length of stay (ILOS). RESULTS AND DISCUSSION: A total of 58 patients were included in the study. The SQ and IV groups were comparable in terms of age, gender, injury severity, revised trauma, Glasgow coma scores and type of trauma (blunt vs. penetrating). There was no significant difference between the two treatment groups in the ILOS (3 vs. 2 days, P = 0·084), hospital length of stay (8 vs. 6, P = 0·09), ventilator support days (6 vs. 3, P = 0·98), requirement for blood transfusion (P = 0·66), rates of infections (P = 0·70), acute kidney injury (P = 0·99) and mortality (P = 0·61). WHAT IS NEW AND CONCLUSION: There was no difference between SQ and IV insulin therapy in the ILOS in non-diabetic trauma patients.

Contrasting gray and white matter changes in preclinical Huntington disease: an MRI study.

BACKGROUND: In Huntington disease (HD), substantial striatal atrophy precedes clinical motor symptoms. Accordingly, neuroprotection should prevent major cell loss before such symptoms arise. To evaluate neuroprotection, biomarkers such as MRI measures are needed. This requires first establishing the best imaging approach. METHODS: Using a cross-sectional design, we acquired T1-weighted and diffusion-weighted scans in 39 preclinical (pre-HD) individuals and 25 age-matched controls. T1-weighted scans were analyzed with gross whole-brain segmentation and voxel-based morphometry. Analysis of diffusion-weighted scans used skeleton-based tractography. For all imaging measures, we compared pre-HD and control groups and within the pre-HD group we examined correlations with estimated years to clinical onset. RESULTS: Pre-HD individuals had lower gross gray matter (GM) and white matter (WM) volume. Voxel-wise analysis demonstrated local GM volume loss, most notably in regions consistent with basal ganglia-thalamocortical pathways. By contrast, pre-HD individuals showed widespread reductions in WM integrity, probably due to a loss of axonal barriers. Both GM and WM imaging measures correlated with estimated years to onset. CONCLUSIONS: Using automated, observer-independent methods, we found that GM loss in pre-HD was regionally specific, while WM deterioration was much more general and probably the result of demyelination rather then axonal degeneration. These findings provide important information about the nature, relative staging, and topographic specificity of brain changes in pre-HD and suggest that combining GM and WM imaging may be the best biomarker approach. The empirically derived group difference images from this study are provided as regions-of-interest masks for improved sensitivity in future longitudinal studies.

Impaired conscious and preserved unconscious inhibitory processing in recent onset schizophrenia.

BACKGROUND: Impairments in inhibitory function have been found in studies of cognition in schizophrenia. These have been linked to a failure to adequately maintain the task demands in working memory. As response inhibition is known to occur in both voluntary and involuntary processes, an important question is whether both aspects of response inhibition are specifically impaired in people with schizophrenia. METHOD: The subjects were 33 patients presenting with a first episode of psychosis (27 with schizophrenia and six with schizo-affective disorder) and 24 healthy controls. We administered two motor response tasks: voluntary response inhibition was indexed by the stop-signal task and involuntary response inhibition by the masked priming task. We also administered neuropsychological measures of IQ and executive function to explore their associations with response inhibition. RESULTS: Patients with schizophrenia compared to healthy controls showed significantly increased duration of the voluntary response inhibition process, as indexed by the stop-signal reaction time (SSRT). By contrast, there were no group differences on the pattern of priming on the masked priming task, indicative of intact involuntary response inhibition. Neuropsychological measures revealed that voluntary response inhibition is not necessarily dependent on working memory. CONCLUSIONS: These data provide evidence for a specific impairment of voluntary response inhibition in schizophrenia.