Fine-tuning the gut ecosystem: the current landscape and outlook of artificial microbiome therapeutics

The gut microbiome is acknowledged as a key determinant of human health, and technological progress in the past two decades has enabled the deciphering of its composition and functions and its role in human disorders. Therefore, manipulation of the gut microbiome has emerged as a promising therapeutic option for communicable and non-communicable disorders. Full exploitation of current therapeutic microbiome modulators (including probiotics, prebiotics, and faecal microbiota transplantation) is hindered by several factors, including poor precision, regulatory and safety issues, and the impossibility of providing reproducible and targeted treatments. Artificial microbiota therapeutics (which include a wide range of products, such as microbiota consortia, bacteriophages, bacterial metabolites, and engineered probiotics) have appeared as an evolution of current microbiota modulators, as they promise safe and reproducible effects, with variable levels of precision via different pathways. We describe the landscape of artificial microbiome therapeutics, from those already on the market to those still in the pipeline, and outline the major challenges for positioning these therapeutics in clinical practice.

The role of faecal microbiota transplantation in chronic noncommunicable disorders

The gut microbiome plays a key role in influencing several pathways and functions involved in human health, including metabolism, protection against infection, and immune regulation. Perturbation of the gut microbiome is recognised as a pathogenic factor in several gastrointestinal and extraintestinal disorders, and is increasingly considered as a therapeutic target in these conditions. Faecal microbiota transplantation (FMT) is the transfer of the microbiota from healthy screened stool donors into the gut of affected patients, and is a well-established and highly effective treatment for recurrent Clostridioides difficile infection. Despite the mechanisms of efficacy of FMT not being fully understood, it has been investigated in several chronic noncommunicable disorders, with variable results. This review aims to give an overview of mechanisms of efficacy of FMT in chronic noncommunicable disorders, and to paint the current landscape of its investigation in these medical conditions, including inflammatory bowel disease (IBD), chronic liver disorders, and also extraintestinal autoimmune conditions.

Impact of COVID-19 in individuals with and without pre-existent digestive disorders with a particular focus on elderly patients

Coronavirus disease 2019 (COVID-19) has several extrapulmonary symptoms. Gastrointestinal (GI) symptoms are among the most frequent clinical manifestations of COVID-19, with severe consequences reported in elderly patients. Furthermore, the impact of COVID-19 on patients with pre-existing digestive diseases still needs to be fully elucidated, particularly in the older population. This review aimed to investigate the impact of COVID-19 on the GI tract, liver, and pancreas in individuals with and without previous digestive diseases, with a particular focus on the elderly, highlighting the distinctive characteristics observed in this population. Finally, the effectiveness and adverse events of the anti-COVID-19 vaccination in patients with digestive disorders and the peculiarities found in the elderly are discussed.

Histamine-producing bacteria and their role in gastrointestinal disorders

INTRODUCTION

Gut microbiota produces thousands of metabolites, which have a huge impact on the host health. Specific microbial strains are able to synthesize histamine, a molecule with a crucial role in many physiologic and pathologic mechanisms of the host. This function is mediated by the histidine decarboxylase enzyme (HDC) that converts the amino acid histidine to histamine.

AREAS COVERED

This review summarizes the emerging data on histamine production by gut microbiota, and the effect of bacterial-derived histamine in different clinical contexts, including cancer, irritable bowel syndrome, and other gastrointestinal and extraintestinal pathologies. This review will also outline the impact of histamine on the immune system and the effect of probiotics that can secrete histamine. Search methodology: we searched the literature on PubMed up to February 2023.

EXPERT OPINION

The potential of modulating gut microbiota to influence histamine production is a promising area of research, and although our knowledge of histamine-secreting bacteria is still limited, recent advances are exploring their diagnostic and therapeutical potential. Diet, probiotics, and pharmacological treatments directed to the modulation of histamine-secreting bacteria may in the future potentially be employed in the prevention and management of several gastrointestinal and extraintestinal disorders.

The Influence of Helicobacter pylori on Human Gastric and Gut Microbiota

Helicobacter pylori is a Gram-negative bacterium that is able to colonize the human stomach, whose high prevalence has a major impact on human health, due to its association with several gastric and extra-gastric disorders, including gastric cancer. The gastric microenvironment is deeply affected by H. pylori colonization, with consequent effects on the gastrointestinal microbiota, exerted via the regulation of various factors, including gastric acidity, host immune responses, antimicrobial peptides, and virulence factors. The eradication therapy required to treat H. pylori infection can also have detrimental consequences for the gut microbiota, leading to a decreased alpha diversity. Notably, therapy regimens integrated with probiotics have been shown to reduce the negative effects of antibiotic therapy on the gut microbiota. These eradication therapies combined with probiotics have also higher rates of eradication, when compared to standard treatments, and are associated with reduced side effects, improving the patient's compliance. In light of the deep impact of gut microbiota alterations on human health, the present article aims to provide an overview of the complex interaction between H. pylori and the gastrointestinal microbiota, focusing also on the consequences of eradication therapies and the effects of probiotic supplementation.

The role of diet in shaping human gut microbiota

Gut microbiota plays a fundamental role within human health, and exerts key functions within the human body. Diet is one of the most powerful modulators of gut microbiota functions and composition. This complex interplay involves also the immune system and the intestinal barrier, highlighting the central role of diet in the pathogenesis and treatment of multiple diseases. In this review article we will paint the landscape of the effects of specific dietary nutrients, and of the detrimental or beneficial outcomes of different dietary patterns, on the composition of human gut microbiota. Moreover, we will discuss the potential application of diet as a therapeutic modulator of gut microbiota, including cutting-edge ways of exploitation, including the use of dietary components as adjuvants to promote microbial engraftment after fecal microbiota transplantation, or personalized nutritional approaches, targeted to the patient microbiome.

Targeting IL12/23 in ulcerative colitis: update on the role of ustekinumab

As our comprehension of the pathogenic mechanisms of inflammatory bowel disease (IBD) increases, the therapeutic armamentarium for its treatment can expand, and novel target therapies join the treatment pipeline. Interleukin (IL)-12 and IL23 are two key cytokines responsible for promoting and perpetuating bowel inflammation in IBD. Ustekinumab is a monoclonal antibody directed against the shared p40 subunit of both cytokines, and it was recently approved for the treatment of ulcerative colitis (UC). In the pivotal phase III UNIFI trial, ustekinumab showed a superiority over placebo in both clinical and endoscopic outcomes; furthermore, it was characterized by a favorable safety profile, with a similar rate of adverse events as compared with placebo. Recent evidence from real-life experiences have started accumulating, generally confirming the effectiveness and safety figures emerged from the registration studies. However, most of these observational studies enrolled multirefractory patients; moreover, comparative data with other target therapies are lacking, leaving physicians without clear indications about the appropriate positioning of ustekinumab in the therapeutic pipeline for UC. This review examines the basis of targeting IL12-23 in UC therapy and summarizes the data from both clinical trials and real-life studies, to highlight the main evidence already available and the research gaps that need to be filled for the optimal usage of ustekinumab in UC.

Psychopathological and temperamental features of Late Onset versus Early Onset Bipolar Disorder

Introduction

Age at onset of type-I bipolar disorder (BD-I) typically averages 12-24 years, is older among patients with type-II-BD (BD-II), even though generally before 50-years-old (EOBD). Clinical observation of late-onset BD (LOBD) posed some questions regarding a differential phenotypic/psychopathological manifestations and affective temperaments between LOBD vs EOBD.

Objectives

A case-control pilot-study was carried out to investigate psychopathological, clinical and temperamental features of a psychogeriatric cohort of LOBD and EOBD subjects.

Methods

Out of 74 enrolled patients, 64 patients (31 EOBD, 33 LOBD) were included and administered an ad hoc socio-demographic datasheet, BPRS, CGI, GAF, HAM-D, GDS, MSRS, MRS, MOCA and TEMPS-M.

Results

LOBD is significantly associated with higher rates of BD-II diagnosis (X2 = 26.1, p<.001), depressive (p=0.05) and mixed states (p=0.011), higher comorbid anxiety levels and depressive affective temperament (p<.001); while clinical manifestations of geriatric EOBD is significantly associated with higher endocrinological (X2 = 7.815, p=.005) and metabolic comorbidity (X2 = 6.896, p=.009), a diagnosis of BD-I, manic episodes and hyperthymic (p=.001) affective temperaments. GDS and MSRS total scores were significantly higher in LOBD (respectively, p<.001 and p=.008).

Conclusions

Further studies with larger sample sizes and a control group should verify whether LOBD is a distinct psychopathological entity from EOBD and evaluate differences (if any) in terms of prognosis and treatment between EOBD and LOBD.

Disclosure

No significant relationships.

Pathoplastic effects of metabolic disorders in severe mental illness

Introduction

Patients with severe mental illness (SMI) have a higher risk of weight gain, dyslipidemia and insulin-resistance. It was observed that insulin resistance has a pathoplastic effect: in Schizophrenia it was associated with a greater severity of negative symptoms, whereas in Bipolar Disorder it was associated with more chronicity and rapid cycling. Moreover a correlation was observed between obesity and a worse outcome in Bipolar Disorder type I.

Objectives

We aimed at assessing the influence of dysmetabolisms on clinical characteristics in patients with SMI.

Methods

We recruited 78 patients with SMI consecutively hospitalized in the Psychiatry Clinic of the Ospedali Riuniti of Ancona, Italy. We administered a checklist for socio-demographic and clinical features (diagnosis, age of onset, illness duration, number of episodes, number of episodes per year, suicidal attempts and comorbidities), and evaluated the following metabolic parameters: weight, height, BMI, abdominal circumference, blood pressure, total cholesterol, HDL, triglycerides, glycemia and insulinemia. We determined insulin-resistance according to the HOMA-IR model. We performed bivariate Pearson correlations to compare metabolic and socio-demographic/clinical parameters.

Results

The analyses showed positive correlations between BMI and disease duration (P = 0.003), and BMI and the number of episodes (P = 0.022). Furthermore, a positive correlation was found between HOMA-IR and the number of episodes per year (P = 0.008). The associations remained statistically significant after controlling for age through partial correlations.

Conclusions

Weight gain and insulin-resistance in severe mental illness are associated with a more severe SMI, as suggested by the greater number of acute episodes.

Disclosure

No significant relationships.

The role of feedback projections in feature tuning and neuronal excitability in the early primate visual system

A general assumption in visual neuroscience is that basic receptive field properties such as orientation and direction selectivity are constructed within intrinsic neuronal circuits and feedforward projections. In addition, it is assumed that general neuronal excitability and responsiveness in early visual areas is to a great extent independent of feedback input originating in areas higher in the stream. Here, we review the contribution of feedback projections from MT, V4 and pulvinar to the receptive field properties of V2 neurons in the anesthetized and paralyzed monkey. Importantly, our results contradict both of these assumptions. We separately inactivated each of these three brain regions using GABA pressure injections, while simultaneously recording V2 single unit activity before and hours after inactivation. Recordings and GABA injections were carried out in topographically corresponding regions of the visual field. We outline the changes in V2 activity, responsiveness and receptive field properties for early, mid and late post-injection phases. Immediately after injection, V2 activity is globally suppressed. Subsequently, there is an increase in stimulus-driven relative to spontaneous neuronal activity, which improves the signal-to-noise coding for the oriented moving bars. Notably, V2 tuning properties change substantially relative to its pre-injection selectivity profile. The resulting increase or decrease in selectivity could not be readily predicted based on the selectivity profile of the inactivated site. Finally, V2 activity rebounds before returning to it pre-injection profile Our results show that feedback projections profoundly impact neuronal circuits in early visual areas, and may have been heretofore largely underestimated in their physiological role.

Intestinal Permeability, Inflammation and the Role of Nutrients

The interaction between host and external environment mainly occurs in the gastrointestinal tract, where the mucosal barrier has a critical role in many physiologic functions ranging from digestion, absorption, and metabolism. This barrier allows the passage and absorption of nutrients, but at the same time, it must regulate the contact between luminal antigens and the immune system, confining undesirable products to the lumen. Diet is an important regulator of the mucosal barrier, and the cross-talk among dietary factors, the immune system, and microbiota is crucial for the modulation of intestinal permeability and for the maintenance of gastrointestinal tract (GI) homeostasis. In the present review, we will discuss the role of a number of dietary nutrients that have been proposed as regulators of inflammation and epithelial barrier function. We will also consider the metabolic function of the microbiota, which is capable of elaborating the diverse nutrients and synthesizing products of great interest. Better knowledge of the influence of dietary nutrients on inflammation and barrier function can be important for the future development of new therapeutic approaches for patients with mucosal barrier dysfunction, a critical factor in the pathogenesis of many GI and non-GI diseases.

GABA-induced inactivation of Cebus apella V2 neurons: effects on orientation tuning and direction selectivity

We investigated the GABA-induced inactivation of V2 neurons and terminals on the receptive field properties of this area in an anesthetized and paralyzed Cebus apella monkey. Extracellular single-unit activity was recorded using tungsten microelectrodes in a monkey before and after pressure-injection of a 0.25 or 0.5 M GABA solution. The visual stimulus consisted of a bar moving in 8 possible directions. In total, 24 V2 neurons were studied before and after blocker injections in 4 experimental sessions following GABA injection into area V2. A group of 10 neurons were studied over a short period. An additional 6 neurons were investigated over a long period after the GABA injection. A third group of 8 neurons were studied over a very long period. Overall, these 24 neurons displayed an early (1-20 min) significant general decrease in excitability with concomitant changes in orientation or direction selectivity. GABA inactivation in area V2 produced robust inhibition in 80% and a significant change in directional selectivity in 60% of the neurons examined. These GABA projections are capable of modulating not only levels of spontaneous and driven activity of V2 neurons but also receptive field properties such as direction selectivity.

Changes of ongoing activity in Cebus monkey perirhinal cortex correlate with behavioral performance

A Cebus apella monkey weighing 4 kg was trained in a saccadic eye movement task and while the animal performed the task we recorded the extracellular activity of perirhinal cortical neurons. Although the task was very simple and maintained at a constant level of difficulty, we observed considerable changes in the performance of the monkey within each experimental session. The behavioral states responsible for such variation may be related to arousal, motivation or attention of the animal while engaged in the task. In approximately 20% (16/82) of the units recorded, long-term direct or inverse correlations could be demonstrated between the monkey's behavioral state and the cells' ongoing activity (independent of the visual stimulation or of the specific behavior along a trial). The perirhinal cortex and other medial temporal structures have long been associated with normal memory function. The data presented here were interpreted in terms of recent reports focusing on the subcortical afferents to temporal lobe structures and their possible role in controlling arousal, motivation, or attention.

Effects of inactivation of the lateral pulvinar on response properties of second visual area cells in Cebus monkeys

1. In the present study, we investigated the influence of the pulvinar nucleus upon response properties of single cells in the second visual area (V2) of Cebus monkeys. The method used consisted of the inactivation of a portion of the lateral pulvinar by GABA injections while studying the response properties of cells in V2 at the same visuotopic location as that of the inactivation. 2. After GABA injection in the pulvinar, most cells in V2 (67%) showed changes in spontaneous and/or stimulus-driven activities. Contrary to the effect found with inactivation of the striate cortex, which promotes a reduction in the response of V2 neurons, we found that the main effect of pulvinar inactivation was an increment in stimulus-driven responses of V2 cells (39% of units studied). A reduction of responses was observed in 27% of units. 3. A change in orientation and/or direction selectivity was found in 91% of cells after inactivation of the pulvinar. Most commonly, the orientation selectivity of a neuron was decreased during pulvinar inactivation. 4. The inactivation results indicate that the pulvinar projections have a modulatory effect on the activity of V2 cells.

Connectional and neurochemical subdivisions of the pulvinar in Cebus monkeys

Based on cytoarchitectonic criteria, the primate pulvinar nucleus has been subdivided into medial (PM), lateral (PL), and inferior (PI) regions. However, these subdivisions show no correlation with those established by electrophysiological, immunocytochemical, or neuroanatomical tracer studies. In this work, we studied the connections of the pulvinar nucleus of Cebus monkey with visual areas V1, V2, V4, MT, and PO by means of retrograde fluorescent tracers injected into these areas. Based on the projection zones to cortical visual areas, the visual portion of the pulvinar of Cebus monkey was subdivided into three subregions: P1, P2, and P3, similar to those described in the macaque (Ungerleider et al., 1984). In Cebus, P1 includes the centrolateral portion of traditionally defined PI and adjacent portion of PL. P2 is located in the dorsal portion of PL and P3 includes the medial portion of PI and extends dorsally into adjacent PL and PM. In addition, we studied the histology of the pulvinar using multiple criteria, such as cytoarchitecture and myeloarchitecture; histochemistry for cytochrome oxidase, NADPH-diaphorase, and acetylcholinesterase; and immunocytochemistry for two calcium-binding proteins, calbindin and parvalbumin, and for a neurofilament recognized by the SMI-32 antibody. Some of these stains, mainly calbindin, showed additional subdivisions of the Cebus pulvinar, beyond the traditional PI, PL, and PM. Based on this immunohistochemical staining, the border of PI is moved dorsally above the brachium of the superior colliculus and PI can be subdivided in five regions (PI(P), PI(M), PI(C), PI(L), and PI(LS)). Regions P1, P2, and P3 defined based on efferent connections with cortical visual areas are not architectonically/neurochemically homogeneous. Rather they appear to consist of further chemoarchitectonic subdivisions. These distinct histochemical regions might be related to different functional modules of visual processing within one connectional area.

Intracellular ascorbic acid enhances the DNA single-strand breakage and toxicity induced by peroxynitrite in U937 cells

A well-established protocol to increase the intracellular content of ascorbic acid was used to investigate the effects of the vitamin on DNA single-strand breakage and toxicity mediated by authentic peroxynitrite (ONOO(-)) in U937 cells. This protocol involved exposure for 60 min to 100 microM dehydroascorbic acid, which was taken up by the cells and converted into ascorbic acid via a GSH-independent mechanism. At the time of exposure to ONOO(-), which was performed in fresh saline immediately after loading with dehydroascorbic acid, the vitamin present in the cells was all in its reduced form. It was found that, in cells that are otherwise ascorbate-deficient, an increase in their ascorbic acid content does not prevent, but rather enhances, the DNA-damaging and lethal responses mediated by exogenous ONOO(-). These results therefore suggest that acute supplementation of ascorbic acid can be detrimental for individuals with pathologies associated with a decrease in ascorbic acid and in which ONOO(-) is known to promote deleterious effects.

Quercetin prevents glutathione depletion induced by dehydroascorbic acid in rabbit red blood cells

Exposure of rabbit red blood cells to dehydroascorbic acid (DHA) caused a significant decline in glutathione content which was largely prevented by quercetin, whereas it was insensitive to various antioxidants, iron chelators or scavengers of reactive oxygen species. This response was not mediated by chemical reduction of either extracellular DHA or intracellular glutathione disulfide. In addition, the flavonoid did not affect the uptake of DHA or its reduction to ascorbic acid. Rather, quercetin appeared to specifically stimulate downstream events promoting GSH formation.

Calcium-dependent mitochondrial formation of species promoting strand scission of genomic DNA in U937 cells exposed to tert-butylhydroperoxide: the role of arachidonic acid

Treatment of U937 cells with a sublethal concentration of tert-butylhydroperoxide generates DNA single strand breakage in U937 cells and this response is increased by caffeine, ATP, pyruvate or antimycin A. As we previously reported (Guidarelli, Clementi, Brambilla and Cantoni, (1997) Biochem. J. 328, 801-806), the enhancing effects of antimycin A are mediated by inhibition of complex III and the ensuing formation of superoxides and hydrogen peroxide in a reaction in which ubisemiquinone serves as an electron donor. Active electron transport was required in pyruvate-supplemented cells since the increased genotoxic response occurred as a consequence of enforced mitochondrial Ca2+ accumulation, a process driven by the increased electrochemical gradient. The enhancing effects of caffeine or ATP were also the consequence of mitochondrial Ca2+ accumulation but these responses were independent on electron transport. The increased formation of DNA lesions resulting from exposure to tert-butylhydroperoxide associated with the Ca2+-mobilizing agents or the respiratory substrate was mediated by arachidonic acid generated by Ca2+-dependent activation of phospholipase A2. Melittin, a potent phospholipase A2 activator, and reagent arachidonic acid mimicked the effects of caffeine, ATP or pyruvate on the tert-butylhydroperoxide-induced DNA single strand breakage.

Products of the phospholipase A(2) pathway mediate the dihydrorhodamine fluorescence response evoked by endogenous and exogenous peroxynitrite in PC12 cells

A short-term exposure of PC12 cells to tert-butylhydroperoxide promotes a rapid oxidation of dihydrorhodamine sensitive to nitric oxide synthase inhibitors and peroxynitrite scavengers. This response was not directly caused by peroxynitrite, but rather appeared to be mediated by peroxynitrite-dependent activation of phospholipase A(2). The following lines of evidence support this inference: (i) the peroxynitrite-dependent dihydrorhodamine fluorescence response was blunted by low concentrations of two structurally unrelated phospholipase A(2) inhibitors; (ii) under similar conditions, the phospholipase A(2) inhibitors prevented release of arachidonic acid; (iii) low levels of arachidonic acid restored the dihydrorhodamine fluorescence response in nitric oxide synthase- as well as phospholipase A(2)-inhibited cells; (iv) the dihydrorhodamine fluorescence response induced by authentic peroxynitrite was also blunted by phospholipase A(2) inhibitors and restored upon addition of reagent arachidonic acid. We conclude that endogenous, or exogenous, peroxynitrite does not directly oxidize dihydrorhodamine in intact cells. Rather, peroxynitrite appears to act as a signalling molecule promoting release of arachidonic acid, which in turn leads to formation of species causing the dihydrorhodamine fluorescence response.

Essential role of the mitochondrial respiratory chain in peroxynitrite-induced strand scission of genomic DNA

A large body of experimental evidence suggests that DNA damage and cytotoxicity mediated by peroxynitrite are linked by a causal relationship and important events in various pathological conditions. In the present study, we investigated the mechanism whereby peroxynitrite causes DNA single strand breakage in intact cells and found that the respiratory chain plays a pivotal role in this response. In particular, peroxynitrite mediates inhibition of complex III and, under these conditions, electrons are directly transferred from ubisemiquinone to molecular oxygen. Hydrogen peroxide produced by the dismutation of superoxides is the species mediating the peroxynitrite-dependent DNA cleavage.

In vitro effects of 50 Hz magnetic fields on oxidatively damaged rabbit red blood cells

The aim of this study was to investigate the effects of 50 Hz magnetic fields (0.2-0.5 mT) on rabbit red blood cells (RBCs) that were exposed simultaneously to the action of an oxygen radical-generating system, Fe(II)/ascorbate. Previous data obtained in our laboratory showed at the exposure of rabbit erythrocytes or reticulocytes to Fe(II)/ascorbate hexokinase inactivation, whereas the other glycolytic enzymes do not show any decay. We also observed depletion of reduced glutathione (GSH) content with a concomitant intracellular and extracellular increase in oxidized glutathione (GSSG) and a decrease in energy charge. In this work we investigated whether 50 Hz magnetic fields could influence the intracellular impairments that occur when erythrocytes or reticulocytes are exposed to this oxidant system, namely, inactivation of hexokinase activity, GSH depletion, a change in energy charge, and hemoglobin oxidation. The results obtained indicate the a 0.5 mT magnetic field had no effect on intact RBCs, whereas it increased the damage with Fe(II)/ascorbate to a 0.5 mT magnetic field induced a significant further decay in hexokinase activity (about 20%) as well as a twofold increase in methemoglobin production compared with RBCs that were exposed to the oxidant system alone. Although further studies will be needed to determine the physiological implications of these data, the results reported in this study demonstrate that the effects of the magnetic fields investigated are able to potentiate the cellular damage induced in vitro by oxidizing agents.

Arachidonic acid induces calcium-dependent mitochondrial formation of species promoting strand scission of genomic DNA

Both the phospholipase A(2) activator melittin and reagent arachidonic acid (AA) are poor inducers of DNA single strand breaks in U937 cells. These responses, however, were dramatically increased by the calcium-mobilizing agent caffeine (Cf) or by the respiratory substrate pyruvate via a mechanism that involved enforced mitochondrial Ca(2+) accumulation and that was sensitive to lipoxygenase inhibitors. In permeabilized cells, the DNA damage generated by AA in combination with either Cf, L-malate or CaCl(2) was blunted by catalase. AA generated DNA strand scission also in HeLa cells supplemented with pyruvate via a mechanism identical to that observed in U937 cells. This response was associated with an enforced formation of free radical species. These results demonstrate that mitochondria play a pivotal role in the DNA-damaging response evoked by AA and provide the bases for a calcium-dependent mechanism whereby the AA produced during inflammatory processes may affect various pathologic conditions, including carcinogenesis.

Dehydroascorbic acid irreversibly inhibits hexokinase activity

The oxidized form of vitamin C (dehydroascorbic acid, DHA) completely and irreversibly inactivates recombinant human hexokinase type I, in a pseudo-first order fashion. The inactivation reaction occurs without saturation, indicating that DHA does not form a reversible complex with hexokinase. Further characterization of this response revealed that the inactivation does not require oxygen and that dithiothreitol, while able to prevent the DHA-mediated loss of enzyme activity, failed to restore the activity of the DHA-inhibited enzyme. Inactivation was not associated with cleavage of the peptide chain or cross-linking. The decay in enzymatic activity was however both dependent on deprotonation of a residue with an alkaline pKa and associated with covalent binding of DHA to the protein. In addition, inactivation of hexokinase decreased or increased, respectively, in the presence of the substrates glucose or MgATP. Finally, amino acid analysis of the DHA-modified hexokinase revealed a decrease of cysteine residues. Taken together, the above results are consistent with the possibility that covalent binding of the reagent with a thiol group of cysteine is a critical event for the DHA-mediated loss of hexokinase activity.

Filling-in in topographically organized distributed networks

We propose a framework for understanding visual perception based on a topographically organized, functionally distributed network. In this proposal the extraction of shape boundaries starts at retinal ganglion cells with concentric receptive fields. This information, relayed through the lateral geniculate nucleus, creates a neural representation of negative and positive boundaries in a set of topographically connected and organized visual areas. After boundary extraction, several processes involving contrast, brightness, texture and motion extraction take place in subsequent visual areas in different cortical modules. Following these steps of processing, filling-in processes at different levels, within each area, and in separate channels, propagate locally to transform boundary representations onto surfaces representations. These partial representations of the image propagate back and forth in the network, yielding a neural representation of the original image. We propose that completion takes places in a wide cortical circuit that heavily relies on V1, where long-range information helps determine contour responses at specific topographically organized locations. Neural representations of illusory contours would emerge in circuits involving primarily area V2. The neural representation of filling-in of a peripheral stimulus in a dynamic surround (such as in texture filling-in) would depend on circuits involving primarily cells in areas V2 and V3, and would include competitive mechanisms required for figure to ground segregation. Finally, we suggest that multiple representations of the stimulus engage competitive mechanisms that select the "most likely hypothesis". Such choice behavior would rely on winner-take-all mechanisms capable of constructing a single neural representation of perceived objects.

Internal globus pallidus discharge is nearly suppressed during levodopa-induced dyskinesias

The functional status of the globus pallidus internal segment (GPi) plays a key role in mediating the effects of antiparkinsonian drugs. During long-term levodopa therapy, patients develop abnormal movements, dyskinesias, the pathophysiological basis of which is poorly understood. We recorded single cells in the GPi of parkinsonian monkeys continuously through the "off" and "on" states, and 10 to 15 minutes later during "on with or without dyskinesias," depending on two doses of levodopa. The transition from the "off" to the "on" state was characterized by a decrease (most cells), no change, or an increase in firing rate of individual cells. During dyskinesias, firing rates declined profoundly in almost all cells, with decrements as low as 97% in individual cells. These changes occurred only when dyskinesias were present. The difference in GPi activity between "on" and "on with dyskinesias" suggests that normal motor function in Parkinson's disease critically depends on fine tuning of the basal ganglia output. Dyskinesias result from an imbalanced low GPi discharge, a circumstance that may be susceptible to development of new therapeutic approaches.

Substrates of hexokinase, glucose-6-phosphate dehydrogenase, and glyceraldehyde-3-phosphate dehydrogenase prevent the inhibitory response induced by ascorbic acid/iron and dehydroascorbic acid in rabbit erythrocytes

Exposure of intact rabbit erythrocytes or erythrocyte lysates to ascorbic acid/FeCl3 in a glucose-free saline promoted a rapid decline in reduced glutathione and this response was paralleled by inactivation of hexokinase. Under the same conditions, the activity of the enzymes glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase did not show appreciablevariations in intact cells, but was severely inhibited in the cell-free system. Similar results were obtained by replacing ascorbic acid/FeCl3 with dehydroascorbic acid. In addition, both treatments effectively inhibited the activity of purified hexokinase as well as those of glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. Further studies using the cell-free system indicated that the inhibition of enzyme activities elicited by either of the two treatments was effectively counteracted by the specific substrates of these enzymes. The fact that the hexokinase substrate glucose freely permeates the plasma membrane, unlike the substrates of glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphatedehydrogenase, explains the selective inhibition of hexokinase observed in intact cells. The above results also indicate that dehydroascorbic acid is an inhibitor of these enzymes and strongly suggest that it is at least in part responsible for the effects mediated by the cocktail ascorbic acid/FeCl3.

Hexokinase inactivation induced by ascorbic acid/Fe(II) in rabbit erythrocytes is independent of glutathione-reductive processes and appears to be mediated by dehydroascorbic acid

Recent studies performed in our laboratory demonstrated that rabbit red blood cell hexokinase was remarkably inhibited by the cocktail ascorbic acid/Fe(II) (Stocchi et al., 1994, Arch. Biochem. Biophys. 311, 160-167) and that the formation of dehydroascorbic acid was a key event in this process (Fiorani et al., 1996, Arch. Biochem. Biophys, 334, 357-361). The present study was undertaken to determine the final hexokinase-inactivating species using cell-free extract as a model. Our results demonstrate superimposable kinetics of hexokinase decay promoted by either ascorbic acid/Fe(II) or dehydroascorbic acid in erythrocyte lysates in which the reduced glutathione (GSH) levels were variously manipulated. In particular, neither removal nor addition of this tripeptide was able to significantly alter the rate or extent of hexokinase inhibition. Thus, GSH-reductive processes are dispensable events in the process of hexokinase inhibition promoted by ascorbic acid/Fe(II) in red blood cells. As a consequence, dehydroascorbic acid appears to be the species which directly inhibits hexokinase. This inference is further supported by the observation that addition of dehydroascorbic acid to the purified enzyme leads to a remarkable inhibition in its activity.

Role of dehydroascorbate in rabbit erythrocyte hexokinase inactivation induced by ascorbic acid/Fe(II)

In this study we investigated the species involved in the process of hexokinase inactivation induced by ascorbic acid/Fe(II) in rabbit erythrocytes. Our results suggest a model in which divalent iron is first oxidized to the trivalent state and then triggers the oxidation of ascorbic acid. The H202 formed during this process accelerates the formation of dehydroascorbic acid, which appears to be necessary and sufficient to induce hexokinase inactivation. This model was validated by showing that: (a) H202-decomposing enzymes, unlike scavengers of the hydroxyl radicals, reduced the extent of hexokinase inactivation; (b) when H202 was used instead of ascorbate/Fe(II), it was unable, even at very high concentrations, to inhibit hexokinase activity; (c) replacing Fe(II) with either Fe(III) or H202 resulted in comparable levels of ascorbic acid-induced hexokinase inactivation; (d) expression of maximal hexokinase inhibiting activity was also triggered via enzyme-catalyzed oxidation of ascorbic acid or direct addition of dehydroascorbic acid; (e) the level of dehydroascorbic acid, which was actively generated in the external medium upon addition of ascorbic acid/Fe(II), increased as a function of time. Taken together, these results demonstrate that the process of hexokinase inactivation induced by ascorbic acid/Fe(II) is mediated by dehydroascorbate and that iron and H202 have the sole function of accelerating its formation.

The phosphorylation state of MAP-kinases modulates the cytotoxic response of smooth muscle cells to hydrogen peroxide

Micromolar concentrations of hydrogen peroxide induced the phosphorylation of mitogen-activated protein (MAP) kinases and a lethal response in growth-arrested smooth muscle cells (A7r5). The H202-induced phosphorylation of MAP-kinases was markedly lower in the presence of protein tyrosine kinase (PTK) inhibitors or in protein kinase C (PKC) down-regulated cells. Similarly, the toxicity of H202 was diminished by concomitant addition of either PKC or PTK inhibitors and was also lower in PKC down-regulated cells. These results are consistent with the possibility that phosphorylation of MAP-kinases is a critical event in the toxic response of cultured smooth muscle cells to H202.

Effect of 50 Hz sinusoidal electric and/or magnetic fields on the rate of repair of DNA single strand breaks in cultured mammalian cells exposed to three different carcinogens: methylmethane sulphonate, chromate and 254 nm U.V. radiation

Treatment of cultured mammalian cells with three different carcinogens, namely methylmethane sulphonate (MMS), chromate and 254 U.V. radiation, produces DNA single strand breaks (SSB) in cultured mammalian cells. The rate of removal of these lesions is not affected by exposure to 50 Hz electric (0.2 - 20 kV/m), magnetic (0.0002-0.2 mT), or combined electric and magnetic fields. These results indicate that, under the experimental conditions utilized in this study, 50 Hz electric, magnetic and electromagnetic fields (over a wide range of intensities) do not affect the machinery involved in the repair of DNA SSBs generated by different carcinogens in three different cultured mammalian cell lines, making it unlikely that field exposure enhances the ability of these carcinogens to induce transformation via inhibition of DNA repair.

Purification and characterization of the carboxyl-domain of human hexokinase type III expressed as fusion protein

In mammalian tissues hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) exists as four isoenzymes encoded by distinct genes. These proteins are homologous and are organized in two homologous domains, with the exception of hexokinase type IV which has only one. This organization is believed to be the result of a duplication and tandem fusion event involving the gene encoding for the ancestral hexokinase. In this study, we cloned the carboxyl-domain of human hexokinase type III and expressed it in Escherichia coli as a glutathione S-transferase fusion protein, using the pGEX-2T expression vector. The recombinant protein showed catalytic activity. A comparative study of the kinetic properties of the expressed carboxyl-domain and the enzyme partially purified from human lymphocytes is also shown. The results now allow a better understanding of the role of the carboxyl-domain in determining the catalytic properties of the enzyme.

Rabbit brain hexokinase: evidence for the presence of two distinct molecular forms

In mammalian brain tissue, most hexokinase is bound to the mitochondria and only a small amount of the enzyme is present in soluble form. In this study we report that, in rabbit brain, hexokinase is present in two distinct molecular forms, which we designated HKH and HKL, both of which are separable using hydrophobic interaction or anion-exchange chromatography. These two molecular forms can be detected when hexokinase is prepared at pH 7.4, whereas at pH 10.0 only the more hydrophobic form, HKH, is present. The two subtypes of hexokinase do not show significant differences in Km values for glucose and ATP, in Ki values for glucose-6-phosphate or in their molecular weights. HKH is able to rebind mitochondrial membranes, while HKL has lost this ability, suggesting that the hydrophobic peptide at the N-terminal has been removed. The susceptibility of the N-terminal peptide to proteolysis is completely inhibited by using antiproteolytic compounds, such as leupeptin or E-64. The results reported in this paper suggest that a cysteine protease, probably belonging to a the class of cathepsins, may be involved in the processing of bindable hexokinase to the non-bindable form in rabbit brain, and that the activity of this protease is pH-dependent.

The effect of 50 Hz sinusoidal electric and/or magnetic fields on the rate of repair of DNA single/double strand breaks in oxidatively injured cells

Exposure of cultured mammalian cells to 50 Hz electric (0.2-20 kV/m), magnetic (0.002-2 G), or combined electric and magnetic fields did not affect the rate of repair of DNA single strand breaks (SSB) induced by hydrogen peroxide. The same lack of effect was observed on the repair of both DNA SSBs and DNA double strand breaks (DSBs) in cells treated with the cocktail hydrogen peroxide/L-Histidine. These results indicate that exposure to electric and/or magnetic fields does not affect the machinery involved in the repair of DNA lesions in oxidatively injured cells thus suggesting that it is unlikely that field exposure might induce changes in the response of the cells to the tumor promoting or carcinogenic effects elicited by reactive oxygen species.

Hydrogen peroxide-and fetal bovine serum-induced DNA synthesis in vascular smooth muscle cells: positive and negative regulation by protein kinase C isoforms

Hydrogen peroxide and fetal bovine serum stimulate DNA synthesis in growth-arrested smooth muscle cells with remarkably similar kinetics and cell density dependence. However, while stimulation with fetal bovine serum results in cell proliferation, that by H2O2 is followed by cell death. Depletion of conventional and novel protein kinase C isoforms, resulting from a long treatment with phorbol-12-myristate-13-acetate, further increases H2O2-induced DNA synthesis. On the other hand, the specific protein kinase C inhibitor calphostin C abolished the increased DNA synthesis promoted by fetal bovine serum or H2O2. H2O2 increases protein kinase C activity in smooth muscle cells. This effect is markedly reduced, but not abolished, by down-regulation of the alpha, delta and epsilon protein kinase C isoforms. Thus, the zeta isoform of protein kinase C, which is not down-regulated, may be responsible for the residual H2O2 stimulation of protein kinase C. In conclusion, the results obtained show that H2O2 stimulates protein kinase C activity and DNA synthesis in growth-arrested smooth muscle cells: these events are not followed by cell proliferation but rather by cell death. This H2O2 stimulated DNA synthesis appears to be negatively controlled by alpha, delta and epsilon isoforms and positively controlled by the zeta isoform of protein kinase C.

High-performance capillary electrophoretic separation of proteins and peptides using a bonded hydrophilic phase capillary

High performance capillary electrophoresis (HPCE) was applied to the separation of protein and peptide mixtures with molecular masses ranging from 1300 to 96000 Da using a new bonded hydrophilic phase capillary, CElect-P150. This coated capillary reduces the interaction between proteins and silanol groups in capillary walls, allowing a complete recovery of the proteins and peptides of interest. HPCE was also used for the analysis of a complex mixture of tryptic fragments and to monitor the process of enzymatic digestion. Moreover, using a CElect-P150 capillary, highly reproducible analysis was possible without preconditioning the capillary with acid or basic solutions before each new analysis.

Mitochondria-bound hexokinase from rabbit reticulocytes is resistant to the inactivation induced by Fe(II)/ascorbate

Exposure of rabbit reticulocytes to Fe(II)/ascorbate induced a pronounced decay in hexokinase activity. In reticulocytes, this enzyme is present in at least three different molecular forms, Ia, Ia* and Ib, sub-types of hexokinase type I, which show different intracellular distribution. Hexokinase Ia and Ib are soluble, whereas hexokinase Ia* is almost entirely bound to the mitochondria. Anion exchange chromatography of hexokinase from intact reticulocytes exposed to Fe(II)/ascorbate revealed a selective inactivation of forms Ia and Ib, whereas the form Ia* did not show any decay. Binding to the mitochondrial membrane seems to be responsible for the observed resistance of the form Ia* to the inactivation elicited by Fe(II)/ascorbate. Indeed, by using a cell-free system in which hexokinase Ia* was solubilized using Triton X-100, the decay in hexokinase activity induced by iron/ascorbate involved all three enzymatic forms.

Simultaneous high-performance capillary electrophoretic determination of reduced and oxidized glutathione in red blood cells in the femtomole range

This paper describes a high-performance capillary electrophoretic (HPCE) method which allows a quick, simultaneous and quantitative determination of reduced (GSH) and oxidized (GSSG) glutathione in mammalian red blood cells using a Supelco-bonded hydrophilic phase capillary CElect-P150. The extraction procedure of GSH and GSSG from erythrocytes using Microcon-10 membranes is very simple and allows a correct evaluation of these compounds present in the red blood cells. Furthermore, the HPCE method does not require removal of the excess N-ethylmaleimide used to block the glutathione in its reduced state, making the simultaneous evaluation of GSH and GSSG possible in a very short time (ca. 4 min), with a sensitivity at femtomole level.

Inactivation of rabbit red blood cell hexokinase activity promoted in vitro by an oxygen-radical-generating system

Rabbit red blood cell hexokinase (EC 2.7.1.1) has been shown to be inactivated in vitro by incubating intact erythrocytes in the presence of an oxygen-radical-generating system represented by ascorbate and iron. It was interesting to note that among the glycolytic enzymes, only hexokinase was found to be susceptible to the action of oxygen radicals, suggesting that the loss of activity of this enzyme may be one of the first signals of cellular damage in rabbit red blood cells. This statement is supported by the fact that, under the experimental conditions used, we did not observe any significant plasma membrane lipid peroxidation nor intracellular proteolysis. Furthermore, mature erythrocytes are unable to synthesize hexokinase as well as other proteins de novo; therefore, the inactivation of this enzyme, which is both the first and one of the rate-limiting enzymes of the glycolytic pathway, could play an important role in determining metabolic impairment of red blood cells, with possible physiological implications. We also investigated the effect of various radical scavengers and antioxidants (glucose, vitamin E, dithiothreitol, flavonoids) which are able to influence the inactivation of hexokinase activity to different extents. Finally, under the experimental conditions used (90 min of incubation at 37 degrees C), we did not observe any difference in the hemolysis of rabbit red blood cells incubated in the presence or absence of ascorbate and iron (hemolysis was about 1% after 90 min of incubation), suggesting that the system used was able to furnish information about the cellular damage produced by oxygen radicals without provoking cell lysis.

Cell density-dependent regulation of ATP levels during the growth cycle of cultured Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells show a cell density-dependent modification of ATP levels during the growth cycle. Cells were seeded at a density of 500,000 cells/75 cm2 flask in 10 ml of growth medium and at various time intervals, samples were taken and assayed for cell number, for adenine (ATP, ADP, AMP) and pyridine (NAD+, NADP+) nucleotide levels and for the activity of some glycolytic enzymes. Glucose consumption was also evaluated. Experimental results indicated that the rate of cell growth was exponential for up to the 4th day of culture after which the cell number remained pratically unchanged up to the 9th day. Under these experimental conditions we found that, whereas the intracellular levels of NAD+ and some glycolytic enzymes were not significantly affected, a drop in ATP content was apparent after 48 hr of culture. The decline in ATP levels progressively increased, reaching a maximum after 4 days of culture, and then remained unchanged. In order to evaluate whether this effect on ATP was determined by a reduced availability of nutritional factors or was really a function of cell density, we also performed experiments similar to those reported above, with the exception that the cells were grown in 40 ml of culture medium. Under these experimental conditions, the exponential growth was longer (in comparison with the cell growth in 10 ml of medium) and a plateau was reached after 6 days of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Cortical afferents of visual area MT in the Cebus monkey: possible homologies between New and Old World monkeys

Cortical projections to the middle temporal (MT) visual area were studied by injecting the retrogradely transported fluorescent tracer Fast Blue into MT in adult New World monkeys (Cebus apella). Injection sites were selected based on electrophysiological recordings, and covered eccentricities from 2-70 deg, in both the upper and lower visual fields. The position and laminar distribution of labeled cell bodies were correlated with myeloarchitectonic boundaries and displayed in flat reconstructions of the neocortex. Topographically organized projections were found to arise mainly from the primary, second, third, and fourth visual areas (V1, V2, V3, and V4). Coarsely topographic patterns were observed in transitional V4 (V4t), in the parieto-occipital and parieto-occipital medial areas (PO and POm), and in the temporal ventral posterior area (TVP). In addition, widespread or nontopographic label was found in visual areas of the superior temporal sulcus (medial superior temporal, MST, and fundus of superior temporal, FST), annectent gyrus (dorsointermediate area, DI; and dorsomedial area, DM), intraparietal sulcus (lateral intraparietal, LIP; posterior intraparietal, PIP; and ventral intraparietal, VIP), and in the frontal eye field (FEF). Label in PO, POm, and PIP was found only after injections in the representation of the peripheral visual field (> 10 deg), and label in V4 and FST was more extensive after injections in the central representation. The projections from V1 and V2 originated predominantly from neurons in supragranular layers, whereas those from V3, V4t, DM, DI, POm, and FEF consisted of intermixed patches with either supragranular or infragranular predominance. All of the other projections were predominantly infragranular. Invasion of area MST by the injection site led to the labeling of further pathways, including substantial projections from the dorsal prelunate area (DP) and from an ensemble of areas located along the medial wall of the hemisphere. In addition, weaker projections were observed from the parieto-occipital dorsal area (POd), area 7a, area prostriata, the posterior bank of the arcuate sulcus, and areas in the anterior part of the lateral sulcus. Despite the different nomenclatures and areal boundaries recognized by different models of simian cortical organization, the pattern of projections to area MT is remarkably similar among primates. Our results provide evidence for the existence of many homologous areas in the extrastriate visual cortex of New and Old World monkeys.

Induction/repair of strand breakage in mature and nascent DNA of cultured Chinese hamster ovary cells exposed to hydrogen peroxide

Treatment of cultured mammalian cells with hydrogen peroxide results in the production of extensive DNA damage. Strand breakage was produced at the level of either nascent or mature DNA and the former target appeared slightly more resistant than the latter. Although inhibitors of the enzyme poly(ADP-ribose) polymerase similarly retarded the repair of such lesions, removal of DNA strand breaks was much slower for the newly synthesized DNA as compared to mature DNA.

Electric and/or magnetic field effects on DNA structure and function in cultured human cells

Exposure of cultured K562 cells to 50 Hz electric (0.2-20 kV/m), magnetic (0.002-2 G), or combined electric and magnetic fields for up to 24 h did not result in the production of detectable DNA lesions, as assayed by the filter elution technique. The rate of cell growth was also unaffected as well as the intracellular ATP and NAD+ levels. These results indicate that, under the experimental conditions utilized in this study, 50 Hz electric, magnetic and electromagnetic fields are not geno- and cyto-toxic in cultured mammalian cells.

Laminar, columnar and topographic aspects of ocular dominance in the primary visual cortex of Cebus monkeys

The representation of the two eyes in striate cortex (V1) of Cebus monkeys was studied by electrophysiological single-unit recordings in normal animals and by morphometric analysis of the pattern of ocular dominance (OD) stripes, as revealed by cytochrome oxidase histochemistry in V1 flat-mounts of enucleated animals. Single-unit recordings revealed that the large majority of V1 neurons respond to the stimulation of either eye but are more strongly activated by one of them. As in other species of monkey, neurons with preference for the stimulation of the same eye are grouped in columns 300-400 microns wide, spanning all cortical layers. Monocular neurons are clustered in layer IVc, specially in its deeper half (IVc-beta), and constitute less than 10% of the population of other layers. Neurons with equal responses to each eye are more commonly found in layer V than elsewhere in V1. In the supragranular layers and in granular layer IVc-alpha neurons strongly dominated by one of the eyes tend to be broadly tuned for orientation, while binocularly balanced neurons tend to be sharply tuned for this parameter. No such correlation was detected in the infragranular layers, and most neurons in layer IVc-beta responded regardless of stimulus orientation. Ocular dominance stripes are present throughout most of V1 as long, parallel or bifurcating bands alternately dominated by the ipsi- or the contralateral eye. They are absent from the cortical representations of the blind spot and the monocular crescent. The domains of each eye occupy nearly equal portions of the surface of binocular V1, except for the representation of the periphery, where the contralateral eye has a larger domain, and a narrow strip along the border of V1 with V2, where either eye may predominate. The orderliness of the pattern of stripes and the relationship between stripe arrangement and the representation of the visual meridians vary with eccentricity and polar angle but follow the same rules in different animals. These results demonstrate that the laminar, columnar and topographic distribution of neurons with different degrees of OD in V1 is qualitatively similar in New- and Old World monkeys of similar sizes and suggest that common ancestry, rather than parallel evolution, may account for the OD phenotypes of contemporaneous simians.

[Transitory myocardial ischemia appearing after cranial trauma. Description of a case]

The authors describe a case of ECG alterations, indicating myocardial ischaemia, in a patient affected by mild cranial trauma. Both cycloergometric test and angiography of coronary vessels were normal. The authors conclude that every patient affected by brain injury should be monitored by the Cardiological Department, because of the high frequency of myocardial damage and lethal arrhythmias in these patients.

Inhibition of Chinese hamster ovary cell DNA synthesis by hydrogen peroxide

The DNA synthesis inhibitory effect of hydrogen peroxide has been examined under a number of experimental conditions. Results have indicated that the effect of the oxidant is more pronounced when the treatment is performed at 37 degrees C than at 4 degrees C and in low density as compared to high density cultures. In addition, similar levels of inhibition were achieved by measuring the incorporation of radiolabelled thymidine in the presence of, or following treatment with, the oxidant. Although early events seem to be responsible for the decreased rate of DNA synthesis, it would appear that hydrogen peroxide does not alter thymidine extracellularly and/or decrease the transport of the nucleoside across the plasma membrane, which may actually be slightly augmented. Thus, the previously illustrated results may represent an underestimate of the actual capacity of the oxidant to reduce DNA synthesis. This inference is further supported by the fact that the effect of hydrogen peroxide appears markedly enhanced in cells preloaded with the radiolabelled precursor. A temporal relationship seems to exist between the steady state level of DNA single strand breaks and the extent of DNA synthesis inhibition by hydrogen peroxide. The oxidant has no effect on DNA chain elongation. In conclusion, data presented in this paper suggest that early events, involving selective effects on replicon initiation, mediate the DNA synthesis inhibitory effect of hydrogen peroxide.