Renal carbonic anhydrases are involved in the reabsorption of endogenous nitrite

Acetazolamide and human carbonic anhydrases: retrospect, review and discussion of an intimate relationship

Acetazolamide (AZM) is a strong pharmacological sulphonamide-type (R-SO2-NH2, pKa 7.2) inhibitor of the activity of several carbonic anhydrase (CA) isoforms, notably of renal CA II (Ki, 12 nM) and CA IV (Ki, 74 nM). AZM is clinically used for about eighty years in various diseases including epilepsy and glaucoma. Pharmacological AZM increases temporarily the urinary excretion of bicarbonate (HCO3-) and sodium ions (Na+) and sustainably the urinary pH. AZM is excreted almost unchanged over several hours at high rates in the urine. Closely parallel concentrations of circulating and excretory AZM are observed upon administration of therapeutical doses of AZM. In a proof-of-principle study, we investigated the effects of the ingestion of a 250-mg AZM-containing tablet by a healthy volunteer on the urinary excretion of organic and inorganic substances over 5 h (range, 0, 0.5, 1, 1.5, 2, 3, 4, 5 h). Measured analytes included: AZM, amino acids and their metabolites such as guanidinoacetate, i.e. the precursor of creatine, of asymmetrically (ADMA) and symmetrically (SDMA) dimethylated arginine, nitrite (O = N-O-, pKa 3.4) and nitrate (O2N-O-, pKa -1.37), the major metabolites of nitric oxide (NO), the C-H acidic malondialdehyde (MDA; (CHO)2CH2, pKa 4.5), and creatinine for correction of analytes excretion. All analytes were measured by validated isotopologues using gas chromatography-mass spectrometry (GC-MS) methods. AZM excretion in the urine reached its maximum value after 2 h and was fairly stable for the next 3 h. Time series analysis by the ARIMA method was performed. AZM ingestion increased temporarily the urinary excretion of the amino acids Leu + Ile, nitrite and nitrate, decreased temporarily the urinary excretion of other amino acids. AZM decreased sustainably the urinary excretion of MDA, a biomarker of oxidative stress (i.e. lipid peroxidation). Whether this decrease is due to inhibition of the excretion of MDA or attenuation of oxidative stress by AZM is unknown. The acute and chronic effects of AZM on the urinary excretion of electrolytes and physiological substances reported in the literature are discussed in depth in the light of its extraordinary pharmacokinetics and pharmacodynamics. Tolerance development/drug resistance to AZM in chronic use and potential mechanisms are also addressed.

GC-MS and GC-MS/MS measurement of malondialdehyde (MDA) in clinical studies: Pre-analytical and clinical considerations

Malondialdehyde (MDA; 1,3-propanedial, OHC-CH2-CHO) is one of the most frequently measured biomarkers of oxidative stress in plasma and serum. L-Arginine (Arg) is the substrate of nitric oxide synthases (NOS), which convert L-arginine to nitric oxide (NO) and L-citrulline. The Arg/NO pathway comprises several members, including the endogenous NOS-activity inhibitor asymmetric dimethylarginine (ADMA) and its major metabolite dimethyl amine (DMA), and nitrite and nitrate, the major NO metabolites. Reliable measurement of MDA and members of the Arg/NO pathway in plasma, serum, urine and in other biological samples, such as saliva and cerebrospinal fluid, is highly challenging both for analytical and pre-analytical reasons. In our group, we use validated gas chromatography-mass spectrometry (GC-MS) and gas chromatography-tandem mass spectrometry (GC-MS/MS) methods for the quantitative determination in clinical studies of MDA as a biomarker of oxidative stress, and various Arg/NO metabolites that describe the status of this pathway. Here, the importance of pre-analytical issues, which has emerged from the use of GC-MS and GC-MS/MS in clinico-pharmacological studies, is discussed. Paradigmatically, two studies on the long-term oral administration of L-arginine dihydrochloride to patients suffering from peripheral arterial occlusive disease (PAOD) or coronary artery disease (CAD) were considered. Pre-analytical issues that were addressed include blood sampling, plasma or serum storage, study design (notably in long-term studies), and the alternative of measuring MDA in human urine.

Circulating and Urinary Concentrations of Malondialdehyde in Aging Humans in Health and Disease: Review and Discussion

(1) Background: Malondialdehyde (MDA) is a major and stable product of oxidative stress. MDA circulates in the blood and is excreted in the urine in its free and conjugated forms, notably with L-lysine and L-serine. MDA is the most frequently measured biomarker of oxidative stress, namely lipid peroxidation. Oxidative stress is generally assumed to be associated with disease and to increase with age. Here, we review and discuss the literature concerning circulating and excretory MDA as a biomarker of lipid peroxidation in aging subjects with regard to health and disease, such as kidney disease, erectile dysfunction, and COVID-19. (2) Methods: Scientific articles, notably those reporting on circulating (plasma, serum) and urinary MDA, which concern health and disease, and which appeared in PubMed were considered; they formed the basis for evaluating the potential increase in oxidative stress, particularly lipid peroxidation, as humans age. (3) Results and Conclusions: The results reported in the literature thus far are contradictory. The articles considered in the present study are not supportive of the general view that oxidative stress increases with aging. Many functions of several organs, including the filtration efficiency of the kidneys, are physiologically reduced in men and women as they age. This effect is likely to result in the apparent "accumulation" of biomarkers of oxidative stress, concomitantly with the "accumulation" of biomarkers of an organ's function, such as creatinine. How free and conjugated MDA forms are transported in various organs (including the brain) and how they are excreted in the urine via the kidney is not known, and investigating these questions should be the objective of forthcoming studies. The age- and gender-related increase in circulating creatinine might be a useful factor to be taken into consideration when investigating oxidative stress and aging.

The effect of acetazolamide on the retinal and choroidal vasculature of the macula and the optic disc using OCT angiography

INTRODUCTION AND OBJECTIVES

To investigate the effect of acetazolamide (AZ) on the retinal and choroidal ocular microvasculature in the macula and radial peripapillary capillaries (RPC) of the optic disc with OCT Angiography (OCTA).

MATERIALS AND METHODS

Nine-month observational cross-sectional study. Forty-five eyes from 45 healthy participants who underwent cataract surgery were recruited. Macular retina and choriocapillaris vessel density (VD) and RPC VD in the optic disc area were compared before and 60min after 250mg acetazolamide per os. Intraocular pressure (IOP) and systemic blood pressure (BP) were also measured before each scan.

RESULTS

Mean age was 73.1±6.9 years. VDs in the superficial (SCP) and deep (DCP) capillary plexus of the retina and the choriocapillaris (CC) in the macular area showed no significant change (P>.5, for all parameters). VD in the RPC showed no significant change with AZ (P>.5, for all parameters). Foveal and parafoveal thickness increased from 248.98 (±23.89) to 250.33 (±23.74) and from 311.62 (±16.53) to 311.98 (±16.38) (P<.001 and P=.046), respectively. IOP decreased from 13.2 (±3.0) mmHg to 11.8 (±3.2) mmHg (P<.001), while systolic and diastolic BP decreased from 144.8 (±21.8) to 137.7 (±19.0) and from 80.0 (±12.7) to 76.2 (±11.7) (P=.021 and P=.030), respectively.

CONCLUSIONS

OCTA imaging did not reveal any significant changes in the VD of the optic disc or the retinal and choroidal VD in the macula with oral AZ one hour after its administration in otherwise healthy participants who underwent cataract surgery.

GC-MS Analysis of Biological Nitrate and Nitrite Using Pentafluorobenzyl Bromide in Aqueous Acetone: A Dual Role of Carbonate/Bicarbonate as an Enhancer and Inhibitor of Derivatization

Carbon dioxide (CO₂) and carbonates, which are widely distributed in nature, are constituents of inorganic and organic matter and are essential in vegetable and animal organisms. CO₂ is the principal greenhouse gas in the atmosphere. In human blood, CO₂/HCO₃⁻ is an important buffering system. Inorganic nitrate (ONO₂⁻) and nitrite (ONO⁻) are major metabolites and abundant reservoirs of nitric oxide (NO), an endogenous multifunctional signaling molecule. Carbonic anhydrase (CA) is involved in the reabsorption of nitrite and nitrate from the primary urine. The measurement of nitrate and nitrite in biological samples is of particular importance. The derivatization of nitrate and nitrite in biological samples alongside their ¹⁵N-labeled analogs, which serve as internal standards, is a prerequisite for their analysis by gas chromatography–mass spectrometry (GC-MS). A suitable derivatization reagent is pentafluorobenzyl bromide (PFB-Br). Nitrate and nitrite are converted in aqueous acetone to PFB-ONO₂ and PFB-NO₂, respectively. PFB-Br is also useful for the GC-MS analysis of carbonate/bicarbonate. This is of particular importance in conditions of pharmacological CA inhibition, for instance by acetazolamide, which is accompanied by elevated concomitant excretion of nitrate, nitrite and bicarbonate, as well as by urine alkalization. We performed a series of experiments with exogenous bicarbonate (NaHCO₃) added to human urine samples (range, 0 to 100 mM), as well as with endogenous bicarbonate resulting from the inhibition of CA activity in healthy subjects before and after ingestion of pharmacological acetazolamide. Our results indicate that bicarbonate enhances the derivatization of nitrate with PFB-Br. In contrast, bicarbonate decreases the derivatization of nitrite with PFB-Br. Bicarbonate is not a catalyst, but it enhances PFB-ONO₂ formation and inhibits PFB-NO₂ formation in a concentration-dependent manner. The effects of bicarbonate are likely to result from its reaction with PFB-Br to generate PFB-OCOOH. Nitrate reacts with concomitantly produced PFB-OCOOH to form PFB-ONO₂ in addition to the direct reaction of nitrate with PFB-Br. By contrast, nitrite does not react with PFB-OCOOH to form PFB-NO₂. Sample acidification by small volumes of 20 wt.% aqueous acetic acid abolishes the effects of exogenous and endogenous bicarbonate on nitrite measurement.

Nitrite in breast milk: roles in neonatal pathophysiology

Dietary nitrate has beneficial effects on health maintenance and prevention of lifestyle-related diseases in adulthood by serving as an alternative source of nitric oxide (NO) through the enterosalivary nitrate-nitrite-NO pathway, particularly when endogenous NO generation is lacking due to vascular endothelial dysfunction. However, this pathway is not developed in the early postnatal period due to a lack of oral commensal nitrate-reducing bacteria and less saliva production than in adults. To compensate for the decrease in nitrite during this period, colostrum contains the highest amount of nitrite compared with transitional, mature, and even artificial milk, suggesting that colostrum plays an important role in tentatively replenishing nitrite, in addition to involving a nutritional aspect, until the enterosalivary nitrate-nitrite-NO pathway is established. Increasing evidence demonstrates that breast milk rich in nitrite can be effective in the prevention of neonatal infections and gastrointestinal diseases such as infantile hypertrophic pyloric stenosis and necrotizing enterocolitis, suggesting that breastfeeding is advantageous for newborns at risk, given the physiological role of nitrite in the early postnatal period. IMPACT: The aim of this review is to discuss the physiological roles of nitrite in breast milk and its implications for neonates. Nitrite in breast milk may compensate for the decrease in nitrite during the early neonatal period until the enterosalivary nitrate-nitrite-nitric oxide pathway is established. Breast milk rich in nitrite may be effective in the prevention of neonatal infections and gastrointestinal diseases by providing nitric oxide bioavailability.

Nitrous anhydrase activity of carbonic anhydrase II: cysteine is required for nitric oxide (NO) dependent phosphorylation of VASP in human platelets

The carbonic anhydrase (CA) family does not only catalyse the reversible hydration of CO₂ to bicarbonate, but it also possesses esterase and phosphatase activity. Recently, bovine CA II and human CA II have been reported to convert inorganic nitrite (O=N-O⁻) to nitric oxide (NO) and nitrous anhydride (N₂O₃). Given the ability of NO to mediate vasodilation and inhibit platelet aggregation, this CA II activity would represent a bioactivation of nitrite. There are contradictory reports in the literature and the physiological role of CA II nitrite bioactivation is still disputed. Here, we provide new experimental data in support of the nitrous anhydrase activity of CA II and the key role L-cysteine in the bioactivation of nitrite by CA II. Using washed human platelets and by measuring VASP phosphorylation we provide evidence that exogenous nitrite (10 µM) is bioactivated to NO in a manner strongly depending on L-cysteine (100 and 200 µM). The process is not inhibitable by acetazolamide, a potent CA inhibitor. The contradictory results of recently published studies in this area are thoroughly discussed.

Nitric oxide signalling in kidney regulation and cardiometabolic health

The prevalence of cardiovascular and metabolic disease coupled with kidney dysfunction is increasing worldwide. This triad of disorders is associated with considerable morbidity and mortality as well as a substantial economic burden. Further understanding of the underlying pathophysiological mechanisms is important to develop novel preventive or therapeutic approaches. Among the proposed mechanisms, compromised nitric oxide (NO) bioactivity associated with oxidative stress is considered to be important. NO is a short-lived diatomic signalling molecule that exerts numerous effects on the kidneys, heart and vasculature as well as on peripheral metabolically active organs. The enzymatic L-arginine-dependent NO synthase (NOS) pathway is classically viewed as the main source of endogenous NO formation. However, the function of the NOS system is often compromised in various pathologies including kidney, cardiovascular and metabolic diseases. An alternative pathway, the nitrate-nitrite-NO pathway, enables endogenous or dietary-derived inorganic nitrate and nitrite to be recycled via serial reduction to form bioactive nitrogen species, including NO, independent of the NOS system. Signalling via these nitrogen species is linked with cGMP-dependent and independent mechanisms. Novel approaches to restoring NO homeostasis during NOS deficiency and oxidative stress have potential therapeutic applications in kidney, cardiovascular and metabolic disorders.

Evidence for placental-derived iron-nitrosyls in the circulation of the fetal lamb and against a role for nitrite in mediating the cardiovascular transition at birth

Circulating metabolites of nitric oxide, such as nitrite, iron nitrosyls (FeNO), and nitrosothiols, have vasodilatory bioactivity. In both human and sheep neonates, plasma concentrations of these NO metabolite (NOx) concentrations fall >50% within minutes after birth, raising the possibility that circulating NOx plays a role in maintaining low fetal vascular resistance and in the cardiovascular transition at birth. To test whether the fall in plasma NOx concentrations at birth is due to either ligation of the umbilical cord or oxygenation of the fetus to newborn levels, plasma NOx concentrations were measured during stepwise delivery of near-term fetal lambs. When fetal lambs were intubated and mechanically ventilated with 100% O₂ to oxygenate the arterial blood while still in utero with the umbilical circulation still intact, there was no change in plasma NOx levels. In contrast, when the umbilical cord was ligated while fetal lambs were mechanically ventilated with O₂ levels that maintained fetal arterial blood gases, plasma NOx levels decreased by nearly 50%. Characterization of the individual NOx species in plasma revealed that the overall fall in NOx at birth was attributable mainly to FeNO compounds. Finally, when the typical fall in NOx after birth was prevented by intravenous nitrite infusion, birth-related changes in blood pressure, heart rate, and carotid flow changes were little affected, suggesting the cardiovascular transition at birth is not dependent on a fall in plasma NOx. In conclusion, this study shows FeNO is released from the placenta and that its decline accounts for most of the measured fall in plasma NOx at birth.

Carbonic anhydrase II does not regulate nitrite-dependent nitric oxide formation and vasodilation

BACKGROUND AND PURPOSE

Although it has been reported that bovine carbonic anhydrase CAII is capable of generating NO from nitrite, the function and mechanism of CAII in nitrite-dependent NO formation and vascular responses remain controversial. We tested the hypothesis that CAII catalyses NO formation from nitrite and contributes to nitrite-dependent inhibition of platelet activation and vasodilation.

EXPERIMENT APPROACH

The role of CAII in enzymatic NO generation was investigated by measuring NO formation from the reaction of isolated human and bovine CAII with nitrite using NO photolysis-chemiluminescence. A CAII-deficient mouse model was used to determine the role of CAII in red blood cell mediated nitrite reduction and vasodilation.

KEY RESULTS

We found that the commercially available purified bovine CAII exhibited limited and non-enzymatic NO-generating reactivity in the presence of nitrite with or without addition of the CA inhibitor dorzolamide; the NO formation was eliminated with purification of the enzyme. There was no significant detectable NO production from the reaction of nitrite with recombinant human CAII. Using a CAII-deficient mouse model, there were no measurable changes in nitrite-dependent vasodilation in isolated aorta rings and in vivo in CAII^-/- , CAII^+/- , and wild-type mice. Moreover, deletion of the CAII gene in mice did not block nitrite reduction by red blood cells and the nitrite-NO-dependent inhibition of platelet activation.

CONCLUSION AND IMPLICATIONS

These studies suggest that human, bovine and mouse CAII are not responsible for nitrite-dependent NO formation in red blood cells, aorta, or the systemic circulation.

GC-NICI-MS analysis of acetazolamide and other sulfonamide (R-SO2-NH2) drugs as pentafluorobenzyl derivatives [R-SO2-N(PFB)2] and quantification of pharmacological acetazolamide in human urine

Acetazolamide (molecular mass (MM), 222) belongs to the class of sulfonamides (R-SO₂-NH₂) and is one of the strongest pharmacological inhibitors of carbonic anhydrase activity. Acetazolamide is excreted unchanged in the urine. Here, we report on the development, validation and biomedical application of a stable-isotope dilution GC-MS method for the reliable quantitative determination of acetazolamide in human urine. The method is based on evaporation to dryness of 50 μL urine aliquots, base-catalyzed derivatization of acetazolamide (d₀-AZM) and its internal standard [acetylo-²H₃]acetazolamide (d₃-AZM) in 30 vol% pentafluorobenzyl (PFB) bromide in acetonitrile (60 min, 30 °C), reconstitution in toluene (200 μL) and injection of 1-μL aliquots. The negative-ion chemical ionization (NICI) mass spectra (methane) of the PFB derivatives contained several intense ions including [M]^‒ at m/z 581 for d₀-AZM and m/z 584 for d₃-AZM, suggesting derivatization of their sulfonamide groups to form N,N-dipentafluorobenzyl derivatives (R-SO₂-N(PFB)₂), i.e., d₀-AZM-(PFB)₂ and d₃-AZM-(PFB)₂, respectively. Quantification was performed by selected-ion monitoring of m/z 581 and 83 for d₀-AZM-(PFB)₂ and m/z 584 and 86 for d₃-AZM-(PFB)₂. The limits of detection and quantitation of the method were determined to be 300 fmol (67 pg) and 1 μM of acetazolamide, respectively. Intra- and inter-assay precision and accuracy for acetazolamide in human urine samples in pharmacologically relevant concentration ranges were determined to be 0.3%–4.2% and 95.3%–109%, respectively. The method was applied to measure urinary acetazolamide excretion after ingestion of a 250 mg acetazolamide-containing tablet (Acemit®) by a healthy volunteer. Among other tested sulfonamide drugs, methazolamide (MM, 236) was also found to form a N,N-dipentafluorobenzyl derivative, whereas dorzolamide (MM, 324) was hardly detectable. No GC-MS peaks were obtained from the PFB bromide derivatization of hydrochlorothiazide (MM, 298), xipamide (MM, 355), indapamide and metholazone (MM, 366 each) or brinzolamide (MM, 384). We demonstrate for the first time that sulfonamide drugs can be derivatized with PFB bromide and quantitated by GC-MS. Sulfonamides with MM larger than 236 are likely to be derivatized by PFB bromide but to lack thermal stability.

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The sulfonamide group of acetazolamide was derivatized for the first time with pentafluorobenzyl bromide.

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Other sulfonamides were also derivatized for the first time with pentafluorobenzyl bromide.

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Pentafluorobenzyl derivatives of acetazolamide and other sulfonamides are useful for GC-MS.

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The validated method was used to quantify pharmacological acetazolamide in human urine.

Therapeutic value of stimulating the nitrate-nitrite-nitric oxide pathway to attenuate oxidative stress and restore nitric oxide bioavailability in cardiorenal disease

Cardiovascular disorders including hypertension and associated renal disease are major health problems affecting more than 1.5 billion people worldwide. Apart from nonmodifiable factors such as ageing, family history and gender, both sedentary lifestyle and unhealthy dietary habits are considered as major risk factors. The disorders are interrelated suggesting common pathological pathways. Mechanistically, oxidative stress and compromised function of the nitric oxide synthase (NOS) system leading to endothelial dysfunction and reduction in nitric oxide (NO) bioavailability have been widely implicated and associated with development and progression of disease. New strategies that correct this redox imbalance and increase NO bioactivity may have major clinical implications. The inorganic anions, nitrate and nitrite, are endogenously formed by oxidization of NOS-derived NO, but there are also high amounts of nitrate in our daily diet. In this regard, accumulated evidence over the past two decades demonstrates that these anions can be recycled back to NO and other bioactive nitrogen oxides, thus offering an attractive alternative strategy for therapeutic exploitation. In this review, we describe how dietary stimulation of the nitrate-nitrite-NO pathway affects cardiovascular and renal functions in health and disease via modulation of oxidative stress and NO bioavailability. Clinical studies addressing potential effects on the renal system are still limited, but blood pressure-lowering effects of nitrate supplementation have been demonstrated in healthy and hypertensive subjects as well as in patients with chronic kidney disease. However, larger clinical studies are warranted to reveal whether chronic nitrate treatment can slow-down the progression of cardiorenal disease and associated complications.

Renal Soluble Guanylate Cyclase Is Downregulated in Sunitinib-Induced Hypertension

Background

The tyrosine kinase inhibitor sunitinib causes hypertension associated with reduced nitric oxide (NO) availability, elevated renal vascular resistance, and decreased fractional sodium excretion. We tested whether (1) nitrate supplementation mitigates sunitinib‐induced hypertension and NO contributes less to renal vascular resistance as well as fractional sodium excretion regulation in sunitinib‐treated rats than in controls; and (2) renal soluble guanylate cyclase (sGC) is downregulated and sGC activation lowers arterial pressure in rats with sunitinib‐induced hypertension.

Methods and Results

Arterial pressure responses to nitrate supplementation and the effects of systemic and intrarenal NO synthase (NOS) inhibition on renal hemodynamics and fractional sodium excretion were assessed in sunitinib‐treated rats and controls. Renal NOS and sGC mRNA as well as protein abundances were determined by quantitative polymerase chain reaction and Western blot. The effect of the sGC activator cinaciguat on arterial pressure was investigated in sunitinib‐treated rats. Nitrate supplementation did not mitigate sunitinib‐induced hypertension. Endothelium‐dependent reductions in renal vascular resistance were similar in control and sunitinib‐treated animals without and with systemic NOS inhibition. Selective intrarenal NOS inhibition lowered renal medullary blood flow in control but not in sunitinib‐treated rats without significant effects on fractional sodium excretion. Renal cortical sGC mRNA and sGC α₁‐subunit protein abundance were less in sunitinib‐treated rats than in controls, and cinaciguat effectively lowered arterial pressure by 15‐20 mm Hg in sunitinib‐treated rats.

Conclusions

Renal cortical sGC is downregulated in the presence of intact endothelium‐dependent renal vascular resistance regulation in developing sunitinib‐induced hypertension. This suggests that sGC downregulation occurs outside the renal vasculature, increases renal sodium retention, and contributes to nitrate resistance of sunitinib‐induced hypertension.

Applications of carbonic anhydrases inhibitors in renal and central nervous system diseases

INTRODUCTION

There are tissues and organs, among which kidneys and the central nervous system (CNS), rich in various isoforms of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). Their role is to regulate pH, to provide bicarbonate or H⁺ ions for electrolyte secretion and possibly a metabolic one. Considering these two systems, CA inhibitors are clinically used mainly as diuretics and antiepileptics, but novel applications in the management of drug-induced renal injury, sleep apnea, migraine, lowering intracranial pressure, cognitive impairment, neuropathic pain, and cerebral ischemia have emerged.

AREAS COVERED

The various classes of clinically used/investigational CA inhibitors and their applications in the management of renal and CNS - connected diseases is reviewed. A patent and literature review covering the period 2013-2018 is presented.

EXPERT OPINION

Both kidneys and CNS are rich in many CA isoforms (CAIs), present also in high amounts. Their inhibition and activation has pharmacological applications, already exploited for diuretic and antiepileptic drugs for decades. New applications were demonstrated in the last years for the CAIs in the management of idiopathic intracranial hypertension, cerebral ischemia, neuropathic pain, avoiding the disruption of blood-brain barrier, and prevention/treatment of migraine, and for the activators for cognition enhancement and the possible treatment of posttraumatic shock and phobias.

Mechanisms underlying blood pressure reduction by dietary inorganic nitrate

Nitric oxide (NO) importantly contributes to cardiovascular homeostasis by regulating blood flow and maintaining endothelial integrity. Conversely, reduced NO bioavailability is a central feature during natural ageing and in many cardiovascular disorders, including hypertension. The inorganic anions nitrate and nitrite are endogenously formed after oxidation of NO synthase (NOS)-derived NO and are also present in our daily diet. Knowledge accumulated over the past two decades has demonstrated that these anions can be recycled back to NO and other bioactive nitrogen oxides via serial reductions that involve oral commensal bacteria and various enzymatic systems. Intake of inorganic nitrate, which is predominantly found in green leafy vegetables and beets, has a variety of favourable cardiovascular effects. As hypertension is a major risk factor of morbidity and mortality worldwide, much attention has been paid to the blood pressure reducing effect of inorganic nitrate. Here, we describe how dietary nitrate, via stimulation of the nitrate-nitrite-NO pathway, affects various organ systems and discuss underlying mechanisms that may contribute to the observed blood pressure-lowering effect.

Carbonic anhydrase is not a relevant nitrite reductase or nitrous anhydrase in the lung

KEY POINTS

Carbonic anhydrase (CA) inhibitors such as acetazolamide inhibit hypoxic pulmonary vasoconstriction (HPV) in humans and other mammals, but the mechanism of this action remains unknown. It has been postulated that carbonic anhydrase may act as a nitrous anhydrase in vivo to generate nitric oxide (NO) from nitrite and that this formation is increased in the presence of acetazolamide. Acetazolamide reduces HPV in pigs without evidence of any NO generation, whereas nebulized sodium nitrite reduces HPV by NO formation; however; combined infusion of acetazolamide with sodium nitrite inhalation did not further increase exhaled NO concentration over inhaled nitrite alone in pigs exposed to alveolar hypoxia. We conclude that acetazolamide does not function as either a nitrous anhydrase or a nitrite reductase in the lungs of pigs, and probably other mammals, to explain its vasodilating actions in the pulmonary or systemic circulations.

ABSTRACT

The carbonic anhydrase (CA) inhibitors acetazolamide and its structurally similar analogue methazolamide prevent or reduce hypoxic pulmonary vasoconstriction (HPV) in dogs and humans in vivo, by a mechanism unrelated to CA inhibition. In rodent blood and isolated blood vessels, it has been reported that inhibition of CA leads to increased generation of nitric oxide (NO) from nitrite and vascular relaxation in vitro. We tested the physiological relevance of augmented NO generation by CA from nitrite with acetazolamide in anaesthetized pigs during alveolar hypoxia in vivo. We found that acetazolamide prevents HPV in anaesthetized pigs, as in other mammalian species. A single nebulization of sodium nitrite reduces HPV, but this action wanes in the succeeding 3 h of hypoxia as nitrite is metabolized and excreted. Pulmonary artery pressure reduction and NO formation as measured by exhaled gas concentration from inhaled sodium nitrite were not increased by acetazolamide during alveolar hypoxia. Thus, our data argue against a physiological role of carbonic anhydrase as a nitrous anhydrase or nitrite reductase as a mechanism for its inhibition of HPV in the lung and blood in vivo.

Results, meta-analysis and a first evaluation of UNOxR, the urinary nitrate-to-nitrite molar ratio, as a measure of nitrite reabsorption in experimental and clinical settings

We recently found that renal carbonic anhydrase (CA) is involved in the reabsorption of inorganic nitrite (NO₂^-), an abundant reservoir of nitric oxide (NO) in tissues and cells. Impaired NO synthesis in the endothelium and decreased NO bioavailability in the circulation are considered major contributors to the development and progression of renal and cardiovascular diseases in different conditions including diabetes. Isolated human and bovine erythrocytic CAII and CAIV can convert nitrite to nitrous acid (HONO) and its anhydride N₂O₃ which, in the presence of thiols (RSH), are further converted to S-nitrosothiols (RSNO) and NO. Thus, CA may be responsible both for the homeostasis of nitrite and for its bioactivation to RSNO/NO. We hypothesized that enhanced excretion of nitrite in the urine may contribute to NO-related dysfunctions in the renal and cardiovascular systems, and proposed the urinary nitrate-to-nitrite molar ratio, i.e., U_NOxR, as a measure of renal CA-dependent excretion of nitrite. Based on results from clinical and experimental animal studies, here, we report on a first evaluation of U_NOxR. We determined U_NOxR values in preterm neonates, healthy children, and adults, in children suffering from type 1 diabetes mellitus (T1DM) or Duchenne muscular dystrophy (DMD), in elderly subjects suffering from chronic rheumatic diseases, type 2 diabetes mellitus (T2DM), coronary artery disease (CAD), or peripheral arterial occlusive disease (PAOD). We also determined U_NOxR values in healthy young men who ingested isosorbide dinitrate (ISDN), pentaerythrityl tetranitrate (PETN), or inorganic nitrate. In addition, we tested the utility of U_NOxR in two animal models, i.e., the LEW.1AR1-iddm rat, an animal model of human T1DM, and the APOE*3-Leiden.CETP mice, a model of human dyslipidemia. Mean U_NOxR values were lower in adult patients with rheumatic diseases (187) and in T2DM patients of the DALI study (74) as compared to healthy elderly adults (660) and healthy young men (1500). The intra- and inter-variabilities of U_NOxR were of the order of 50% in young and elderly healthy subjects. U_NOxR values were lower in black compared to white boys (314 vs. 483, P = 0.007), which is in line with reported lower NO bioavailability in black ethnicity. Mean U_NOxR values were lower in DMD (424) compared to healthy (730) children, but they were higher in T1DM children (1192). ISDN (3 × 30 mg) decreased stronger U_NOxR compared to PETN (3 × 80 mg) after 1 day (P = 0.046) and after 5 days (P = 0.0016) of oral administration of therapeutically equivalent doses. In healthy young men who ingested NaNO₃ (0.1 mmol/kg/d), U_NOxR was higher than in those who ingested the same dose of NaCl (1709 vs. 369). In LEW.1AR1-iddm rats, mean U_NOxR values were lower than in healthy rats (198 vs. 308) and comparable to those in APOE*3-Leiden.CETP mice (151).

GC-MS determination of nitrous anhydrase activity of bovine and human carbonic anhydrase II and IV

The most widely recognized activity of the large family of the metalloenzyme carbonic anhydrases (CAs) is the diffusion-controlled hydration of CO₂ to HCO₃^- and one proton, and the less rapid dehydration of HCO₃^- to CO₂: CO₂ + H₂O ⇆ HCO₃^- + H⁺. CAs also catalyze the reaction of water with other electrophiles such as aromatic esters, sulfates and phosphates, thus contributing to lending to CAs esterase, sulfatase and phosphatase activity, respectively. Renal CAII and CAIV are involved in the reabsorption of nitrite, the autoxidation product of the signalling molecule nitric oxide (NO): 4 NO + O₂ + 2 H₂O → 4 ONO^- + 4 H⁺. Bovine and human CAII and CAIV have been reported to exert nitrite reductase and nitrous anhydride activity: 2 NO₂^- + 2 H⁺ ⇆ [2 HONO] ⇆ N₂O₃ + H₂O. In the presence of L-cysteine, NO may be formed. In the literature, these issues are controversial, mainly due to analytical shortcomings, i.e., the inability to detect authentic HONO and N₂O₃. Here, we present a gas chromatography-mass spectrometry (GC-MS) assay to unambiguously detect and quantify the nitrous anhydrase activity of CAs. The assay is based on the hydrolysis of N₂O₃ in H₂¹⁸O to form ON¹⁸O^- and ¹⁸ON¹⁸O^-. After pentafluorobenzyl bromide derivatization and electron capture negative-ion chemical ionization of the pentafluorobenzyl nitro derivatives, quantification is performed by selected-ion monitoring of the anions with mass-to-charge (m/z) ratios of 46 (ONO^-), m/z 48 (ON¹⁸O^- and ¹⁸ONO^-), m/z 50 (¹⁸ON¹⁸O^-) and m/z 47 (O¹⁵NO^-, internal standard).

Acetazolamide Reduces Blood Pressure and Sleep-Disordered Breathing in Patients With Hypertension and Obstructive Sleep Apnea: A Randomized Controlled Trial

STUDY OBJECTIVES

The carbonic anhydrase inhibitor acetazolamide (AZT) modulates blood pressure at high altitude and reduces sleep-disordered breathing in patients with obstructive sleep apnea (OSA). We aimed to investigate the treatment effect of AZT and in combination with continuous positive airway pressure (CPAP) on blood pressure in patients with hypertension and OSA.

METHODS

In a prospective, randomized, three-way crossover study, 13 male patients with hypertension and moderate to severe OSA (age 64 ± 7 years, body mass index 29 ± 4 kg/m², and mean apnea-hypopnea index 37 ± 23 events/h) received AZT, CPAP, or AZT plus CPAP for 2-week periods. Antihypertensive medication was washed out. Office and 24-hour blood pressure, arterial stiffness, polygraphic sleep study data, and blood chemistry were compared.

RESULTS

AZT alone and AZT plus CPAP, but not CPAP alone, reduced office mean arterial pressure compared to baseline (-7 [95% CI -11 to -4], -7 [95% CI -11 to -4] and -1 [95% CI -5 to 4] mmHg, respectively; repeated- measures analysis of variance (RM-ANOVA; P = .015). Aortic systolic pressure and augmentation index, assessed by radial artery oscillatory tonometry, were unaffected by CPAP but decreased after AZT and AZT plus CPAP (RM-ANOVA P = .030 and .031, respectively). The apnea-hypopnea index was significantly reduced in all three treatment arms, most prominently by AZT plus CPAP (RM-ANOVA P = .003). The reduction of venous bicarbonate concentration following AZT was correlated with the change of apnea-hypopnea index (r = 0.66, P = .013).

CONCLUSIONS

AZT reduced blood pressure, vascular stiffness, and sleep-disordered breathing in patients with OSA and comorbid hypertension. Carbonic anhydrase inhibition may constitute a potential target for drug therapy in patients with sleep apnea and comorbid hypertension.

CLINICAL TRIAL REGISTRATION

Registry: ClinicalTrials.gov; Identifier: NCT02220803; Title: A Short Term Open, Randomized Cross-over Trial Exploring the Effect of Carbonic Anhydrase Inhibition by Acetazolamide on Sleep Apnea Associated Hypertension and Vascular Dysfunction; URL: https://clinicaltrials.gov/ct2/show/NCT02220803 and Registry: EU Clinical Trials Register; EudraCT Number: 2013-004866-33; Title: A short term open, randomized cross over trial exploring the effect of carbonic anhydrase inhibition by acetazolamide on sleep apnea associated hypertension; URL: https://www.clinicaltrialsregister.eu/ctr-search/search?query=2013-004866-33.

Measurement of S -Nitrosoglutathione in Plasma by Liquid Chromatography-Tandem Mass Spectrometry

This chapter describes an ultraperformance liquid chromatographic-tandem mass spectrometric (UPLC-MS/MS) method for the quantitative determination of S-nitrosoglutathione (GSNO) in human plasma. S-[¹⁵N]Nitrosoglutathione (GS¹⁵NO) serves as the internal standard. The protocol involves inactivation of plasma γ-glutamyltransferase activity by serine-borate, stabilization of GSNO with EDTA, and avoidance of S-transnitrosylation reactions by blocking SH groups with N-ethylmaleimidide (NEM). Fresh blood is treated with NEM/serine-borate/EDTA, plasma is spiked with GS¹⁵NO (50 nM), ultrafiltered (cutoff 10 kDa) and 10-μL aliquots of ultrafiltrate are analyzed by UPLC-MS/MS in the positive electrospray ionization (ESI+) mode. LC is performed on a Nucleoshell column using isocratic (0.5 mL/min) elution with acetonitrile-20 mM ammonium formate (70:30, v/v), pH 7. Quantification is performed by selected-reaction monitoring the mass transition m/z 337 ([M+H]⁺) → m/z 307 ([M+H-¹⁴NO]^+●) for GSNO and m/z 338 ([M+H]⁺) → m/z 307 ([M+H-¹⁵NO]^+●) for GS¹⁵NO. Matrix effects are outweighed by the internal standard GS¹⁵NO. The lower limit of quantitation (LOQ) is 2.8 nM.

S-Nitroso-N-acetyl-L-cysteine ethyl ester (SNACET) and N-acetyl-L-cysteine ethyl ester (NACET)-Cysteine-based drug candidates with unique pharmacological profiles for oral use as NO, H2S and GSH suppliers and as antioxidants: Results and overview

S-Nitrosothiols or thionitrites with the general formula RSNO are formally composed of the nitrosyl cation (NO⁺) and a thiolate (RS⁻), the base of the corresponding acids RSH. The smallest S-nitrosothiol is HSNO and derives from hydrogen sulfide (HSH, H₂S). The most common physiological S-nitrosothiols are derived from the amino acid L-cysteine (CysSH). Thus, the simplest S-nitrosothiol is S-nitroso-L-cysteine (CysSNO). CysSNO is a spontaneous potent donor of nitric oxide (NO) which activates soluble guanylyl cyclase to form cyclic guanosine monophosphate (cGMP). This activation is associated with multiple biological actions that include relaxation of smooth muscle cells and inhibition of platelet aggregation. Like NO, CysSNO is a short-lived species and occurs physiologically at concentrations around 1 nM in human blood. CysSNO can be formed from CysSH and higher oxides of NO including nitrous acid (HONO) and its anhydride (N₂O₃). The most characteristic feature of RSNO is the S-transnitrosation reaction by which the NO⁺ group is reversibly transferred to another thiolate. By this way numerous RSNO can be formed such as the low-molecular-mass S-nitroso-N-acetyl-L-cysteine (SNAC) and S-nitroso-glutathione (GSNO), and the high-molecular-mass S-nitrosol-L-cysteine hemoglobin (HbCysSNO) present in erythrocytes and S-nitrosol-L-cysteine albumin (AlbCysSNO) present in plasma at concentrations of the order of 200 nM. All above mentioned RSNO exert NO-related biological activity, but they must be administered intravenously. This important drawback can be overcome by lipophilic charge-free RSNO. Thus, we prepared the ethyl ester of SNAC, the S-nitroso-N-acetyl-L-cysteine ethyl ester (SNACET), from synthetic N-acetyl-L-cysteine ethyl ester (NACET). Both NACET and SNACET have improved pharmacological features over N-acetyl-L-cysteine (NAC) and S-nitroso-N-acetyl-L-cysteine (SNAC), respectively, including higher oral bioavailability. SNACET exerts NO-related activities which can be utilized in the urogenital tract and in the cardiovascular system. NACET, with high oral bioavailability, is a strong antioxidant and abundant precursor of GSH, unlike its free acid N-acetyl-L-cysteine (NAC). Here, we review the chemical and pharmacological properties of SNACET and NACET as well as their analytical chemistry. We also report new results from the ingestion of S-[¹⁵N]nitroso-N-acetyl-L-cysteine ethyl ester (S¹⁵NACET) demonstrating the favorable pharmacological profile of SNACET.

Comprehensive analysis of the L-arginine/L-homoarginine/nitric oxide pathway in preterm neonates: potential roles for homoarginine and asymmetric dimethylarginine in foetal growth

L-Arginine (Arg) and L-homoarginine (hArg) are precursors of nitric oxide (NO), a signalling molecule with multiple important roles in human organism. In the circulation of adults, high concentrations of asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) and low concentrations of hArg emerged as cardiovascular risk factors. Yet, the importance of the Arg/hArg/NO pathway, especially of hArg and ADMA, in preterm neonates is little understood. We comprehensively investigated the Arg/hArg/NO pathway in 106 healthy preterm infants (51 boys, 55 girls) aged between 23 + 6 and 36 + 1 gestational weeks. Babies were divided into two groups: group I consisted of 31 babies with a gestational age of 23 + 6 - 29 + 6 weeks; group II comprised 75 children with a gestational age of 30 + 0 - 36 + 1 weeks. Plasma and urine concentrations of ADMA, SDMA, hArg, Arg, dimethylamine (DMA) which is the major urinary ADMA metabolite, as well as of nitrite and nitrate, the major NO metabolites, were determined by GC-MS and GC-MS/MS methods. ADMA and hArg plasma levels, but not the hArg/ADMA molar ratio, were significantly higher in group II than in group I: 895 ± 166 nM vs. 774 ± 164 nM (P = 0.001) for ADMA and 0.56 ± 0.04 µM vs. 0.48 ± 0.08 µM (P = 0.010) for hArg. There was no statistical difference between the groups with regard to urinary ADMA (12.2 ± 4.6 vs 12.8 ± 3.6 µmol/mmol creatinine; P = 0.61) and urinary SDMA. Urinary hArg, ADMA, SDMA correlated tightly with each other. Urinary excretion of DMA was slightly higher in group I compared to group II: 282 ± 44 vs. 247 ± 35 µmol/mmol creatinine (P = 0.004). The DMA/ADMA molar ratio in urine was tendentiously higher in neonates of group I compared to group II: 27 ± 13 vs. 20 ± 5 (P = 0.065). There were no differences between the groups with respect to Arg in plasma and to nitrite and nitrate in plasma and urine. In preterm neonates, ADMA and hArg biosynthesis increases with gestational age without remarkable changes in the hArg/ADMA ratio or NO biosynthesis. Our study suggests that ADMA and hArg are involved in foetal growth.

Carbonic anhydrases are producers of S-nitrosothiols from inorganic nitrite and modulators of soluble guanylyl cyclase in human platelets

Nitric oxide (NO), S-nitrosoglutathione (GSNO) and S-nitrosocysteine are highly potent signaling molecules, acting both by cGMP-dependent and cGMP-independent mechanisms. The NO metabolite nitrite (NO2 (-)) is a major NO reservoir. Hemoglobin, xanthine oxidoreductase and carbonic anhydrase (CA) have been reported to reduce/convert nitrite to NO. We evaluated the role and the physiological importance of CA for an extra-platelet CA/nitrite/NO/cGMP pathway in human platelets. Authentic NO was analyzed by an NO-sensitive electrode. GSNO and GS(15)NO were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). cGMP was determined by LC-MS/MS or RIA. In reduced glutathione (GSH) containing aqueous buffer (pH 7.4), human and bovine erythrocytic CAII-mediated formation of GSNO from nitrite and GS(15)NO from (15)N-nitrite. In the presence of L-cysteine and GSH, this reaction was accompanied by NO release. Incubation of nitrite with bovine erythrocytic CAII and recombinant soluble guanylyl cyclase resulted in cGMP formation. Upon incubation of nitrite with bovine erythrocytic CAII and washed human platelets, cGMP and P-VASP(S239) were formed in the platelets. This study provides the first evidence that extra-platelet nitrite and erythrocytic CAII may modulate platelet function in a cGMP-dependent manner. The new nitrite-dependent CA activity may be a general principle and explain the cardioprotective effects of inorganic nitrite in the vasculature. We propose that nitrous acid (ONOH) is the primary CA-catalyzed reaction product of nitrite.

Evidence of the chemical reaction of (18)O-labelled nitrite with CO2 in aqueous buffer of neutral pH and the formation of (18)OCO by isotope ratio mass spectrometry

Inorganic nitrite (NO2(-), ON-O(-) ←→ (-)O-NO) is the autoxidation product of nitric oxide (NO). Nitrite can also be formed from inorganic nitrate (ONO2(-)), the major oxidation product of NO in erythrocytes, by the catalytic action of bacterial nitrate reductase in gut and oral microflora. Nitrite can be reduced to NO by certain cellular proteins and enzymes, as well as in the gastric juice under acidic conditions. Hemoglobin, xanthine oxidoreductase and carbonic anhydrase (CA) have been reported to convert nitrite to NO. Renal CA isoforms are involved in the reabsorption of nitrite and may, therefore, play an important role in NO homeostasis. Yet, the mechanisms underlying the action of CA on nitrite are incompletely understood. The nitrate/nitrite system is regarded as a reservoir of NO. We have recently shown that nitrite reacts chemically with carbon dioxide (CO2), the regular substrate of CA. The present communication reports a stable isotope ratio mass spectrometry (IRMS) study on the reaction of NO2(-) and CO2 performed in 50 mM HEPES buffer of pH 7.4 at 37 °C. By using (18)O-labelled nitrite ((18)ON-O(-)/(-18)O-NO) and CO2 we observed formation of (18)O-labelled CO2. This finding is an unequivocal evidence of the chemical reaction of (18)ON-O(-)/(-18)O-NO with CO2. The reaction is rapid and involves nucleophilic attack of the negatively charged nitrite via one of its oxygen atoms on the partially positively charged CO2 molecule to form the putative intermediate (18)ON-O-CO2(-)/(-)O2C-(18)O-NO. The by far largest fraction of this intermediate decomposes back to (18)ON-O(-)/(-18)O-NO and CO2. A very small fraction of the intermediate, however, rearranges and finally decomposes to form (18)OCO and nitrite. This reaction is slower in the presence of an isolated erythrocytic CA isoform II. In summary, NO2(-), CO2 and CA are ubiquitous. The chemical reaction of NO2(-) with CO2 and its modulation by CA isoforms may play important roles in the transport of nitrite in red blood cells, the kidney and other cells and organs.

Changes in plasma and urinary nitrite after birth in premature infants at risk for necrotizing enterocolitis

BACKGROUND

Plasma nitrite serves as a reservoir of nitric oxide (NO) bioactivity. Because nitrite ingestion is markedly lower in newborns than adults, we hypothesized plasma nitrite levels would be lower in newborns than in adults, and that infants diagnosed with necrotizing enterocolitis (NEC), a disease characterized by ischemia and bacterial invasion of intestinal walls, would have lower levels of circulating nitrite in the days prior to diagnosis.

METHODS

Single blood and urine samples were collected from 9 term infants and 12 adults, 72 preterm infants every 5 d for 3 wk, and from 13 lambs before and after cord occlusion.

RESULTS

Nitrite fell 50% relative to cord levels in the first day after birth; and within 15 min after cord occlusion in lambs. Urinary nitrite was higher in infants than adults. Plasma and urinary nitrite levels in infants who developed NEC were similar to those of preterm control infants on days 1 and 5, but significantly elevated at 15 and 20 d after birth.

CONCLUSION

Plasma nitrite falls dramatically at birth while newborn urinary nitrite levels are significantly greater than adults. Acute NEC is associated with elevated plasma and urinary nitrite levels.

Discovery and microassay of a nitrite-dependent carbonic anhydrase activity by stable-isotope dilution gas chromatography-mass spectrometry

The intrinsic activity of carbonic anhydrase (CA) is the hydration of CO2 to carbonic acid and its dehydration to CO2. CA may also function as esterase and phosphatase. Recently, we demonstrated that renal CA is mainly responsible for the reabsorption of nitrite (NO2(-)) which is the most abundant reservoir of the biologically highly potent nitric oxide (NO). By means of a stable-isotope dilution GC-MS method, we discovered a novel CA activity which strictly depends upon nitrite. We found that bovine erythrocytic CAII (beCAII) catalyses the incorporation of (18)O from H2 (18)O into nitrite at pH 7.4. After derivatization with pentafluorobenzyl bromide, gas chromatographic separation and mass spectrometric analysis, we detected ions at m/z 48 for singly (18)O-labelled nitrite ((16)O=N-(18)O(-)/(18)O=N-(16)O(-)) and at m/z 50 for doubly (18)O-labelled nitrite ((18)O=N-(18)O(-)) in addition to m/z 46 for unlabelled nitrite. Using (15)N-labelled nitrite ((15)NO2 (-), m/z 47) as an internal standard and selected-ion monitoring of m/z 46, m/z 48, m/z 50 and m/z 47, we developed a GC-MS microassay for the quantitative determination of the nitrite-dependent beCAII activity. The CA inhibitors acetazolamide and FC5 207A did not alter beCAII-catalysed formation of singly and doubly (18)O-labelled nitrite. Cysteine and the experimental CA inhibitor DIDS (a diisothiocyanate) increased several fold the beCAII-catalysed formation of the (18)O-labelled nitrite species. Cysteine, acetazolamide, FC5 207A, and DIDS by themselves had no effect on the incorporation of (18)O from H2 (18)O into nitrite. We conclude that erythrocytic CA possesses a nitrite-dependent activity which can only be detected when nitrite is used as the substrate and the reaction is performed in buffers of neutral pH values prepared in H2 (18)O. This novel CA activity, i.e., the nitrous acid anhydrase activity, represents a bioactivation of nitrite and may have both beneficial (via S-nitrosylation and subsequent NO release) and possibly adverse (via C- and N-nitrosylation) effects in living organisms.

Increased Carbonic Anhydrase Activity is Associated with Sleep Apnea Severity and Related Hypoxemia

STUDY OBJECTIVES

The catalytic function of the enzyme carbonic anhydrase (CA) plays a fundamental role in carbon dioxide (CO2), proton (H(+)), and bicarbonate (HCO3(-)) homeostasis. Hypoxia and tissue acidosis have been proposed to increase physiological CA activity in various compartments of the body. We hypothesized that CA activity in blood is upregulated in patients with obstructive sleep apnea (OSA).

DESIGN

Cross-sectional analysis of a sleep clinic cohort.

SETTINGS

Sleep laboratory at a university hospital.

PARTICIPANTS

Seventy referred patients with suspected OSA (48 males, age 54 ± 13 y, apnea-hypopnea index (AHI) median [interquartile range] 21 [8-41] n/h).

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

In-laboratory cardiorespiratory polygraphy was used to assess OSA. CA activity was determined by an in vitro assay that quantifies the pH change reflecting the conversion of CO2 and H2O to HCO3(-) and H(+). CA activity was positively associated with AHI and 4% oxygen desaturation index (ODI4) (Spearman correlation r = 0.44 and 0.47, both P < 0.001). The associations (CA activity versus logAHI and CA versus logODI4) were independent of sex, age, body mass index, presleep oxygen saturation, nocturnal oxygen saturation, hypertension status, and use of diuretic medication in two generalized linear models (P = 0.007 and 0.011, respectively). Sitting diastolic blood pressure was associated with CA activity after adjustment of sex, age, body mass index, mean oxygen saturation, and AHI (P = 0.046).

CONCLUSIONS

Carbonic anhydrase (CA) activity increased with apnea-hypopnea index and related nocturnal hypoxemia measures in patients with obstructive sleep apnea (OSA). Altered CA activity may constitute a component that modulates respiratory control and hemodynamic regulation in patients with OSA.

Inhibitory effects of nitrite on the reactions of bovine carbonic anhydrase II with CO2 and bicarbonate consistent with zinc-bound nitrite

Carbonic anhydrase (CA) is a zinc enzyme that catalyzes hydration of carbon dioxide (CO2) and dehydration of bicarbonate in red blood cells, thus facilitating CO2 transport and excretion. Bovine CA II may also react with nitrite to generate nitric oxide, although nitrite is a known inhibitor of the CO2 hydration reaction. To address the potential in vivo interference of these reactions and the nature of nitrite binding to the enzyme, we here investigate the inhibitory effect of 10-30 mM nitrite on Michaelis-Menten kinetics of CO2 hydration and bicarbonate dehydration by stopped-flow spectroscopy. Our data show that nitrite significantly affects the apparent dissociation constant KM for CO2 (11 mM) and bicarbonate (221 mM), and the turnover number kcat for the CO2 hydration (1.467 × 10(6) s(-1)) but not for the bicarbonate dehydration (7.927 × 10(5) s(-1)). These effects demonstrate mixed and competitive inhibition for the reaction with CO2 and bicarbonate, respectively, and are consistent with nitrite binding to the active site zinc. The high apparent dissociation constant found here for CO2, bicarbonate and nitrite (16-120 mM) are all overall consistent with published data and reveal a large capacity of free enzyme available for binding each of the three substrates at their in vivo levels, with little or no significant interference among reactions. The low affinity of the enzyme for nitrite suggests that the in vivo interaction between red blood cell CA II and nitrite requires compartmentalization at the anion exchanger protein of the red cell membrane to be physiologically relevant.

Sulfonamides and their isosters as carbonic anhydrase inhibitors

Molecules containing the sulfonamide group (R-SO2NH2) as well as its structurally related isosters, sulfamido (R-NH-SO2NH2) and sulfamato (R-O-SO2NH2), constitute the most important class of inhibitors acting on the metalloenzyme carbonic anhydrase (EC 4.2.1.1). Despite their presence in the literature, in general the reports lack of a clear and organic overview linking the main structural features of the clinically used inhibitors with the therapeutic aspects. The current review is intended to highlight the structural basis of the interactions of sulfonamide-like groups within the active site of the carbonic anhydrases and will summarize the clinical use of the most interesting molecules for the treatment of relevant pathologies, such as glaucoma, obesity, cancer and CNS-affecting diseases.

Safety of carbonic anhydrase inhibitors

INTRODUCTION

Carbonic anhydrase (CA) inhibitors have an impressive safety record despite the multiple functions that CA isozymes serve because they are not fully inhibited with most dosing. While reducing the targeted CA-dependent process sufficiently for disease control, residual activity and uncatalyzed rates in combination with compensations are adequate to avoid lethal consequences. Some drugs have in vitro selectivity differences against the 13 active isozymes, but none are convincingly selective in vivo or clinically. Efforts to synthesize selective inhibitors should result in safer drugs with fewer side effects.

AREAS COVERED

This review will focus on approved drugs with CA-inhibiting activity, whether used directly for this purpose or others. Side effects are discussed in relation to various organ systems and the disease being treated. Causes of side effects are considered, and strategies for symptom reduction are given.

EXPERT OPINION

Common side effects of paresthesias, dyspepsia, lassitude and fatigue in 30 - 40% of patients are generally tolerable or abate, but if not can be partially relieved by bicarbonate supplementation. The most important safety concerns are severe acidosis, respiratory failure and encephalopathy in patients with renal, pulmonary and hepatic disease where caution is critical, as is also the case in persons with sulfa drug allergies.

Carbonic anhydrase inhibitors and high altitude illnesses

Carbonic anhydrase (CA) inhibitors, particularly acetazolamide, have been used at high altitude for decades to prevent or reduce acute mountain sickness (AMS), a syndrome of symptomatic intolerance to altitude characterized by headache, nausea, fatigue, anorexia and poor sleep. Principally CA inhibitors act to further augment ventilation over and above that stimulated by the hypoxia of high altitude by virtue of renal and endothelial cell CA inhibition which oppose the hypocapnic alkalosis resulting from the hypoxic ventilatory response (HVR), which acts to limit the full expression of the HVR. The result is even greater arterial oxygenation than that driven by hypoxia alone and greater altitude tolerance. The severity of several additional diseases of high attitude may also be reduced by acetazolamide, including high altitude cerebral edema (HACE), high altitude pulmonary edema (HAPE) and chronic mountain sickness (CMS), both by its CA-inhibiting action as described above, but also by more recently discovered non-CA inhibiting actions, that seem almost unique to this prototypical CA inhibitor and are of most relevance to HAPE. This chapter will relate the history of CA inhibitor use at high altitude, discuss what tissues and organs containing carbonic anhydrase play a role in adaptation and maladaptation to high altitude, explore the role of the enzyme and its inhibition at those sites for the prevention and/or treatment of the four major forms of illness at high altitude.

Vascular effects of dietary nitrate (as found in green leafy vegetables and beetroot) via the nitrate-nitrite-nitric oxide pathway

The discovery that dietary (inorganic) nitrate has important vascular effects came from the relatively recent realization of the 'nitrate-nitrite-nitric oxide (NO) pathway'. Dietary nitrate has been demonstrated to have a range of beneficial vascular effects, including reducing blood pressure, inhibiting platelet aggregation, preserving or improving endothelial dysfunction, enhancing exercise performance in healthy individuals and patients with peripheral arterial disease. Pre-clinical studies with nitrate or nitrite also show the potential to protect against ischaemia-reperfusion injury and reduce arterial stiffness, inflammation and intimal thickness. However, there is a need for good evidence for hard endpoints beyond epidemiological studies. Whilst these suggest reduction in cardiovascular risk with diets high in nitrate-rich vegetables (such as a Mediterranean diet), others have suggested possible small positive and negative associations with dietary nitrate and cancer, but these remain unproven. Interactions with other nutrients, such as vitamin C, polyphenols and fatty acids may enhance or inhibit these effects. In order to provide simple guidance on nitrate intake from different vegetables, we have developed the Nitrate 'Veg-Table' with 'Nitrate Units' [each unit being 1 mmol of nitrate (62 mg)] to achieve a nitrate intake that is likely to be sufficient to derive benefit, but also to minimize the risk of potential side effects from excessive ingestion, given the current available evidence. The lack of data concerning the long term effects of dietary nitrate is a limitation, and this will need to be addressed in future trials.

Diuretics with carbonic anhydrase inhibitory action: a patent and literature review (2005 - 2013)

INTRODUCTION

The benzothiadiazines and high ceiling diuretics (hydrochlorothiazide, hydroflumethiazide, quinethazone, metolazone, chlorthalidone, indapamide, furosemide and bumetanide) contain primary sulfamoyl moieties acting as zinc-binding groups in the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). These drugs are widely used clinically and were recently shown to weakly inhibit isoforms CA I and II, but to possess stronger activity against isoforms involved in other important pathologies, for example, obesity, cancer, epilepsy and hypertension.

AREAS COVERED

The class of clinically used diuretics, with CA inhibitory properties, is the main topic of the review. A patent literature review covering the period from 2005 to 2013 is presented.

EXPERT OPINION

This section presents an overview of the patent literature in the sulfonamide diuretic field. Most of the patents deal with the combination of diuretic sulfonamide CA inhibitors with other agents useful in the management of cardiovascular diseases and obesity. Such combinations exert a better therapeutic activity compared to similar diuretics that do not inhibit CAs, raising the question of the polypharmacological and drug repositioning effects of these old drugs. These effects seem to be due to the potent inhibition of such drugs against CA isoforms present in kidneys and blood vessels, which explain both the blood pressure lowering effects as well as organ-protective activity of the drugs. An explanation of these data is provided by the fact that inhibition of the renal CAs leads to a large increase of the nitrite excretion in urine, suggesting that renal CAs are involved in nitrite reabsorption in humans. Important lessons for the drug design of sulfonamide CA inhibitors (CAIs) can be drawn from these data.