Evidence that L-arginine is a key amino acid in sickle cell anemia — A preliminary report

Hydroxyurea improves nitric oxide bioavailability in humanized sickle cell mice

Despite advancements in disease management, sickle cell nephropathy, a major contributor to mortality and morbidity in patients, has limited therapeutic options. Previous studies indicate hydroxyurea, a commonly prescribed therapy for sickle cell disease (SCD), can reduce renal injury in SCD but the mechanisms are uncertain. Because SCD is associated with reduced nitric oxide (NO) bioavailability, we hypothesized that hydroxyurea treatment would improve NO bioavailability in the humanized sickle cell mouse. Humanized male 12-wk-old sickle (HbSS) and genetic control (HbAA) mice were treated with hydroxyurea or regular tap water for 2 wk before renal and systemic NO bioavailability as well as renal injury were assessed. Untreated HbSS mice exhibited increased proteinuria, elevated plasma endothelin-1 (ET-1), and reduced urine concentrating ability compared with HbAA mice. Hydroxyurea reduced proteinuria and plasma ET-1 levels in HbSS mice. Untreated HbSS mice had reduced plasma nitrite and elevated plasma arginase concentrations compared with HbAA mice. Hydroxyurea treatment augmented plasma nitrite and attenuated plasma arginase in HbSS mice. Renal vessels isolated from HbSS mice also had elevated nitric oxide synthase 3 (NOS3) and arginase 2 expression compared with untreated HbAA mice. Hydroxyurea treatment did not alter renal vascular NOS3, however, renal vascular arginase 2 expression was significantly reduced. These data support the hypothesis that hydroxyurea treatment augments renal and systemic NO bioavailability by reducing arginase activity as a potential mechanism for the improvement on renal injury seen in SCD mice.

Biochemical investigation of the upstream anti-sickling mechanisms of soursop (Annona muricata): 15-acetyl guanacone as an inhibitor of deoxyhaemoglobin polymerisation

Current sickle cell disease (SCD) therapies are limited and inefficient. The ethnomedicinal values of Annona muricata in the treatment of SCD, leading to this present research. Leaves and fruits of Annona muricata were processed using solvent extraction and partitioning; aqueous, chloroform and ethyl acetate fractions. In vitro (anti-oxidant and anti-sickling), in silico, quantitative (amino acids) and kinetic simulation experiments were done. 15-acetyl guanacone, was used, in silico against 2,3-bisphosphoglycerate (2, 3-BPG) mutase and deoxyhaemoglobin. The ethyl acetate and chloroform fractions better NO^● scavengers, iron-chelators and ferric reducing. In vitro unsickling (UT₅₀) had ethyl acetate = 5 h and methanol = 7 h. Chloroform fraction had EC₅₀ 1.00 mg/mL (EC₅₀ = 546 mg/mL) to 10.00 mg/mL (EC₅₀ = 99 mg/mL). EC₅₀ and IC₅₀ of ethyl acetate fraction had steady-decrease. At higher concentration, chloroform fraction had higher Bmax (1.48 × 10²¹ U/mL) and higher Kd (3.66 × 10¹⁹ mg/mL), whereas, at a lower concentration, the ethyl acetate fraction demonstrated higher Bmax (7.23 × 10¹² U/mL) and lower Kd (2.12 × 10¹¹ mg/mL); The relative affinity (BP) of chloroform fraction increased progressively with concentration. The amino acid profile revealed rich concentrations glycine, valine, leucine, lysine, phenylalanine, histidine, arginine, and tryptophan. From the in silico experiments, 15-acetyl guanacone specifically targeted the A and B chains, with greater affinity for the beta subunit. This suggested that 15-acetyl guanacone might be able to prevent the polymerisation of deoxyHbSS, induce an allosteric conformational change that increases the oxygen affinity, and decrease the cellular 2, 3-BPG concentration.Communicated by Ramaswamy H. Sarma.

Acquired Amino Acid Deficiencies: A Focus on Arginine and Glutamine

Nonessential amino acids are synthesized de novo and therefore not diet dependent. In contrast, essential amino acids must be obtained through nutrition since they cannot be synthesized internally. Several nonessential amino acids may become essential under conditions of stress and catabolic states when the capacity of endogenous amino acid synthesis is exceeded. Arginine and glutamine are 2 such conditionally essential amino acids and are the focus of this review. Low arginine bioavailability plays a pivotal role in the pathogenesis of a growing number of varied diseases, including sickle cell disease, thalassemia, malaria, acute asthma, cystic fibrosis, pulmonary hypertension, cardiovascular disease, certain cancers, and trauma, among others. Catabolism of arginine by arginase enzymes is the most common cause of an acquired arginine deficiency syndrome, frequently contributing to endothelial dysfunction and/or T-cell dysfunction, depending on the clinical scenario and disease state. Glutamine, an arginine precursor, is one of the most abundant amino acids in the body and, like arginine, becomes deficient in several conditions of stress, including critical illness, trauma, infection, cancer, and gastrointestinal disorders. At-risk populations are discussed together with therapeutic options that target these specific acquired amino acid deficiencies.

Arginine metabolism in vascular biology and disease

Arginine metabolism plays a major role in cardiovascular physiology and pathophysiology, largely via nitric oxide (NO)-dependent processes. It is becoming increasingly apparent, however, that arginine metabolic enzymes other than the NO synthases can also play important roles via both NO-dependent and -independent processes. There are three sources of arginine in vivo and at least five mammalian enzymes or enzyme families that utilize arginine as substrate. Changes in arginine availability or in production of the different end products of the various arginine metabolic pathways can have distinct and profound physiologic consequences. However, our knowledge regarding the complex interplay between these pathways at the level of the whole body, specific tissues, and even individual cells, is incomplete. This review will highlight recent findings in this area that may suggest additional avenues of investigation that will allow a fuller understanding of cardiovascular physiology in health and disease.

New strategies for the treatment of pulmonary hypertension in sickle cell disease : the rationale for arginine therapy

Nitric oxide (NO) is inactivated in sickle cell disease (SCD), while bioavailability of arginine, the substrate for NO synthesis, is diminished. Impaired NO bioavailability represents the central feature of endothelial dysfunction, and is a key factor in the pathophysiology of SCD. Inactivation of NO correlates with the hemolytic rate and is associated with erythrocyte release of cell-free hemoglobin and arginase during hemolysis. Accelerated consumption of NO is enhanced further by the inflammatory environment of oxidative stress that exists in SCD. Based upon its critical role in mediating vasodilation and cell growth, decreased NO bioavailability has also been implicated in the pathogenesis of pulmonary arterial hypertension (PHT). Secondary PHT is a common life-threatening complication of SCD that also occurs in most hereditary and chronic hemolytic disorders. Aberrant arginine metabolism contributes to endothelial dysfunction and PHT in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and PHT. Decompartmentalization of hemoglobin into plasma consumes endothelial NO and thus drives a metabolic requirement for arginine, whose bioavailability is further limited by arginase activity. New treatments aimed at maximizing both arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, or oral arginine supplementation may represent novel therapeutic strategies.

Citrulline Supplementation Improves Organ Perfusion and Arginine Availability under Conditions with Enhanced Arginase Activity

Enhanced arginase-induced arginine consumption is believed to play a key role in the pathogenesis of sickle cell disease-induced end organ failure. Enhancement of arginine availability with l-arginine supplementation exhibited less consistent results; however, l-citrulline, the precursor of l-arginine, may be a promising alternative. In this study, we determined the effects of l-citrulline compared to l-arginine supplementation on arginine-nitric oxide (NO) metabolism, arginine availability and microcirculation in a murine model with acutely-enhanced arginase activity. The effects were measured in six groups of mice (n = 8 each) injected intraperitoneally with sterile saline or arginase (1000 IE/mouse) with or without being separately injected with l-citrulline or l-arginine 1 h prior to assessment of the microcirculation with side stream dark-field (SDF)-imaging or in vivo NO-production with electron spin resonance (ESR) spectroscopy. Arginase injection caused a decrease in plasma and tissue arginine concentrations. l-arginine and l-citrulline supplementation both enhanced plasma and tissue arginine concentrations in arginase-injected mice. However, only the citrulline supplementation increased NO production and improved microcirculatory flow in arginase-injected mice. In conclusion, the present study provides for the first time in vivo experimental evidence that l-citrulline, and not l-arginine supplementation, improves the end organ microcirculation during conditions with acute arginase-induced arginine deficiency by increasing the NO concentration in tissues.

Infusion of hemolyzed red blood cells within peripheral blood stem cell grafts in patients with and without sickle cell disease

Peripheral blood stem cell (PBSC) infusions are associated with complications such as elevated blood pressure and decreased creatinine clearance. Patients with sickle cell disease experience similar manifestations, and some have postulated release of plasma-free hemoglobin with subsequent nitric oxide consumption as causative. We sought to evaluate whether the infusion of PBSC grafts containing lysed red blood cells (RBCs) leads to the toxicity observed in transplant subjects. We report a prospective cohort study of 60 subjects divided into 4 groups based on whether their infusions contained dimethyl sulfoxide (DMSO) and lysed RBCs, no DMSO and fresh RBCs, DMSO and no RBCs, or saline. Our primary end point, change in maximum blood pressure compared with baseline, was not significantly different among groups. Tricuspid regurgitant velocity and creatinine levels also did not differ significantly among groups. Our data do not support free hemoglobin as a significant contributor to toxicity associated with PBSC infusions. This study was registered at clinicaltrials.gov (NCT00631787).

Arginine metabolism in vascular biology and disease

Arginine metabolism plays a major role in cardiovascular physiology and pathophysiology, largely via nitric oxide (NO)-dependent processes. It is becoming increasingly apparent, however, that arginine metabolic enzymes other than the NO synthases can also play important roles via both NO-dependent and -independent processes. There are three sources of arginine in vivo and at least five mammalian enzymes or enzyme families that utilize arginine as substrate. Changes in arginine availability or in production of the different end products of the various arginine metabolic pathways can have distinct and profound physiologic consequences. However, our knowledge regarding the complex interplay between these pathways at the level of the whole body, specific tissues, and even individual cells, is incomplete. This review will highlight recent findings in this area that may suggest additional avenues of investigation that will allow a fuller understanding of cardiovascular physiology in health and disease.

Simplified method to assay total plasma peroxidase activity and ferriheme products in sickle cell anemia, with initial results in assessing clinical severity in a trial with citrulline therapy

A method using dianisidine is described to measure promptly the total plasma peroxidase (POX) activity of methemoglobin and other ferrihemes. Methemoglobin (35 mg/dL) is used as POX standard. Three-minute POX activities and total POX concentrations measured by a classic benzidine method were compared in a three-patient trial with citrulline supplementation. High POX values became progressively lower. In two of the patients, 3-minute POX activities were reduced more than total concentrations. Oral citrulline reduced harmful plasma ferriheme levels. Free ferriheme also exhibited POX activity. POX levels may be useful to judge severity in sickle cell anemia and to monitor therapeutic efficacy.

Hydroxyurea and arginine therapy: impact on nitric oxide production in sickle cell disease

PURPOSE

Recent data suggest that hydroxyurea (HU) increases the production of nitric oxide (NO), a potent vasodilator. NO is normally metabolized from l-Arginine (Arg). However, in vitro and animal experiments suggest that HU is the NO donor itself. In contrast, a recent study indicates that nitric oxide synthase (NOS) may play a role. Since adults with sickle cell disease (SCD) are Arg-deficient, Arg availability may limit the ability of HU to maximally impact NO production if an NOS mechanism is involved. The authors have previously shown that Arg supplementation alone induces a paradoxical decrease in NO metabolite (NO(x)) production.

METHODS

The authors studied the effects of HU and Arg supplementation on NO(x) production. HU alone or HU + Arg was administered to patients with SCD at steady state, and sequential levels of Arg, serum NO(x) and exhaled NO were followed over 4 hours.

RESULTS

After HU + Arg, all patients demonstrated a significant increase in serum NO(x) production within 2 hours. When the same patients were treated with HU alone (5.1 +/- 2 micromol/L), a mixed response occurred. NO(x) levels increased in four patients and decreased in one patient (-23.3 micromol/L).

CONCLUSIONS

While Arg alone does not increase serum NO(x) production in SCD patients at steady state, it does when given together with HU. Hence, co-administration of Arg with HU may augment the NO(x) response in SCD and improve utilization of Arg in patients at steady state.

Arginine therapy: a new treatment for pulmonary hypertension in sickle cell disease?

Pulmonary hypertension is a life-threatening complication of sickle cell disease. L-Arginine is the nitrogen donor for synthesis of nitric oxide, a potent vasodilator that is deficient during times of sickle cell crisis. This deficiency may play a role in pulmonary hypertension. The enzyme arginase hydrolyzes arginine to ornithine and urea, and thus, it may compete with nitric oxide synthase, leading to decreased nitric oxide production. Nitric oxide therapy by inhalation has improved pulmonary hypertension associated with acute chest syndrome in sickle cell disease, and several studies demonstrate therapeutic benefits of arginine therapy for primary and secondary pulmonary hypertension. We sought to determine the effects of arginine therapy on pulmonary hypertension in patients with sickle cell disease. Arginase activity was also determined. Oral arginine produced a 15.2% mean reduction in estimated pulmonary artery systolic pressure (63.9 +/- 13 to 54.2 +/- 12 mm Hg, p = 0.002) after 5 days of therapy in 10 patients. Arginase activity was elevated almost twofold (p = 0.07) in patients with pulmonary hypertension and may limit arginine bioavailability. With limited treatment options and a high mortality rate for patients with sickle cell disease who develop pulmonary hypertension, arginine is a promising new therapy that warrants further investigation.