Alexander Bollenbach

Bio: Alexander Bollenbach is an academic researcher from Hannover Medical School. The author has contributed to research in topics: Asymmetric dimethylarginine & Derivatization. The author has an hindex of 8, co-authored 34 publications receiving 246 citations.

Asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA) and homoarginine (hArg): the ADMA, SDMA and hArg paradoxes.

Abstract: NG-Methylation of l-arginine (Arg) residues in certain proteins by protein arginine methyltransferases and subsequent proteolysis yields NG-monomethyl-l-arginine (MMA), NG,NG-dimethyl-l-arginine (asymmetric dimethylarginine, ADMA) and NG,N′G-dimethyl-l-arginine (symmetric dimethylarginine, SDMA). Biological MMA, ADMA and SDMA occur as free acids in the nM-range and as residues of proteins of largely unknown quantity. Arginine:glycine amidinotransferase (AGAT) catalyzes the synthesis of L-homoarginine (hArg) from free Arg and l-lysine. Biological hArg is considered to occur exclusively as free acid in the lower µM-range. Nitric oxide synthase (NOS) catalyzes the conversion of Arg (high affinity) and hArg (low affinity) to nitric oxide (NO) which is a pleiotropic signaling molecule. MMA, ADMA and SDMA are inhibitors (MMA > ADMA ≫ SDMA) of NOS activity. Slightly elevated ADMA and SDMA concentrations and slightly reduced hArg concentrations in the circulation are associated with many diseases including diabetes mellitus. Yet, this is paradox: (1) free ADMA and SDMA are weak inhibitors of endothelial NOS (eNOS) which is primarily responsible for NO-related effects in the cardiovascular system, with free hArg being a poor substrate for eNOS; (2) free ADMA, SDMA and hArg are not associated with oxidative stress which is considered to induce NO-related endothelial dysfunction. This ADMA/SDMA/hArg paradox may be solved by the assumption that not the free acids but their precursor proteins exert biological effects in the vasculature, with hArg antagonizing the effects of NG-methylated proteins.

Development and validation of GC-MS methods for the comprehensive analysis of amino acids in plasma and urine and applications to the HELLP syndrome and pediatric kidney transplantation: evidence of altered methylation, transamidination, and arginase activity.

Abstract: We developed and validated gas chromatography–mass spectrometry (GC–MS) methods for the simultaneous measurement of amino acids and their metabolites in 10-µL aliquots of human plasma and urine. De novo synthesized trideutero-methyl esters were used as internal standards. Plasma proteins were precipitated by acidified methanol and removed by centrifugation. Supernatants and native urine were evaporated to dryness. Amino acids were first esterified using 2 M HCl in methanol and then amidated using pentafluoropropionic anhydride for electron-capture negative-ion chemical ionization. Time programmes were used for the gas chromatograph oven and the selected-ion monitoring of specific anions. The GC–MS methods were applied in clinical studies on the HELLP syndrome and pediatric kidney transplantation (KTx) focusing on l-arginine-related pathways. We found lower sarcosine (N-methylglycine) and higher asymmetric dimethylarginine (ADMA) plasma concentrations in HELLP syndrome women (n = 7) compared to healthy pregnant women (n = 5) indicating altered methylation. In plasma of pediatric KTx patients, lower guanidinoacetate and homoarginine concentrations were found in plasma but not in urine samples of patients treated with standard mycophenolate mofetil-based immunosuppression (MMF; n = 22) in comparison to matched patients treated with MMF-free immunosuppression (n = 22). On average, the global arginine bioavailability ratio was by about 40% lower in the MMF group compared to the EVR group (P = 0.004). Mycophenolate, the major pharmacologically active metabolite of MMF, is likely to inhibit the arginine:glycine amidinotransferase (AGAT), and to enhance arginase activity in leukocytes and other types of cell of MMF-treated children.

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

Abstract: 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., UNOxR, 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 UNOxR. We determined UNOxR 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 UNOxR values in healthy young men who ingested isosorbide dinitrate (ISDN), pentaerythrityl tetranitrate (PETN), or inorganic nitrate. In addition, we tested the utility of UNOxR 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 UNOxR 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 UNOxR were of the order of 50% in young and elderly healthy subjects. UNOxR 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 UNOxR 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 UNOxR 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), UNOxR was higher than in those who ingested the same dose of NaCl (1709 vs. 369). In LEW.1AR1-iddm rats, mean UNOxR values were lower than in healthy rats (198 vs. 308) and comparable to those in APOE*3-Leiden.CETP mice (151).

Effects of single and combined metformin and l-citrulline supplementation on l-arginine-related pathways in Becker muscular dystrophy patients: possible biochemical and clinical implications

Abstract: The L-arginine/nitric oxide synthase (NOS) pathway is considered to be altered in muscular dystrophy such as Becker muscular dystrophy (BMD). We investigated two pharmacological options aimed to increase nitric oxide (NO) synthesis in 20 male BMD patients (age range 21-44 years): (1) supplementation with L-citrulline (3 × 5 g/d), the precursor of L-arginine which is the substrate of neuronal NO synthase (nNOS); and (2) treatment with the antidiabetic drug metformin (3 × 500 mg/d) which activates nNOS in human skeletal muscle. We also investigated the combined use of L-citrulline (3 × 5 g/d) and metformin (3 × 500 mg/d). Before and after treatment, we measured in serum and urine samples the concentration of amino acids and metabolites of L-arginine-related pathways and the oxidative stress biomarker malondialdehyde (MDA). Compared to healthy subjects, BMD patients have altered NOS, arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT) pathways. Metformin treatment resulted in concentration decrease of arginine and MDA in serum, and of homoarginine (hArg) and guanidinoacetate (GAA) in serum and urine. L-Citrulline supplementation resulted in considerable increase of the concentrations of amino acids and creatinine in the serum, and in their urinary excretion rates. Combined use of metformin and L-citrulline attenuated the effects obtained from their single administrations. Metformin, L-citrulline or their combination did not alter serum nitrite and nitrate concentrations and their urinary excretion rates. In conclusion, metformin or L-citrulline supplementation to BMD patients results in remarkable antidromic changes of the AGAT and GAMT pathways. In combination, metformin and L-citrulline at the doses used in the present study seem to abolish the biochemical effects of the single drugs in slight favor of L-citrulline.

Preeclampsia: Risk Factors, Diagnosis, Management, and the Cardiovascular Impact on the Offspring.

Abstract: Hypertensive disorders of pregnancy affect up to 10% of pregnancies worldwide, which includes the 3%–5% of all pregnancies complicated by preeclampsia. Preeclampsia is defined as new onset hypertension after 20 weeks’ gestation with evidence of maternal organ or uteroplacental dysfunction or proteinuria. Despite its prevalence, the risk factors that have been identified lack accuracy in predicting its onset and preventative therapies only moderately reduce a woman’s risk of preeclampsia. Preeclampsia is a major cause of maternal morbidity and is associated with adverse foetal outcomes including intra-uterine growth restriction, preterm birth, placental abruption, foetal distress, and foetal death in utero. At present, national guidelines for foetal surveillance in preeclamptic pregnancies are inconsistent, due to a lack of evidence detailing the most appropriate assessment modalities as well as the timing and frequency at which assessments should be conducted. Current management of the foetus in preeclampsia involves timely delivery and prevention of adverse effects of prematurity with antenatal corticosteroids and/or magnesium sulphate depending on gestation. Alongside the risks to the foetus during pregnancy, there is also growing evidence that preeclampsia has long-term adverse effects on the offspring. In particular, preeclampsia has been associated with cardiovascular sequelae in the offspring including hypertension and altered vascular function.

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

Abstract: 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 4211) Their role i

The African Prospective study on the Early Detection and Identification of Cardiovascular disease and Hypertension (African-PREDICT): Design, recruitment and initial examination.

Abstract: BackgroundGlobally hypertension is stabilising, but in sub-Saharan Africa the incidence of hypertension remains on an increase. Although this might be attributed to poor healthcare and ineffective ...

Role of L-Arginine in Nitric Oxide Synthesis and Health in Humans.

Abstract: As a functional amino acid (AA), L-arginine (Arg) serves not only as a building block of protein but also as an essential substrate for the synthesis of nitric oxide (NO), creatine, polyamines, homoarginine, and agmatine in mammals (including humans). NO (a major vasodilator) increases blood flow to tissues. Arg and its metabolites play important roles in metabolism and physiology. Arg is required to maintain the urea cycle in the active state to detoxify ammonia. This AA also activates cellular mechanistic target of rapamycin (MTOR) and focal adhesion kinase cell signaling pathways in mammals, thereby stimulating protein synthesis, inhibiting autophagy and proteolysis, enhancing cell migration and wound healing, promoting spermatogenesis and sperm quality, improving conceptus survival and growth, and augmenting the production of milk proteins. Although Arg is formed de novo from glutamine/glutamate and proline in humans, these synthetic pathways do not provide sufficient Arg in infants or adults. Thus, humans and other animals do have dietary needs of Arg for optimal growth, development, lactation, and fertility. Much evidence shows that oral administration of Arg within the physiological range can confer health benefits to both men and women by increasing NO synthesis and thus blood flow in tissues (e.g., skeletal muscle and the corpora cavernosa of the penis). NO is a vasodilator, a neurotransmitter, a regulator of nutrient metabolism, and a killer of bacteria, fungi, parasites, and viruses [including coronaviruses, such as SARS-CoV and SARS-CoV-2 (the virus causing COVID-19). Thus, Arg supplementation can enhance immunity, anti-infectious, and anti-oxidative responses, fertility, wound healing, ammonia detoxification, nutrient digestion and absorption, lean tissue mass, and brown adipose tissue development; ameliorate metabolic syndromes (including dyslipidemia, obesity, diabetes, and hypertension); and treat individuals with erectile dysfunction, sickle cell disease, muscular dystrophy, and pre-eclampsia.

The Biological Axis of Protein Arginine Methylation and Asymmetric Dimethylarginine

Abstract: Protein post-translational modifications (PTMs) in eukaryotic cells play important roles in the regulation of functionalities of the proteome and in the tempo-spatial control of cellular processes. Most PTMs enact their regulatory functions by affecting the biochemical properties of substrate proteins such as altering structural conformation, protein–protein interaction, and protein–nucleic acid interaction. Amid various PTMs, arginine methylation is widespread in all eukaryotic organisms, from yeasts to humans. Arginine methylation in many situations can drastically or subtly affect the interactions of substrate proteins with their partnering proteins or nucleic acids, thus impacting major cellular programs. Recently, arginine methylation has become an important regulator of the formation of membrane-less organelles inside cells, a phenomenon of liquid–liquid phase separation (LLPS), through altering π-cation interactions. Another unique feature of arginine methylation lies in its impact on cellular physiology through its downstream amino acid product, asymmetric dimethylarginine (ADMA). Accumulation of ADMA in cells and in the circulating bloodstream is connected with endothelial dysfunction and a variety of syndromes of cardiovascular diseases. Herein, we review the current knowledge and understanding of protein arginine methylation in regards to its canonical function in direct protein regulation, as well as the biological axis of protein arginine methylation and ADMA biology.