Relevance of allosteric conformations and homocarnosine concentration on carnosinase activity

Low Plasma Carnosinase-1 Activity in Patients with Left Ventricular Systolic Dysfunction: Implications for Carnosine Therapy in Heart Failure

Therapeutic efficacy of histidyl dipeptides such as carnosine is hampered by circulating carnosinase-1 (CN1), which catalyzes carnosine's hydrolysis and degradation. Prior reports suggest that oral carnosine may improve cardiometabolic parameters in patients with heart failure (HF), but whether CN1 activity is affected by HF is unknown. Here, we measured CN1 content and carnosine degradation rate (CDR) in preoperative plasma samples from a cohort of patients (n = 138) undergoing elective cardiac surgery to determine whether plasma CN1 and/or CDR varied with left ventricular (LV) systolic dysfunction. CN1 content was normally distributed in the cohort, but plasma CDR displayed a quasi-bimodal distribution into high- (>2 nmol/(h*μL)) and low-activity (≤2 nmol/(h*μL)) clusters. Multivariable analysis confirmed female sex, diabetes and LV systolic dysfunction was associated with the low-activity CDR cluster. Although CN1 content did not differ, logistic regression analysis revealed that CDR and CN1-specific activity (CDR/CN1 content) was significantly lower in patients with both moderate (ejection fraction, EF ≥ 35 to <50%) and severe LV systolic dysfunction (EF < 35%) compared with patients in the normal range (EF ≥ 50%). These findings suggest that plasma CN1 activity is regulated by factors independent of expression, and that a decline in LV systolic function is associated with low CN1 activity. Further studies are needed to delineate specific mechanisms controlling CN1 expression and activity, which will facilitate the development of carnosine and other histidyl dipeptide therapies for cardiometabolic disorders such as HF.

Assessing carnosinase 1 activity for diagnosing congenital disorders of glycosylation

Diagnosing Congenital Disorders of Glycosylation (CDG) is challenging due to clinical heterogeneity and the limited sensitivity of the classic serum transferrin isoelectric focusing (IEF) or capillary zone electrophoresis test. This study investigates the potential of using the glycoprotein carnosinase 1 (CN1) activity as a diagnostic marker for CDG patients. CN1 activity was measured photometrically in serum from 81 genetically confirmed CDG patients and healthy individuals. While the IEF transferrin method detected 77 patients, four remained undetected. In healthy individuals, serum CN1 activity ranged from 0.1 to 6.4 μmol/ml/h depending on age, with mean CN1 activities up to four-fold higher than in CDG patients. CDG patients´ CN1 activities never exceeded 2,04 μmol/ml/h. Using the 25th percentile to differentiate between groups, the test performance varied by age. For children over 10 years old, the sensitivity and specificity were 96 % and 83 %, respectively. For those under 10, sensitivity and specificity dropped to 71 % and to 64 %. However, CN1 activity successfully identified three of four patients with normal IEF patterns. Although mean CN1 activity in CDG patients is significantly lower than in healthy controls, the test's reliability for classic CDG diagnosis is limited, as the diagnosis is usually made at a young age. Nevertheless, it is a simple, cost-effective assay that can complement classic tests, especially in settings with limited access to complex methods or for patients with normal transferrin patterns but suspicious for CDG.

State of the Art in the Development of Human Serum Carnosinase Inhibitors

Human serum carnosinase is an enzyme that operates the preferential hydrolysis of dipeptides with a C-terminus histidine. Only higher primates excrete such an enzyme in serum and cerebrospinal fluid. In humans, the serum hydrolytic rate has high interindividual variability owing to gene polymorphism, although age, gender, diet, and also diseases and surgical interventions can modify serum activity. Human genetic diseases with altered carnosinase activity have been identified and associated with neurological disorders and age-related cognitive decline. On the contrary, low peripheral carnosinase activity has been associated with kidney protection, especially in diabetic nephropathy. Therefore, serum carnosinase is a druggable target for the development of selective inhibitors. However, only one molecule (i.e., carnostatine) has been discovered with the purpose of developing serum carnosinase inhibitors. Bestatin is the only inhibitor reported other than carnostatine, although its activity is not selective towards serum carnosinase. Herein, we present a review of the most critical findings on human serum carnosinase, including enzyme expression, localization and substrate selectivity, along with factors affecting the hydrolytic activity, its implication in human diseases and the properties of known inhibitors of the enzyme.

Human carnosinases: A brief history, medicinal relevance, and in silico analyses

Carnosine, an endogenous dipeptide, has been found to have a plethora of medicinal properties, such as antioxidant, antiageing, and chelating effects, but with one downside: a short half-life. Carnosinases and two hydrolytic enzymes, which remain enigmatic, are responsible for these features. Hence, here we emphasize why research is valuable for better understanding crucial concepts like ageing, neurodegradation, and cancerogenesis, given that inhibition of carnosinases might significantly prolong carnosine bioavailability and allow its further use in medicine. Herein, we explore the literature regarding carnosinases and present a short in silico analysis aimed at elucidating the possible recognition pattern between CN1 and its ligands.

Binding Modes of Carnostatine, Homocarnosine, and Ophidine to Human Carnosinase 1

Carnosine (CAR), anserine (ANS), homocarnosine (H-CAR), and ophidine (OPH) are histidine-containing dipeptides that show a wide range of therapeutic properties. With their potential physiological effects, these bioactive dipeptides are considered as bioactive food components. However, such dipeptides display low stability due to their rapid degradation by human serum carnosinase 1 (CN1). A dimeric CN1 hydrolyzes such histidine-containing compounds with different degrees of reactivities. A selective CN inhibitor, carnostatine (CARN), was reported to effectively inhibit CN's activity. To date, the binding mechanisms of CAR and ANS have been recently reported, while no clear information about H-CAR, OPH, and CARN binding is available. Thus, in this work, molecular dynamics simulations were employed to elucidate the binding mechanism of H-CAR, OPH, and CARN. Among all, the amine end and imidazole ring are the main players for trapping all of the ligands in a pocket. OPH shows the poorest binding affinity, while CARN displays the tightest binding. Such firm binding is due to the longer amine chain and the additional hydroxyl (-OH) group of CARN. H-CAR and CARN are analogous, but the absence of the -OH moiety in H-CAR significantly enhances its mobility, resulting in the reduction in binding affinity. For OPH which is an ANS analogue, the methylated imidazole ring destroys the OPH-CN1 interaction network at this region, consequentially leading to the poor binding ability. An insight into how CN recognizes and binds its substrates obtained here will be useful for designing an effective strategy to prolong the lifetime of CAR and its analogues after ingestion.

Exploring the structural and dynamic differences between human carnosinase I (CN1) and II (CN2)

Human carnosinases (CNs) are dimeric dipeptidases in the metallopeptidase M20 family. Two isoforms of carnosinases (Zn²⁺ -containing carnosinase 1 (CN1) found in serum and Mn²⁺ -carnosinase 2 (CN2) in tissue) were identified. Both CNs cleave histidine-containing (Xaa-His) dipeptides such as carnosine where CN2 was found to accept a broader spectrum of substrates. A loss of CN function, resulting in a high carnosine concentration, reduces risk for diabetes and neurological disorders. Although several studies on CN activities and its Michaelis complex were conducted, all shed the light on CN1 activity where the CN2 data is limited. Also, the molecular details on CN1 and CN2 similarity and dissimilarity in structure and function remain unclear. Thus, in this work, molecular dynamics (MD) simulations were employed to study structure and dynamics of human CN1 and CN2 in comparison. The results show that the different catalytic ability of both CNs is due to their pocket size and environment. CN2 can accept a wider range of substrate due to the wider mouth of a binding pocket. The L1 loop seems to play a role in gating activity. Comparing to CN2, CN1 provides more electronegative entrance, more wettability, and higher stability of catalytic metal ion-pair in the active site which allow more efficient water-mediated catalysis. The microscopic understanding obtained here can serve as a basis for CN inhibition strategies resulting in higher carnosine levels and consequently mitigating complications associated with diseases such as diabetes and neurological disorder.

Molecular insights into the binding of carnosine and anserine to human serum carnosinase 1 (CN1)

Carnosine (CAR) and anserine (ANS) are histidine-containing dipeptides that show the therapeutic properties and protective abilities against diabetes and cognitive deficit. Both dipeptides are rich in meat products and have been used as a supplement. However, in humans, both compounds have a short half-life due to the rapid degradation by dizinc carnosinase 1 (CN1) which is a hurdle for its therapeutic application. To date, a comparative study of carnosine- and anserine-CN1 complexes is limited. Thus, in this work, molecular dynamics (MD) simulations were performed to explore the binding of carnosine and anserine to CN1. CN1 comprises 2 chains (Chains A and B). Both monomers are found to work independently and alternatingly. The displacement of Zn2+ pair is found to disrupt the substrate binding. CN1 employs residues from the neighbour chain (H235, T335, and T337) to form the active site. This highlights the importance of a dimer for enzymatic activity. Anserine is more resistant to CN 1 than carnosine because of its bulky and dehydrated imidazole moiety. Although both dipeptides can direct the peptide oxygen to the active Zn2+ which can facilitate the catalytic reaction, the bulky methylated imidazole on anserine promotes various poses that can retard the hydrolytic activity in contrast to carnosine. Anserine is likely to be the temporary competitive inhibitor by retarding the carnosine catabolism.

Correlation between serum carnosinase concentration and renal damage in diabetic nephropathy patients

Diabetic nephropathy (DN) is one of the major complications of diabetes and contributes significantly towards end-stage renal disease. Previous studies have identified the gene encoding carnosinase (CN-1) as a predisposing factor for DN. Despite this fact, the relationship of the level of serum CN-1 and the progression of DN remains uninvestigated. Thus, the proposed study focused on clarifying the relationship among serum CN-1, indicators of renal function and tissue injury, and the progression of DN. A total of 14 patients with minimal changes disease (MCD) and 37 patients with DN were enrolled in the study. Additionally, 20 healthy volunteers were recruited as control. Further, DN patients were classified according to urinary albumin excretion rate into two groups: DN with microalbuminuria (n = 11) and DN with macroalbuminuria (n = 26). Clinical indicators including urinary protein components, serum carnosine concentration, serum CN-1 concentration and activity, and renal biopsy tissue injury indexes were included for analyzation. The serum CN-1 concentration and activity were observed to be the highest, but the serum carnosine concentration was the lowest in DN macroalbuminuria group. Moreover, within DN group, the concentration of serum CN-1 was positively correlated with uric acid (UA, r = 0.376, p = 0.026) and serum creatinine (SCr, r = 0.399, p = 0.018) and negatively correlated with serum albumin (Alb, r = − 0.348, p = 0.041) and estimated glomerular filtration rate (eGRF, r = − 0.432, p = 0.010). Furthermore, the concentration of serum CN-1 was discovered to be positively correlated with indicators including 24-h urinary protein–creatinine ratio (24 h-U-PRO/CRE, r = 0.528, p = 0.001), urinary albumin-to-creatinine ratio (Alb/CRE, r = 0.671, p = 0.000), urinary transferrin (TRF, r = 0.658, p = 0.000), retinol-binding protein (RBP, r = 0.523, p = 0.001), N-acetyl-glycosaminidase (NAG, r = 0.381, p = 0.024), immunoglobulin G (IgG, r = 0.522, p = 0.001), cystatin C (Cys-C, r = 0.539, p = 0.001), beta-2-microglobulin (β2-MG, r = 0.437, p = 0.009), and alpha-1-macroglobulin (α1-MG, r = 0.480, p = 0.004). Besides, in DN with macroalbuminuria group, serum CN-1 also showed a positive correlation with indicators of fibrosis, oxidative stress, and renal tubular injury. Taken together, our data suggested that the level of CN-1 was increased as clinical DN progressed. Thus, the level of serum CN-1 might be an important character during the occurrence and progression of DN. Our study will contribute significantly to future studies focused on dissecting the underlying mechanism of DN.

Development of a direct LC-ESI-MS method for the measurement of human serum carnosinase activity

Carnosine (β-alanyl-L-histidine) is a natural peptide that have been described as a potential pharmacological agent owing to some positive outcomes from several pharmacological tests in animal models of human diseases. However, carnosine has limited activity in humans since the peptide upon absorption is rapidly hydrolyzed in the serum by the enzyme carnosinase (i.e. CN1; E.C. 3.4.13.20). Over the years the main approaches aimed at limiting carnosine hydrolysis have been focused on obtaining CN1-stable derivatives with an increased bioavailability and unmodified or enhanced activity. Only recently the hypothesis of co-administration of carnosine and selective inhibitors of CN1 have been proposed. Such an approach requires reliable methods for screening the effect on carnosine hydrolysis rate operated by CN1 in a throughput scale allowing to test from few compounds up to whole compound libraries. The only assay with such features available in literature relies on ortho-phtalaldehyde (OPA) derivatization of the hydrolysis product (i.e. histidine), followed by a fluorimetric read. Herein, we propose an alternative method based on a direct measurement of the residual substrate by liquid chromatography-mass spectrometry (LC-MS). The assay demonstrated to be reliable since gave results comparable to literature data concerning the hydrolysis rate of carnosine as determined into human serum. Moreover, the method was quite flexible and easily adaptable to a substrate change, as demonstrated by the measurement of the hydrolysis rate of all the natural analogs of carnosine. In this context the data collected for anserine suggest that our method looked more reliable and substrate change can undergo an underestimation of hydrolytic activity in OPA -based assays.

Carnosine and Lung Disease

Carnosine (β-alanyl-L-histidine) is a small dipeptide with numerous activities, including antioxidant effects, metal ion chelation, proton buffering capacity, and inhibitory effects on protein carbonylation and glycation. Carnosine has been mostly studied in organs where it is abundant, including skeletal muscle, cerebral cortex, kidney, spleen, and plasma. Recently, the effect of supplementation with carnosine has been studied in organs with low levels of carnosine, such as the lung, in animal models of influenza virus or lipopolysaccharide-induced acute lung injury and pulmonary fibrosis. Among the known protective effects of carnosine, its antioxidant effect has attracted increasing attention for potential use in treating lung disease. In this review, we describe the in vitro and in vivo biological and physiological actions of carnosine. We also report our recent study and discuss the roles of carnosine or its related compounds in organs where carnosine is present in only small amounts (especially the lung) and its protective mechanisms.

Dietary GABA induces endogenous synthesis of a novel imidazole peptide homocarnosine in mouse skeletal muscles

Carnosine (β-alanyl-L-histidine) is an imidazole dipeptide present at high concentrations in skeletal muscles, where it plays a beneficial role. However, oral intake of carnosine or β-alanine to increase skeletal muscle carnosine levels has disadvantages such as low efficiency and side effects. Therefore, we proposed homocarnosine (γ-aminobutyryl-L-histidine) as a novel alternative imidazole peptide for skeletal muscle based on its structural similarity to carnosine. To induce endogenous homocarnosine synthesis in skeletal muscles, mice were fed a basal diet mixed with 0, 0.5, 2, or 5% γ-aminobutyric acid (GABA) for 6 weeks. As expected, in the control group (0% GABA), GABA and homocarnosine were present in trace concentrations. Skeletal muscle homocarnosine levels were significantly increased in the 2% and 5% GABA intake groups (tenfold, P < 0.01 and 53-fold, P < 0.01; respectively) relative to those of the control group, whereas 0.5% GABA intake induced no such effect. GABA intake had no effect on the levels of carnosine, anserine, and β-alanine. Vigabatrin (inhibitor of GABA transaminase (GABA-T)) administration to mice receiving 2% GABA intake for 2 weeks led to GABA-T inhibition in the liver. Subsequently, a 43-fold increase in circulating GABA levels and a tendency increase in skeletal muscle homocarnosine levels were observed. Therefore, skeletal muscle homocarnosine synthesis can be induced by supplying its substrate GABA in tissues. As GABA availability is tightly regulated by GABA-T via GABA degradation, inhibitors of GABA or β-alanine degradation could be novel potential interventions for increasing skeletal muscle imidazole dipeptides.

CNDP1 knockout in zebrafish alters the amino acid metabolism, restrains weight gain, but does not protect from diabetic complications

The gene CNDP1 was associated with the development of diabetic nephropathy. Its enzyme carnosinase 1 (CN1) primarily hydrolyzes the histidine-containing dipeptide carnosine but other organ and metabolic functions are mainly unknown. In our study we generated CNDP1 knockout zebrafish, which showed strongly decreased CN1 activity and increased intracellular carnosine levels. Vasculature and kidneys of CNDP1-/- zebrafish were not affected, except for a transient glomerular alteration. Amino acid profiling showed a decrease of certain amino acids in CNDP1-/- zebrafish, suggesting a specific function for CN1 in the amino acid metabolisms. Indeed, we identified a CN1 activity for Ala-His and Ser-His. Under diabetic conditions increased carnosine levels in CNDP1-/- embryos could not protect from respective organ alterations. Although, weight gain through overfeeding was restrained by CNDP1 loss. Together, zebrafish exhibits CN1 functions, while CNDP1 knockout alters the amino acid metabolism, attenuates weight gain but cannot protect organs from diabetic complications.

Carnosinase concentration, activity, and CNDP1 genotype in patients with type 2 diabetes with and without nephropathy

This study assessed if serum carnosinase (CNDP1) activity and concentration in patients with type 2 diabetes mellitus (T2D) with diabetic nephropathy (DN) differs from those without nephropathy. In a cross-sectional design 127 patients with T2D with DN ((CTG)₅ homozygous patients n = 45) and 145 patients with T2D without nephropathy ((CTG)₅ homozygous patients n = 47) were recruited. Univariate and multivariate regression analyses were performed to predict factors relevant for serum CNDP1 concentration. CNDP1 (CTG)₅ homozygous patients with T2D with DN had significantly lower CNDP1 concentrations (30.4 ± 18.3 vs 51.2 ± 17.6 µg/ml, p < 0.05) and activity (1.25 ± 0.5 vs 2.53 ± 1.1 µmol/ml/h, p < 0.05) than those without nephropathy. This applied for patients with DN on the whole, irrespective of (CTG)₅ homozygosity. In the multivariate regression analyses, lower serum CNDP1 concentrations correlated with impaired renal function and to a lesser extend with the CNDP1 genotype (95% CI of regression coefficients: eGFR: 0.10-1.94 (p = 0.001); genotype: - 0.05 to 5.79 (p = 0.055)). Our study demonstrates that serum CNDP1 concentrations associate with CNDP1 genotype and renal function in patients with T2D. Our data warrant further studies using large cohorts to confirm these findings and to delineate the correlation between low serum CNDP1 concentrations and renal function deterioration in patients with T2D.

Serum Carnosinase-1 and Albuminuria Rather than the CNDP1 Genotype Correlate with Urinary Carnosinase-1 in Diabetic and Nondiabetic Patients with Chronic Kidney Disease

BACKGROUND

Carnosinase-1 (CN-1) can be detected in 24 h urine of healthy individuals and patients with type 2 diabetes (T2DM). We aimed to assess whether urinary CN-1 is also reliably measured in spot urine and investigated its association with renal function and the albumin/creatinine ratio (ACR). We also assessed associations between the CNDP1 (CTG) _n genotype and CN-1 concentrations in serum and urine.

METHODS

Patients with T2DM (n = 85) and nondiabetic patients with chronic kidney disease (CKD) (n = 26) stratified by albuminuria (ACR ≤ 300 mg/g or ACR > 300 mg/g) recruited from the nephrology clinic and healthy subjects (n = 24) were studied.

RESULTS

Urinary CN-1 was more frequently detected and displayed higher concentrations in patients with ACR > 300 mg/g as compared to those with ACR ≤ 300 mg/g irrespective of the baseline disease (T2DM: 554 ng/ml [IQR 212-934 ng/ml] vs. 31 ng/ml [IQR 31-63 ng/ml] (p < 0.0001) and nondiabetic CKD: 197 ng/ml [IQR 112-739] vs. 31 ng/ml [IQR 31-226 ng/ml] (p = 0.015)). A positive correlation between urinary CN-1 and ACR was found (r = 0.68, p < 0.0001). Multivariate linear regression analysis revealed that ACR and serum CN-1 concentrations but not eGFR or the CNDP1 genotype are independent predictors of urinary CN-1, explaining 47% of variation of urinary CN-1 concentrations (R ² = 0.47, p < 0.0001).

CONCLUSION

These results confirm and extend previous findings on urinary CN-1 concentrations, suggesting that assessment of CN-1 in spot urine is as reliable as in 24 h urine and may indicate that urinary CN-1 in macroalbuminuric patients is primarily serum-derived and not locally produced.

Detection of carnosinase-1 in urine of healthy individuals and patients with type 2 diabetes: correlation with albuminuria and renal function

Low serum carnosinase (CN-1) concentrations are associated with low risk for development of diabetic nephropathy (DN) in patients with type 2 diabetes (T2D). Although CN-1 is expressed in the kidney, urinary CN-1 (CNU) excretion and its pathological relevance in patients with T2D have not been investigated to date. The present study therefore assessed the extent of CNU excretion in healthy subjects (n = 243) and in patients with T2D (n = 361) enrolled in the DIAbetes and LifEstyle Cohort Twente-1 (DIALECT-1) in relation to functional renal parameters. CNU was detected in a high proportion of healthy individuals, 180 (74%); median CNU excretion was 0.25 mg/24 h [(IQR 0-0.65 mg/24 h]. In patients with T2D the prevalence and extent of CNU increased in parallel with albuminuria (r = 0.59, p < 0.0001; median CNU 0.1 vs 0.2 vs 1.5 mg/24 h, p < 0.0001; prevalence of CNU 61 vs. 81 vs. 97% p < 0.05 in normo- (n = 241), micro- (n = 80) and macroalbuminuria (n = 40), respectively). Patients with estimated glomerular filtration rate (eGFR) < 30 ml/min/1.73 m² displayed higher median CNU excretion rates in comparison to patients with preserved eGFR (> 90 ml/min/1.73 m²) (1.36 vs 0.13 mg/24 h, p < 0.05). Backward stepwise multivariate linear regression analysis revealed albuminuria, eGFR and glycosuria to be independent factors of CNU excretion rates, all together explaining 37% of variation of CNU excretion rates (R² = 0.37, p < 0.0001). These results show for the first time that CN-1 can be detected in urine and warrants prospective studies to assess the relevance of CNU for renal function deterioration in diabetes patients.

Carnosinase, diabetes mellitus and the potential relevance of carnosinase deficiency

Carnosinase (CN1) is a dipeptidase, encoded by the CNDP1 gene, that degrades histidine-containing dipeptides, such as carnosine, anserine and homocarnosine. Loss of CN1 function (also called carnosinase deficiency or aminoacyl-histidine dipeptidase deficiency) has been reported in a small number of patients with highly elevated blood carnosine concentrations, denoted carnosinaemia; it is unclear whether the variety of clinical symptoms in these individuals is causally related to carnosinase deficiency. Reduced CN1 function should increase serum carnosine concentrations but the genetic basis of carnosinaemia has not been formally confirmed to be due to CNDP1 mutations. A CNDP1 polymorphism associated with low CN1 activity correlates with significantly reduced risk for diabetic nephropathy, especially in women with type 2 diabetes, and may slow progression of chronic kidney disease in children with glomerulonephritis. Studies in rodents demonstrate antiproteinuric and vasculoprotective effects of carnosine, the precise molecular mechanisms, however, are still incompletely understood. Thus, carnosinemia due to CN1 deficiency may be a non-disease; in contrast, carnosine may potentially protect against long-term sequelae of reactive metabolites accumulating, e.g. in diabetes and chronic renal failure.

Allosteric inhibition of carnosinase (CN1) by inducing a conformational shift

In humans, low serum carnosinase (CN1) activity protects patients with type 2 diabetes from diabetic nephropathy. We now characterized the interaction of thiol-containing compounds with CN1 cysteine residue at position 102, which is important for CN1 activity. Reduced glutathione (GSH), N-acetylcysteine and cysteine (3.2 ± 0.4, 2.0 ± 0.3, 1.6 ± 0.2 µmol/mg/h/mM; p < .05) lowered dose-dependently recombinant CN1 (rCN1) efficiency (5.2 ± 0.2 µmol/mg/h/mM) and normalized increased CN1 activity renal tissue samples of diabetic mice. Inhibition was allosteric. Substitution of rCN1 cysteine residues at position 102 (Mut1^C102S) and 229 (Mut2^C229S) revealed that only cysteine-102 is influenced by cysteinylation. Molecular dynamic simulation confirmed a conformational rearrangement of negatively charged residues surrounding the zinc ions causing a partial shift of the carnosine ammonium head and resulting in a less effective pose of the substrate within the catalytic cavity and decreased activity. Cysteine-compounds influence the dynamic behaviour of CN1 and therefore present a promising option for the treatment of diabetes.

Carnosine and Homocarnosine Degradation Mechanisms by the Human Carnosinase Enzyme CN1: Insights from Multiscale Simulations

The endogenous dipeptide l-carnosine, and its derivative homocarnosine, prevent and reduce several pathologies like amytrophic lateral sclerosis (ALS), Alzheimer's disease, and Parkinson's disease. Their beneficial action is severely hampered because of the hydrolysis by carnosinase enzymes, in particular the human carnosinase, hCN1. This belongs to the metallopeptidase M20 family, where a cocatalytic active site is formed by two Zn(2+) ions, bridged by a hydroxide anion. The protein may exist as a monomer and as a dimer in vivo. Here we used hybrid quantum mechanics/molecular mechanics simulations based on the dimeric apoenzyme's structural information to predict the Michaelis complexes with l-carnosine and its derivative homocarnosine. On the basis of our calculations, we suggest that (i) l-carnosine degradation occurs through a nucleophilic attack of a Zn(2+)-coordinated bridging moiety for both monomer and dimer. This mechanistic hypothesis for hCN1 catalysis differs from previous proposals, while it is in agreement with available experimental data. (ii) The experimentally measured higher affinity of homocarnosine for the enzyme relative to l-carnosine might be explained, at least in part, by more extensive interactions inside the monomeric and dimeric hCN1's active site. (iii) Hydrogen bonds at the binding site, present in the dimer but absent in the monomer, might play a role in the experimentally observed higher activity of the dimeric form. Investigations of the enzymatic reaction are required to establish or disprove this hypothesis. Our results may serve as a basis for the design of potent hCN1 inhibitors.

Monoclonal Antibody RYSK173 Recognizes the Dinuclear Zn Center of Serum Carnosinase 1 (CN-1): Possible Consequences of Zn Binding for CN-1 Recognition by RYSK173

BACKGROUND AND AIMS

The proportion of serum carnosinase (CN-1) recognized by RYSK173 monoclonal antibody negatively correlates with CN-1 activity. We thus hypothesized that the epitope recognized by RYSK173 is accessible only in a catalytically incompetent conformation of the zinc dependent enzyme and we mapped its position in the CN-1 structure. Since patients with kidney failure are often deficient in zinc and other trace elements we also assessed the RYSK173 CN-1 proportion in serum of these patients and studied the influence of hemodialysis hereon in relation to Zn2+ and Cu2+ concentration during hemodialysis.

METHODS AND RESULTS

Epitope mapping using myc-tagged CN-1 fragments and overlapping peptides revealed that the RYSK173 epitope directly contributes to the formation of the dinuclear Zn center in the catalytic domain of homodimeric CN-1. Binding of RYSK173 to CN-1 was however not influenced by addition of Zn2+ or Cu2+ to serum. In serum of healthy controls the proportion of CN-1 recognized by RYSK173 was significantly lower compared to end-stage renal disease (ESRD) patients (1.12 ± 0.17 vs. 1.56 ± 0.40% of total CN-1; p<0.001). During hemodialysis the relative proportion of RYSK173 CN-1 decreased in parallel with increased serum Zn2+ and Cu2+ concentrations after dialysis.

CONCLUSIONS

Our study clearly indicates that RYSK173 recognizes a sequence within the transition metal binding site of CN-1, thus supporting our hypothesis that metal binding to CN-1 masks the epitope. The CN-1 RYSK173 proportion appears overall increased in ESRD patients, yet it decreases during hemodialysis possibly as a consequence of a relative increase in transition metal bound enzyme.

Aerobic and resistance training do not influence plasma carnosinase content or activity in type 2 diabetes

A particular allele of the carnosinase gene (CNDP1) is associated with reduced plasma carnosinase activity and reduced risk for nephropathy in diabetic patients. On the one hand, animal and human data suggest that hyperglycemia increases plasma carnosinase activity. On the other hand, we recently reported lower carnosinase activity levels in elite athletes involved in high-intensity exercise compared with untrained controls. Therefore, this study investigates whether exercise training and the consequent reduction in hyperglycemia can suppress carnosinase activity and content in adults with type 2 diabetes. Plasma samples were taken from 243 males and females with type 2 diabetes (mean age = 54.3 yr, SD = 7.1) without major microvascular complications before and after a 6-mo exercise training program [4 groups: sedentary control (n = 61), aerobic exercise (n = 59), resistance exercise (n = 63), and combined exercise training (n = 60)]. Plasma carnosinase content and activity, hemoglobin (Hb) A1c, lipid profile, and blood pressure were measured. A 6-mo exercise training intervention, irrespective of training modality, did not decrease plasma carnosinase content or activity in type 2 diabetic patients. Plasma carnosinase content and activity showed a high interindividual but very low intraindividual variability over the 6-mo period. Age and sex, but not Hb A1c, were significantly related to the activity or content of this enzyme. It can be concluded that the beneficial effects of exercise training on the incidence of diabetic complications are probably not related to a lowering effect on plasma carnosinase content or activity.

Intrinsic carnosine metabolism in the human kidney

Histidine-containing dipeptides like carnosine and anserine have protective functions in both health and disease. Animal studies suggest that carnosine can be metabolized within the kidney. The goal of this study was to obtain evidence of carnosine metabolism in the human kidney and to provide insight with regards to diabetic nephropathy. Expression, distribution, and localization of carnosinase-1 (CNDP1), carnosine synthase (CARNS), and taurine transporters (TauT) were measured in human kidneys. CNDP1 and CARNS activities were measured in vitro. CNDP1 and CARNS were located primarily in distal and proximal tubules, respectively. Specifically, CNDP1 levels were high in tubular cells and podocytes (20.3 ± 3.4 and 15 ± 3.2 ng/mg, respectively) and considerably lower in endothelial cells (0.5 ± 0.1 ng/mg). CNDP1 expression was correlated with the degradation of carnosine and anserine (r = 0.88 and 0.81, respectively). Anserine and carnosine were also detectable by HPLC in the renal cortex. Finally, TauT mRNA and protein were found in all renal epithelial cells. In diabetic patients, CNDP1 seemed to be reallocated to proximal tubules. We report compelling evidence that the kidney has an intrinsic capacity to metabolize carnosine. Both CNDP1 and CARNS are expressed in glomeruli and tubular cells. Carnosine-synthesizing and carnosine-hydrolyzing enzymes are localized in distinct compartments in the nephron and increased CNDP1 levels suggest a higher CNDP1 activity in diabetic kidneys.

Carnosine metabolism in diabetes is altered by reactive metabolites

Carnosinase 1 (CN1) contributes to diabetic nephropathy by cleaving histidine-dipeptides which scavenge reactive oxygen and carbonyl species and increase nitric oxide (NO) production. In diabetic mice renal CN1 activity is increased, the regulatory mechanisms are unknown. We therefore analysed the in vitro and in vivo regulation of CN1 activity using recombinant and human CN1, and the db/db mouse model of diabetes. Glucose, leptin and insulin did not modify recombinant and human CN1 activity in vitro, glucose did not alter renal CN1 activity of WT or db/db mice ex vivo. Reactive metabolite methylglyoxal and Fenton reagent carbonylated recombinant CN1 and doubled CN1 efficiency. NO S-nitrosylated CN1 and decreased CN1 efficiency for carnosine by 70 % (p < 0.01), but not for anserine. Both CN1 cysteine residues were nitrosylated, the cysteine at position 102 but not at position 229 regulated CN1 activities. In db/db mice, renal CN1 mRNA and protein levels were similar as in non-diabetic controls, CN1 efficiency 1.9 and 1.6 fold higher for carnosine and anserine. Renal carbonyl stress was strongly increased and NO production halved, CN1 highly carbonylated and less S-nitrosylated compared to WT mice. GSH and NO2/3 concentrations were reduced and inversely related with carnosine degradation rate (r = -0.82/-0.85). Thus, reactive metabolites of diabetes upregulate CN1 activity by post-translational modifications, and thus decrease the availability of reactive metabolite-scavenging histidine dipeptides in the kidney in a positive feedback loop. Interference with this vicious circle may represent a new therapeutic target for mitigation of DN.

Carnosine decreases IGFBP1 production in db/db mice through suppression of HIF-1

IGF binding protein 1 (IGFBP1) is a member of the binding proteins for the IGF with an important role in glucose homeostasis. Circulating IGFBP1 is derived essentially from the liver where it is mainly regulated negatively by insulin. Carnosine, a natural antioxidant, has been shown to improve metabolic control in different animal models of diabetes but its mechanisms of action are still not completely unraveled. We therefore investigate the effect of carnosine treatment on the IGFBP1 regulation in db/db mice. Db/db mice and heterozygous non-diabetic mice received for 4 weeks regular water or water supplemented with carnosine. Igfbp1 mRNA expression in the liver was evaluated using qPCR and the protein levels in plasma by western blot. Plasma IGF1 and insulin were analyzed using immunoassays. HepG2 cells were used to study the in vitro effect of carnosine on IGFBP1. The modulation of hypoxia inducible factor-1 alpha (HIF-1α) which is the central mediator of hypoxia-induction of IGFBP1 was analyzed using: WB, reporter gene assay and qPCR. Carnosine decreased the circulating IGFBP1 levels and the liver expression Igfbp1, through a complex mechanism acting both directly by suppressing the HIF-1α-mediated IGFBP1 induction and indirectly through increasing circulating insulin level followed by a decrease in the blood glucose levels and increased the plasma levels or IGF1. Reduction of IGFBP1 in diabetes through insulin-dependent and insulin-independent pathways is a novel mechanism by which carnosine contributes to the improvement of the metabolic control in diabetes.

Muscle histidine-containing dipeptides are elevated by glucose intolerance in both rodents and men

OBJECTIVE

Muscle carnosine and its methylated form anserine are histidine-containing dipeptides. Both dipeptides have the ability to quench reactive carbonyl species and previous studies have shown that endogenous tissue levels are decreased in chronic diseases, such as diabetes.

DESIGN AND METHODS

Rodent study: Skeletal muscles of rats and mice were collected from 4 different diet-intervention studies, aiming to induce various degrees of glucose intolerance: 45% high-fat feeding (male rats), 60% high-fat feeding (male rats), cafeteria feeding (male rats), 70% high-fat feeding (female mice). Body weight, glucose-tolerance and muscle histidine-containing dipeptides were assessed. Human study: Muscle biopsies were taken from m. vastus lateralis in 35 males (9 lean, 8 obese, 9 prediabetic and 9 newly diagnosed type 2 diabetic patients) and muscle carnosine and gene expression of muscle fiber type markers were measured.

RESULTS

Diet interventions in rodents (cafeteria and 70% high-fat feeding) induced increases in body weight, glucose intolerance and levels of histidine-containing dipeptides in muscle. In humans, obese, prediabetic and diabetic men had increased muscle carnosine content compared to the lean (+21% (p>0.1), +30% (p<0.05) and +39% (p<0.05), respectively). The gene expression of fast-oxidative type 2A myosin heavy chain was increased in the prediabetic (1.8-fold, p<0.05) and tended to increase in the diabetic men (1.6-fold, p = 0.07), compared to healthy lean subjects.

CONCLUSION

Muscle histidine-containing dipeptides increases with progressive glucose intolerance, in male individuals (cross-sectional). In addition, high-fat diet-induced glucose intolerance was associated with increased muscle histidine-containing dipeptides in female mice (interventional). Increased muscle carnosine content might reflect fiber type composition and/or act as a compensatory mechanism aimed at preventing cell damage in states of impaired glucose tolerance.

Carnosine treatment in combination with ACE inhibition in diabetic rats

In humans, we reported an association of a certain allele of carnosinase gene with reduced carnosinase activity and absence of nephropathy in diabetic patients. CN1 degrades histidine dipeptides such as carnosine and anserine. Further, we and others showed that treatment with carnosine improves renal function and wound healing in diabetic mice and rats. We now investigated the effects of carnosine treatment alone and in combination with ACE inhibition, a clinically established nephroprotective drug in diabetic nephropathy. Male Sprague-Dawley rats were injected i.v. with streptozotocin (STZ) to induce diabetes. After 4 weeks, rats were unilaterally nephrectomized and randomized for 24 weeks of treatment with carnosine, lisinopril or both. Renal CN1 protein concentrations were increased under diabetic conditions which correlated with decreased anserine levels. Carnosine treatment normalized CN1 abundance and reduced glucosuria, blood concentrations of glycosylated hemoglobin (HbA1c), carboxyl-methyl lysine (CML), N-acetylglucosamine (GlcNac; all p<0.05 vs. non-treated STZ rats), reduced cataract formation (p<0.05) and urinary albumin excretion (p<0.05), preserved podocyte number (p<0.05) and normalized the increased renal tissue CN1 protein concentration. Treatment with lisinopril had no effect on HbA1C, glucosuria, cataract formation and CN1 concentration, but reduced albumin excretion rate more effectively than carnosine treatment (p<0.05). Treatment with both carnosine and lisinopril combined the effects of single treatment, albeit without additive effect on podocyte number or albuminuria. Increased CN1 amount resulted in decreased anserine levels in the kidney. Both carnosine and lisinopril exert distinct beneficial effects in a standard model of diabetic nephropathy. Both drugs administered together combine the respective effects of single treatment, albeit without exerting additive nephroprotection.

Physiology and pathophysiology of carnosine

Carnosine (β-alanyl-l-histidine) was discovered in 1900 as an abundant non-protein nitrogen-containing compound of meat. The dipeptide is not only found in skeletal muscle, but also in other excitable tissues. Most animals, except humans, also possess a methylated variant of carnosine, either anserine or ophidine/balenine, collectively called the histidine-containing dipeptides. This review aims to decipher the physiological roles of carnosine, based on its biochemical properties. The latter include pH-buffering, metal-ion chelation, and antioxidant capacity as well as the capacity to protect against formation of advanced glycation and lipoxidation end-products. For these reasons, the therapeutic potential of carnosine supplementation has been tested in numerous diseases in which ischemic or oxidative stress are involved. For several pathologies, such as diabetes and its complications, ocular disease, aging, and neurological disorders, promising preclinical and clinical results have been obtained. Also the pathophysiological relevance of serum carnosinase, the enzyme actively degrading carnosine into l-histidine and β-alanine, is discussed. The carnosine system has evolved as a pluripotent solution to a number of homeostatic challenges. l-Histidine, and more specifically its imidazole moiety, appears to be the prime bioactive component, whereas β-alanine is mainly regulating the synthesis of the dipeptide. This paper summarizes a century of scientific exploration on the (patho)physiological role of carnosine and related compounds. However, far more experiments in the fields of physiology and related disciplines (biology, pharmacology, genetics, molecular biology, etc.) are required to gain a full understanding of the function and applications of this intriguing molecule.

Copper(II)-chelating homocarnosine glycoconjugate as a new multifunctional compound

Homocarnosine is an endogenous dipeptide distributed in cerebral regions and cerebrospinal fluid. Homocarnosine may serve as an antioxidant, free radical scavenger, neurotransmitter, buffering system and metal chelating agent, especially for copper(II) and zinc(II). The homeostasis of homocarnosine is regulated by carnosinases; the serum-circulating isoform of these metallodipeptidases partially hydrolyses homocarnosine in the blood. The enzyme activity is also inhibited by homocarnosine itself in a dose-dependent manner. We synthesized a new multifunctional homocarnosine derivative with trehalose, a disaccharide that possesses several beneficial properties, among which the inhibition of protein aggregation (i.e. Aβ amyloid and polyglutamine proteins) involved in widespread neurodegenerative disorders. We studied the copper(II) binding features of the new conjugate by means of potentiometric and spectroscopic techniques (UV-visible and circular dichroism) and the superoxide dismutase-like activity of the copper(II) complexes with homocarnosine and its trehalose conjugate was evaluated. The inhibitory effect of the new homocarnosine derivative on the carnosinase activity and its effects on Aβ aggregation were also investigated.

Low plasma carnosinase activity promotes carnosinemia after carnosine ingestion in humans

A polymorphism in the carnosine dipeptidase-1 gene ( CNDP1), resulting in decreased plasma carnosinase activity, is associated with a reduced risk for diabetic nephropathy. Because carnosine, a natural scavenger/suppressor of ROS, advanced glycation end products, and reactive aldehydes, is readily degraded in blood by the highly active carnosinase enzyme, it has been postulated that low serum carnosinase activity might be advantageous to reduce diabetic complications. The aim of this study was to examine whether low carnosinase activity promotes circulating carnosine levels after carnosine supplementation in humans. Blood and urine were sampled in 25 healthy subjects after acute supplementation with 60 mg/kg body wt carnosine. Precooled EDTA-containing tubes were used for blood withdrawal, and plasma samples were immediately deproteinized and analyzed for carnosine and β-alanine by HPLC. CNDP1 genotype, baseline plasma carnosinase activity, and protein content were assessed. Upon carnosine ingestion, 8 of the 25 subjects (responders) displayed a measurable increase in plasma carnosine up to 1 h after supplementation. Subjects with no measurable increment in plasma carnosine (nonresponders) had ∼2-fold higher plasma carnosinase protein content and ∼1.5-fold higher activity compared with responders. Urinary carnosine recovery was 2.6-fold higher in responders versus nonresponders and was negatively dependent on both the activity and protein content of the plasma carnosinase enzyme. In conclusion, low plasma carnosinase activity promotes the presence of circulating carnosine upon an oral challenge. These data may further clarify the link among CNDP1 genotype, carnosinase, and diabetic nephropathy.

Different conformational forms of serum carnosinase detected by a newly developed sandwich ELISA for the measurements of carnosinase concentrations

Serum carnosinase (CN-1) measurements are at present mainly performed by assessing enzyme activity. This method is time-consuming, not well suited for large series of samples and can be discordant to measurements of CN-1 protein concentrations. To overcome these limitations, we developed sandwich ELISA assays using different anti-CN-1 antibodies, i.e., ATLAS (polyclonal IgG) and RYSK173 (monoclonal IgG1). With the ATLAS-based assay, similar amounts of CN-1 were detected in serum and both EDTA and heparin plasma. The RYSKS173-based assay detected CN-1 in serum in all individuals at significantly lower concentrations compared to the ATLAS-based assay (range: 0.1-1.8 vs. 1-50 μg/ml, RYSK- vs. ATLAS-based, P<0.01). CN-1 detection with the RYSK-based assay was increased in EDTA plasma, albeit at significantly lower concentrations compared to ATLAS. In heparin plasma, CN-1 was also poorly detected with the RYSK-based assay. Addition of DTT to serum increased the detection of CN-1 in the RYSK-based assay almost to the levels found in the ATLAS-based assay. Both ELISA assays were highly reproducible (R: 0.99, P<0.01 and R: 0.93, P<0.01, for the RYSK- and ATLAS-based assays, respectively). Results of the ATLAS-based assay showed a positive correlation with CN-1 activity (R: 0.62, P<0.01), while this was not the case for the RYSK-based assay. However, there was a negative correlation between CN-1 activity and the proportion of CN-1 detected in the RYSK-based assay, i.e., CN-1 detected with the RYSK-based assay/CN-1 detected with the ATLAS-based assay × 100% (Spearman-Rang correlation coefficient: -0.6, P<0.01), suggesting that the RYSK-based assay most likely detects a CN-1 conformation with low CN-1 activity. RYSK173 and ATLAS antibodies reacted similarly in Western blot, irrespective of PNGase treatment. Binding of RYSK173 in serum was not due to differential N-glycosylation as demonstrated by mutant CN-1 cDNA constructs. In conclusion, our study demonstrates a good correlation between enzyme activity and CN-1 protein concentration in ELISA and suggests the presence of different CN-1 conformations in serum. The relevance of these different conformations is still elusive and needs to be addressed in further studies.

Carnosine treatment largely prevents alterations of renal carnosine metabolism in diabetic mice

Recently, we identified an allelic variant of human carnosinase 1 (CN1) that results in increased enzyme activity and is associated with susceptibility for diabetic nephropathy in humans. Investigations in diabetic (db/db) mice showed that carnosine ameliorates glucose metabolism effectively. We now investigated the renal carnosinase metabolism in db/db mice. Kidney CN1 activity increased with age and was significantly higher in diabetic mice compared to controls. Increased CN1 activity did not affect renal carnosine levels, but anserine concentrations were tenfold lower in db/db mice compared to controls (0.24±0.2 vs. 2.28±0.3 nmol/mg protein in controls; p<0.001). Homocarnosine concentrations in kidney tissue were low in both control and db/db mice (below 0.1 nmol/mg protein, p=n.s.). Carnosine treatment for 4 weeks substantially decreased renal CN1 activity in diabetic mice (0.32±0.3 in non-treated db/db vs. 0.05±0.05 μmol/mg/h in treated db/db mice; p<0.01) close to normal activities. Renal anserine concentrations increased significantly (0.24±0.2 in non-treated db/db vs. 5.7±1.2 μmol/mg/h in treated db/db mice; p<0.01), while carnosine concentrations remained unaltered (53±6.4 in non-treated vs. 61±15 nmol/mg protein in treated db/db mice; p=n.s.). Further, carnosine treatment halved proteinuria and reduced vascular permeability to one-fifth in db/db mice. In renal tissue of diabetic mice carnosinase activity is significantly increased and anserine concentrations are significantly reduced compared to controls. Carnosine treatment largely prevents the alterations of renal carnosine metabolism.

Anserine inhibits carnosine degradation but in human serum carnosinase (CN1) is not correlated with histidine dipeptide concentration

BACKGROUND

We reported an association of a particular allele of the carnosinase (CNDP1 Mannheim) gene with reduced serum carnosinase (CN1) activity and absence of nephropathy in diabetic patients. Carnosine protects against the adverse effects of high glucose levels but serum carnosine concentration was generally low.

METHODS

We measured the concentration of two further histidine dipeptides, anserine and homocarnosine, via HPLC. CN1 activity was measured fluorometically and for concentration we developed a capture ELISA.

RESULTS

We found an association between the CNDP1 Mannheim allele and reduced serum CN1 activity for all three dipeptides but no correlation to serum concentrations although anserine and homocarnosine inhibited carnosinase activity. Patients with liver cirrhosis have low CN1 activity (0.24 ± 0.17 μmol/ml/h, n=7 males; normal range: 3.2 ± 1.1, n=104; p<0.05) and CN1 concentrations (2.3 ± 1.5 μg/ml; normal range: 24.9 ± 8.9, p<0.05) but surprisingly, histidine dipeptide concentrations in serum are not increased compared to controls.

CONCLUSIONS

Serum histidine dipeptide concentrations are not correlated to CN1 activity. The protective effect of low CN1 activity might be related either to turnover of CN1 substrates or a protective function of dipeptides might be localized in other tissues.

N-glycosylation of carnosinase influences protein secretion and enzyme activity: implications for hyperglycemia

OBJECTIVE

The (CTG)(n) polymorphism in the serum carnosinase (CN-1) gene affects CN-1 secretion. Since CN-1 is heavily glycosylated and glycosylation might influence protein secretion as well, we tested the role of N-glycosylation for CN-1 secretion and enzyme activity. We also tested whether CN-1 secretion is changed under hyperglycemic conditions.

RESULTS

N-glycosylation of CN-1 was either inhibited by tunicamycin in pCSII-CN-1-transfected Cos-7 cells or by stepwise deletion of its three putative N-glycosylation sites. CN-1 protein expression, N-glycosylation, and enzyme activity were assessed in cell extracts and supernatants. The influence of hyperglycemia on CN-1 enzyme activity in human serum was tested in homozygous (CTG)(5) diabetic patients and healthy control subjects. Tunicamycin completely inhibited CN-1 secretion. Deletion of all N-glycosylation sites was required to reduce CN-1 secretion efficiency. Enzyme activity was already diminished when two sites were deleted. In pCSII-CN-1-transfected Cos-7 cells cultured in medium containing 25 mmol/l d-glucose, the immature 61 kilodaltons (kDa) CN-1 immune reactive band was not detected. This was paralleled by an increased GlcNAc expression in cell lysates and CN-1 expression in the supernatants. Homozygous (CTG)(5) diabetic patients had significantly higher serum CN-1 activity compared with genotype-matched, healthy control subjects.

CONCLUSIONS

We conclude that apart from the (CTG)(n) polymorphism in the signal peptide of CN-1, N-glycosylation is essential for appropriate secretion and enzyme activity. Since hyperglycemia enhances CN-1 secretion and enzyme activity, our data suggest that poor blood glucose control in diabetic patients might result in an increased CN-1 secretion even in the presence of the (CTG)(5) allele.