Carnosinase deficiency: a new variant with high residual activity

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.

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.

A CTG polymorphism in the CNDP1 gene determines the secretion of serum carnosinase in Cos-7 transfected cells

Recently, we demonstrated that a polymorphism in exon 2 of the serum carnosinase (CNDP1) gene is associated with susceptibility to developing diabetic nephropathy. Based on the number of CTG repeats in the signal peptide, five different alleles coding for 4, 5, 6, 7, or 8 leucines (4L-8L) are known. Diabetic patients without nephropathy are homozygous for the 5L allele more frequently than those with nephropathy. Since serum carnosinase activity correlates with CNDP1 genotype, we hypothesized in the present study that secretion of serum carnosinase is determined by the CNDP1 genotype. To test this hypothesis, we transfected Cos-7 cells with different CNDP1 constructs varying in CTG repeats and assessed the expression of CNDP1 protein in cell extracts and supernatants. Our results demonstrate that CNDP1 secretion is significantly higher in cells expressing variants with more than five leucines in the signal peptide. Hence, our data might explain why individuals homozygous for the 5L allele have low serum carnosinase activity. Because carnosine, the natural substrate for carnosinase, exerts antioxidative effects and inhibits ACE activity and advanced glycation end product formation, our results support the finding that diabetic patients homozygous for CNDP1 5L are protected against diabetic nephropathy.

Inborn errors of carnosine and homocarnosine metabolism

Serum carnosinase deficiency with carnosinuria has been reported in 23 children with neurological signs and/or mental retardation. In adults four cases in one family had serum carnosinase deficiency, carnosinuria, and in addition elevated homocarnosine in CSF and in the brain. The mother was one of these cases but had no clinical symptoms; however her three children have spastic paraparesis, retinitis pigmentosa and mental retardation. Serum carnosinase deficiency alone is not the cause of the neurological symptoms. When two of the affected children consumed carnosine, anserine or homocarnosine, they metabolized these compounds much less rapidly than did two normal control individuals.

Urinary excretion of 1-methylhistidine: a qualitative indicator of exogenous 3-methylhistidine and intake of meats from various sources

In order to investigate whether the urinary excretion of 1-methylhistidine (1MH) might serve as an objective indicator of meat ingestion and exogenous 3-methylhistidine (3MH) intake, healthy subjects were fed an ovolactovegetarian diet. At five-day intervals they were given meat of different origin and 24-hour urinary excretions of 1MH and 3MH were determined. After beef intake there was a marked increase of 3MH and 1MH excretion. The elimination curves were found to follow first-order kinetics and to indicate similar elimination rates. 1MH was present in ten different types of meat analyzed. A strong linear relationship was found between increase in 3MH and 1MH excretion and the amount of chicken, pork, or plaice ingested. IMH may serve as an objective indicator of meat and exogenous 3MH intake, since it is present in meat, and, regardless of source, shows similar dose-independent kinetics, and has similar half-life to 3MH.

Inherited differences in mouse kidney carnosinase activity

Carnosinase is a peptidase which cleaves B-alanyl-L-histidine (carnosine) and closely related dipeptides. Its activity in kidney cytosol of various mouse strains varies more than 50-fold. The highest activity occurs in random-bred CD-1 and inbred NZB/BINJ mice, while it is barely detectable in BALB/cJ, C57BL/6J, and AU/SsJ among others. Carnosinase is immunologically and enzymologically identical in all high-activity strains. This is the first report of quantitative interstrain differences in carnosinase activity. No other peptidase activity has been reported which exhibits the same strain distribution shown here. In matings and backcrosses between the NZB/BINJ and the BALB/cJ strains, the levels of kidney carnosinase activity in the progeny behave as a classical Mendelian trait.

Human serum carnosinase: characterization, distinction from cellular carnosinase, and activation by cadmium

Human serum carnosinase was assayed using a simple and sensitive fluorometric method. Under optimum conditions, the average adult serum hydrolyzed 42 mu mol of carnosine per ml per hour, about 17 times the average activity reported in the literature. Cadmium was twice as effective as manganese as an activator of this enzyme. Serum carnosinase was found to be different in many respects from cellular carnosinase. For example, the serum isozyme hydrolyzed homocarnosine, whereas the cellular carnosinase did not. The apparent molecular weight of serum carnosinase was 160 000, while that of the cellular isozyme was 90 000. Although it has been reported that serum contains two molecular forms of carnosinase, only one form was detected using several electrophoretic methods and two ion exchange chromatography procedures. The concentration of serum carnosinase varied greatly between individuals. Little or no enzyme was detected in children below 10 months in age. Thereafter, the average concentration of carnosinase increased gradually to reach the adult range at age 13-15.