Effect of phenylalanine and its metabolites on ATP diphosphohydrolase activity in synaptosomes from rat cerebral cortex

Chemically induced acute model of sarcosinemia in wistar rats

In the present study, we developed an acute chemically induced model of sarcosinemia in Wistar rats. Wistar rats of 7, 14 and 21 postpartum days received sarcosine intraperitoneally in doses of 0.5 mmol/Kg of body weight three time a day at intervals of 3 h. Control animals received saline solution (NaCl 0.85 g%) in the same volume (10 mL/Kg of body weight). The animals were killed after 30 min, 1, 2, 3 or 6 h after the last injection and the brain and the blood were collected for sarcosine measurement. The results showed that plasma and brain sarcosine concentrations achieved levels three to four times higher than the normal levels and decreased in a time-dependent way, achieving normal levels after 6 hours. Considering that experimental animal models are useful to investigate the pathophysiology of human disorders, our model of sarcosinemia may be useful for the research of the mechanisms of neurological dysfunction caused by high tissue sarcosine levels.

DNA damage induced by phenylalanine and its analogue p-chlorophenylalanine in blood and brain of rats subjected to a model of hyperphenylalaninemia

Phenylketonuria (PKU) is a disease caused by a deficiency of phenylalanine hydroxylase (PAH), resulting in an accumulation of phenylalanine (Phe) in the brain tissue, cerebrospinal fluid, and other tissues of PKU patients. Considering that high levels of Phe are associated with neurological dysfunction and that the mechanisms underlying the neurotoxicity in PKU remain poorly understood, the main objective of this study was to investigate the in vivo and in vitro effects of Phe on DNA damage, as determined by the alkaline comet assay. The results showed that, compared to control group, the levels of DNA migration were significantly greater after acute administration of Phe, p-chlorophenylalanine (p-Cl-Phe, an inhibitor of PAH), or a combination thereof in cerebral cortex and blood, indicating DNA damage. These treatments also provoked increase of carbonyl content. Additionally, when Phe or p-Cl-Phe was present in the incubation medium, we observed an increase in the frequency and index of DNA damage in the cerebral cortex and blood, without affecting lactate dehydrogenase (LDH) release. Our in vitro and in vivo findings indicate that DNA damage occurs in the cerebral cortex and blood of rats receiving Phe, suggesting that this mechanism could be, at least in part, responsible for the neurological dysfunction in PKU patients.

Arginine exposure alters ectonucleotidase activities and morphology of zebrafish larvae (Danio rerio)

Hyperargininemia is an inborn error of metabolism (IEM) characterized by tissue accumulation of arginine (Arg). Mental retardation and other neurological features are common symptoms in hyperargininemic patients. Considering purinergic signaling has a crucial role from the early stages of development and underlying mechanisms of this disease are poorly established, we investigated the effect of Arg administration on locomotor activity, morphological alterations, and extracellular nucleotide hydrolysis in larvae and adult zebrafish. We showed that 0.1 mM Arg was unable to promote changes in locomotor activity. In addition, 7-day-post-fertilization (dpf) larvae treated with Arg demonstrated a decreased body size. Arg exposure (0.1 mM) promoted an increase in ATP, ADP, and AMP hydrolysis when compared to control group. These findings demonstrated that Arg might affect morphological parameters and ectonucleotidase activities in zebrafish larvae, suggesting that purinergic system is a target for neurotoxic effects induced by Arg.

Behavioral and neurochemical effects of proline

Proline is an amino acid with an essential role for primary metabolism and physiologic functions. Hyperprolinemia results from the deficiency of specific enzymes for proline catabolism, leading to tissue accumulation of this amino acid. Hyperprolinemic patients can present neurological symptoms and brain abnormalities, whose aetiopathogenesis is poorly understood. This review addresses some of the findings obtained, mainly from animal studies, indicating that high proline levels may be associated to neuropathophysiology of some disorders. In this context, it has been suggested that energy metabolism deficit, Na(+),K(+)-ATPase, kinase creatine, oxidative stress, excitotoxicity, lipid content, as well as purinergic and cholinergic systems are involved in the effect of proline on brain damage and spatial memory deficit. The discussion focuses on the relatively low antioxidant defenses of the brain and the vulnerability of neural tissue to reactive species. This offers new perspectives for potential therapeutic strategies for this condition, which may include the early use of appropriate antioxidants as a novel adjuvant therapy, besides the usual treatment based on special diets poor in proline.

Development of an animal model for chronic mild hyperhomocysteinemia and its response to oxidative damage

The purpose of this study was to develop a chronic chemically induced model of mild hyperhomocysteinemia in adult rats. We produced levels of Hcy in the blood (30μM), comparable to those considered a risk factor for the development of neurological and cardiovascular diseases, by injecting homocysteine subcutaneously (0.03μmol/g of body weight) twice a day, from the 30th to the 60th postpartum day. Controls received saline in the same volumes. Using this model, we evaluated the effect of chronic administration of homocysteine on redox status in the blood and cerebral cortex of adult rats. Reactive oxygen species and thiobarbituric acid reactive substances were significantly increased in the plasma and cerebral cortex, while nitrite levels were reduced in the cerebral cortex, but not in the plasma, of rats subjected to chronic mild hyperhomocysteinemia. Homocysteine was also seen to disrupt enzymatic and non-enzymatic antioxidant defenses in the blood and cerebral cortex of rats. Since experimental animal models are useful for understanding the pathophysiology of human diseases, the present model of mild hyperhomocysteinemia may be useful for the investigation of additional mechanisms involved in tissue alterations caused by homocysteine.

Trypanosoma evansi: Activities of adenine nucleotide degradation enzymes in cerebral cortex of infected rats

This study aimed to evaluate the activities of the ectoenzymes NTPDase and 5'-nucleotidase in synaptosomes from cerebral cortex of rats experimentally infected with Trypanosoma evansi. The animals were divided in four groups (n=10) according to the time and degree of parasitemia (groups A, B, C and D). The animals from group A were euthanized on day 3 (low parasitemia), group B on day 5 (high parasitemia) and group C on day 15 (low parasitemia). Group D consisted of healthy rats (not-infected, n=15) and were divided in three periods (n=5) in order to compare with the infected groups. After euthanasia, cerebral cortex was removed for the preparation of synaptosomes and enzymatic assays. Group A showed no changes in enzymatic activities compared with control. The hydrolysis of ATP, ADP and AMP by the enzymes NTPDase and 5'-nucleotidase were increased (P<0.05) in group B (38%, 140% and 61%, respectively) when compared with control. In the group C it was observed a decreased (22%) hydrolysis of ATP when compared with control group. The activities of NTPDase and 5'-nucleotidase in synaptosomes alters the acute phase of the disease when the number of circulating parasites is high, thus the change observed is probably due to the parasitemia.

NTPDase and 5'-nucleotidase activities of synaptosomes from hippocampus of rats subjected to hyperargininemia

ATP is an important excitatory neurotransmitter and adenosine acts as a neuromodulatory structure inhibiting neurotransmitters release in the central nervous system. Since the ecto-nucleotidase cascade that hydrolyzes ATP to adenosine is involved in the control of brain functions and previous studies realized in our laboratory have recently reported that acute administration of Arg decreases the NTPDase and 5'-nucleotidase activities of rat blood serum, in the present study we investigated the effect of arginine administration on NTPDase and 5'-nucleotidase activities by synaptosomes from hippocampus of rats. First, sixty-days-old rats were treated with a single or a triple intraperitoneal injection of arginine (0.8 g/Kg) or an equivalent volume of 0.9% saline solution (control) and were killed 1 h later. Second, rats received an intracerebroventricular injection of 1.5 mM arginine solution or saline (5 microL) and were killed 1 h later. We also tested the in vitro effect of arginine (0.1-1.5 mM) on nucleotide hydrolysis in synaptosomes from rat hippocampus. Results showed that intraperitoneal arginine administration did not alter nucleotide hydrolysis. On the other hand, arginine administered intracerebroventricularly reduced ATP (32%), ADP (30%) and AMP (21%) hydrolysis, respectively. In addition, arginine added to the incubation medium, provoked a decrease on ATP (19%), ADP (17%) and AMP (23%) hydrolysis, respectively. Furthermore, kinetic studies showed that the inhibitory effect of arginine was uncompetitive in relation to ATP, ADP and AMP. In conclusion, according to our results it seems reasonable to postulate that arginine alters the cascade involved in the extracellular degradation of ATP to adenosine.

Proline induces alterations on nucleotide hydrolysis in synaptosomes from cerebral cortex of rats

In the present study we investigated the in vivo (acute and chronic) and in vitro effects of proline on NTPDase and 5'-nucleotidase activities in synaptosomes obtained from cerebral cortex of rats. For acute administration, 29-day-old rats received one subcutaneous injection of proline (18.2 micromol/g body weight) or an equivalent volume of 0.9% saline solution (control) and were killed 1 h later. For chronic treatment, buffered proline was injected subcutaneously into rats twice a day at 10 h intervals from the 6th to the 28th day of age. Rats were killed 12 h after the last injection. Results showed that acute and chronic proline administration provoked a reduction (25%) of ATP hydrolysis, but did not alter ADP and AMP hydrolysis. We also verified the in vitro effect of proline (3.0 microM-1.0 mM) on nucleotide hydrolysis in synaptosomes from cerebral cortex of rats. In contrast to the in vivo studies, it was not observed any statistically significant alteration on ATP, ADP and AMP hydrolysis. In conclusion, according to our results, it seems reasonable to postulate that proline administration alters the hydrolysis of ATP and probably affects the responses mediated by adenine nucleotides in the central nervous system of proline treated rats.

NTPDase and 5'-nucleotidase activities in physiological and disease conditions: new perspectives for human health

Extracellular nucleotides and nucleosides act as signaling molecules involved in a wide spectrum of biological effects. Their levels are controlled by a complex cell surface-located group of enzymes called ectonucleotidases. There are four major families of ectonucleotidases, nucleoside triphosphate diphosphohydrolases (NTPDases/CD39), ectonucleotide pyrophosphatase/phosphodiesterases (E-NPPs), alkaline phosphatases and ecto-5'-nucleotidase. In the last few years, substantial progress has been made toward the molecular identification of members of the ectonucleotidase families and their enzyme structures and functions. In this review, there is an emphasis on the involvement of NTPDase and 5'-nucleotidase activities in disease processes in several tissues and cell types. Brief background information is given about the general characteristics of these enzymes, followed by a discussion of their roles in thromboregulatory events in diabetes, hypertension, hypercholesterolemia and cancer, as well as in pathological conditions where platelets are less responsive, such as in chronic renal failure. In addition, immunomodulation and cell-cell interactions involving these enzymes are considered, as well as ATP and ADP hydrolysis under different clinical conditions related with alterations in the immune system, such as acute lymphoblastic leukemia (ALL), B-chronic lymphocytic leukemia (B-CLL) and infections associated with human immunodeficiency virus (HIV). Finally, changes in ATP, ADP and AMP hydrolysis induced by inborn errors of metabolism, seizures and epilepsy are discussed in order to highlight the importance of these enzymes in the control of neuronal activity in pathological conditions. Despite advances made toward understanding the molecular structure of ectonucleotidases, much more investigation will be necessary to entirely grasp their role in physiological and pathological conditions.

Investigation of oxidative stress parameters in treated phenylketonuric patients

Phenylketonuria (PKU) is the most frequent disturbance of amino acid metabolism being caused by severe deficiency of phenylalanine hydroxylase activity. Untreated PKU patients present severe mental retardation whose pathophysiology is not completely estabilished. Despite the low-Phe diet, a considerable number of phenylketonuric patients present a mild to moderate psychomotor delay and decreased cognitive functions. In the present study we evaluated various parameters of oxidative stress namely thiobarbituric acid-reactive species (TBA-RS), total antioxidant reactivity (TAR) and activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) in two groups of treated PKU patients, one with well controlled and the other with high Phe blood levels in order to investigate whether blood Phe concentrations could be correlated with the extend of oxidative stress. We initially verified a marked increase of TBA-RS, and a decrease of TAR in plasma, as well as a reduction of erythrocyte GSH-Px activity which were similar in both groups of PKU patients, when compared to controls of similar ages. In contrast, CAT and SOD activities were not altered in PKU patients. These results show that oxidative stress occurs in PKU patients and that this pathogenic process is probably not directly correlated to Phe blood levels.

Proline induces alterations in nucleotide hydrolysis in rat blood serum

The main objective of the present study was to evaluate the in vivo (acute and chronic) and in vitro effects of proline on serum nucleotide hydrolysis. For acute administration, 29-day-old rats received one subcutaneous injection of proline (18.2 (micromol/g body weight) or an equivalent volume of 0.9% saline solution (control) and were sacrificed 1 h, 3 h or 12 h later. Results showed that acute proline administration provoked a decrease in ATP (42%) and ADP (49%) hydrolysis when rats were sacrificed 1 h after the injection. Furthermore, in rats killed 3 h and 12 h after acute injection, no change in nucleotide hydrolysis were observed. For chronic treatment, buffered proline was injected subcutaneously twice a day at 10 h intervals from the 6(th) to the 28(th) day of age. Rats were sacrificed 3 h or 12 h after the last injection. Chronic administration of proline did not alter the nucleotide hydrolysis when the rats were killed 12 h after the last injection, but decreased ATP (15%) and ADP (32%) hydrolysis when rats were sacrificed 3 h after the last injection. The in vitro effect of proline (3.0 microM - 1.0 mM) on serum nucleotide hydrolysis was also investigated; results showed that 1.0 mM of proline significantly increased ATP (45%), ADP (55%) and AMP (49%) hydrolysis. The data indicate that proline in vivo and in vitro alters nucleotide hydrolysis, which may be involved in the pathogeny of hyperprolinemic patients.

Chemically induced model of hypermethioninemia in rats

In the present study, we developed a chronic chemically induced model of hypermethioninemia in rats. We induced elevated concentrations of methionine in the blood by injecting subcutaneously methionine (1.34-2.68 micromol/g of body weight) to developing animals of various ages. Brain methionine concentrations were approximately 1.25 micromol/g wet tissue ( approximately 1.0mM). We then injected the same doses of methionine to young rats twice a day at 8h intervals from the 6(th) to the 28(th) postpartum day. Controls received saline in the same volumes. The body, brain and hippocampus of rats were weighed after treatment and showed that hypermethioninemic animals had no differences in these parameters, when compared to the control group, suggesting that methionine did not cause malnutrition in the rats. Considering that experimental animal models are useful to understand the pathophysiology of human disease, the present model of hypermethioninemia may contribute to the investigation of the mechanisms of brain damage caused by high tissue methionine levels.

Oxidative stress in patients with phenylketonuria

Phenylketonuria (PKU) is an autossomal recessive disease caused by phenylalanine-4-hydroxylase deficiency, which is a liver-specific enzyme that catalyzes the hydroxylation of l-phenylalanine (Phe) to l-tyrosine (Tyr). The deficiency of this enzyme leads to the accumulation of Phe in the tissues and plasma of patients. The clinical characterization of this disease is mental retardation and other neurological features. The mechanisms of brain damage are poorly understood. Oxidative stress is observed in some inborn errors of intermediary metabolism owing to the accumulation of toxic metabolites leading to excessive free radical production and may be a result of restricted diets on the antioxidant status. In the present study we evaluated various oxidative stress parameters, namely thiobarbituric acid-reactive species (TBA-RS) and total antioxidant reactivity (TAR) in the plasma of PKU patients. The activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were also measured in erythrocytes from these patients. It was observed that phenylketonuric patients present a significant increase of plasma TBA-RS measurement, indicating a stimulation of lipoperoxidation, as well as a decrease of plasma TAR, reflecting a deficient capacity to rapidly handle an increase of reactive species. The results also showed a decrease of erythrocyte GSH-Px activity. Therefore, it is presumed that oxidative stress is involved in the pathophysiology of the tissue damage found in PKU.

A longitudinal study of antioxidant status in phenylketonuric patients

OBJECTIVES

To investigate the implications of the three main factors of the antioxidant system reported in relation to oxidative damage in phenylketonuric patients: selenium, ubiquinone-10 (Q10) and antioxidant enzymes over 3 years of metabolic follow-up.

DESIGN AND METHODS

Longitudinal study of 46 phenylketonuric patients (age range: 6 months-34 years). Antioxidants were measured by atomic absorption spectrophotometric, chromatographic and spectrophotometric procedures.

RESULTS

Plasma selenium concentrations in phenylketonuria (PKU) were not different from those of a healthy population. Decreased plasma Q10 concentrations were mainly related to the dietary control and the age of patients. Erythrocyte catalase activity was significantly decreased in PKU while the other enzyme activities were not different from those of a healthy population.

CONCLUSION

Selenium status is not impaired in phenylketonuric patients under dietary treatment. Q10 values tend to decrease with increased patient age. Catalase activity was negatively associated with plasma phenylalanine values.

Hyperphenylalaninemia reduces creatine kinase activity in the cerebral cortex of rats

Phenylketonuria (PKU) is a metabolic disorder accumulating phenylalanine (Phe) and its metabolites in plasma and tissues of the patients. Considering that phenylalanine is the main neurotoxic metabolite, and brain energy homeostasis seems to be affected in phenylketonuria, our main objective was to investigate the effect of acute and chronic hyperphenylalaninemia (HPA) on creatine kinase (CK) activity in brain cortex of Wistar rats. Hyperphenylalaninemia was induced by subcutaneous administration of 5.2 micromol phenylalanine + 2.4 micromol alpha-methylphenylalanine (phenylalanine hydroxylase (PAH) inhibitor)/g of body weight. We also investigated the in vitro effect of phenylalanine and/or alpha-methylphenylalanine on creatine kinase activity in the brain cortex of non-treated rats. The results showed that phenylalanine significantly inhibited creatine kinase activity in vitro and reduced the enzyme activity in vivo. Considering the importance of creatine kinase for the maintenance of energy homeostasis in brain, if this enzyme inhibition also occurs in phenylketonuric patients, it is possible that creatine kinase inhibition may be one of the mechanisms by which phenylalanine is neurotoxic in phenylketonuria.

Inhibition of the mitochondrial respiratory chain by phenylalanine in rat cerebral cortex

Phenylketonuria (PKU) is biochemically characterized by the accumulation of phenylalanine (Phe) and its metabolites in tissues of affected children. Neurological damage is the clinical hallmark of PKU, and Phe is considered the main neurotoxic metabolite in this disorder. However, the mechanisms of neurotoxicity are poorly known. The main objective of the present work was to measure the activities of the mitochondrial respiratory chain complexes (RCC) and succinate dehydrogenase (SDH) in brain cortex of Wistar rats subjected to chemically induced hyperphenylalaninemia (HPA). We also investigated the in vitro effect of Phe on SDH and RCC activities in the cerebral cortex of 22-day-old rats. HPA was induced by subcutaneous administration of 2.4 micromol/g body weight alpha-methylphenylalanine, a phenylalanine hydroxylase inhibitor, once a day, plus 5.2 microM/g body weight phenylalanine, twice a day, from the 6th-21st postnatal day. The results showed a reduction of SDH and complex I + III activity in brain cortex of rats subjected to HPA. We also verified that Phe inhibited the in vitro activity of complexes I + III, possibly by competition with NADH. Considering the importance of SDH and RCC for the maintenance of energy supply to brain, our results suggest that energy deficit may contribute to the Phe neurotoxicity in PKU.

Experimental hyperphenylalaninemia provokes oxidative stress in rat brain

Tissue accumulation of L-phenylalanine (Phe) is the biochemical hallmark of human phenylketonuria (PKU), an inherited metabolic disorder clinically characterized by mental retardation and other neurological features. The mechanisms of brain damage observed in this disorder are poorly understood. In the present study we investigated some oxidative stress parameters in the brain of rats with experimental hyperphenylalaninemia. Chemiluminescence, total radical-trapping antioxidant potential (TRAP), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities were measured in the brain of the animals. We observed that chemiluminescence is increased and TRAP is reduced in the brain of hyperphenylalaninemic rats. Similar data were obtained in the in vitro experiments using Phe at various concentrations. CAT activity was significantly inhibited by Phe in vitro and in vivo, whereas GSH-Px activity was reduced in vivo but not in vitro and SOD activity was not altered by any treatment. The results indicate that oxidative stress may be involved in the neuropathology of PKU. However, further studies are necessary to confirm and extend our findings to the human condition and also to determine whether an antioxidant therapy may be of benefit to these patients.

Phenylalanine and phenylpyruvate inhibit ATP diphosphohydrolase from rat brain cortex

The main objective of the present study was to characterize the inhibition by phenylalanine and phenylpyruvate of ATP diphosphohydrolase activity in synaptosomes from the brain cortex of rats. This enzyme participates together with a 5'-nucleotidase in adenosine formation from the neurotransmitter, ATP, in the synaptic cleft. The inhibition of ATP diphosphohydrolase was competitive for nucleotide hydrolysis but 5'-nucleotidase was not affected by these metabolites. Furthermore, the two substances inhibited enzyme activity by acting at the same binding site. If the enzyme inhibition observed in vitro also occurs in the brain of PKU patients, it may promote an increase in ATP levels in the synaptic cleft. In this case, the neurotoxicity of ATP could possibly be one of the mechanisms leading to the characteristic brain damage of phenylketonuria.

Alanine prevents the decrease of Na+,K+-ATPase activity in experimental phenylketonuria

Our objective was to investigate the effect of alanine administration on Na+,K+-ATPase activity in cerebral cortex of rats subjected to chemically-induced phenylketonuria. Wistar rats were treated from the 6th to the 28th day of life with subcutaneous injections of either 2.6 micromol alanine or 5.2 micromol phenylalanine plus 2.6 micromol alpha-methylphenylalanine per g body weight or phenylalanine plus alpha-methylphenylalanine plus alanine in the same doses or equivalent volumes of 0.15 M saline. The animals were killed on the 29th or 60th day of life. Synaptic plasma membrane from cerebral cortex was prepared for Na+,K+-ATPase activity determination. The results showed that alanine injection prevents the decrease of Na+,K+-ATPase activity in animals subjected to experimental phenylketonuria. Therefore, in case the same effects are achieved with ingested alanine, it is possible that alanine supplementation may be an important dietary adjuvant for phenylketonuric patients.

Sensitivity of ATPase-ADPase activities from synaptic plasma membranes of rat forebrain to lipid peroxidation in vitro and the protective effect of vitamin E

The in vitro effects of membrane lipid peroxidation on ATPase-ADPase activities in synaptic plasma membranes from rat forebrain were investigated. Treatment of synaptic plasma membranes with an oxidant generating system (H(2)0(2)/Fe(2+)/ascorbate) resulted in lipid peroxidation and inhibition of the enzyme activity. Besides, trolox as a water soluble vitamin E analogue totally prevented lipid peroxidation and the inhibition of enzyme activity. These results demonstrate the susceptibility of ATPase-ADPase activities of synaptic plasma membranes to free radicals and suggest that the protective effect against lipid peroxidation by trolox prevents the inhibition of enzyme activity. Thus, inhibition of ATPase-ADPase activities of synaptic plasma membranes in cerebral oxidative stress probably is related to lipid peroxidation in the brain.