Plasma selenium levels in treated phenylketonuric patients

Oxidative stress in phenylketonuria: what is the evidence?

Phenylketonuria (PKU) is an inborn error of amino acid metabolism caused by severe deficiency of phenylalanine hydroxylase activity, leading to the accumulation of phenylalanine and its metabolites in blood and tissues of affected patients. Phenylketonuric patients present as the major clinical feature mental retardation, whose pathomechanisms are poorly understood. In recent years, mounting evidence has emerged indicating that oxidative stress is possibly involved in the pathology of PKU. This article addresses some of the recent developments obtained from animal studies and from phenylketonuric patients indicating that oxidative stress may represent an important element in the pathophysiology of PKU. Several studies have shown that enzymatic and non-enzymatic antioxidant defenses are decreased in plasma and erythrocytes of PKU patients, which may be due to an increased free radical generation or secondary to the deprivation of micronutrients which are essential for these defenses. Indeed, markers of lipid, protein, and DNA oxidative damage have been reported in PKU patients, implying that reactive species production is increased in this disorder. A considerable set of data from in vitro and in vivo animal studies have shown that phenylalanine and/or its metabolites elicit reactive species in brain rodent. These findings point to a disruption of pro-oxidant/antioxidant balance in PKU. Considering that the brain is particularly vulnerable to oxidative attack, it is presumed that the administration of appropriate antioxidants as adjuvant agents, in addition to the usual treatment based on restricted diets or supplementation of tetrahydrobiopterin, may represent another step in the prevention of the neurological damage in PKU.

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.

Effect of docosahexaenoic acid administration on plasma lipid profile and metabolic parameters of children with methylmalonic acidaemia

AIM

To evaluate the effect of administration of docosahexaenoic acid (DHA) on dyslipidaemia, plasma fatty acid composition and metabolic parameters of children with isolated methylmalonic acidaemia (MMA) (McKusick 25100).

METHODS

Four children (3 male, 1 female) with MMA (mut(0)), participated in a crossover, randomized study of DHA administration (25 mg/kg per day, divided into three daily doses). The control group comprised 56 healthy children, aged 10+/- 2.7 years, (51 male, 5 female), who were followed in our clinic owing to possible familial risk of cardiovascular disease.

RESULTS

The comparison of plasma fatty acid composition of children with MMA versus control children demonstrated that the patients had significantly higher values for oleic acid (p = 0.004) and linolenic acid (p = 0.008). No differences were observed in the levels of DHA and arachidonic acid. Plasma concentrations of insulin, glycine, ammonia, total cholesterol and cholesterol fractions did not change with DHA administration. No significant changes were observed in urinary excretion of methylmalonic acid. As expected, the percentage of DHA and n-3 fatty acids in plasma increased significantly after therapy (p = 0.005 and 0.014, respectively). The most remarkable result was a decrease of plasma levels of triglycerides after DHA therapy (p = 0.014).

CONCLUSION

As previously found in normal children, dietary supplementation with DHA decreases the triglyceride levels, normalizing the hypertriglyceridaemia of these children without any evidence of short-term adverse effects.

Oxidative stress induced by phenylketonuria in the rat: Prevention by melatonin, vitamin E, and vitamin C

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the phenylalanine hydroxylation system and is characterized by a block in the conversion of phenylalanine (PHE) to tyrosine. We examined the effects of maternal hyperphenylalaninemia on the morphological and biochemical development of pup rat brain and cerebellum. In our model of PKU we evaluated a number of markers of oxidative stress such as Ehrlich adducts formation, lipid peroxidation, as well as the levels of reduced and oxidized glutathione, and the activities of the enzymes glutathione peroxidase and glutathione reductase. We also studied the expression of heme-oxigenase-1 and mitogen-activated protein kinase 1/2 (MAPK 1/2) as additional markers of oxidative stress. We demonstrate that PKU strongly increased most of the oxidative stress markers studied and induced significant morphological damage. We also showed that daily administration of melatonin (20 mg/kg BW), vitamin E (30 mg/kg BW), and vitamin C (30 mg/kg BW) until delivery prevented the oxidative biomolecular damage in the rat brain and cerebellum. Although no significant differences were observed among the antioxidants studied, it should be noted that the doses of melatonin were less than those for vitamins E and C. We conclude that PKU induces a clear state of oxidative stress that is somehow involved in the brain and body damage occurring in this inborn error. Moreover, melatonin and other antioxidants are capable of preventing completely the damage induced by PKU.

Oxidative stress in a phenylketonuria animal model

Oxidative stress is seen in various metabolic disorders for unknown reasons. Oxidative stress is defined as an imbalance between pro-oxidant and antioxidant status in favor of the former. This study investigated whether oxidative stress exists in phenylketonuria (PKU) using the BTBR-Pah(enu2) animal model for PKU. Animals (14-24 weeks old) were sacrificed and brain and red blood cells (RBCs) were obtained aseptically. The lipid peroxidation by-product, evaluated as malondialdehyde (MDA), was significantly higher in the brains and RBCs of PKU animals (n = 6) than in controls (n = 6). Glutathione/glutathione disulfide, a good indicator for tissue thiol status, was significantly decreased both in the brains and RBCs. Some antioxidant enzymes were also analyzed in RBCs, including glucose-6-phosphate dehydrogenase (G6PD), which provides the RBC's main reducing power, reduced nicotinamide adenine dinucleotide phosphate (NADPH), and catalase detoxifies H2O2 by catalyzing its reduction to O2 and H2O. Both catalase and G6PD were significantly increased in the RBCs of PKU animals.

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.

Antioxidant status in hyperphenylalaninemia

Abnormal oxidative stress was observed in some inborn errors of metabolism owing to the accumulation of toxic metabolites leading to excessive free radical production and to the influence of restricted diets on the antioxidant status. Erythrocyte antioxidant enzymes activities and tocopherol concentrations were measured in a group of phenylketonuric (n = 42) and mild-hyperphenylalaninemic (n = 28) patients compared with 45 age-matched controls. We also determined plasma selenium levels in these groups. We also evaluated the possible relationship between antioxidant status and neuropsychological disorders. Erythrocyte glutathione peroxidase (GSH-Px) activity was significantly lower (P < 0.001) in both phenylketonuric and mild-hyperphenylalaninemic patients compared with the control group, but no differences were observed between the two groups of patients. Neuropsychological disturbances were more frequent in the group of PKU patients with low GSH-Px activity than in PKU patients with normal GSH-Px. Low GSH-Px activity might be explained in phenylketonuria as a result of a selenium deficiency caused by a poor selenium intake or absorption, but not in mild hyperphenylalaninemic patients with free diet. Selenium levels were normal in both groups of patients, so low glutathione peroxidase activity in both phenylketonuric and hyperphenylalaninemic groups might be influenced by other factors, such as the consequences of an unbalanced amino acid profile, common to both conditions.

Inborn errors of metabolism with a protein-restricted diet: effect on polyunsaturated fatty acids

Previous studies have shown that phenylketonuric patients display a deficiency in long-chain polyunsaturated fatty acids. A study has now been performed on 13 cases (5 with methylmalonic acidaemia and 8 with urea cycle disorders) whose dietary treatment also implies a limitation in protein-rich food. Plasma and red-cell phospholipid fatty acid profiles were studied. The most relevant results were a lower percentage of docosahexaenoic acid in plasma and red-cell phospholipids (0.91% +/- 0.53% vs 2.88% +/- 1.17% and 2.07% +/- 0.92% vs 3.62% +/- 1.01% (p < 0.001)) and a lower percentage of arachidonic acid in plasma (5.22% +/- 2.02% vs 8.3% +/- 2.11% (p < 0.001)). A long-chain polyunsaturated acid deficiency has also been confirmed in this group of metabolic patients and a dietary supplement is recommended since this population is subject to a special risk factor with regard to adequate psychomotor development. By extrapolating these data to the general population, the possibility can be inferred that long-chain polyunsaturated fatty acids are semi-essential in infant nutrition far beyond the breast-feeding period.

Selenium status in infants and children with phenylketonuria and in maternal phenylketonuria

The selenium status was investigated in 87 patients of the German Collaborative Study of Phenylketonuria (PKU) (mean age 9.7 years). The selenium values and glutathione peroxidase activity in plasma and erythrocytes were negatively correlated to the quality of dietary management (mean plasma phenylalanine value). Despite a low selenium state, the children showed no clinical sign of deficiency and almost all biochemical parameters checked were normal. In the low selenium state thyroxine values are increased and decline during selenium supplementation, whereas tri-iodothyronine and thyroid stimulating hormone levels remain unchanged. The reduction in glutathione peroxidase activity in plasma was more pronounced than in the erythrocytes pointing to a different availability of both enzymes for selenium. In addition we estimated the selenium status in 29 women with PKU during pregnancy. In 32 healthy pregnant women we observed a decrease in plasma selenium values and the glutathione peroxidase activity in the third trimester, whereas the erythrocyte glutathione peroxidase activity remained stable. In contrast to the healthy women we found in the PKU group a steady decrease of all selenium parameters tested during the whole pregnancy. During the long-term low-dose selenium supplementation in PKU children the glutathione peroxidase activity of plasma and erythrocytes increased. They reached a similar plateau after the application of inorganic or organic selenium compounds. In contrast the selenium values of plasma and whole blood showed only a plateau after the application of sodium selenite. The supplementation with low doses of selenium in the form of selenomethionine increased the plasma and whole blood selenium values constantly within the first 9 months. Therefore selenomethionine supplementation cannot be recommended.

Nutrient intakes of adolescents with phenylketonuria and infants and children with maple syrup urine disease on semisynthetic diets

Adequacy of nutrient intakes of adolescents with and without phenylketonuria (PKU) and infants and children with and without maple syrup urine disease (MSUD) were assessed using 3-day diet records sorted by disease and by age of the subject. Mean intakes of all nutrients were greater than two-thirds of the Recommended Dietary Allowances (RDA) or Estimated Safe and Adequate Daily Dietary Intakes (ESADDI) for all adolescents studied, with the exception of selenium (Se) in PKU adolescents, which averaged 27.8 micrograms. For adolescents with PKU, > 50% of the RDA or ESADDI for all nutrients was provided by elemental or modified protein hydrolysate medical foods, except for vitamin A in children aged 11-15 years and Se in children 11-18 years. Mean nutrient intakes of all infants and children were greater than two-thirds of the RDA or ESADDI for all nutrients except Se in MSUD children aged 1-11 years, where intakes ranged from 6.4 to 13.2 micrograms (21-66% of the RDA). The medical foods provided for most of the RDA and ESADDI recommendations, with the exception of Se in MSUD children.

Selenium, glutathione peroxidase (GSH-Px) and lipid peroxidation products before and after selenium supplementation

Treated phenylketonuric (PKU) children are at risk of selenium deficiency. We have studied 15 treated PKU children and 30 control children. We observed significantly lower (P less than 0.0005) plasma and erythrocyte selenium, as well as significantly lower (P less than 0.0005) plasma and erythrocyte glutathione peroxidase activities (GSH-Px) in PKU children than in controls. The lipid peroxidation products, evaluated as plasma malondialdehyde (MDA), was higher (P less than 0.0005) in PKU children than in controls. Specific oral sodium selenite supplementation (Selenium: 0.13 mumol/kg/day) resulted in a rapid increase of plasma selenium and GSH-Px activity, and after 10 days and 1 month respectively significant difference is no longer observed between PKU children and controls values. Statistically significant differences in erythrocyte selenium, erythrocyte GSH-Px activity and plasma MDA between PKU and control children disappear after respectively 2 months, 4 months and 6 months of selenium supplementation.

Decreased selenium intake and low plasma selenium concentrations leading to clinical symptoms in a child with propionic acidaemia

A child with biotin-non-responsive propionic acidaemia treated with a propiogenic amino acid-restricted diet presented with an elevated blood mean corpuscular volume (MCV) of 93.1 fl, indicative of macrocytosis, and unusual hair texture with hypopigmentation. Plasma selenium concentration at this time was subnormal (45.9 micrograms/L), and calculated dietary selenium intake was 4.7 micrograms/day. After 4 months of selenium supplementation (50 micrograms/day) plasma selenium concentration normalized (97.7 micrograms/L) in conjunction with a reduced MCV (84.0 fl) and a dramatic improvement in hair growth, colour and length. Two additional periods off and on selenium supplementation, of varying time intervals, resulted in similar clinical changes. We conclude that these clinical changes were due to a deficient intake of dietary selenium.

In vitro OKT3-induced mitogenesis in selenium-deficient patients on a diet for phenylketonuria

Patients with phenylketonuria (PKU) are frequently deficient in the essential trace element selenium (Se), because of their very low protein diet. Using two approaches to investigate T-cell response to proliferative signaling, viz, mitogenesis caused by the monoclonal antibody OKT3 and the plant lectin phytohaemagglutinin (PHA), we demonstrated significantly reduced responses to optimal concentrations of OKT3 in a group of PKU patients with reduced serum Se compared with a normal group (p = 0.0005) and with a group of PKU patients whose serum Se was normal (p = 0.0023). The response of the Se-deficient group to optimal levels of PHA did not differ from that of the normal controls or from that of Se-normal PKU patients. A dose-dependent relationship between serum Se levels and mitogenic response was evident for OKT3 (r = 0.34, p = 0.0154), but not for PHA (r = -0.02, p = 0.9086). We suggest that the reduced response to OKT3 mitogenesis in Se-deficient PKU patients is possibly the consequence of impaired Se-dependent metabolic activity, which affects mitogenic signaling via the T cell antigen receptor (TCR/CD3) complex.

Effect of selenium supplementation on the distribution of selenium among plasma proteins of a patient with maple syrup urine disease

Selenium (Se) status was studied in a patient with classical maple syrup urine disease (MSUD) receiving Se supplement. The basal plasma Se concentration was 0.06 mumol/l increasing to 2.1 mumol/l after 40 days of supplementation. When the plasma Se distribution was analysed by gel filtration, a major peak was seen close to the high molecular weight proteins with a second peak in the albumin region. When the Se dose was decreased in a stepwise manner from 50 micrograms/day to 25 micrograms/day and then to 17 micrograms/day plasma Se decreased, but the proportion of plasma Se in the two protein peaks did not change. In a healthy girl not supplemented with Se, the proportion of plasma Se in the albumin region was somewhat lower. In the MSUD patient glutathione peroxidase activity was initially low, and increased ten-fold during Se supplementation. The study indicates that the Se requirement for plasma glutathione peroxidase activity was fulfilled at the lowest dose of Se used and that Se is incorporated into several plasma proteins after supplementation.

Selenium: clinical significance and analytical concepts

Selenium is an essential trace element in humans and animals. Its only established function in humans is the antioxidant activity of glutathione peroxidase, a selenoenzyme. Severe prolonged deficiency may cause a fatal cardiomyopathy. Iatrogenic causes of selenium deficiency include parenteral and enteral nutrition. Low plasma selenium is also found in malabsorption, cystic fibrosis, rheumatoid arthritis, neoplasia, and other varied clinical disorders. Death has resulted from a single massive ingestion of selenium, while chronic excessive intake causes skin, nail, and hair pathology. Extreme geographical variation in population blood and urine selenium levels and a marked age-specific variation in population reference intervals are important factors in understanding selenium nutrition. Nutritional requirements, biological availability, and metabolism are discussed in relation to geographical, age, and method variability. Sampling, processing procedures, and methods for selenium quantitation are reviewed. Selenium content in different biological matrices and reference values for pediatric, adult, and obstetric populations are provided.