Increased Vigilance and Dopamine Synthesis by Large Doses of Tyrosine or Phenylalanine Restriction in Phenylketonuria

ABSTRACT. In a group of 9 patients with classical phenylketonuria (PKU) aged 15–24 years we examined the effect of phenylalanine restricted diet on vigilance, as judged by the continuous visual reaction times, and neurotransmitter synthesis, as judged by cerebrospinal fluid (CSF) homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) levels. HVA and 5-HIAA levels decreased significantly with increase in plasma phenylalanine concentration on free diet ( p <0.01 and p <0.0005 respectively). Vigilance improved on phenylalanine restricted diet in 6 of the 7 patients with abnormally long reaction times on free diet. Addition of tyrosine (160 mg/kg/24 h) to the free diet resulted in an increased HVA/5-HIAA ratio in CSF in the six patients examined. In 14 patients on free diet supplemented with tyrosine, an improvement in vigilance (reaction times at the 90 percentile) was seen in all 12 patients with values longer than the normal mean (264 msec) ( p <0.001). Tyrosine treatment may be a therapeutical alternative when phenylalanine restriction is impractical.     

Increased vigilance and dopamine synthesis by large doses of tyrosine or phenylalanine restriction in phenylketonuria

In a group of 9 patients with classical phenylketonuria (PKU) aged 15-24 years we examined the effect of phenylalanine restricted diet on vigilance, as judged by the continuous visual reaction times, and neurotransmitter synthesis, as judged by cerebrospinal fluid (CSF) homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) levels. HVA and 5-HIAA levels decreased significantly with increase in plasma phenylalanine concentration on free diet (p less than 0.01 and p less than 0.0005 respectively). Vigilance improved on phenylalanine restricted diet in 6 of the 7 patients with abnormally long reaction times on free diet. Addition of tyrosine (160 mg/kg/24 h) to the free diet resulted in an increased CSF in the six patients examined. In 14 patients on free diet supplemented with tyrosine, an improvement in vigilance (reaction times at the 90 percentile) was seen in all 12 patients with values longer than the normal mean (264 msec) (p less than 0.001). Tyrosine treatment may be a therapeutical alternative when phenylalanine restriction is impractical.     

The neurochemistry of phenylketonuria

The mechanisms by which deficiency of hepatic phenylalanine hydroxylase causes central nervous system disease are reviewed. The neurological disease appears to be secondary to increased concentrations of phenylalanine and a decrease in the concentrations of other large neutral amino acids, especially methionine and tyrosine, within the central nervous system. This causes a deficiency of the neurotransmitter dopamine, reduced protein synthesis and demyelination. Similar mechanisms appear to be operating when blood phenylalanine concentrations are in the range expected for early continuously treated phenylketonuria. Conclusion The severe brain disease found in phenylketonuria is caused by a raised blood phenylalanine content which increases the brain free phenylalanine and decreases the concentration of other large neutral amino acids. Brain protein synthesis is decreased, myelin turnover is increased and there are abnormalities in amine neurotransmitter systems.     

Case studies of the effect of tyrosine administration in children with phenylketonuria on cognitive processes]

Eight patients with phenylketonuria and low protein nutrition were treated in a double blind crossover study for the time of 3 month with 150 mg tyrosine per kg body weight and day and placebo thereafter or vice versa. The concentration of phenylalanine in serum was not influenced by the administration of tyrosine whereas tyrosine in serum markedly increased. Psychological tests were 7 times repeated in monthly intervals. From the study it is to suggest that there is an improvement of the test results caused by an effect of training due to the repeated tests and an additional improvement caused by tyrosine.     

Effects of Maternal Phenylalanine or Tyrosine Hydroxylase Inhibition on Postnatal Maturation of Catecholamine and Amino Acid Metabolism in Rats

Effects of the inhibition of maternal phenylalanine (Phe) or tyrosine (Tyr) hydroxylase on the postnatal maturation of catecholamine, thyroxine and amino acid metabolisms were examined in young rats. The intraperitoneal injections of 100 mg/kg p-chlorophenylalanine three times during the second gestational week together with 0.2% Phe solution as drinking water produced marked reduction in birth weight, body weight gain, brain weight, cerebral Tyr hydroxylase activity, adrenal epinephrine, renal norepinephrine and free amino acids in plasma and brain as well as delayed maturation of thermoregulation in the offspring. In contrast, the administration of 100 mg/kg α-methyl-p-tyrosine with 0.2% Tyr supplementation did not affect general growth, but only decreased cerebral Tyr hydroxylase and serum thyroxine level, while cerebral dopamine was increased. These results indicate that maternal Phe hydroxylase inhibition induces more severe growth failure including catecholamine and amino acid metabolism than Try hydroxylase inhibition in postnatal development.     

Dietary management of oculocutaneous tyrosinemia in an 11-year-old child

An 11-year-old girl with keratitis and plantar keratosis had tyrosinemia. The concentration of tyrosine in the plasma was 16.5 mg/dL. Dietary intake of phenylalanine and tyrosine was systematically varied, and the plasma concentrations of tyrosine and nitrogen balance were studied. It was necessary to achieve a total intake of phenylalanine and tyrosine less than 100 mg/kg/day to obtain plasma concentrations of tyrosine of less than 10 mg/dL. After dietary therapy was started, the keratitis resolved promptly, and the patient remained asymptomatic during a period of 16 months in which the mean plasma concentration of tyrosine was 11.1 mg/dL. The dietary management of a child at this age presents a different problem from that of a young infant. It can be successfully pursued at home, as well as in the carefully regulated environment of a clinical research center.     

Pregnancy in phenylketonuria: dietary treatment aimed at normalising maternal plasma phenylalanine concentration.

The transport characteristics of the placenta, which favour higher phenylalanine concentrations in the fetus than in the mother, and regression data of head circumference at birth against phenylalanine concentration at conception in maternal phenylketonuria (PKU), suggest that treatment of maternal PKU should ideally aim to maintain plasma phenylalanine concentration within the normal range throughout pregnancy. A patient with classical PKU was treated from before conception by aiming to maintain plasma phenylalanine concentration within the range 50-150 mumol/l and tyrosine within the range 60-90 mumol/l. The diet was supplemented with phenylalanine-free amino acids (100-180 g/day) and tyrosine (0-5 g/day). Plasma amino acid concentrations were monitored weekly by amino acid analyser. Dietary phenylalanine intake ranged from 6 mg/kg/day at conception to 30 mg/kg/day at delivery. Normal weight gain and fetal growth were maintained throughout the pregnancy. A normal baby was born at term with a head circumference of 35.5 cm; at 1 year of age no abnormality is detectable. These results show that with careful monitoring and compliance it is possible, and may be advisable, to maintain plasma phenylalanine concentration within the normal range in the management of PKU pregnancy.     

Tyrosine supplementation in phenylketonuria: diurnal blood tyrosine levels and presumptive brain influx of tyrosine and other large neutral amino acids

Tyrosine supplementation has not consistently been found to improve neuropsychologic function in phenylketonuria (PKU), possibly because of failure to achieve adequate levels of tyrosine in the brain. OBJECTIVES: To evaluate blood levels achieved after tyrosine supplementation in treated PKU and calculate brain influxes of tyrosine and other large neutral amino acids before and with tyrosine supplementation. STUDY DESIGN: Ten subjects with PKU receiving a phenylalanine-restricted diet were studied over 48 hours; each received tyrosine supplementation (300 mg/kg) on day 2. Plasma phenylalanine and tyrosine were measured every 2 hours, and all free amino acids were measured every 6 hours. Brain influxes of tyrosine and other large neutral amino acids were calculated. RESULTS: Plasma tyrosine levels were low normal at baseline. With supplementation there was a substantial but unsustained rise in plasma tyrosine. Calculated brain influx of tyrosine was 27% +/- 19% of normal before supplementation, increasing to 90% +/- 58% of normal with supplementation. Nevertheless, calculated influx remained less than 70% of normal at 50% of the time points. The calculated brain influxes of all other large neutral amino acids except tryptophan were 20% to 40% of normal before and with tyrosine supplementation. CONCLUSIONS: Tyrosine supplementation in the diet for PKU produces marked but nonsustained increases in plasma tyrosine levels, with calculated brain influx that often remains suboptimal. This could explain the lack of consistent neuropsychologic benefit with tyrosine supplementation.     

Combined tetrahydrobiopterin-phenylalanine loading test in the detection of partially defective biopterin synthesis

Deficiency in the synthesis of biopterin causes neonatal hyperphenylalaninemia. We report a 10-year-old girl of normal appearance with a partial defect in biopterin synthesis, normal intelligence and normal serum phenylalanine levels (95 mol/l) (1.6 mg/dl). During her 1st year of life srum phenylalanine levels were 250 mol/l (4 mg/dl) and phenylalanine loading performed at 6 months and 1 year of age was not followed by an increase in serum tyrosine. At 9 years of age she had developed a severely abnormal EEG with focal spike activity but no observable clinical abnormalities. Determination of urinary pterins showed abnormal low levels of biopterin and high levels of neopterin. Phenylalanine loading combined with oral administration of tetrahydrobiopterin (BH₄) was followed by a normal increase in serum tyrosine and a normal decrease in serum phenylalanine.Considering the importance of BH₄ for the synthesis of dopamine, catecholamines, and serotonin we suggest that these cases should be followed carefully. If neurological symptoms appear, e.g., epilepsy, it may be worthwhile to consider treatment with BH₄ and neurotransmitter precursors.     

Amino acid and protein requirements in a preterm infant with classic phenylketonuria.

A preterm infant with classic phenylketonuria required rather less than 90 mg/kg of phenylalanine and between 270 and 290 mg/kg tyrosine daily to achieve a rate of weight gain of around 20 g/kg per day. Using Lofenalac as the low phenylalanine food, the intake of tyrosine, an essential amino acid for patients with phenylketonuria seemed to be limiting in respect of growth.     

Metabolism of intravenous phenylalanine by babies born before 33 weeks of gestation

Random urine samples collected weekly from 22 infants of 25-32 weeks of gestation were analyzed by capillary gas chromatography-mass spectrometry to define the normal organic acid profile. Increased excretion of phenolic acid derivatives of phenylalanine and tyrosine was found in 21 samples from 13 babies during established parenteral nutrition. Compared with 53 samples collected during milk feeding, phenyllactic acid, 4-hydroxy-3-methoxyphenyllactic acid, and N-acetyltyrosine were excreted significantly more often and 4-hydroxyphenyllactic and 4-hydroxyphenylpyruvic acids at significantly higher concentrations. The mean daily intake of phenylalanine (197 mg/kg) was significantly higher and that of tyrosine (22 mg/kg) significantly lower during parenteral nutrition. Three cyclohexanediol isomers were identified which might have derived from phenylalanine or one of its metabolites.     

Antenatal treatment with corticosteroids affects mRNA expression of dopamine D1 and D2 receptors in the striatum of developing rabbit

The expression levels of dopamine D1 and D2 receptor mRNA have been investigated using Northern blot analysis in developing rabbit striatum after antenatal exposure to betamethasone. Pregnant rabbits were given either 0.1 mg/kg betamethasone or 0.1 mg/kg saline doses twice within 24 h. Dopamine D1 receptor mRNA levels were found significantly higher in the fetuses exposed to antenatal betamethasone than in the saline-treated controls. After birth, dopamine D1 and D2 receptor mRNA levels were both significantly lower in 1- and 25-day-old treated pups but recovered to normal levels in adulthood. These data suggest that antenatal exposure to betamethasone can lead to lasting abnormalities.     

Protein metabolism in phenylketonuria and Lesch-Nyhan syndrome

Animal and in vitro studies have implicated decreased protein synthesis in the pathogenesis of tissue damage in phenylketonuria (PKU) and of growth failure in Lesch-Nyhan syndrome. Protein turnover was measured in vivo in ten young adult subjects with classical PKU, two subjects with hyperphenylalaninemia, and three children with Lesch-Nyhan syndrome using techniques based on continuous infusions of [13C]leucine and, in Lesch-Nyhan subjects, [2H5]phenylalanine. The PKU subjects had various degrees of dietary phenylalanine restriction and plasma phenylalanine levels at the time of study ranged from 450-1540 mumol/L (mean 1106). Plasma phenylalanine in the two hyperphenylalaninemic subjects was 533 and 402 mumol/L. Rates of protein synthesis in all PKU subjects (mean 3.71 g/kg/24 h, range 2.68-5.10, [13C]leucine as tracer) were in a range similar to or above control values (mean 2.97, range 2.78-3.22, n = 6), as were rates of protein catabolism (PKU mean 4.23 g/kg/24 h, range 3.15-5.45; controls 3.64, 3.50-3.91). Protein turnover values in hyperphenylalaninemia were also similar to those in controls. With [13C]leucine as tracer, both mean protein synthesis and catabolism values in Lesch-Nyhan subjects (mean 4.80 and 5.64 g/kg/24 h, respectively) were higher than values in control children matched for protein intake (synthesis 4.32 +/- 0.74 (SD) and catabolism 4.85 +/- 0.57 (g/kg/24 h, n = 5). Similar results were obtained in Lesch-Nyhan subjects using [2H5]phenylalanine as tracer. These results suggest that protein turnover is not decreased in either PKU or Lesch-Nyhan syndrome. This conclusion is inconsistent with the hypothesis that tissue damage in PKU results from impaired protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that phenylalanine may not provide the full needs for aromatic amino acids in children

Phenylalanine is nutritionally classified as an indispensable amino acid and can be converted to tyrosine by phenylalanine hydroxylation. The initial goal of the present study was to determine the aromatic amino acid (phenylalanine plus tyrosine) requirements in healthy children fed a diet without tyrosine by using the indicator amino acid oxidation (IAAO) method using lysine as the indicator amino acid. Healthy school-age children (n = 5) were fed in random order a diet with eight graded intakes of phenylalanine without tyrosine. The requirement was determined by the rate of recovery of CO2 from L-[1-C]lysine oxidation (FCO2). Phenylalanine (total aromatic amino acid) requirement, in the absence of tyrosine, for children was determined to be 28 mg/kg/d, which was only 64% of the adult requirement, which is biologically absurd. A possible reason for the lower estimate of phenylalanine requirement could be lower phenylalanine hydroxylation rate in children, which is supported by the finding of lower urinary tyrosine/phenylalanine ratios in children compared with adults. In conclusion, this study indicates that phenylalanine may not provide the total needs for aromatic amino acids in children fed an amino acid-based diet without tyrosine.     

Plasma concentration and acute clinical effects of docarpamine,orally active dopamine prodrug,in infants

Currently, there are no data available on the optimal doses and efficacy of docarpamine in infants. In the present study, three doses of docarpamine, 15.0–20.4 (19.0 ± 1.9; mean ± SD) mg/kg per dose every 8 h to 10 infants suffering heart failure. Age and bodyweight were from 1 to 4 (1.4 ± 1) months and 2960–5160 (3350 ± 872) g, respectively. In all infants, plasma concentrations of free dopamine were measured 1, 2 and 3 h after the first administration. Heart rate and systolic blood pressure were examined before and at the same time as the first administration. In seven infants, the 24 h urinary output and urinary excretion of electrolytes and creatinine before and during docarpamine were measured. Peak plasma concentration of free dopamine (ng/mL) was achieved after 1 or 2 h of administration, 0–163.1 (37.9 ± 47.2) and 0–105.0 (37.8 ± 39.3), respectively. The concentration had decreased rapidly by 3 h to 0–34.2 (12.4 ± 11.0). Both heart rate (b.p.m.) and blood pressure (mmHg) tended to increase from 120–154 (140 ± 15) and 56–90 (76 ± 11) to a peak of 124–162 (148 ± 14) and 70–92 (79 ± 8), respectively (P = 0.197, P = 0.289). There were no significant changes in urinary output or excreta. Oral docarpamine of 15–20 mg/kg per dose can achieve plasma free concentrations of dopamine that increase heart rate and systolic blood pressure.     

Atypical phenylketonuria due to biopterin deficiency. Early treatment with tetrahydrobiopterin and neurotransmitter precursors, trials of monotherapy

This report presents the first case where an infant with tetrahydrobiopterin deficiency has been identified by screening of newborns with hyperphenylalaninemia for tetrahydrobiopterin deficiency. Therapy with L-Dopa, 5-hydroxytryptophan, Carbidopa and tetrahydrobiopterin was started at the age of seven weeks while the child received a normal diet. At that time already muscular hypotonia was observed. The girl, now 2 1/2 years old, shows slight muscular hypotonia and hypomotility, short periods of hypertonic extension of the limbs, and retardation of sensomotor and mental development of about 6-8 months. Monotherapy with tetrahydrobiopterin dihydrochloride, 20-40 mg/kg b.w., diminished the muscular hypotonia. The effect lasted however for only about 1 day. While urinary serotonin and phenylalanine remained normal for at least 3 days and neopterin was only slightly elevated, urinary free dopamine however remained low. Similar results were obtained after 1',2'-diacetyl tetrahydrobiopterin dihydrochloride administration, 20 mg/kg b.w.     

The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirement

Although tyrosine is considered indispensable during the neonatal period, its poor solubility has limited its inclusion in parenteral amino acid solutions to less than 1% of total amino acids. Dipeptides of tyrosine are highly soluble, have been shown to be well used and safe in animal models and humans, and, therefore, may be used as an effective means of providing tyrosine in the parenterally fed neonate. The goal of the present study was to determine the tyrosine requirement of the parenterally fed neonate receiving graded intakes of glycyl-L-tyrosine as a source of tyrosine. Thirteen infants receiving adequate energy (340 +/- 38 kJ. kg(-1).d(-1)) and protein (2.4 +/- 0.4 g.kg(-1).d(-1)) were randomized to receive parenteral nutrition with one of five graded levels of glycyl-L-tyrosine. The mean requirement and safe level of intake were estimated using a 1-(13)C-phenylalanine tracer and linear regression cross-over analysis that identified a break point in the response of label appearance in breath CO(2) (F(13)CO(2)) and phenylalanine oxidation to graded tyrosine intake. Based on the mean estimates of whole-body phenylalanine oxidation, the tyrosine mean requirement and safe level of intake were found to be 74 mg.kg(-1). d(-1) and 94 mg.kg(-1).d(-1), respectively. This represents 3.1 and 3.9% of total amino acids, respectively, considerably higher than levels found in present commercially available pediatric amino acid solutions. These data raise concern regarding the adequacy of aromatic amino acid intake in the parenterally fed neonate.     

Tetrahydrobiopterin loading test in hyperphenylalaninemia.

Some cases of primary hyperphenylalaninemia are not caused by the lack of phenylalanine hydroxylase, but by the lack of its cofactor tetrahydrobiopterin. These patients are not clinically responsive to a phenylalanine-restricted diet, but need specific substitution therapy. Thus, it became necessary to examine all newborns screened as positive with the Guthrie test for tetrahydrobiopterin deficiency. Methods based on urinary pterin or on specific enzyme activity measurements are limited in their availability, and the simplest method, based on the lowering of serum phenylalanine after loading with cofactor, was discouraged by the finding that some dihydropteridine reductase-deficient patients were unresponsive. The preliminary observation that this limitation could be overcome by increasing the dose of the administered cofactor prompted us to reevaluate the potential of the tetrahydrobiopterin loading test in hyperphenylalaninemia. Fifteen patients, eight with ultimate diagnosis of phenylketonuria, three with 6-pyruvoyl tetrahydropterin synthase-, and four with dihydropteridine reductase-deficiency, have been examined by administering synthetic tetrahydrobiopterin both orally, at doses of 7.5 and 20 mg/kg, and i.v., at a dose of 2 mg/kg. All the tetrahydrobiopterin-deficient patients, unlike those with phenylketonuria, responded to the oral dose of 20 mg/kg cofactor by lowering their serum phenylalanine concentration markedly below baseline to an extent easily detectable by Guthrie cards. This method allows for a simple screening method when enzyme or pterin studies are not available.     

PLASMA CONCENTRATIONS OF DIAZEPAM AND N-DESMETHYLDIAZEPAM IN NEWBORN INFANTS AFTER INTRAVENOUS, INTRAMUSCULAR, RECTAL AND ORAL ADMINISTRATION

ABSTRACT. Five newborn infants (birth weight 2900–3600 g) were given diazepam (Valium®, LaRoche) for convulsive disorders in 4 equal doses intravenously, intramuscularly, rectally and orally with at least 24 hours intervals. Three infants were given doses of 1 mg diazepam/kg body weight, and 2 0.5 mg/kg. The parenteral solution of the drug was given intravenously, intramuscularly and rectally. Powder of tablets was given orally. After intravenous administration very high peak values of plasma-diazepam concentration were obtained (5775–10800 ng/ml after 1 mg/kg, 2750 and 6450 ng/ml after 0.5 mg/kg). Next to intravenous administration rectal administration caused the most rapid increase in plasma-diazepam concentration. Presumed anticonsulsive concentrations (150–300 ng/ml) were obtained within 5 min with 1 mg/kg as well as 0.5 mg/kg rectally. Rectal administration therefore could be a suitable treatment for seizures in the newborn infant. Accumulation of the main depressive metabolite N-desmethyldiazepam occurred in all infants. This phenomenon must be taken into account when repeated doses of diazepam are administered.     

Plasma concentrations of diazepam and N-desmethyldiazepam in newborn infants after intravenous, intramuscular, rectal and oral administration.

Five newborn infants (birth weight 2900--3600 g) were given diazepam (Valium, LaRoche) for convulsive disorders in 4 equal doses intravenously, intramuscularly, rectally and orally with at least 24 hours intervals. Three infants were given doses of 1 mg diazepam/kg body weight, and 2 0.5 mg/kg. The parenteral solution of the drug was given intravenously, intramuscularly and rectally. Powder of tablets was given orally. After intravenous administration very high peak values of plasma-diazepam concentration were obtained (5775--10800 ng/ml after 1 mg/kg, 2750 and 6450 ng/ml after 0.5 mg/kg). Next to intravenous administration rectal administration caused the most rapid increase in plasma-diazepam concentration. Presumed anticonsulsive concentrations (150--300 ng/ml) were obtained within 5 min with 1 mg/kg as well as 0.5 mg/kg rectally. Rectal administration therefore could be a suitable treatment for seizures in the newborn infant. Accumulation of the main depressive metabolite N-desmethyldiazepam occurred in all infants. This phenomenon must be taken into account when repeated doses of diazepam are administered.