Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria

An examination of the blood-brain barrier in health and disease

The brain is protected from bloodborne toxins by the walls of the brain capillaries. The capillaries make up the primary part of the so-called blood-brain barrier (BBB). This is an exclusive barrier that precludes a large number of substances from entering the brain. This is because of its specific structural and biochemical properties that arise from interactions of a number of cell types. This review introduces the concept of the BBB to the practitioner. It examines the elements that are presently understood to be necessary for its formation. Finally, the influence of the BBB on disease is examined. This will enable the practitioner to have a comprehensive understanding of the effect the presence or absence of the BBB has on central nervous system health.

Branched-chain amino acids and brain function

Branched-chain amino acids (BCAAs) influence brain function by modifying large, neutral amino acid (LNAA) transport at the blood-brain barrier. Transport is shared by several LNAAs, notably the BCAAs and the aromatic amino acids (ArAAs), and is competitive. Consequently, when plasma BCAA concentrations rise, which can occur in response to food ingestion or BCAA administration, or with the onset of certain metabolic diseases (e.g., uncontrolled diabetes), brain BCAA concentrations rise, and ArAA concentrations decline. Such effects occur acutely and chronically. Such reductions in brain ArAA concentrations have functional consequences: biochemically, they reduce the synthesis and the release of neurotransmitters derived from ArAAs, notably serotonin (from tryptophan) and catecholamines (from tyrosine and phenylalanine). The functional effects of such neurochemical changes include altered hormonal function, blood pressure, and affective state. Although the BCAAs thus have biochemical and functional effects in the brain, few attempts have been made to characterize time-course or dose-response relations for such effects. And, no studies have attempted to identify levels of BCAA intake that might produce adverse effects on the brain. The only "model" of very high BCAA exposure is a very rare genetic disorder, maple syrup urine disease, a feature of which is substantial brain dysfunction but that probably cannot serve as a useful model for excessive BCAA intake by normal individuals. Given the known biochemical and functional effects of the BCAAs, it should be a straightforward exercise to design studies to assess dose-response relations for biochemical and functional effects and, in this context, to explore for adverse effect thresholds.

State regulation and response inhibition in children with ADHD and children with early- and continuously treated phenylketonuria: an event-related potential comparison

BACKGROUND

The presentation rate of stimuli plays an important role in explaining the performance inefficiency in children with ADHD. In general, children with ADHD have been found to perform more poorly in conditions of relatively slow event rates as compared with fast and moderate event rates. The state regulation hypothesis states that these children have problems in correcting their energetic state necessary to counteract a performance decrement, which requires extra effort allocation. In this study, we investigated state regulation in children with ADHD and used children with early- and continuously treated phenylketonuria (PKU) as a clinical contrast group.

METHOD

We measured the parietal P3 during a Go/No-Go task that incorporated a condition with a fast and a slow presentation rate.

RESULTS

We were able to show that children with ADHD, relative to controls, responded more slowly and more variably in the slow condition only, which was accompanied by a smaller P3, suggesting less effort allocation. In contrast, the children with PKU did not show a state regulation deficit. The PKU group showed prolonged stimulus evaluation processing, as indexed by P3 latency, compared to controls and children with ADHD. In addition, they made more errors of commission than the controls and the ADHD group.

CONCLUSIONS

Our electrophysiological data support the state regulation hypothesis of ADHD. Only the children with PKU had more problems in inhibiting pre-potent responding than controls, which is in accord with the prefrontal dysfunction hypothesis of PKU.

Adult phenylketonuria

Newborn screening for phenylketonuria began 35 to 40 years ago in most industrialized countries. Because of this initiative, which resulted in early institution of phenylalanine-restricted diets, there are now many young adults with this disease who have normal or near-normal intellectual function. In North America alone, 200 patients with phenylketonuria enter adulthood every year. Most expert panels recommend following a phenylalanine-restricted "diet for life." However, there are few adult physicians dedicated to continuing care of this group, with the possible exception of maternal phenylketonuria. Up to 10% of adults with classic phenylketonuria, and possibly 50% of those with milder variants, may not need treatment; after adolescence, intelligence does not appear to deteriorate, at least into early adulthood, even if diet therapy is discontinued or not in good control. However, neuropsychological and psychosocial problems develop frequently, needing focused and intensive support by health care providers. New investigative methods and treatment options are on the horizon. There is an urgent need for physicians who will orchestrate the care of adults with phenylketonuria.

1H MR chemical shift imaging detection of phenylalanine in patients suffering from phenylketonuria (PKU)

Short echo time single voxel methods were used in previous MR spectroscopy studies of phenylalanine (Phe) levels in phenylketonuria (PKU) patients. In this study, apparent T2 relaxation time of the 7.3-ppm Phe multiplet signal in the brain of PKU patients was assessed in order to establish which echo time would be optimal. 1H chemical shift imaging (CSI) examinations of a transverse plain above the ventricles of the brain were performed in 10 PKU patients and 11 persons not suffering from PKU at 1.5 T, using four echo times (TE 20, 40, 135 and 270 ms). Phe was detectable only when the signals from all CSI voxels were summarized. In patients suffering from PKU the T2 relaxation times of choline, creatine and N-acetyl aspartate (NAA) were similar to those previously reported for healthy volunteers (between 200 and 325 ms). The T2 of Phe in brain tissue was 215 +/- 120 ms (standard deviation). In the PKU patients the brain tissue Phe concentrations were 141 +/- 69 microM as opposed to 58 +/- 23 microM in the persons not suffering from PKU. In the detection of Phe, MR spectroscopy performed at TE 135 or 270 ms is not inferior to that performed at TE 20 or 40 ms (all previous studies). Best results were obtained at TE=135 ms, relating to the fact that at that particular TE, the visibility of a compound with a T2 of 215 ms still is good, while interfering signals from short-TE compounds are negligible.

A preliminary report on dopamine system reactivity in PKU: acute effects of haloperidol on neuropsychological, physiological, and neuroendocrine functions

BACKGROUND

Classic phenylketonuria (PKU) is due to an inborn error of metabolism resulting in an inability to metabolize the amino acid phenylalanine. To avoid mental retardation, affected individuals observe a phenylalanine-restricted diet. When dietary control is poor, deficits in prefrontally mediated cognitive functions have been observed. It has been suggested that these deficits are due to disruptions in the mesocortical dopamine system that projects to the prefrontal cortex.

METHODS

In this study, dopamine system reactivity was examined in individuals with PKU, relative to age-matched controls, using the non-specific DA antagonist haloperidol, in a repeated measures placebo-controlled design. Outcome variables included neuroendocrine, physiological, and cognitive measures.

RESULTS

Regardless of drug condition, PKU participants differed from control participants in their blood phenylalanine and tyrosine levels, and in their times to complete measures of attention and working memory. Also, relative to placebo, haloperidol influenced several variables irrespective of group status, including serum prolactin secretion, times to complete attention and working memory tasks, and accuracy of working memory performance. An interaction between group and drug condition was observed for the digit span task, where PKU participants exhibited greater relative impairments on haloperidol. When composite indices of impairment were derived, PKU participants demonstrated selective disruption in executive function on haloperidol relative to control subjects.

CONCLUSIONS

Findings are consistent with the presence of frontostriatal dysfunction in PKU but are less consistent with the notion that PFC dopamine function is specifically affected.

Developmental timing of exposure to elevated levels of phenylalanine is associated with ADHD symptom expression

This study addresses attention deficit hyperactivity disorder (ADHD), with a focus on how the timing of a known biological insult affects ADHD symptom expression. The sample consists of children exposed to elevated levels of phenylalanine, either postnatally as in Phenylketonuria (PKU; n = 46) or prenatally as in Maternal PKU (MPKU; n = 15). Non-hyperphenylalaninemic siblings of children with PKU (n = 18) serve as controls. Results indicate that elevated levels of phenylalanine are associated with ADHD symptoms. The manifestations of the symptom expression are dependent on exposure timing: prenatal exposure is associated with a higher likelihood of expressing hyperactive/impulsive symptoms and postnatal exposure is associated with a higher likelihood of expressing inattentive symptoms. This toxicity is dose-dependent and higher levels of phenylalanine appear more detrimental.

Mechanisms of Disease: The Blood-Brain Barrier

OBJECTIVE

The blood-brain barrier (BBB) is often perceived as a passive membrane. However, evidence has demonstrated that the BBB plays an active role in normal homeostasis and in certain disease processes.

METHODS

Approximately 300 peer-reviewed publications that discussed normal or abnormal BBB function were reviewed.

RESULTS

The role of the BBB and how it contributes to disorders of the central nervous system vary, depending on the specific disease process.

CONCLUSION

In health and disease and extending to old age, endothelial cells, neurons, and glia constitute a neurovascular unit that regulates the BBB. Advances toward penetrating the BBB must account for both normal and abnormal functions of the neurovascular unit.

The role of drug transporters at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic interface between the blood and the brain. It eliminates (toxic) substances from the endothelial compartment and supplies the brain with nutrients and other (endogenous) compounds. It can be considered as an organ protecting the brain and regulating its homeostasis. Until now, many transport systems have been discovered that play an important role in maintaining BBB integrity and brain homeostasis. In this review, we focus on the role of carrier- and receptor-mediated transport systems (CMT, RMT) at the BBB. These include CMT systems, such as P-glycoprotein, multidrug-resistance proteins 1-7, nucleoside transporters, organic anion transporters, and large amino-acid transporters; RMT systems, such as the transferrin-1 and -2 receptors; and the scavenger receptors SB-AI and SB-BI.

Timing is everything: executive functions in children exposed to elevated levels of phenylalanine

This study addresses how the timing of a known biological insult affects the developmental progression of executive functions. The sample consisted of children exposed to elevated levels of phenylalanine, either postnatally, as in phenylketonuria (PKU; n = 46), or prenatally, as in maternal PKU (n = 15). Nonhyperphenylanemic siblings of children with PKU (n = 18) served as controls. Results indicated that elevated levels of phenylalanine are toxic to the neurological systems that manage executive functions and cognitive tempo. This toxicity is dose dependent, with higher levels of phenylalanine being more detrimental. Executive function difficulties noted in PKU are consistent with attention deficit hyperactivity disorder (ADHD)-inattentive type, whereas maternal PKU offspring had executive function difficulties consistent with ADHD-combined type.

Motor function under lower and higher controlled processing demands in early and continuously treated phenylketonuria

This study examined motor control in 61 early and continuously treated patients with phenylketonuria (PKU) and 69 control participants, aged 7 to 14 years. The pursuit task demanded concurrent planning and execution of unpredictable movements, whereas the tracking task required a highly automated circular movement that could be planned in advance. PKU patients showed significantly poorer motor control in both tasks compared with control participants. Deficits were particularly observed for younger patients (age < 11 years). Differences between control participants and PKU patients were significantly greater in the pursuit task compared with the tracking task, indicating more serious deficits when a higher level of controlled processing is required. Correlations with historical phenylalanine levels indicated a later maturation of the level of control required by the pursuit task compared with the tracking task.

L-phenylalanine selectively depresses currents at glutamatergic excitatory synapses

To explore the hypothesis that L-phenylalanine (L-Phe) depresses glutamatergic synaptic transmission and thus contributes to brain dysfunction in phenylketonuria (PKU), the effects of L-Phe on spontaneous and miniature excitatory postsynaptic currents (s/mEPSCs) in rat and mouse hippocampal and cerebrocortical cultured neurons were studied using the patch-clamp technique. L-Phe depressed the amplitude and frequency of both N-methyl-D-aspartate (NMDA) and non-NMDA components of glutamate receptor (GluR) s/mEPSCs. The IC(50) of L-Phe to inhibit non-NMDAR mEPSC frequency was 0.98 +/- 0.13 mM, a brain concentration seen in classical PKU. In contrast, D-Phe had a significantly smaller effect, whereas L-leucine, an amino acid that competes with L-Phe for brain transporter, had no effect on mEPSCs. Unlike GluR s/mEPSCs, GABA receptor mIPSCs were not attenuated by L-Phe. A high extracellular concentration of glycine prevented the attenuation by L-Phe of NMDAR current, activated by exogenous agonist, and of NMDAR s/mEPSC amplitude, but not of NMDAR s/mEPSC frequency. On the other hand, L-Phe significantly depressed non-NMDAR current activated by low but not high concentrations of exogenous agonists. Glycine-independent attenuation of NMDAR s/mEPSC frequency suggests decreased presynaptic glutamate release caused by L-Phe, whereas decreased amplitudes of NMDAR and non-NMDAR s/mEPSCs are consistent with competition of L-Phe for the glycine- and glutamate-binding sites of NMDARs and non-NMDARs, respectively. The finding that GluR activity is significantly depressed at conditions characteristic of classical PKU indicates a potentially important contribution of impaired GluR function to PKU-related mental retardation and provides important insights into the potential physiological consequences of impaired GluR function.

Cerebral energy metabolism in phenylketonuria: findings by quantitative In vivo 31P MR spectroscopy

Both severe impairments of brain development in untreated infants and acute reversible neurotoxic effects on brain function are clinical features of phenylketonuria (PKU). For determining whether impairments of cerebral energy metabolism play a role in the pathophysiology of PKU, quantitative in vivo 31P magnetic resonance spectroscopy (MRS) was performed in a supratentorial voxel of 11 adult PKU patients and controls. Peak areas of inorganic phosphate; phosphocreatine; alpha-, beta-, and gamma-ATP; NAD; phosphomonoesters; phosphodiesters; and a broad phospholipid signal were converted to millimolar concentrations. Mg2+, pH, ADP, the phosphorylation potential, and the relative velocity of oxidative metabolism V/Vmax were derived. Clinical evaluation included mutation analysis, neurologic investigation, intelligence testing, magnetic resonance imaging, and concurrent plasma and brain phenylalanine (Phe), the last by 1H-MRS. Phe loading was performed in five patients with an oral dose of 100 mg/kg body wt L-Phe monitored by spectral EEG analysis. Under steady-state conditions, 31P-MRS revealed normal values for ATP, phosphocreatine, NAD, phosphomonoesters, phosphodiesters, Mg2+, and pH in PKU. ADP (+11%) and the phosphorylation potential (+22%) were increased. Peak areas of inorganic phosphate (-22%) and phospholipid (-8%) were decreased. ADP correlated with concurrent plasma (r = 0.65) and brain (r = 0.55) Phe. During the Phe load, blood Phe levels increased steeply. EEG revealed slowing of background activity. The phosphorylation potential decreased, whereas ADP and V/Vmax increased. In vivo 31P-MRS demonstrated subtle abnormalities of cerebral energy metabolism in PKU in steady-state conditions that were accentuated by a Phe load, indicating a link between Phe neurotoxicity and imbalances of cerebral energy metabolism.

Inhibition of prepotent responding and attentional flexibility in treated phenylketonuria

Inhibition of prepotent responding and attentional flexibility were assessed in 58 early and continuously treated phenylketonuria (PKU) patients and 69 controls, age 7 to 14 years. A computerized task was used requiring participants to process consecutive stimuli according to various attentional sets. Analysis of error rate suggested poorer inhibition of prepotent responding in PKU patients compared with controls. No influence of concurrent plasma phenylalanine (phe) was shown, neither in the younger (age < 11 years) nor in the older participants (age > or = 11 years). Analysis of error rate provided strong evidence for poorer attentional flexibility in PKU patients compared with controls. The difference between attentional flexibility in controls and PKU patients could mainly be attributed to younger PKU patients, with concurrent phe levels higher than 360 micromol/L. Younger PKU patients with phe levels below 360 micromol/L performed at the same level as age-matched controls. Performance of PKU patients was strongly associated with phe levels in age periods during the first 10 years of life, which are characterized by a strong development of executive functioning (ages 2-7 and age 9). High phe levels during these age periods could delay development of inhibitory control and attentional flexibility. With regard to treatment, analyses with lifetime and concurrent phe levels support strict dietary control throughout the first decade of life, after which the phe-restricted diet can be relaxed. However, based on the evidence that development of specific executive functions continues until approximately age 12, it is recommended to maintain phe levels below 360 micromol/L throughout early adolescence.

[Morphology and physiology of the blood-brain barrier]

The blood-brain barrier (BBB) is a complex biological system that consists of endothelial cells, pericytes and astrocytes, which are involved in the induction and maintenance of its physiological and ultrastructural characteristics. The BBB plays a primordial role in isolating the cerebral parenchyma as well as in controlling brain homeostasis by its selective permeability to nutriments and other molecules flowing through the cerebral microcapillaries. A better knowledge of this system is crucial in order to improve the efficiency of brain penetration by drugs, and in order to prevent BBB opening, leading to brain edema, in physiopathological situations such as brain ischemia, trauma or inflammatory processes.

Phenylalanine can be detected in brain tissue of healthy subjects by 1H magnetic resonance spectroscopy

Transport of phenylalanine (Phe) and the other large neutral amino acids across the blood-brain barrier plays a crucial role in the pathogenesis of phenylketonuria (PKU). Thus, investigation of Phe transport kinetics by means of proton magnetic resonance spectroscopy (1H MRS) became an important research area in the mid 1990s. As 1H MRS measurements of brain phenylalanine are restricted to tissue concentrations above 100-150 micromol/kg wet weight, this approach was possible only in PKU patients, and comparison with healthy controls was not achieved. Using standardized single-dose oral Phe loading in three healthy subjects, it was shown that Phe values increase steeply, peak at about 1 h post load, and decrease thereafter. In a single case study, repetitive Phe loading was then performed to achieve a plateau of high blood Phe concentrations for several hours. It was demonstrated that detection and monitoring of brain Phe concentrations is feasible by means of 1H MRS. This approach constitutes a prerequisite for describing carrier kinetics in health.

Dietary threonine reduces plasma phenylalanine levels in patients with hyperphenylalaninemia

BACKGROUND

In order to achieve normal intellectual development, the plasma phe-nylalanine (PHE) levels of patients with hyperphenylalaninemia should not exceed toxic levels. This goal is usually accomplished by employing special diets in which the patient's protein intake is in the form of PHE-free mixtures of amino acids. There is evidence from our own observations in animals and a preliminary observation in patients with hyperphenylalaninemia that supplemental dietary threonine (THR) might decrease plasma PHE concentrations.

METHODS

In this placebo-controlled crossover study, the effect of supplemental oral THR on the plasma amino acid concentrations of 12 patients with hyperphenylalaninemia was investigated. Before starting the first treatment period of this cross-over study, the patients were randomly assigned to one of two groups supplemented either with approximately 50 mg THR/kg per day or with a similar amount of maltodextrin as placebo. After a feeding period of 8 weeks and a wash-out period of 8 weeks, the supplements were crossed over and the study continued for an additional 8 weeks. Blood was obtained at the start and the end of each supplementation period.

RESULTS

Dietary THR supplementation of approximately 50 mg/kg per day resulted in a significant decrease of plasma PHE levels ( P = 0.0234). There was a close positive correlation between plasma and urinary PHE concentrations ( P < 0.001) indicating that the lower plasma PHE levels in the THR supplemented patients were not caused by higher urinary excretion of PHE.

CONCLUSIONS

The data of the present study show that oral THR supplementation has a clear plasma-PHE-reducing effect but they do not allow any conclusion about the mechanisms responsible for the observed effect. Although it seems attractive on the basis of the present data to use THR supplementation in patients with hyperphenylalaninemia, the mechanism of the observed effect should be clarified before introduction of such a treatment in these patients.

Deficits in brain serotonin synthesis in a genetic mouse model of phenylketonuria

Although hyperphenylalaninemia causes neurological disturbances and mental retardation, the neuropathological effects of phenylalanine excess are still poorly understood. Brain serotonin depletion may play a major role in such disturbances and is a possible target for feasible pharmacotherapies. In the present study, we investigated hyperphenylalaninemia-related brain serotonin depletion using a genetic mouse model of phenylketonuria, the Pah(enu2) mutant. Mutant mice showed severe depletion of whole brain serotonin, a mild reduction in the brain level of tryptophan, its amino acid precursor, and major deficits in the brain level of 5-hydroxytryptophan, the second rate-limiting factor in serotonin synthesis. These results suggest that interference with brain 5-hydroxytryptophan synthesis may be the major cause of serotonin deficits in hyperphenylalaninemia.

Short-term dietary interventions in children and adolescents with treated phenylketonuria: effects on neuropsychological outcome of a well-controlled population

This study addressed two questions: is there an effect of dietary interventions that induce relatively small changes in phenylalanine (Phe) concentration on neuropsychological outcome of early- and continuously treated phenylketonuria (PKU) patients, and are there differences in effects for PKU children and adolescents? To answer the first question, the effect of a short-term dietary intervention (1-2 weeks) was compared for patients whose Phe concentrations increased vs those whose Phe concentrations decreased. Controls were tested twice to control for learning effects. To answer the second question, the effect of dietary interventions was examined in younger patients (aged 7-10 years) and older patients (aged 11-14 years). The effect of dietary interventions was determined with three neuropsychological tasks: one requiring sustained attention; a second demanding maintenance in working memory; and a third in which complex operations were performed in working memory. Relatively small fluctuations in Phe concentration were found to influence neuropsychological task performance of PKU patients. Patients whose Phe concentrations decreased at the second assessment showed generally more improvement than controls. Patients whose Phe concentrations had increased showed minimal improvement or deterioration of task performance. The strongest effects were observed when sustained attention and manipulation of working memory content were required. There were some indications of a greater sensitivity of younger patients to fluctuations in Phe concentration.

The neuropsychological profile of early and continuously treated phenylketonuria: orienting, vigilance, and maintenance versus manipulation-functions of working memory

In this paper, we review neuropsychological test results of early and continuously treated Phenylketonuria (PKU) patients. To increase insight into the neuropsychological profile of this population, we have attempted to place the results within an attentional network model [Images of the mind, 1994], which proposes interacting but dissociable attentional networks for orienting, vigilance, and executive control of attention. Executive control of attention is discussed against the background of the process-specific theory of working memory (WM) [Handbook of neuropsychology, 1994], which postulates a distinction between the 'maintenance'-function of WM and the 'manipulation and monitoring'-function. Neuropsychological results are presented for 67 early and continuously treated PKU patients and 73 controls aged 7-14 years. Four neuropsychological tasks were employed to measure orienting, mnemonic processing, interference suppression, and top-down control in visual search. No differences were found in orienting and the maintenance-function of WM. In addition to previously reported impairments in sustained attention/vigilance and inhibition of prepotent responding, PKU patients exhibited deficits when top-down control was required in a visual search task, but showed no impairment when interference suppression was required. It is discussed how the specific neuropsychological impairments in PKU may be a consequence of mid-dorsolateral prefrontal cortex (DLPFC) dysfunctioning due to deficiencies in catecholamine modulation.

Phenylketonuria in adulthood: a collaborative study

During 1967-1983, the Maternal and Child Health Division of the Public Health Services funded a collaborative study of 211 newborn infants identified on newborn screening as having phenylketonuria (PKU). Subsequently, financial support was provided by the National Institute of Child Health and Human Development (NICHD). The infants were treated with a phenylalanine (Phe)-restricted diet to age 6 years and then randomized either to continue the diet or to discontinue dietary treatment altogether. One hundred and twenty-five of the 211 children were then followed until 10 years of age. In 1998, NICHD scheduled a Consensus Development Conference on Phenylketonuria and initiated a study to follow up the participants from the original Collaborative Study to evaluate their present medical, nutritional, psychological, and socioeconomic status. Fourteen of the original clinics (1967-1983) participated in the Follow-up Study effort. Each clinic director was provided with a list of PKU subjects who had completed the original study (1967-1983), and was asked to evaluate as many as possible using a uniform protocol and data collection forms. In a subset of cases, magnetic resonance imaging and spectroscopy (MRI/MRS) were performed to study brain Phe concentrations. The medical evaluations revealed that the subjects who maintained a phenylalanine-restricted diet reported fewer problems than the diet discontinuers, who had an increased rate of eczema, asthma, mental disorders, headache, hyperactivity and hypoactivity. Psychological data showed that lower intellectual and achievement test scores were associated with dietary discontinuation and with higher childhood and adult blood Phe concentrations. Abnormal MRI results were associated with higher brain Phe concentrations. Early dietary discontinuation for subjects with PKU is associated with poorer outcomes not only in intellectual ability, but also in achievement test scores and increased rates of medical and behavioural problems.

Phenylketonuria: no specific frontal lobe-dependent neuropsychological deficits of early-treated patients in comparison with diabetics

Neuropsychologic studies have shown that even phenylketonuric patients treated early suffer from phenylalanine-related deficits in all age periods, from childhood to adulthood. This study was performed to determine whether phenylketonuric children show specific frontal lobe-dependent deficits when compared with diabetic patients. The comparative study included 42 phenylketonuric patients, 10 to 18 y of age [mean 14.7 (years, months), SD 2.9], and 42 diabetic patients matched for sex, age, and socioeconomic status. Patients were assessed for intelligence quotient (Culture Fair Intelligence Test), information processing (Wisconsin Card Sorting Test, Trail-Making Test), and selective (Stroop task) as well as sustained attention (Test d-2). Phenylketonuric patients had significantly poorer results than the diabetic patients. Within all tests, however, this was due to reduced performance speed, not to deficits in specific functions. Patients did not show deficits in insight and learning. The selection abilities and the sustained attention of the phenylketonuric patients were not impaired. Performance speed and blood phenylalanine levels were negatively correlated. Elevated phenylalanine levels may cause an imbalance in neurotransmitter metabolism. However, this seems to refer to a global neurotoxic effect rather than to specific effects on the dopaminergic system, which would affect specifically the activation of the frontal lobes.

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.

Individual blood-brain barrier phenylalanine transport determines clinical outcome in phenylketonuria

Different clinical outcomes in spite of comparable dietary controls are well known in patients with phenylketonuria. Currently, reasons for this phenomenon are unknown. Kinetic investigations in 15 patients with classic phenylketonuria were performed using in vivo nuclear magnetic resonance spectroscopy before and after an oral phenylalanine load (100 mg/kg body weight). Patients' brain phenylalanine concentrations were quite different in spite of similar blood phenylalanine levels. Interindividual variations of the apparent transport Michaelis constant, K(t,app), covered a range from 0.10 to 1.03 mmol/L. The ratio of the maximal transport velocity, Tmax, over the intracerebral consumption rate, Vmet, varied between 2.61 and 14.0. Both parameters as well as the preload brain phenylalanine levels correlated significantly with the degree of cerebral white matter abnormalities on magnetic resonance images. Correlations of K(t,app), Tmax/Vmet, and the preload brain phenylalanine levels with patients' intelligence scores approached significance. In conclusion, blood-brain barrier phenylalanine transport characteristics and the resultant brain phenylalanine levels seem to be causative factors for the individual clinical outcome in phenylketonuria. This observation may lead to individual dietary recommendations in the future.

National Institutes of Health Consensus Development Conference Statement: phenylketonuria: screening and management, October 16-18, 2000

OBJECTIVE

To provide health care providers, patients, and the general public with a responsible assessment of currently available data regarding screening for and management of phenylketonuria (PKU).

PARTICIPANTS

A nonfederal, nonadvocate, 14-member panel representing the fields of pediatrics, genetics, human development, public policy, nursing, and molecular physiology and including patient representatives. In addition, 19 experts in pediatrics, medical genetics, psychology, pediatric neurology, biochemical and molecular genetics, and gene therapy presented data to the panel and to a conference audience of 312.

EVIDENCE

The literature was searched using Medline for January 1980 through July 2000, and an extensive bibliography of 3394 references was provided to the panel. Experts prepared abstracts for their conference presentations with relevant citations from the literature. Scientific evidence was given precedence over clinical anecdotal experience.

CONSENSUS PROCESS

The panel, answering predefined questions, developed its conclusions based on the scientific evidence presented in open forum and the scientific literature. The panel composed a draft statement, which was read in its entirety and circulated to the experts and the audience for comment. Thereafter, the panel resolved conflicting recommendations and released a revised statement at the end of the conference. The panel finalized the revisions within a few weeks after the conference. The draft statement was made available on the World Wide Web immediately after its release at the conference and was updated with the panel's final revisions. The statement is available at http://consensus.nih.gov.

CONCLUSIONS

Genetic testing for PKU has been in place for almost 40 years and has been very successful in preventing severe mental retardation in thousands of children and adults. Metabolic control is necessary across the lifespan of individuals with PKU. A comprehensive, multidisciplinary, integrated system is needed to delivery of care to individuals with PKU. Greatly needed are consistency and coordination between screening, treatment, data collection, and patient support programs. There should be equal access to culturally sensitive, age-appropriate treatment programs. Ethically sound, specific policies for storage, ownership, and use in future studies of archived samples remaining from PKU testing should be established. Research into the pathophysiology of PKU and relationship to genetic, neural, and behavioral variation is strongly encouraged. Uniform policies must be established to remove financial barriers to the acquisition of medical foods and modified low-protein foods and to provide access to support services needed to maintain metabolic control in individuals with PKU. Research on nondietary alternative treatments for PKU is strongly encouraged. To achieve optimal statistical power and cross-cultural applicability, it will be beneficial to use data acquired via national and international collaboration.phenylketonuria, hyperphenylalanimea, phenylketonuria screening, phenylalanine-restricted diet, maternal phenylketonuria, newborn screening, phenylalanine monitoring, phenylketonuria outcomes.

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.

Assessment of adult phenylketonuria

Phenylketonuria (PKU) has been detected on the newborn screening programme since the 1960s. Although it is recognised that dietary treatment is successful in avoiding the severe mental retardation associated with untreated PKU, the long-term outcome for adults remains unclear. The Medical Research Council recommends that the diet be followed for life. This paper discusses the relevance of the findings of neurological deterioration, neuropsychological problems and brain imaging in adults with PKU. It suggests an approach to follow-up for adults with PKU including neurological assessments, awareness of nutritional deficiencies, educational requirements and the risks of maternal PKU.

Quantitative (1)H magnetic resonance spectroscopy of myoglobin de- and reoxygenation in skeletal muscle: reproducibility and effects of location and disease

1H-magnetic resonance spectroscopy ((1)H-MRS) of deoxymyoglobin (DMb) provides a means to noninvasively monitor the oxygenation state of human skeletal muscle in work and disease. As shown in this work, it also offers the opportunity to measure the absolute tissue content of DMb, the basic oxygen consumption of resting muscle, and the reperfusion characteristics after release of a pressure cuff. The methodology to determine these tissue properties simultaneously at two positions along the calf is presented. The obtained values are in agreement with invasive determinations. The reproducibility of the (1)H-MRS measurements is established for healthy controls and patients with peripheral arterial disease (PAD). A location dependence in axial direction, as well as differences between controls and patients are demonstrated for all parameters. The reoxygenation time in particular is expected to provide a means to quantitatively monitor therapies aimed at improving muscular perfusion in these patients.

Leucine suppresses acid-induced protein wasting in L6 rat muscle cells

BACKGROUND

Metabolic acidosis induces protein wasting in skeletal muscle cells, accompanied by decreased glycolysis and compensatory increased consumption of other metabolic fuels, implying that protein wasting arises from fuel starvation and might be rectified by fuel supplements. Design To test this hypothesis, total protein and protein degradation (release of 14C-phenylalanine) were measured in L6 skeletal muscle cells cultured in Eagle's Minimum Essential Medium at pH 7.1-7.5 for 3 days with metabolic inhibitors or metabolic fuel supplements.

RESULTS

Inducing metabolic fuel starvation with inhibitors (1 mmol L(-1) 2-deoxyglucose or 0.1 mmol L(-1) KCN [potassium cyanide]) failed to stimulate protein degradation or net protein wasting under nonacidaemic conditions (pH 7.5). Conversely metabolic fuel supplements (1 mmol L(-1) octanoate, pyruvate or alanine) failed to increase the protein content of the cultures at any pH tested, in spite of significant consumption of the fuels by the cells. Only leucine (1-3 mmol L(-1)) increased protein content and suppressed protein degradation in opposition to the catabolic effect of acidaemia (pH 7.1). Conclusion Leucine exerts a beneficial anabolic effect on cultured skeletal muscle cells in the face of metabolic acidaemia. The failure of other metabolic fuels to do this, and of the metabolic inhibitors to exert a catabolic effect, suggests that leucine acts as a specific modulator of protein turnover and not as a nonspecific source of carbon for oxidation as a fuel.

Variability of blood-brain ratios of phenylalanine in typical patients with phenylketonuria

Blood-brain ratios (BBR) of phenylalanine (Phe) were determined by quantitative in vivo 1H magnetic resonance spectroscopy (1H-MRS) in 17 adult patients with early-treated phenylketonuria who were randomly selected from a sample of 75 adults. Measurements were performed in all patients during steady-state conditions. The BBR showed a unimodal distribution with a mean of 4.0 (range 3.3 to 4.5). Blood-brain ratios were comparable for subgroups of patients with genotypes classified as severe, moderate, or mild and for patients on different types of diets. Brain Phe concentrations showed a strong linear correlation with blood Phe values (r = 0.93, P < 0.001). There were no saturation effects for blood Phe values up to 1.8 mmol/L, and a local regression analysis did not confirm increasing BBR for increasing blood Phe values. The intellectual outcome (Wechsler Adult Intelligence Scale) was correlated with long-term dietary control (r = -0.65, P < 0.05), fluctuation of blood Phe values during treatment (r = -0.60, P < 0.05), and concurrent blood and brain Phe concentration. The severity of white matter changes visible on magnetic resonance images (MRI) was increased with high blood and brain Phe concentrations but failed to reach statistical significance. No correlation was found between BBR values, intelligence quotient, and MRI grade. Based on the assumption that BBR show intraindividual stability, the current data do not support the hypothesis that blood-brain barrier transport of Phe is a key explanatory factor for outcome variability in the vast majority of "typical" patients with phenylketonuria.

Phenylketonuria: tyrosine beyond the phenylalanine-restricted diet

Controversies exist on the role of tyrosine in the pathogenesis of phenylketonuria (PKU) and, consequently, on the therapeutic role of tyrosine. This review examines data and theoretical considerations on the role of tyrosine in the pathogenesis and treatment of PKU. It is concluded that treatment with tyrosine alone to replace the phenylalanine-restricted diet cannot be justified. A treatment with large neutral amino acids (LNAA) including tyrosine to restore the balance in the transport of phenylalanine and other LNAA across the blood-brain barrier deserves further investigation. Such studies should prove the safety and the efficacy of such a treatment, finding the optimal dose of all LNAA, disclosing the correct age to start and the way to monitor treatment biochemically.

Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets

Treatment of phenylketonuria (PKU) consists of restriction of natural protein and provision of a protein substitute that lacks phenylalanine but is enriched in tyrosine. Large and unexplained differences exist, however, in the tyrosine enrichment of the protein substitutes. Furthermore, some investigators advise providing extra free tyrosine in addition to the tyrosine-enriched protein substitute, especially in the treatment of maternal PKU. In this article, we discuss tyrosine concentrations in blood during low-phenylalanine, tyrosine-enriched diets and the implications of these blood tyrosine concentrations for supplementation with tyrosine. We conclude that the present method of tyrosine supplementation during the day is far from optimal because it does not prevent low blood tyrosine concentrations, especially after an overnight fast, and may result in largely increased blood tyrosine concentrations during the rest of the day. Both high tyrosine enrichment of protein substitutes and extra free tyrosine supplementation may not be as safe as considered at present, especially to the fetus of a woman with PKU. The development of dietary compounds that release tyrosine more slowly could be beneficial. We advocate decreasing the tyrosine content of protein substitutes to approximately 6% by wt (6 g/100 g protein equivalent) at most and not giving extra free tyrosine without knowing the diurnal variations in the blood tyrosine concentration and having biochemical evidence of a tyrosine deficiency. We further advocate that a better daily distribution of the protein substitute be achieved by improving the palatability of these products.

Clinical significance of brain phenylalanine concentration assessed by in vivo proton magnetic resonance spectroscopy in phenylketonuria

Recent studies using in vivo proton magnetic resonance spectroscopy (1H MRS) have suggested that plasma phenylalanine (Phe) may not be a reliable indicator of brain Phe level in subjects with phenylketonuria (PKU). Interindividual variation in cerebral Phe can contribute to the phenotypic variability of the disease. We report the results of the direct assessment of brain Phe by 1H MRS in 10 off-diet PKU patients (aged 15.5-30.5 years), 4 detected and treated early, 6 late. In a single patient, brain Phe was evaluated before and 15 days after diet discontinuation. FLAIR MRI and 1H MRS were performed in the same setting by a 1.5 T clinical MR scanner. MR images were scored according to the extent of the lobar white-matter hyperintensity. Brain 1H MRS Phe signal (resonating at 7.36 ppm) was evaluated as a ratio to the creatine+phosphocreatine signal. Brain Phe was correlated with clinical, biochemical and MRI findings. Results were as follows. (1) An abnormal concentration of brain Phe was detected in all 10 PKU subjects (ranging from 0.030 to 0.074), associated with a wide interindividual variability of concurrent plasma Phe (ranging from 724 to 2800 micromol/L). (2) In late-detected subjects, brain Phe concentration correlated with clinical phenotype better than did plasma Phe. The discrepancy between brain and plasma Phe was relevant from a clinical point of view in two cases: in one, a late-detected patient with normal mental development, a high level of plasma Phe was associated with a relatively low concentration of brain Phe; in the other, a late-detected subject with severe neurological impairment, a very high level of brain Phe was associated with plasma Phe compatible with the diagnosis of mild PKU. (3) White-matter alterations were detected in all patients. FLAIR MRI sequences disclosed an involvement of optic chiasma and tracts in 7 subjects. No correlation was found between white-matter alterations and concurrent brain Phe concentrations. (4) In the only case assessed under different intake of Phe, the relevant increase of brain Phe paralleled the concurrent increase of plasma Phe, showing that 1H MRS can be a useful tool in evaluating the individual vulnerability of PKU patients to different values of plasma Phe.

Metabolism of carnitine in phenylacetic acid-treated rats and in patients with phenylketonuria

The effect of metabolites accumulating in phenylketonuria (PKU) was investigated on carnitine metabolism in rats and in patients with PKU. Of phenylacetic acid (PEAA), phenylpyruvic acid and homogentisic acid the PEAA was found to be the most effective in inhibiting carnitine biosynthesis in rats. Following 60 min, a single intraperitoneal dose of PEAA the relative conversion rate, i. e. the hydroxylation, of tracer [Me-(3)H]butyrobetaine to [Me-(3)H]carnitine decreased from 62.2+/-6.00% to 39.4+/-5.11% (means+/-S.E.M., P<0.01) in the liver, in the only organ doing this conversion in rats. The conversion of loading amount of unlabeled butyrobetaine to carnitine was also markedly reduced. The impaired hydroxylation of butyrobetaine was reflected by a reduced free and total carnitine levels in the liver and a reduced total carnitine concentration in the plasma. PEAA decreased the hepatic level of glutamic acid and alpha-ketoglutaric acid (alpha-KG), suggesting a mechanism for the reduced flux through the butyrobetaine hydroxylase enzyme, because alpha-KG is an obligatory co-enzyme. In the plasma and urine of PKU patients on unrestricted diet, markedly decreased total carnitine levels were detected. In the liver of PEAA-treated rats and urine of PKU patients, a novel carnitine derivative, phenacetyl-carnitine was verified by HPLC and gas chromatography-mass spectrometry.

"Hypotyrosinemia" in phenylketonuria

It has been postulated that the significant incidence of learning disabilities in well-treated patients with phenylketonuria (PKU) may be due, in part, to reduced production of neurotransmitters as a result of deficient tyrosine transport across the neuronal cell membrane. Hypotyrosinemia has been reported in treated and untreated PKU but virtually no data are available. We decided to examine this in our patient population and to compare it with the published norms, patient data from our hospital clinical biochemical laboratory database, and a group of normal children and adolescents in a private pediatric practice. We found that the mean nonfasting plasma tyrosine in 99 classical PKU patients was 41.1 micromol/L, in 26 mild (atypical) PKU patients 53.3 micromol/L, and in 35 non-PKU mild hyperphenylalaninemia patients 66.6 micromol/L. This compared to nonfasting plasma tyrosine levels in 102 non-PKU subjects of 64.0 micromol/L in our hospital biochemistry database, 69.1 micromol/L in 58 volunteers in the private office practice, and 64-78.8 micromol/L in infants, children, and adolescents in the literature review. Our data support the previously undocumented statements in the literature that plasma tyrosine levels are low in PKU.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.