Brain uptake and protein incorporation of amino acids studied in rats subjected to prolonged hyperphenylalaninaemia

Serum levels of neural protein S-100B in phenylketonuria

BACKGROUND

Serum neural protein S-100B concentration is considered as a marker of CNS lesions. Phenylketonuria (PKU) is a metabolic disorder characterized by high phenylalanine (Phe) levels in blood and foci of myelin absence in the CNS of untreated patients.

AIM

To evaluate S-100B blood levels in PKU patients.

METHODS

Twenty-five (N = 25) PKU patients of comparable age, who were diagnosed by neonatal screening and "followed up" in our Inborn Error of Metabolism Department, were divided into two groups: group A (N = 13) with almost normal Phe levels and group B (N = 12) "off diet" with high Phe concentrations. Their MRI examinations were normal 12-14 months before the beginning of the study. Twenty-three (N = 23) healthy children were the controls. Serum S-100B levels, measured with an immunoluminometric assay, were greatly elevated in the group B (0.48 +/- 0.6 microg/l) as compared to those of group A (0.16 +/- 0.4 microg/l, P < 0.001) and controls (0.10 +/- 0.02, P < 0.001). Positive correlation was found between S-100B and Phe blood concentration (r = 0.46, P < 0.01). Foci of myelin absence in MRIs were observed in 1/13 of group A and in 10/12 of group B at the end of this study.

CONCLUSIONS

(a) Serum S-100B protein level, for the first time evaluated in PKU, was positively correlated with Phe blood level in PKU patients. (b) S-100B blood estimation could be a useful peripheral marker of CNS lesions in patients with demyelinated disease such as PKU.

Reduced acetylcholinesterase activity in erythrocyte membranes from patients with phenylketonuria

OBJECTIVE

a) To evaluate acetylcholinesterase (AChE) activities in erythrocyte membranes from phenylketonuric (PKU) patients and controls and to correlate with their plasma phenylalanine (Phe), tyrosine (Tyr), alanine (Ala) and dopamine (DA) levels. b) To determine the in vitro effects of Phe, Ala and Phe plus Ala on their AChE activities.

DESIGN AND METHODS

AChE activities were determined spectrophotometrically in erythrocyte membranes from PKU children (n = 12) adhering to their diet (group A), from 11 "off diet" (group B) and from 23 controls. Their plasma amino acids were evaluated with an amino acid analyser and DA with an HPLC method. Ala (1.8 mM) and/or Phe (1.8 mM) were added in the enzyme incubation medium from controls, whereas only Ala was added in that from group B.

RESULTS

AChE activity (1.19 +/- 0.05 deltaOD/min x mg protein), Tyr (46 +/- 17 micromol/L) and DA (56 +/- 18 micromol/L) were remarkably decreased by about 60% in group B as compared to those of group A (3.01 +/- 0.18 deltaOD/min x mg protein, 115 +/- 39 micromol/L, 137 +/- 29 micromol/L, respectively, p < 0.001) and controls (3.13 +/- 0.16 deltaOD/min x mg protein, 117 +/- 44 micromol/L, 142 +/- 22 micromol/L, respectively, p < 0.001). Phe negatively correlated with AChE activity and positively with plasma Tyr and DA. Ala reversed the inhibited AChE by Phe in erythrocyte membranes from healthy children to control values, whereas no reverse effect was observed on the enzyme activity from PKU patients.

CONCLUSIONS

a) The low levels of DA and its precursor Tyr are due to the high Phe blood levels, as a consequence of the decreased activity of Phe-hydroxylase in the liver of our patients. So, high Phe blood levels inhibit AChE in PKU patients, probably resulting in higher acetylcholine concentrations. b) Determination of AChE in erythrocyte membranes from PKU could be a useful marker for the neurotoxic effects of Phe.

Effects of L-phenylalanine on acetylcholinesterase and Na(+), K(+)-ATPase activities in adult and aged rat brain

The effect of different L-phenylalanine (Phe) concentrations (0.12-1.8 mM) on acetylcholinesterase (AChE), (Na(+), K(+))-ATPase and Mg(2+)-ATPase activities was investigated in homogenates of adult and aged rat whole brain at 37 degrees C. Adult and aged rat experiments were necessary in relation to phenylketonuria (PKU) since phenylketonuric patients usually discontinue their therapeutic special diet when they reach adulthood. Diet discontinuation results in the pathological increase of Phe concentration in plasma and consequently in brain. AChE activity in adult brain homogenates showed a decrease up to 18% (P<0.01) with 0.48--1.8 mM Phe preincubated for 1 h. Adult brain Na(+), K(+)-ATPase was stimulated by 30--35% (P<0.01) in the presence of 0.48--1.8 mM Phe. However, high Phe concentrations were not able to affect the activities of AChE and Na(+), K(+)-ATPase, when preincubated with aged brain homogenate for 3 h. Moreover, high Phe concentrations appeared unable to affect the activity of eel E. electricus pure AChE inhibited about 30% (P<0.001) by the free radical system H(2)O(2)/Fe(2+). Also, the antagonists of alpha- and beta-adrenergic receptors (phenoxybenzamine and propranolol, respectively) inhibited adult rat brain Na(+), K(+)-ATPase activity about 30--40% (P<0.01) and Phe was unable to change this action. It is suggested that: (a) The inhibitory effect of Phe on brain AChE and its stimulatory effect on brain Na(+), K(+)-ATPase are decreased with age; (b) These effects may be influenced by aging factors, such as free radical action and/or reduced density of alpha- and beta-adrenergic receptors in the tissue.

The effect of elevated plasma phenylalanine levels on protein synthesis rates in adult rat brain

Increasing the plasma phenylalanine concentration to levels as high as 0.560-0.870 mM (over ten times normal levels) had no detectable effect on the rate of brain protein synthesis in adult rats. The average rates for 7-week-old rats were: valine, 0.58 +/- 0.05%/h, phenylalanine, 0.59 +/- 0.06%/h, and tyrosine, 0.60 +/- 0.09%/h, or 0.59 +/- 0.06%/h overall. Synthesis rates calculated on the basis of the specific activity of the tRNA-bound amino acid were slightly lower (4% lower for phenylalanine) than those based on the brain free amino acid pool. Similarly, the specific activities of valine and phenylalanine in microdialysis fluid from striatum were practically the same as those in the brain free amino acid pool. Thus the specific activities of the valine and phenylalanine brain free pools are good measures of the precursor specific activity for protein synthesis. In any event, synthesis rates, whether based on the specific activities of the amino acids in the brain free pool or those bound to tRNA, were unaffected by elevated levels of plasma phenylalanine. Brain protein synthesis rates measured after the administration of quite large doses of phenylalanine (> 1.5 mumol/g) or valine (15 mumol/g) were in agreement (0.62 +/- 0.01 and 0.65 +/- 0.01%/h respectively) with the rates determined with infusions of trace amounts of amino acids. Thus the technique of stabilizing precursor-specific activity, and pushing values in the brain close to those of the plasma, by the administration of large quantities of precursor, appears to be valid.

In vitro localization of the protein synthesis defect associated with experimental phenylketonuria

We have used a cell-free system derived from hamster brain to investigate protein synthesis during experimental phenylketonuria. In such a system the elongation inhibitor emetine impeded translation in extracts derived from both treated and control animals. On the other hand the initiation inhibitor aurintricarboxylic acid showed no effects on protein synthesis activity of treated hamsters, although it was severely inhibiting in controls. This suggests that initiation is the altered step in brain protein synthesis failure consecutive to phenylketonuria.

Cerebral glycine content and phosphoserine phosphatase activity in hyperaminoacidemias

Chronic hyperphenylalaninemia maintained with the aid of a suppressor of phenylalanine hydroxylase, alpha-methylphenylalanine, increases the glycine concentration and the phosphoserine phosphatase activity of the developing rat brain but not that of liver or kidney. Similar increases occur after daily injections with large doses of phenylalanine alone, while tyrosine, isoleucine, alanine, proline, and threonine, were without effect. Treatment with methionine, which increases the phosphoserine phosphatase activity of the brain and lowered that of liver and kidney, left the cerebral glycine level unchanged. When varying the degrees of gestational or early postnatal hyperphenylalaninemia, a significant linear correlation was found between the developing brains' phosphoserine phosphatase and glycine concentration. Observations on the uptake of injected glycine and its decline further indicate that coordinated rises in the brain's phosphoserine phosphatase and glycine content associated with experimental hyperphenylalaninemia denote a direct impact of phenylalanine on the intracellular pathway of glycine synthesis in immature animals.

Inhibition of protein and aminoacyl-tRNA synthesis, and binding and transport sites for aromatic amino acids in the brain in vitro with aromatic acids

The influx of [3H]phenylalanine, [3H]tyrosine and [3H]tryptophan into brain slices and synaptosomes, their binding to synaptic membranes and their incorporation into protein and aminoacyl-tRNA were studied in the presence of an excess of a second aromatic amino acid or some other aromatic acid, viz., phenylpyruvate, phenyllactate, phenylacetate, homogentisate, salicylate or benzoate. The influx into brain slices was strongly inhibited by a second aromatic amino acid and in general also by phenylpyruvate and homogentisate, but the effects of these substances upon the influx into synaptosomes were slight. The binding of phenylalanine and tyrosine to the synaptic membranes was affected mainly by phenylpyruvate and homogentisate, and these were also effective in preventing the formation of aminoacyl-tRNA, and thus apparently inhibited the biosynthesis of proteins and polyphenylalanine. In all cases phenyllactate, phenylacetate salicylate and benzoate had virtually no effect. Phenylalanine seemed to be a noncompetitive, and tyrosine a competitive inhibitor, while tryptophan had both properties, as was also the case with phenylpyruvate and homogentisate. Under phenylketonuric conditions high excesses of phenylalanine and phenylpyruvate, and also certain other aromatic compounds, seemed to occupy the cellular transport sites for amino acids on the cellular membranes and prevent the formation of aminoacyl-tRNAs, thus inhibiting brain protein synthesis. The reduced supply of intracellular amino acids and the inhibition of protein synthesis may constitute one reason for the development of biochemical phenylketonuric abnormalities.

Turnover of the fast components of myelin and myelin proteins in experimental hyperphenylalaninaemia. Relevance to termination of dietary treatment in human phenylketonuria

The turnover of myelin and of myelin protein fractions has been measured in the central nervous system of rats who were placed on a hyperphenylalaninaemia-inducing diet (3% L-phenylalanine and 0.12% p-chlorophenylalanine added to the normal laboratory chow) when they were 25 days of age. A considerably increased turnover of the fast component of myelin and of myelin protein fractions was observed, which was not found in weight-matched controls or in controls fed the normal laboratory chow supplemented with 0.12% p-chlorophenylalanine. The increased turnover is therefore due to the hyperphenylalaninaemic condition and not due to the slow-down in growth or the presence of p-chlorophenylalanine. Furthermore, an inhibition of myelin synthesis due to the hyperphenylalaninaemic condition has been observed. Since these effects on myelin metabolism can be demonstrated to occur even when the brain has matured considerably, prudence should be exercised in considering the termination of the dietary treatment of patients with phenylketonuria.

Tryptophan overload in the pregnant rat: effect on brain amino acid levels in in vitro protein synthesis

The concentration of most amino acids was higher in the brains of 19- and 21-day rat fetuses than in their respective mothers. After an intraperitoneal load of tryptophan to the mother, the intracerebral concentration of several amino acids (including leucine) decreased not only in the mothers, but also in their fetuses. The in vitro incorporation of [3H]leucine into proteins in brain postmitochondrial supernatant fractions was enhanced in both the mothers and fetuses after tryptophan administration, but this effect disappeared when protein synthesis was calculated by using specific activities corrected for the amount of unlabeled leucine in the preparation. By this criterion, protein synthesis activity appeared similar in the brains of 19- and 21-day pregnant rats but was higher in their fetuses, especially in the 21-day subjects. Thus, protein synthesis in the brain was not altered by marked changes in the amino acid pool and more profound and prolonged metabolic disturbances must occur to cause permanent damage in the developing brain.

Effect of alpha-methylphenylalanine and phenylalanine on brain polyribosomes and protein synthesis

A chronic hyperphenylalanemia was effectively produced in developing mice by daily administrations of phenylalanine (2 mg/g body wt) and a phenylalanine hydroxylase inhibitor alpha-methyl-D,L-phenylalanine (0.43 mg/g body wt). The presence of alpha-methylphenylalanine in newborn mice inhibited 65-70% of hepatic phenylalanine hydroxylase activity within 12 h. Since this maximum inhibition persisted for 24 h or longer, decreased enzyme activity was maintained by daily administrations. Whereas concentrations of phenylalanine increased approximately 40-fold in both plasma and brain following injection of alpha-methylphenylalanine and phenylalanine, plasma levels of tyrosine were not altered significantly. Concomitant with changes in phenylalanine concentrations we observed the brain polyribosomes' disaggregation, which reached a maximum 3 h after injection and persisted as long as 18 h. Polyribosomes did not become refractory to as many as 10 daily injections of alpha-methylphenylalanine and phenylalanine. In addition to polyribosome disaggregation, chronic hyperphenylalanemia reduced the rates of polypeptide chain elongation on polyribosomes isolated from brain homogenates.

Chronic hyperphenylalaninemia produces cerebral hyperglycinemia in immature rats

The amino acid content of three tissues was measured in 10-day-old rats made hyperphenylalaninemic from age 3 to 10 days by daily injection of phenylalanine plus alpha-methylphenylalanine to inhibit phenylalanine hydroxylase (PAH). At 12 h after the last injection, the concentrations of alanine, valine, methionine, isoleucine, and leucine in the cerebral hemispheres were depressed by 25-50%, whereas that of glycine was elevated 2.3-fold. In the spinal cord, the levels of phosphoserine, methionine, and leucine were decreased by 40-50%, and those of serine and threonine increased by 50%. Tyrosine and phenylalanine concentrations were high in all tissues, 2-3 and 15-30 times normal, respectively; of the amino acids investigated, they were the only ones changed in the liver. Cerebral hyperglycinemia was also produced by chronic treatment with phenylalanine plus p-chlorophenylalanine to inhibit PAH, but not by acute (12 h) hyperphenylalaninemia. An increase in cerebral phosphoserine phosphatase activity was greater in rats treated with phenylalanine plus PAH inhibitor than with inhibitor alone. The content of brain glycine normally declines with age from birth to 15 days; this decrease was prevented by chronic hyperphenylalaninemia. Attempts to reduce the cerebral glycine content of the hyperphenylalaninemic rats were unsuccessful. However, one of the therapeutic protocols, methionine loading, may be useful because it increased the methionine and decreased the phenylalanine contents in the brain.

The effect of hyperphenylalaninaemia on glycine metabolism in developing rat brain

The brains of 3--16-day-old rats that were rendered hyperphenylalaninaemic by daily injections of alpha-methylphenylalanine plus phenylalanine were subjected to biochemical analysis. Fluctuations throughout the treatment period in the concentrations of branched-chain amino acids, methionine and serotonin were in agreement with the known interference of excess plasma phenylalanine with transport. The glycine content, however, became abnormal only by day 5, remained so through the treatment, and the elevation was equally apparent at 4, 8 or 24 h after the last daily injections. On the last day of treatment there were small increases in the taurine, glutamate, aspartate and 4-aminobutyrate concentrations, attributable mainly to the diencephalon or brain stem. After day 3 of treatment there were persistent elevations in the specific activity of phosphoserine phosphatase and glycine synthase (but not serine hydroxymethyltransferase) of the brain in each of the regions analysed. The observations indicate that chronic hyperphenylalaninaemia interferes with the normal regulation of intracerebral glycine metabolism during a critical period of early postnatal development, and suggest that the resulting excess in this amino acid (particularly marked in the cortex) contributes to the behavioural abnormalities that these animals exhibit in later life.

Phenylketonuria: clinical and experimental considerations revealed by the use of animal models

Phenylketonuria is a genetic defect that leads to imbecility, if the diagnosis is not made directly after birth. Since the development of imbecility can be almost totally halted by suitable dietary treatment, phenylketonuria is of more interest to neurochemists than to clinicians. This genetic defect is not known to occur in aminals. It is therefore necessary to develop suitable models for neurochemical analysis. Most successful is the simultaneous application to developing rats of alpha-methyl-phenylalanine (an inhibitor of phenylalanine hydroxylase), together with phenylalanine. With this treatment it is possible to induce changes in the central nervous system which are surprisingly similar to those found in patients with phenylketonuria. This model is therefore of great importance in the analyses of the disturbances of metabolism, which finally causes the severe defects in normal brain function.

A simple reproducible cell-free system for measuring brain protein synthesis

A simple, rapid, sensitive, and reproducible cell-free assay system for studying brain protein synthesis is described. This system uses small amounts of brain postmitochondrial supernatant, making it a convenient screening test when only small amounts of tissue are available. It showed over 95% dependence on Mg2+ and on an energy source. Optimal incorporation occurred under the following conditions: Mg2+ 3 mM; ATP, 0.6 mM; GTP, 0.6 mM; high K+, greater than or equal to 25 mM; Low Na+, less than or equal to 15 mM; pH 7.1-7.5. The rate of amino acid incorporation did not vary with leucine concentrations in vitro up to 1 mM, which obviated the need to measure endogenous leucine concentrations.

Imbalance in the activities of alkaline phosphatase and Na+-K+-ATPase in the brain of experimentally induced phenylketonuric squirrels (Funambulus palmarum)

Phenylketonuric squirrels have shown marked inhibition of alkaline phosphatase in the olfactory lobes and cerebral hemispheres, whereas the Na+-K+-ATPase remained less altered. In the pathogenesis of phenylketonuria inhibition of alkaline phosphatase at the level of "Blood-Brain Barrier" (BBB), leads transport system to impaired functioning.