The effects of hyperphenylalaninaemia on the concentrations of aminoacyl-transfer ribonucleic acid in vivo. A mechanism for the inhibition of neural protein synthesis by phenylalanine

Use of acute hyperphenylalaninemia in rhesus monkeys to examine sensitivity and stability of the L-[1-11C]leucine method for measurement of regional rates of cerebral protein synthesis with PET

We have previously shown by direct comparison with autoradiographic and biochemical measurements that the L-[1-(11)C]leucine positron emission tomography method provides accurate determinations of regional rates of cerebral protein synthesis (rCPS) and the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis derived from arterial plasma. In this study, we examine sensitivity of the method to detect changes in lambda and stability of the method to measure rCPS in the face of these changes. We studied four isoflurane-anesthetized monkeys dynamically scanned with the high resolution research tomograph under control and mild hyperphenylalaninemic conditions. Hyperphenylalaninemia was produced by an infusion of phenylalanine that increased plasma phenylalanine concentrations three- to five-fold. In phenylalanine-infused monkeys, plasma leucine concentrations remained relatively constant, but values of lambda were statistically significantly decreased by 11% to 15%; rCPS was unaffected. Effects on lambda are consistent with competitive inhibition of leucine transport by increased plasma phenylalanine. The effect on lambda shows that competition for the transporter results in a reduction in the fraction of leucine in the precursor pool for protein synthesis coming from plasma. Even under these hyperphenylalaninemic conditions, rCPS remains unchanged due to the compensating increased contribution of leucine from protein degradation to the precursor pool.

Developmental regulation of the rabbit blood-brain barrier LAT1 large neutral amino acid transporter mRNA and protein

The expression of the blood-brain barrier (BBB) LAT1 large neutral amino acid transporter mRNA and protein was investigated in development in rabbits. The BBB LAT1 mRNA was down-regulated with postnatal development. However, the BBB immunoreactive LAT1 protein was unchanged in postnatal development, despite an up-regulation of the BBB GLUT1 glucose transporter protein during this period. The dissociation between LAT1 protein and mRNA levels in development is consistent with posttranscriptional regulation of BBB LAT1 gene expression.

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.

Inhibition of methionine incorporation into brain proteins after the systemic administration of p-chlorophenylalanine and L-5-hydroxytryptophan

The effects of p-chlorophenylalanine (p-CPA) and L-5-hydroxytryptophan (L-5-HTP) on local rates of plasma methionine incorporation into brain proteins were investigated by a quantitative autoradiographic method. The sequential i.v. administration of p-CPA (280 mg/kg, 42 h before the measurement) and L-5-HTP (60 mg/kg, 40 min before the measurement) resulted in an average 82% decrease of plasma methionine incorporation. The two treatments given separately also reduced the rates of plasma methionine incorporation in all the brain areas examined by 33 and 50%, respectively for p-CPA and L-5-HTP. These results indicate that: (1) p-CPA and L-5-HTP, two drugs which affect brain serotonin production in opposite ways, both produce large and general decreases of brain protein synthesis; (2) the administration of L-5-HTP does not restore the p-CPA-induced inhibition of brain protein synthesis but induces further decreases of protein synthesis. These results suggest that the reduction of brain protein synthesis in p-CPA-treated rats is mainly related to high circulating levels of p-CPA and phenylalanine; and that brain serotonin is not the only factor involved in the widespread metabolic changes observed. Such profound alterations of brain metabolism should be considered when interpreting the behavioral and neurochemical effects of p-CPA and L-5-HTP.

Measurement of free intracellular and transfer RNA amino acid specific activity and protein synthesis in rat brain in vivo

Brain protein synthesis was measured in anesthetized adult, male Sprague-Dawley rats by an in situ internal carotid arterial perfusion technique using [3H]leucine. The specific activity of free intracellular leucine and of tRNA leucine were determined by HPLC separation of phenylisothiocyanate (PITC) derivatives of amino acids. The specific activity of the leucyl-tRNA pool rapidly equilibrated with the free intracellular leucine pool within 2 min. The specific activity of the tRNA and free leucine pools in brain reached equilibrium by 10 min. Plasma amino acid specific activity, however, remained threefold higher than the specific activity of tRNA and free leucine pools. Estimates of protein synthesis were 0.62 +/- 0.06 nmol/min/g and were constant between 10 and 30 min of perfusion. The in situ perfusion model for protein synthesis described is a controlled system suited to measurements of protein synthesis in brain that can be applied to the study of brain metabolism under changing physiological conditions.

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.

Characterization of a translational inhibitor isolated from rabbit brain following intravenous administration of d-lysergic acid diethylamide

Intravenous administration of d-lysergic acid diethylamide (LSD) to rabbits results in a transient inhibition of brain protein synthesis in vivo and in vitro. A translational inhibitor that appears in the postribosomal supernatant fraction of cerebral hemispheres following LSD administration was partially purified by gel filtration on Sephadex G-150 and precipitation with 60% ammonium sulfate. This inhibitor, which was proteinaceous, reduced the translational capacity of an initiating cell-free protein synthesis system derived from brain. It also inhibited a messenger RNA-dependent reticulocyte lysate programmed with brain polysomes and a globin-synthesizing reticulocyte lysate system. Addition of the partially purified inhibitor to a brain cell-free protein synthesis system resulted in the decreased formation of ternary complexes as well as 40 and 80S initiation complexes, suggesting that the inhibitor affects an early step in the initiation of protein synthesis in brain.

Control of cell-free protein synthesis by amino acids: effects on tRNA charging

In order to resolve the observation that addition of glutamine and glutamate appears to be of particular importance in enhancing the activity of a cell-free protein synthesis system derived from rat liver (Manchester and Tyobeka, 1980), we have measured the KM of the aminoacyl-tRNA synthetases towards amino acids and the extent of aminoacylation of tRNA under the conditions of our earlier experiments. During incubation of the cell-free system in the presence of an amino acid mixture the extent of acylation to tRNA of 15 amino acids studied showed no clear change from initial time values. When incubation took place in the absence of added amino acids, however, the levels of glutamate and glutamine bound to their appropriate tRNAs dropped more rapidly and to lower levels than for other amino acids except tryptophan. The pronounced drop for these two amino acids does not seem to result from an abnormally high KM value for the synthetases towards the respective amino acids, nor an abnormally low Vmax, but probably from the fact that the amounts of glutamyl and glutaminyl-tRNA in the cell-free system are comparatively low.

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.

The effects of chronic hyperphenylalaninaemia on mouse brain protein synthesis can be prevented by other amino acids

A prolonged elevation in the concentrations of circulating phenylalanine was maintained in newborn mice by daily injections of phenylalanine and a phenylalanine hydroxylase inhibitor, alpha-methylphenylalanine. The result of this chronic hyperphenylalaninaemia was an accumulation of vacant or inactive monoribosomes that persisted for 18 h of each day. An elongation assay in vitro with brain postmitochondrial supernatants demonstrated that, in addition, there was an equally prolonged decrease in the rates of polypeptide-chain elongation by the remaining brain polyribosomes. Analyses of the free amino acid composition in the brains of hyperphenylalaninaemic mice showed a loss of several amino acids from the brain, particularly the large, neutral amino acids, which are co- or counter-transported across plasma membranes with phenylalanine. When a mixture of these amino acids (leucine, isoleucine, valine, threonine, tryptophan, tyrosine, methionine) was injected into hyperphenylalaninaemic mice, there was an immediate cessation of monoribosome accumulation in the brain and there was no inhibition of the rates of polypeptide-chain elongation. Although the concentrations of the large, neutral amino acids in the brain were partially preserved by treatment of hyperphenylalaninaemic mice with the amino acid mixture, the elevated concentrations of phenylalanine remained unaltered. The amino acid mixture had no detectable effect on brain protein synthesis in the absence of the hyperphenylalaninaemic condition.

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.

Progress in experimental phenylketonuria: a critical review

Experimental progress in the development of an accurate and useful model of phenylketonuria (PKU) during the last 15 years is reviewed in detail. From this review it is clear that the recent emergence of models using the combined administration of phenylalanine (phe) and p-chlorophenylalanine (PCPA) constitutes a major success that lays the groundwork for future research into the pathogenesis and treatment of PKU. Biochemical evidence on the pathophysiology of PKU is also briefly reviewed in the context of the behavioral and biochemical adequacy of the models used. It appears that in the past biochemical investigations into PKU have been impaired by use of inadequate models, a situation that should now change if the best of the phe-PCPA models are more widely adopted. New trends in PKU research involve the role of large neutral amino acids other than phe as potential aids in the treatment of PKU and the appearance of a new model based on the use of alpha-methylphenylalanine (AMPhe) combined with phe. It appears that PKU research may be on the brink of a new and productive era as investigations into these promising areas unfold and as new emerge through the full utilization of existing models.

Brain uptake of 11C-methionine in phenylketonuria

The brain uptake of 11C-methionine was studied in 26 children with classical phenylketonuria; one adult was used as a control. Labelled methionine uptake in brain was first measured during a low phenylalanine diet and again one week later after a load of phenylalanine. Ten children aged 1 to 30 months were studied twice at intervals of several months. In children having a phenylalaninemia less than or equal to 0.3 mumoles . ml-1, a decrease in methionine brain uptake was observed with increasing age, with the largest change occurring during the first year of life. After the phenylalanine load, a mean increase in phenylalaninemia by a factor of ten was accompanied by a mean decrease in brain methionine uptake by a factor of two while blood methionine remained unchanged. Brain activity curves increased with time for children younger than one year and having phenylalaninemia less than 0.6 mumoles . ml-1. After the age of 2 most patients had a decreasing curve regardless of the blood phenylalanine level. This study indicates that 11C-methionine brain uptake may be taken as an index of blood barrier permeability to essential amino acids, and of brain maturation. The results obtained suggest that an increase in phenylalaninemia to levels greater than 0.6 mumole . ml-1 induces a modification in brain uptake of amino acids, primarily during the first two years of life.

Characterization of an initiating cell-free protein synthesis system derived from rabbit brain

Protein synthesis in the brain is known to be affected by a wide range of treatments. The detailed analysis of the mechanisms that are involved would be facilitated by the development of cell-free translation systems derived from brain tissue. To date, brain cell-free systems have not been fully characterized to demonstrate a capacity for initiation of translation. The following criteria were utilized to demonstrate that a cell-free protein synthesis system derived from rabbit brain was capable of initiation in vitro: (a) sensitivity of cell-free translation to the initiation inhibitor aurintricarboxylic acid (ATA); (b) binding of [35S]Met-tRNAf to 40S and 80S initiation complexes; (c) incorporation of labeled initiation methionine into high-molecular-weight proteins; and (d) the association of labeled exogenous mRNA with polysomes. The optimum conditions for amino acid incorporation in this system were 4 mM-Mg2+, 140 mM-K+, and pH 7.55. Incorporation was dependent on the addition of ATP, GTP, and an energy-generating system. Cell-free protein synthesis reflected the normal process, since a similar spectrum of proteins was synthesized in vitro and in vivo. This initiating cell-free translation system should have wide application in the analysis of the mechanisms whereby various treatments affect protein synthesis in the brain.

Cerebral ribosomal protein phosphorylation in experimental hyperphenylalaninaemia

Investigations were carried out on the effects of phenylalanine loading on ribosomal protein phosphorylation in cerebral cortices of infant rats. Administration of L-phenylalanine intraperitoneally, in doses of 1 or 2 mg/g body wt., resulted within 30 min in a significant decrease in incorporation of radioactivity from intracisternally administered [32P]Pi into constitutive ribosomal proteins of the cerebral 40S subunit. This phenomenon was not accompanied by significant variations in 32P uptake into the cerebral cytosol. Incorporation of radioactivity into ribosomal proteins of the cerebral 60S subunit exhibited only minor variations under these circumstances. Alterations in the phosphorylation state of cerebral 40S ribosomal proteins induced by phenylalanine loading involved principally the S6 protein, which exists in multiple states of phosphorylation. The proportions of the more highly phosphorylated congeners of this protein were markedly decreased, as detected by two-dimensional electrophoretograms and autoradiographs of the cerebral 40S ribosomal proteins. Phenylalanine loading also altered the relative extent of phosphorylation of the S6 protein in cerebral polyribosomes and monoribosomes. In control animals, the specific radioactivity of 40S proteins in cerebral polyribosomes was five to ten times that of 40S proteins in the monoribosome population. At 1 h after phenylalanine administration, the specific radioactivities of 40S proteins in the two ribosome populations tended to approach equality. These alterations in ribosomal protein phosphorylation were accompanied by a decrease in the proportion of polyribosomes in purified ribosome preparations isolated from cerebral cortices of phenylalanine-treated infant rats. In animals given the higher dose of phenylalanine (2 mg/g body wt.), subsequent administration of a mixture of seven neutral amino acids, which resulted in partial recovery of polyribosomes, also tended to reverse the changes in ribosomal protein phosphorylation. Variations in the activities of ribonuclease enzymes in the cerebral cytosol were also observed under these conditions. Administration of phenylalanine increased the activities of cerebral ribonucleases, whereas subsequent treatment with the amino acid mixture partly reversed this effect. The results suggest that alterations in cerebral ribosomal protein phosphorylation, ribosome aggregation and ribosome function are interrelated in experimental hyperphenylalaninaemia.

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.

Aminoacylation of four tRNA species in lupin (Lupinus luteus) cotyledons

During germination of lupin seeds, the levels of in-vivo tRNA aminoacylation increase in different ways, depending on the species of tRNA. Column chromatography of tRNA on reverse-phase-chromatography (RPC-5) has shown the presence of 4 peaks of isoleucyl-tRNA, 5 of leucyl-tRNA, 5 of lysyl-tRNA, 2 of tyrosyl-tRNA, and 4 of valyl-tRNA. Cochromatography of periodate treated and control tRNA preparations, labeled with radioactive amino acids, indicates identical aminoacylation in vivo of isoaccepting tRNAs during plant development. One isoacceptor of isoleucine tRNA changes its elution profile after periodate treatment.

Control of protein synthesis in mammalian cells by aminoacylation of transfer ribonucleic acid

The dependence of protein synthesis on the intracellular content of aminoacylated tRNA has been studied in mouse ascites tumor cells deprived for various amino acids. A remarkable reduction in net protein synthesis has been found only after a drastic decrease in aminoacylation of tRNA. The quantitative correlation of protein synthesis with the degree of aminoacylation suggests that a moderate amino acid starvation primarily influences the rate of elongation at the codon concerned. These results are in contrast to the findings previously reported for HeLa cells. Some crucial steps during the determination of intracellular aminoacyl-tRNA have been investigated. The reliability of the method employed has been discussed on a theoretical basis.

Postnatal alpha-methylphenylalanine treatment effects on adult mouse locomotor activity and avoidance learning

Neonatal mice were injected for five days with a combination of alpha-methylphenylalanine and phenylalanine to determine the influences of excess phenylalanine during development upon the behavior of these mice as adults. Spontaneous activity, bolus production, passive avoidance learning, simple active avoidance learning and complex active avoidance learning were tested in mice treated at two different postnatal periods. The results show that the treatments during development produced adult behavioral alterations compared to controls. The effects were most pronounced in mice treated in the postnatal period immediately after birth. The behavioral effects can be summarized as increased emotionality and generalized, stimulus-induced activity as well as decreased passive avoidance performance and complex active avoidance performance. These behavioral deficits are consistent with those usually reported in various models of human phenylketonuria.

Effects of p-chlorophenylalanine and alpha-methylphenylalanine on amino acid uptake and protein synthesis in mouse neuroblastoma cells

The phenylalanine analogues p-chlorophenylalanine and alpha-methylphenylalanine were used to inhibit phenylalanine hydroxylase in animal models for phenylketonuria. The present report examines the affects of these analogues on the metabolism of neuroblastoma cells. p-Chlorophenylalanine inhibited growth and was toxic to neuroblastoma cells. Although in vivo this analogue increased cell monoribosomes by 42%, it did not significantly affect poly(U)-directed protein synthesis in vitro. P-Chlorophenylalanine did not compete with phenylalanine or tyrosine for aminoacylation of tRNA and was therefore not substituted for those amino acids in nascent polypeptides. The initial cellular uptake of various large neutral amino acids was inhibited by this analogue but did not affect the flux of amino acids already in the cell; this suggested that an alteration of the cell's amino acid pools was not responsible for the cytotoxicity of the analogues. In contrast with p-chlorophenylalanine, alpha-methylphenylalanine did not exert these direct toxic effects because the administration of alpha-methylphenylalanine in vivo did not affect brain polyribosomes and a comparable concentration of this analogue was neither growth inhibitory nor cytotoxic to neuroblastoma cells in culture. The suitability of each analogue as an inhibitor of phenylalanine hydroxylase in animal models for phenylketonuria is discussed.

Experimentally induced and natural recovery from the effects of phenylalanine on brain protein synthesis

The decrease in the neural polyribosomes produced during hyperphenylalaninemia could not be restored to normal levels by the injection of other single neutral amino acids. All of the neutral amino acids that are transported with phenylalanine were found to produce an alteration of neural polyribosomes similar to that measured with phenylalanine. However, the injection of a balanced mixture of 6 or 7 neutral amino acids could restore the brain polyribosomes to normal states. Although this experimentally induced recovery did not lower brain phenylalanine concentrations, it did restore the acylation levels of methionyl-tRNA, and in particular, the methionyl-tRNA initiator species. This also led to a concomitant stimulation of the elongation rate of brain polypeptide synthesis. A natural recovery of brain polyribosomal levels (occurring 2 h after 1 mg/g phenylalanine is injected) did not appear to represent a real recovery of neural protein metabolism. Phenylalanine concentrations were increased in the brain, the acylation levels of methionyl-tRNA, alanyl-tRNA and the initiator methionyl-tRNA remained altered, and the rate of ribosome translocation was decreased 28%.

The effect of amino acids on protein metabolism as measured in long-term experiments in immature brain explants

In a study of a system suitable for investigating long-term effects on brain protein metabolism, we measured amino-acid incorpration into isolated immature brain explants incubated under sterile conditions up to ten days. Measurements of changes in total proteins, total DNA, cell number during the experiments, and 14C-thymidine incorporation measurements indicated no significant net growth; new cell formation was below 5% in a 5-day period; therefore, amino-acid incorporation was mainly due to protein turnover. The rate of incorporation in our immature brain preparation was similar to that of the adult brain in vivo: by ten days about one-half of the tissue protein turned over. The label incorporated was released in subsequent incubations with cold amino acids. Such release occurred in all subcellular fractions examined. Incorporation was fairly stable; at temperatures below 30 degrees C it rapidly declined, but it was not affected when phenylalanine or the branched chain amino acids (leucine, isoleucine, valine) were elevated in the incubation medium. Brief exposure to low amino-acid media had no effect; longer exposure resulted in tissue damage. Our model system indicates that overall brain protein turnover is not sensitive to such variations in the level of most amino acids, which may occur under various conditions. Protein metabolism of the nervous system occurs at a high rate. A recent long-term labeling method (Lajtha, Latzkovits, and Toth, 1976) gave a best fit to incorporation curves by assuming two compartments for adult brain proteins, one of which (about 6%) has a half-life of 15 hr and the other (94%) has a half-life of ten days. The disappearance of protein-bound label with time under conditions in which all proteins were previously labeled indicated that most, possibly all, proteins in brain are in a dynamic state (Lajtha and Toth, 1966). Incorporation of amino acids was found in all proteins and structures that have been studied to date; myelin proteins previously thought less active are also metabolized at a significant rate (Sabri, Bone, and Davison, 1974; Lajtha, Toth, Fujimoto, and Agrawal, 1977). We have fairly extensive information available in addition to turnover studies about the mechanisms of protein synthesis in brain (Roberts, 1971); protein breakdown was also studied in some detail (Marks and Lajtha, 1971). In contrast to our knowledge about protein metabolism under physiological equilibrium conditions, our information about alterations during functional demands or pathological conditions is scanty. Although a significant amount of work has been reported, largely because of technical difficulties the results are difficult to interpret unequivocally. The present report represents our effort to address some of the obstacles: to develop a system in which influences on long-term incorporation can be studied...