Characterization of experimental phenylketonuria. Augmentation of hyperphenylalaninemia with alpha-methylphenylalanine and p-chlorophenylalanine

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Biochemical, Metabolic, and Behavioral Characteristics of Immature Chronic Hyperphenylalanemic Rats

Phenylketonuria and hyperphenylalanemia are inborn errors in metabolism of phenylalanine arising from defects in steps to convert phenylalanine to tyrosine. Phe accumulation causes severe mental retardation that can be prevented by timely identification of affected individuals and their placement on a Phe-restricted diet. In spite of many studies in patients and animal models, the basis for acquisition of mental retardation during the critical period of brain development is not adequately understood. All animal models for human disease have advantages and limitations, and characteristics common to different models are most likely to correspond to the disorder. This study established similar levels of Phe exposure in developing rats between 3 and 16 days of age using three models to produce chronic hyperphenylalanemia, and identified changes in brain amino acid levels common to all models that persist for ~16 h of each day. In a representative model, local rates of glucose utilization (CMRglc) were determined at 25-27 days of age, and only selective changes that appeared to depend on Phe exposure were observed. CMRglc was reduced in frontal cortex and thalamus and increased in hippocampus and globus pallidus. Behavioral testing to evaluate neuromuscular competence revealed poor performance in chronically-hyperphenylalanemic rats that persisted for at least 3 weeks after cessation of Phe injections and did not occur with mild or acute hyperphenylalanemia. Thus, the abnormal amino acid environment, including hyperglycinemia, in developing rat brain is associated with selective regional changes in glucose utilization and behavioral abnormalities that are not readily reversed after they are acquired.

State-of-the-art 2003 on PKU gene therapy

Phenylketonuria (or PKU) is a well-known and widespread genetic disease for which many countries perform newborn screening, and life-long dietary restriction is still the ultimate and effective therapy. However, the diet is complicated, unpalatable, and expensive. The long-term effects of diet discontinuation in adults, except for the serious adverse effects of maternal hyperphenylalaninemia upon the developing fetus, have not been systematically studied, but cognitive decline and neurologic abnormalities have been anecdotally reported. Thus, alternative approaches for PKU therapy, including gene therapy, must be further explored. Here we summarize past present nonviral and viral gene transfer approaches, both in vitro studies and preclinical animal trials, to delivering the PAH gene into liver or other organs as potential alternatives to life-long phenylalanine-restricted dietary therapy.

Effect of phenylalanine and p-chlorophenylalanine on Na+, K+-ATPase activity in the synaptic plasma membrane from the cerebral cortex of rats

Na+, K+-ATPase activity was measured in synaptic plasma membrane from cerebral cortex of Wistar rats subjected to experimental phenylketonuria, i.e., chemical hyperphenylalaninemia induced by subcutaneous administration of 5.2 micromol phenylalanine / g body weight (twice a day) plus 0.9 micromol p-chlorophenylalanine / g body weight (once a day). The treatment was performed from the 6th to the 14th postpartum day and rats were killed 12 h after the last injection. Synaptic plasma membrane from cerebral cortex was prepared by a discontinuous density sucrose gradient for Na+, K+-ATPase activity determination. The results showed that the enzyme activity was decreased by 30% in animals subjected to experimental phenylketonuria when compared to control. The in vitro effects of the drugs on Na+, K+-ATPase activity were also investigated. Phenylalanine and p-chlorophenylalanine inhibited the enzyme activity and this inhibition was reversed by alanine. In addition, competition between phenylalanine and p-chlorophenylalanine for binding to the enzyme was observed, suggesting a common binding site for these substances. Our results suggest that reduction of Na+, K+-ATPase activity may be one of the mechanisms related to the brain dysfunction observed in human PKU.

Microelectrophoresis and auxilary micromethods

In this retrospect of approximately 30 years of work with micromethods, some of them developed in our own laboratory, their principles and application to different separation problems are described, such as one- and two-dimensional microelectrophoresis in capillaries and microslab gels, isoelectric focusing in capillaries or microslab gels, microchromatography, microphotometry, and microfluorometry for qualitative and quantitative evaluation of separation patterns. In addition, some useful auxiliary methods are also described, e.g., a method for quantitative protein determination in a microliter volume when neither the volume nor the protein content in that volume are known, and methods for the determination of glycoproteins, amino acids, and sugars in the picomole range.

Effects of dietary mixtures of amino acids on fetal growth and maternal and fetal amino acid pools in experimental maternal phenylketonuria

BACKGROUND

Branched-chain amino acids have been reported to improve fetal brain development in a rat model in which maternal phenylketonuria (PKU) is induced by the inclusion of an inhibitor of phenylalanine hydroxylase, DL-p-chlorophenylalanine, and L-phenylalanine in the diet.

OBJECTIVE

We studied whether a dietary mixture of several large neutral amino acids (LNAAs) would improve fetal brain growth and normalize the fetal brain amino acid profile in a rat model of maternal PKU induced by DL-alpha-methylphenylalanine (AMPhe).

DESIGN

Long-Evans rats were fed a basal diet or a similar diet containing 0.5% AMPhe + 3.0% L-phenylalanine (AMPhe + Phe diet) from day 11 until day 20 of gestation in experiments to test various mixtures of LNAAs. Maternal weight gains and food intakes to day 20, fetal body and brain weights at day 20, and fetal brain and fetal and maternal plasma amino acid concentrations at day 20 were measured.

RESULTS

Concentrations of phenylalanine and tyrosine in fetal brain and in maternal and fetal plasma were higher and fetal brain weights were lower in rats fed the AMPhe + Phe diet than in rats fed the basal diet. However, fetal brain growth was higher and concentrations of phenylalanine and tyrosine in fetal brain and in maternal and fetal plasma were lower in rats fed the AMPhe + Phe diet plus LNAAs than in rats fed the diet containing AMPhe + Phe alone.

CONCLUSION

LNAA supplementation of the diet improved fetal amino acid profiles and alleviated most, but not all, of the depression in fetal brain growth observed in this model of maternal PKU.

Effect of hyperphenylalaninemia chemically induced on in vitro incorporation of 32P into cytoskeletal proteins from cerebral cortex of developing rats

We studied the effect of hyperphenylalaninemia on in vitro incorporation of 32P into cytoskeletal proteins from cerebral cortex of rats by injecting l-phenylalanine plus alpha-methylphenylalanine subcutaneously from the 6th to the 14th day postpartum. Chronic hyperphenylalaninemia induced an increased in vitro phosphorylation of the 150-kDa neurofilament subunit and tubulins present in the cytoskeletal fraction at the end of the treatment and 3 days after treatment discontinuation. In addition, when in vitro phosphorylation of the cytoskeletal proteins from treated animals was performed in the presence of the drugs we observed a decreased in vitro incorporation of 32P into these proteins. Thus, the effect of l-phenylalanine plus alpha-methylphenylalanine on the endogenous protein kinase and phosphatase activities was examined and the results demonstrated that these drugs have an inhibitory effect on calcium/calmodulin-dependent protein kinase II and protein phosphatase type 1.

Reduced myelinogenesis and recovery in hyperphenylalaninemic rats. Correlation between brain phenylalanine levels, characteristic brain enzymes for myelination, and brain development

In a previous paper (Burri et al., 1990), we have shown that experimental hyperphenylalaninemia (hyper-Phe) in 3-17 d-old rats leads to reduced myelinogenesis. Such treated rats recover during a 6 w low phenylalanine (Phe) period between days 17 and 59. In order to get more detailed information about the disturbed myelinogenesis and recovery, we measured in hyper-Phe rats the developmental pattern of two brain enzymes typical for myelination, cerebroside sulfotransferase (CST), and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), and other developmental parameters. Further, we correlated brain Phe levels with the brain damage in hyper-Phe rats, and we measured brain acetylcholinesterase (AChE) as a neuronal marker. Experimental hyper-Phe rats, injected between postnatal days 3 and 17 with alpha-methylphenylalanine and phenylalanine, showed a delayed age-dependent increase of CST activity, compared to that of controls. In hyper-Phe rats, CST peak activity was reached 2-4 d later, and was lower than in controls. The age-dependent decrease of the CST activity, however, started in test and control rats at the same time, at day 21. Between days 24 and 59, hyper-Phe rats had normal CST activity. CNP activity in hyper-Phe rats was lower than in controls from day 10 to 35, and recovered to normal values between days 35 and 59. Our results indicate that recovery from reduced myelinogenesis is possible after the period of fast myelination without compensatory increased CST activity. Further, the brain damage in test rats with Phe levels higher than average is more severe than in test rats with Phe levels lower than average; and there is no effect of hyperphenylalaninemia on brain neurons containing AChE.

Cognitive profile of rats exposed to lactational hyperphenylalaninemia: correspondence with human mental retardation

The present study was designed to provide further information on the enduring cognitive effects of experimental phenylketonuria (PKU) in rats, produced by the administration of alpha-methylphenylalanine and phenylalanine on postnatal days 3-21. These rats evidenced: (1) impaired learning set formation, (2) stimulus perseveration, particularly after an error, and (3) difficulty in utilizing the less salient features of their environment in mastering discrimination problems. In contrast, long-term memory function and the ability to form simple associations did not differ from controls. This pattern of intact and impaired cognitive functions bears remarkable similarity to that of mentally retarded humans and neonatally hyperphenylalaninemic rhesus monkeys, thus affirming the use of rats to study mental retardation. In addition, possible reasons for the mildness of the impairments commonly observed in animal models of severe mental retardation syndromes are discussed. We suggest that transfer of learning paradigms that assess the animal's ability to use information acquired in other problems are more likely to uncover significant cognitive impairments in such models than are procedures that test only the animals' ability to solve a single problem.

Biochemical and developmental features of experimental phenylketonuria induced by L-ethionine in suckling rats

Suckling rats were injected subcutaneously with doses of L-ethionine (0.1 mumole/g body wt) at intervals of 12 hr. In the latter group, phenylalanine hydroxylase was effectively inhibited in vivo resulting in hyperphenylalaninemia and phenylketonuria. Due to the well-known sex-specific differences in L-ethionine metabolism female rats were much more affected by chronic administration of L-ethionine. The underlying mechanism of enzyme inhibition by ethionine could be disturbed protein synthesis and impaired protein phosphorylation, which was suggested by pronounced decreases in ATP content in liver. In the high dosage group depletions mainly of the branched-chain amino acids and lysine occurred in serum and brain, whereas the concentrations of methionine and tryptophan were increased. Tyrosine tended to be decreased in the course of hyperphenylalaninemia. Hyperphenylalaninemia and other resulting amino acid imbalances obviously impaired brain development during the early postnatal period. Concomitantly with reductions in protein concentrations, the activity of cathepsin D, a major intralysosomal acid proteinase, was increased in brain, suggesting also higher protein catabolism in brain. Side effects of this treatment, however, were higher mortality, loss of body weight, and a general impression of delayed development, resembling a state of undernutrition to some extent. These obvious side effects of ethionine limit the usefulness of ethionine as a suitable model for classic phenylketonuria in suckling rats.

The development of the muscarinic acetylcholine receptor in normal and hyperphenylalaninemic rat cerebrum

The effect of hyperphenylalaninemia on the development of the muscarinic acetylcholine receptor in rat cerebrum has been studied. Rats were subjected to the hyperphenylalaninemic regimen as of 5 days of age. A gradual and steady decrease in the number of binding sites for L-[3H]quinuclidinylbenzilate was observed, with the white matter more affected than the gray matter. A return to normal blood phenylalanine levels after the age of 21 days does not lead to an increase in this number of binding sites.

Effect of experimental hyperphenylalaninemia induced by dietary phenylalanine plus alpha-methylphenylalanine administration on amino acid concentration in neonatal chick brain, plasma, and liver

Supplementation of 5% phenylalanine plus 0.4% alpha-methylphenylalanine to the standard diet or 1% phenylalanine plus 0.08% alpha-methylphenylalanine to the drinking water produced phenylketonuria-like conditions in 5-day-old chicks. An increase of 10 to 15-fold in the phenylalanine content was observed in plasma or brain of animals after 9 days of both types of treatment. A smaller but significant increase was also observed in liver. However, practically no changes were found in the levels of tyrosine in the same conditions. Thus, the high values of plasma and brain phenylalanine/tyrosine ratio obtained by these treatments were mainly due to an increase in the phenylalanine levels, without increasing those of tyrosine. Chronic hyperphenylalaninemia induced a nonsignificant decrease in the most of amino acid contents in brain, especially after 9 days of treatment, although the levels of glycine and serine were significantly increased. A similar decrease was found in the plasma and liver concentration of various amino acids, although the variations observed in the liver were smaller than those found in plasma and brain.

Cerebral serotonin regulation by phenylalanine analogues and during hyperphenylalaninemia

Severe hyperphenylalaninemia induced in infant rats by 3 days of treatment with p-chlorophenylalanine (p-cl phe) plus phenylalanine (phe) did not lower the tryptophan concentration of the brain, and the cerebral serotonin (5-HT) deficiency was attributable entirely to the known suppression to tryptophan hydroxylase (TPH) by p-cl phe. The decrease in 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) was thus no more pronounced than in rats which, treated with p-cl phe alone, were devoid of hyperphenylalaninemia. Suppression of TPH was found to also underlie the decrease in cerebral 5-HT caused by treatment with alpha-methylphenylalanine (alpha-mephe) alone: a 22% loss of midbrain TPH activity was detectable 24 hr after an injection only, reverted toward the normal during the next 2 days, and was clearly unrelated to the weak competitive inhibition of the enzyme by alpha-mephe in vitro. However, alpha-mephe (unlike p-cl phe), when administered together with phe, did not suppress TPH, nor did it counterbalance the reduction of cerebral tryptophan uptake by excess phe. Thus the 5-HT diminution in the rat model of phenylketonuria produced by treatment with alpha-mephe plus phe was attributable to hyperphenylalaninemia and the inhibition of tryptophan transport to the brain. Injection of tryptophan was found to restore the cerebral 5-HT level in the face of persistently severe hyperphenylalaninemia.

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.

Induction of hyperphenylalaninemia in mice by ethionine and phenylalanine

Female NMRI mice were fed diets containing l-ethionine (0.1 and 0.3% w/w) and phenylalanine (3% w/w), as well as respective control diets. Ethionine, the S-ethylated analog of methionine, was shown to inhibit phenylalanine hydroxylase in vivo, whereby in vitro kinetics remained unaffected. Treatment with ethionine resulted in fatty liver, reduced ATP content of liver, and alterations in serum amino acid concentrations. In the high dosage ethionine group, for instance, concentrations of Ala, Gly, Ser, Met, and Phe were increased whereas concentrations of Lys, Asp, and Pro were decreased. Applying ethionine together with phenylalanine resulted in hyperphenylalaninemia and phenylketonuria. Feeding phenylalanine alone also led to decreased activity of phenylalanine hydroxylase and increased concentration of Phe in serum. Ethionine only had a minimal effect on body weight gain; however, the hyperphenylalaninemic condition induced by application of the high dosage of ethionine and phenylalanine induced severe loss of body weight. A disturbed protein synthesis and protein phosphorylation might be the underlying mechanism of ethionine-induced suppression of phenylalanine hydroxylase.

Lysosomal protein degradation in experimental hyperphenylalaninaemia

The influence of alpha-methylphenylalanine-induced hyperphenylalaninaemia (HYP) on the lysosomal protein degradation system in brain and liver of suckling rats was investigated. In both tissues cathepsin D and L activities, measured at 5, 10 and 15 days post partum (p.p.), exhibited no differences between experimental and control animals. N-Acetyl-beta-D-glucosaminidase (NAGase) activity in brain, measured at 10 and 15 days p.p., was not affected by HYP either. The release of valine and lysine from liver and brain homogenates respectively, serving as a measure for the lysosomal content of degradable proteins, was not influenced by HYP. Lysosomal integrity during incubation of homogenate was monitored by the recovery of NAGase activity in the cytosolic supernatant, and by the relative NAGase activity in total homogenates in the absence of the lysosome disrupting detergent Triton X-100. In conclusion, experimental HYP appears unlikely to influence the lysosomal protein degradation system in brain and liver of suckling rats.

Developmental changes of cerebral phenylalanine uptake from severely elevated blood levels

Brain phenylalanine concentrations at plasma levels raised to that in phenylketonuric subjects were studied in rats from fetal through postnatal life. Suppression of the hepatic phenylalanine hydroxylase with alpha methylphenylalanine, and injections of age-adjusted doses of phenylalanine on the next day, assured the persistence of the same elevation of plasma levels for at least four hours prior to assay. The net phenylalanine uptake determined under these conditions underwent several-fold decreases between the fourth day and the end of the suckling period, and by about the age of 30 days it was as low as in adulthood. The development of transport properties studied here could contribute to the change with age in the vulnerability of the brain to the same degree of hyperphenylalaninemia and, since the cerebral phenylalanine uptake may decrease to non-damaging levels during childhood, it is pertinent to defining the age at which the rigorous diet of phenylketonurics might be safely relaxed.

Effect of hyperphenylalaninemia induced during suckling on brain DNA metabolism in rat pups

We studied DNA metabolism (synthesis and degradation) in brain to investigate the effect of hyperphenylalaninemia induced in rats by treatment with PCPA or alpha MPA plus PHE during suckling (4th-20th days of postnatal age) on cell proliferation and naturally occurring cell death. The incorporation of 14C in DNA as percent of total radioactivity in the tissue, 30 min after administration of [14C]thymidine served as a measure of DNA synthesis in vivo, and the amount of radioactivity recovered in DNA as percent of total 14C in the tissues of 21 day old rats, injected with [14C]thymidine on 2nd day after birth, indicated the turnover (degradation) of DNA. The results showed that the DNA content of cerebellum as well as cerebrum was reduced by treatment with PCPA plus PHE, while treatment with alpha MPA plus PHE had no effect on DNA content in cerebellum but reduced the levels in cerebrum. Treatment with PCPA or alpha MPA plus PHE reduced the synthesis of DNA in cerebrum of 11 day old rats but not in 21 day old rats, and the treatments did not affect DNA synthesis in cerebellum of either 11 or 21 day old rats. The turnover (degradation) of DNA was increased in both cerebellum and cerebrum from rats treated with PCPA plus PHE but alpha MPA plus PHE treatment did not alter the DNA turnover either in cerebellum or in cerebrum. The activity of acid DNase was reduced in both cerebellum and cerebrum from 11 as well as 21 day old rats treated with PCPA plus PHE, but the enzyme activity was not altered in the tissues from rats of both ages treated with alpha MPA plus PHE. The data thus indicate that in rats treated with PCPA plus PHE the reduction in cerebral DNA levels occurs due to reduced synthesis and/or increased turnover (degradation) of DNA but that the reduction in cerebellar DNA may occur only as a result of increased turnover (degradation), and that in rats treated with alpha MPA plus PHE the reduction in cerebral DNA must occur due to reduced synthesis. This suggests that treatment of rats with PCPA plus PHE during suckling inhibits cell proliferation and/or increases naturally occurring cell death in both cerebellum and cerebrum while treatment with alpha MPA plus PHE inhibits only cell proliferation and in cerebrum alone.

PKU, learning, and models of mental retardation

Experimental phenylketonuria was induced in male rats by daily injections of alpha-methylphenylalanine and phenylalanine on postnatal Days 3-31. Beginning at 8 weeks of age, the animals were subjected to a test of observational learning followed by a test of latent learning (two tests of "advantageous" learning). The animals subjected to the PKU treatment early in life showed significant learning deficits in both tests. The importance of these studies lies in the fact that unlike conventional tests of learning, tests of advantageous learning are sensitive to the kinds of biological insults which cause mental retardation in humans. This differential sensitivity evident in studies of animal models of cognitive pathology is analogized to the areas of dysfunction which characterize human mental retardation. Suggestions for the development of appropriate models of intellectual development are made.

Effect of experimental hyperphenylalaninemia on biogenic amine synthesis at later stages of brain development

The effects of experimental hyperphenylalaninemia on catecholamine and serotonin synthesis in brain at a later stage of brain development were investigated. A group of 35-day-old rats treated with normal chow supplemented with 5% Phe + 0.4% alpha-methylphenylalanine, alpha MP, for the previous 10 days showed decreases in dopa, norepinephrine, and epinephrine versus controls. A group treated with a normal diet supplemented with 0.4% alpha MP showed similar decreases and these differences could be attributed to the presence of the phenylalanine hydroxylase and tyrosine hydroxylase inhibitor, alpha MP, rather than the hyperphenylalaninemia condition. No differences in dopamine were observed. Serotonin and 5-hydroxyindoleacetic acid (5HIAA) were decreased 50% in the HyPhe condition and were unaffected in the presence of alpha MP alone, indicating that the decreases in serotonin and 5HIAA were due to the increases in phenylalanine rather than the presence of the inhibitor. These abnormalities in serotonin metabolism at later stages of brain development may be relevant to early discontinuation of dietary therapy in the PKU patient and implies a role in tryptophan supplementation to increase intracerebral serotonin values.

Modulation of cerebral catecholamine concentrations during hyperphenylalaninaemia

Hyperphenylalaninaemia induced by daily injections of alpha-methylphenylalanine plus phenylalanine caused 20-40% decreases in cerebral dopamine (3,4-dihydroxyphenethylamine) and noradrenaline in 7- and 11-day-old rats. alpha-Methylphenylalanine alone as well as phenylalanine alone caused cerebral dopamine depletion. However, the effects were not additive, in that the depletion caused by alpha-methylphenylalanine was greater, not less, than that after treatment with both it and phenylalanine. Increased concentrations of tyrosine in the brain, owing to administered or endogenously formed tyrosine, could overcome the effect of excess phenylalanine on cerebral dopamine content. The fact that the inhibition of tyrosine hydroxylase by phenylalanine (or alpha-methylphenylalanine) in vitro was overcome by tyrosine concentrations similar to those effective in vivo further implicates the tyrosine hydroxylase inhibition as the mechanism underlying the dopamine depletion in hyperphenylalaninaemia. These results provide a theoretical basis for elevation, by tyrosine supplementation, of the cerebral phenylalanine/tyrosine ratio as a possible treatment modality for phenylketonuria.

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.

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.