The effects of phenylalanine on cultured rat embryos

The offspring of mothers with untreated classic phenylketonuria (PKU) have shown a high frequency of microcephaly, mental retardation, pre- and postnatal growth retardation, and birth defects. The aim of this study was to determine whether phenylalanine (phe) is the teratogenic agent in maternal PKU. To observe the direct effects of phe on organogenesis, embryos of 9.5-day pregnant rats were cultured for 48 h in the presence of phe at concentrations of 0.1 to 6.0 mM. Within this range no morphological abnormalities occurred in exposed embryos, when compared to control embryos. However there was a reduction (P less than or equal to 0.05) in embryonic protein content and somite number at the highest concentration of phe (6.0 mM). This does not preclude the longer-term effects of phe at later stages of gestation. To examine phe transport into the embryo in response to elevated serum phe levels, 3H-phe uptake studies were undertaken. These showed that 3H-phe from the culture serum is incorporated rapidly into the free amino acid pools and embryonic protein. At serum concentrations of 1.4 mM or higher, phe saturation occurs in the cellular pools of the embryo. Amino acid analysis of the exocoelomic fluid showed that when embryos were cultured for 48 h in serum containing 3.45 mM phe, the total amino acid concentration was maintained near the control levels (16 mM). Of this, 27% (4.26 mM) was contributed by phe, and all other amino acids, except methionine, were decreased with respect to control levels.

Paired comparisons between early treated PKU children and their matched sibling controls on intelligence and school achievement test results at eight years of age

Early-treated PKU children were compared to their matched non-PKU sibling controls on Wechsler Intelligence Scale for Children (WISC) and Wide Range Achievement Test (WRAT) results at age 8. Fifty-five PKU children had mean WISC Full Scale IQ score of 100, in comparison to a mean of 107 for their matched sibling controls (p = 0.001). Treatment parameters significantly correlated with sibling-PKU IQ score differences included maximum diagnostic phe level (r = 0.244, p = 0.036) and phe levels at age 6 (r = 0.329, p = 0.007) and at age 8 (r = 0.489, p less than 0.0005). Fifty PKU subjects scored significantly lower than their matched sibling controls on standard scores of the WRAT Reading (102 vs. 107, p = 0.016) and Arithmetic (96 vs, 101, p = 0.006) subtests, and lower, but not significantly so, in Spelling (100 vs. 103, p = 0.145). When the sample was grouped according to diet status at age 8, on-diet PKUs scored at or above the level achieved by their siblings on all three scales of the WISC and all three WRAT subtests, whereas the off-diet group scored from 7 to 13 points below their siblings on all measures. These results suggest that PKU children should restrict phe intake at least through their school years.

Neurophysiological, psychological, and nutritional investigations during discontinuation of the phenylalanine-restricted diet in children with classic phenylketonuria

Assessment of cortical sensory and psychological functions in children with classic phenylketonuria during the time period of diet discontinuation did not uncover any significant changes when pre-diet and post-diet termination data were compared. Analysis of relationships among the study variables found that children who had adequate dietary control and who were of average or above average intelligence displayed normal somatosensory evoked potentials (SEP) whereas those children with less adequate dietary control and lower IQ scores had atypical SEP waveforms.

Effects of maternal hyperphenylalaninemia on fetal brain development: a biochemical study

We examined the effects of maternal hyperphenylalaninemia on body and brain growth, and the biochemical maturation of the fetal and neonatal rat brain. Elevated concentrations of plasma phenylalanine were induced in pregnant rats under two experimental conditions from the 14th through the 21st days of gestation. In the first treatment, pregnant rats were injected subcutaneously with alpha-methylphenylalanine (to inhibit maternal liver phenylalanine hydroxylase) at a dosage of 30 mg/100 g body weight plus phenylalanine supplementation (to increase maternal and fetal plasma phenylalanine) at a dosage of 60 mg/100 g body weight two times daily. In the second treatment, pregnant dams were injected with phenylalanine only at a dosage of 65 mg/100 g body weight three times daily. Treatment with alpha-methylphenylalanine/phenylalanine (mPhe/Phe) resulted in a 76% inhibition in the activity of maternal phenylalanine hydroxylase and a 25-fold increase in the mean daily concentration of phenylalanine in the maternal and fetal plasma. Phenylalanine treatment alone resulted in a 15-fold increase in plasma phenylalanine in the maternal and fetal animals. Significant reductions in body and brain weights in the fetal and neonatal rats were found in both treatment groups. Biochemical determinations indicated that the total DNA, RNA, and protein contents of the cerebra were reduced, with the reductions being greater in the mPhe/Phe- than the phenylalanine-treated rats. However, the retardation in body and brain growth of both treatment groups did not appear to be permanent because substantial recovery was noted in the rats after postnatal day 7. These results suggest that exposure of the fetus to high plasma concentrations of phenylalanine caused a delay in the biochemical maturation of the fetal rat brain.

Effects of maternal hyperphenylalaninemia on fetal brain development: a morphological study

We examined the effects of maternal hyperphenylalaninemia on the morphological development of the fetal and neonatal rat brain. High concentrations of phenylalanine were induced in pregnant rats from embryonic days 14 through 21 by subcutaneous injections of alpha-methylphenylalanine (mPhe) (to inhibit maternal phenylalanine hydroxylase) at a dosage of 30 mg/100 g body weight plus phenylalanine (Phe) supplementation (to raise fetal plasma phenylalanine) at a dosage of 60 mg/100 g body weight two times daily at 12-h intervals. This treatment resulted in a significant hyperphenylalaninemia compared with vehicle-injected, pair-fed control rats. At embryonic day 21, mPhe/Phe-treated embryos displayed a reduced thickness of the cortical plate and marginal zone, a decrease in the size of postmitotic neurons, and an increase in the packing density of cells in the cortical plate. There was an increase in the number of pyknotic cells (cell death) and in the reactive microglia in the mPhe/Phe-treated group. During the postnatal period the differences between mPhe/Phe-treated and control rats became fewer and by postnatal day 7 the morphology of the mPhe/Phe-exposed brains was similar to the controls. These morphologic data, in conjunction with our previous biochemical finding of reduced cerebral DNA, RNA, and protein contents of mPhe/Phe-treated rats, indicate that the induced hyperphenylalaninemia caused a significant delay in the development of the cerebral cortex which was able to undergo recovery during the postnatal period.

Biochemical modulation of blood-brain barrier permeability

Hydrophilic substrates necessary for brain function cross the capillary by facilitated diffusion. The facilitation has many features in common with enzyme-catalyzed reactions and is probably subserved by protein entities in the endothelial wall. The proteins act as receptors, recognizing substrate molecules, and as translocators, giving the molecules access to an aqueous path through the endothelium. These receptor-translocators can be saturated, and the transport is subject to competitive inhibition by substrate analogs. Thus, amino acids inhibit the transport of each other, and galactose can inhibit glucose transport in suckling rats. The proteins can be induced, as in the case of ketone transport in starvation, and repressed, as in the case of glucose transport in hyperglycemia. In rats with hyperglycemia for three weeks, the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 mumol/hg/min. An important result of the description is the understanding that rigid distinctions between the function of receptors, translocators, and enzymes is impossible. Understanding of the biochemical properties of facilitated diffusion may help explain a variety of symptoms in many 'inborn errors of metabolism'. This understanding has followed greater, recent insights into the general properties of the blood-brain barrier (45,46,47).

EEG as a possible prognostic tool in phenylketonuria

Clinical EEG and biochemical data were recorded in 10 children with classical PKU and 5 with variant forms of hyperphenylalaninaemias during the first year of life. A semiquantitative evaluation of the EEG showed a high correlation between epileptiform abnormalities and phenylalanine blood levels in the first 90 days of life and therefore with the delay before dietary therapy in PKU children. Although performed on a limited population, such an approach may indicate an additional non-computerized EEG tool for the clinical management of hyperphenylalaninaemias, and suggest some criteria for neurophysiological risk evaluation during the first year of life.

Pattern evoked potentials in phenylketonuria

Humans with untreated phenylketonuria (PKU) are candidates for having misrouted optic projections related to their hypopigmentation. Six tractable PKUs were tested using scalp-recorded evoked potentials. Both pattern reversal and pattern onset/offset (appearance/disappearance) stimuli were used under conditions of binocular and monocular stimulation. Evoked potentials were of normal form and hemispheric symmetry following both monocular and binocular stimulation in 5 of the 6 PKUs. The exception demonstrated a small hemispheric asymmetry upon stimulation of the left eye, but not following stimulation of the right eye. The asymmetry observed in this subject was probably idiosyncratic and not representative of PKUs in general. Four of the 6 PKUs produced reversal. This may be related to incomplete postnatal myelination. There appears to be no detectable optic misrouting in PKU. In spite of total absence of fetal phenylalanine hydroxylase activity, availability of tyrosine and melanin pigment is evidently sufficient during early development of the visual system to permit normal decussation of optic fibers at the chiasm.

Zinc status and growth of children undergoing treatment for phenylketonuria

Children with phenylketonuria (PKU) are treated with semi-synthetic diets restricted in phenylalanine. Low-phenylalanine or phenylalanine-free formulae provide the majority of protein and energy in the diet while phenylalanine requirements are met by low-protein natural foods. Because of the restriction of natural protein sources in this diet, the study assessed the zinc nutrition of 22 treated children with PKU (aged from 1 month to 8 1/2 years) and correlated linear growth with zinc status. The mean (+/- SE) plasma zinc concentration of the PKU population was 66.6 +/- 3.3 micrograms/dl. The mean (+/- SE) hair zinc concentration was 70.2 +/- 11.5 micrograms/g. The mean plasma and hair zinc concentrations of the PKU population were significantly different (p less than 0.05) when compared with mean (+/- SE) normal values of 84.2 +/- 2.9 micrograms/dl and 130.7 +/- 8.3 micrograms/g, respectively. The mean (+/- SD) dietary zinc intake of 10 PKU patients was 8.56 +/- 2.68 mg/day. No significant differences (p less than 0.123) were found when the mean zinc intake was compared with National Academy of Sciences Recommended Dietary Allowance for age of 10 mg/day. No significant correlations were found when plasma and hair zinc concentrations were plotted with height percentiles. Further studies are required to assess the effects of zinc supplementation and the bioavailability of zinc from low-phenylalanine diets.

Phenylketonuric Tetrahymena: phenylalanine hydroxylase mutants and other tyrosine auxotrophs

Nineteen tyrosine auxotrophs of the ciliated protozoan Tetrahymena thermophila have been isolated and biochemically examined. These mutants are defective in the conversion of phenylalanine to tyrosine; this is analogous to the defect that causes phenylketonuria in humans. After nitrosoguanidine mutagenesis and self-fertilization, progeny clones were screened for tyrosine auxotrophy and positively identified by using growth tests and in vivo radiometric assays for phenylalanine-to-tyrosine conversion. Mutants in one complementation group (locus) lacked phenylalanine hydroxylase activity; mutants in three other loci appeared to be deficient in the unconjugated pteridine cofactor that is necessary for the function of the hydroxylase. Another mutant lacked the dihydropteridine reductase activity required to regenerate the reduced form of the pteridine cofactor. Because hydroxylation of tyrosine to dopa and of tryptophan to 5-hydroxytryptophan may require the same cofactor and pterin reductase as phenylalanine hydroxylase, these mutants may also prove useful for the study of the role of catecholamines and serotonin, substances known to be present in Tetrahymena.

[EEG sleep activity in a phenylketonuric child of the 'new variant' type, before and after treatment with 5-HTP and L-DOPA (author's transl)]

The EEG during the sleep of a 'new variant' (having a dihyropteridine reductase deficiency) phenylketonuric child, aged 11 months, has been studied under low phenylalanine and following a substitute treatment with 5-HTP and L-DOPA. Prior to treatment, a paroxystic type of activity composed of spikes and/or polyspikes associated with sharp theta waves was chiefly observed during paradoxical sleep (PS). After a 10 day treatment with 5-HTP and L-DOPA, the EEG was not modified. However, after 6 months of this therapy, the paroxysms disappeared as well as the spindles; the quiet sleep trace was disorganized. The EEG modifications observed could be due to an evolution of the illness, if the substitute treatment was begun too late to go into effect.

EEG pattern in phenylketonuria under early initiated dietary treatment

Nineteen of 161 infants with a confirmed diagnosis of phenylketonuria (PKU) had initially abnormal EEGs obtained within days of diagnosis. The abnormalities consisted of single repetitive or multiple spikes and/or sharp waves, focal or scattered, which rarely occurred in paroxysmal bursts. Initial maximal diagnostic serum phenylalanine (Phe) levels were significantly higher for the group with abnormal EEGs. The mean diagnostic serum Phe level was 56 mg/dL in the group with abnormal EEGs, whereas the mean diagnostic Phe level was 48 mg/dL in the group with normal tracings. The average age at initiation of restrictive dietary treatment was approximately 3 weeks. The initially abnormal EEGs became normal by 1 year of age in all but two patients who had a minimal and a mild abnormality, respectively. Hypsarrhythmic patterns were not seen. Infants with PKU detected early and treated well do not appear to need routine EEG monitoring.

Development of a refractory stage in a dog model for phenylketonuria

Dogs were fed a continuous diet of phenylalanine (Phe) daily and para-chloro-phenylalanine (p-CPhe), an inhibitor of phenylalanine hydroxylase (PH-ase) every second day. It was reported that such a diet produces a sustained hyperphenylalanemia in rats. We have found, however, that in dogs an initial rise in circulating Phe is followed, after a period of time, by a return to normal levels in spite of diet maintenance. PH-ase ws measured in liver samples obtained from dogs before the start of the above diet and at the time when Phe levels returned to normal. It was found that the post-feeding liver sample had 57%-100% less activity than the normal sample. Accordingly, decline in Phe levels cannot be attributed to an increase in PH-ase activity. Experiments are being initiated using labelled Phe, to investigate whether chronic feeding of Phe and p-CPhe resulted in an intestinal block to Phe absorption, which may explain our experimental findings.

Neuropathology of phenylacetate poisoning in rats: an experimental model of phenylketonuria

Results of this investigation indicate that the suckling rat treated with phenylacetate should be a useful new model for studying the pathogenesis of phenylketonuria and neuronal development. Both cerebellar and retinal neurons of postnatally treated rats are vulnerable to the adverse effects of phenylacetate. Morphological changes observed in the cerebellum, retina, and optic nerve of treated animals during the fourth to twenty-first days of life consist of regional reduction in the size of cerebellar vermis lobules IV, V, VIa, and IX, 35 to 40% reduction in thickness of the molecular layer, accumulation of cerebellar external granular cells and retinal neuroblastic cells, fewer parallel fibers in the cerebellar cortex, and fewer myelinated axons in the optic nerve.

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.

Inhibition of growth of cells in culture by L-phenylalanine as a model system for the analysis of phenylketonuria. I. aminoacid antagonism and the inhibition of protein synthesis

Phenylalanine in high concentrations inhibits the growth of mouse A9 cells. Protein synthesis is inhibited earlier and more severely than RNA or DNA synthesis. Phenylalanine inhibits the uptake and decreases the intracellular pool of several amino acids. Certain amino acids added in excess reverse the phenylalanine inhibition. The strongest reversing amino acids appear to function by excluding phenylalanine. The phenylalanine inhibition does not appear to be due to a deficiency of any amino acid, but to the high intracellular phenylalanine concentration and/or an amino acid imbalance resulting from the large ratio of phenylalanine to other amino acids.