The justification theory: the essential nature of the non-essential amino acids

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.

Robert Guthrie and the Trials and Tribulations of Newborn Screening

Routine newborn screening for many disorders is now so ingrained in newborn care that there is no question about whether it should be done. However, acceptance of newborn screening was not guaranteed when Robert Guthrie introduced it for phenylketonuria (PKU). This article describes the professional and personal story of Guthrie, a physician and microbiologist, who veered from cancer research to a commitment to prevent intellectual disability from PKU. It recounts how Guthrie was able to overcome strong opposition to mandatory screening from prominent physicians and medical societies, so that newborn screening for PKU would be routinely performed throughout the developed world, and would eventually form the basis for the (much more) comprehensive screening conducted today.

Randomised controlled trial of tyrosine supplementation on neuropsychological performance in phenylketonuria

OBJECTIVE

To test the efficacy of tyrosine supplementation, as an adjunct to dietary treatment, on neuropsychological test performance in individuals with phenylketonuria.

DESIGN

A randomised controlled trial of tyrosine supplementation using a double blind crossover procedure with three four week phases.

SETTING

The Hospital for Sick Children, Toronto.

PARTICIPANTS

21 individuals with phenylketonuria (ages 6 to 28 years, mean 11.3).

INTERVENTION

Participants were given 100 mg/kg body weight/d of L-tyrosine or L-alanine (placebo).

RESULTS

At baseline, performance on several of the neuropsychological test measures was correlated with tyrosine levels. Dietary supplements of tyrosine increased plasma tyrosine concentrations; however, no change in test performance was found across the tyrosine and placebo phases of the study.

CONCLUSIONS

Tyrosine supplementation of this type does not appear to alter neuropsychological performance in individuals with phenylketonuria.

Inherited metabolic diseases affecting the carrier

The objective of this review is to draw attention to those inherited metabolic traits which are potentially harmful also for the carrier, and to outline preventive measures, at least for obligate heterozygotes, i.e. parents of homozygous children. Concerning carriers of food-dependent abnormalities, early vascular disease in homocystinuria, hyperammonaemic episodes in ornithine transcarbamylase deficiency, presenile cataracts in galactosaemia as well as galactokinase deficiency, spastic paraparesis in X-linked adrenoleukodystrophy, and HELLP syndrome in mothers of babies with long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency have to be mentioned. In the group of food-independent disorders, clinical features in carriers may be paraesthesias and corneal dystrophy in Fabry disease, lens clouding in Lowe syndrome, lung and/or liver diseases in alpha 1-antitrypsin deficiency, and renal stones in cystinuria type II and III. Finally, two monogenic carrier states are known which in pregnant individuals could possibly afflict the developing fetus, i.e. heterozygosity for galactosaemia and for phenylketonuria. Elevated levels of galactose-1-phosphate have been found in red blood cells of infants heterozygous for galactosaemia born to heterozygous mothers. Aspartame in very high doses is reported to increase blood phenylalanine levels in heterozygotes for phenylketonuria, thus being a risk for the fetus of a heterozygous mother. For some of these carrier states preventive measures can be recommended, e.g. restriction of lactose in parents and heterozygous grandparents of children with galactosaemia and galactokinase deficiency as well as transiently in infants heterozygous for galactosaemia, dietary supplementation with monounsaturated fatty acids in symptomatic carriers for X-linked adrenoleukodystrophy, avoidance of smoking and alcohol in heterozygotes for alpha 1-antitrypsin deficiency, avoidance of episodes of dehydration in heterozygotes for cystinuria, and restriction of aspartame in pregnant women.

Gestational carrier--a reproductive haven for offspring of mothers with phenylketonuria (PKU): an alternative therapy for maternal PKU

Maternal phenylketonuria, PKU, has a detrimental effect on embryogenesis. Infant pathology is independent of fetal genotype, but is directly correlated with excessive phenylalaninaemia throughout pregnancy. Although normal children have been delivered by affected mothers who either had benign hyperphenylalaninaemia or in whom strict diet has apparently maintained maternal phenylalaninaemia in the low normal range from before conception, more abnormal than normal births have been reported. In addition, attempts at dietary management are often unsuccessful; most reported cases documented various severe pathological consequences of maternal PKU. Currently available methods provide viable alternative treatment. In vitro fertilization using the parental gametes, followed by implantation of the pre-embryo in a surrogate mother, would avoid a metabolic environment impairing normal development, and therefore should be recommended as alternative therapy for potential mothers with PKU.

Bony changes of PKU neonates unrelated to phenylalanine levels

In 1962 bone abnormalities were described radiographically in phenylketonuria patients. Later, observations were made on PKU infants during the neonatal period, which allowed differentiation between inherited alterations in bone development from those changes due to dietary restriction. Similar changes have been described in other aminoacidurias. Wrist radiographs and serum phenylalanine levels were obtained on 73 patients first seen between 1965 and 1990. All radiographs were taken on the day of referral, when the patient was between 6 and 57 days old. Forty-nine patients were less than 28 days old. Bone abnormalities were present in 56 of 73 (77%) of the children, as compared to 0 of 16 in a control group. The presence or absence of bone abnormality is unrelated to serum phenylalanine level, and to the age of the children at referral. This finding suggests that the cause of these mesodermal changes is not a deviation of a single amino acid in the infant, but that they are caused by an intrauterine amino acid imbalance.

Energy production, intracellular amino acid pools, and protein synthesis in chronic renal disease

Intracellular glycolytic regulating enzyme activities, pyruvate kinase (PK) and phosphofructokinase (PFK), adenylate kinase (AK), energy charge (Ech), free amino acids (ICAA), and protein synthesis (PS) were measured in polymorphonuclear leukocytes--used as a cell model--in 62 adults and 12 children with chronic renal failure, and 66 normal adults and 21 children as comparison controls. In normal subjects, children had significantly lower enzyme activities and cell amino acid levels but similar Ech and higher PS than adults. ICAA concentrations were significantly higher than plasma amino acid concentrations (PAA) in both groups, and the PAA were not correlated with, nor indicative of, the ICAA concentrations. The variance (R2) in PS could be largely accounted for by a combination ("set") of six ICAA, as determined by multivariate analysis. The sets differed in children vs adults, suggesting that different proteins were being synthesized. In the uremic patients, reduced PF, PFK, Ech, most ICAAs and PS were indicative of cellular malnutrition. For the uremic adults, the abnormalities in cell metabolism were modified by therapy--nondialyzed uremics being worst, CAPD patients best and approximately normal, and hemodialyzed intermediate. The uremic CAPD children had reduced, PK, PFK, AK, most ICAA, and PS. Ech was increased. Cellular malnutrition in children with chronic renal failure may contribute to their poor growth.

Intracellular amino acid levels as predictors of protein synthesis

Protein synthesis depends on a complete complement of precursor amino acids, specific acetylating enzymes, tRNA, and so forth. It has been related to metabolism of individual amino acids, eg, valine and leucine; however, the relation of protein synthesis to the ambient concentrations of amino acids in the intracellular and extracellular pools has not been defined. Using the viable, isolated granulocyte (leukocyte) as an in vitro cell model, protein synthesis (incorporation of 4,5-3H-leucine) has been related to simultaneous amino acid concentrations in the cell and plasma by multiple regression analysis. Fifty-five normal neonates and 30 normal adults were studied. Protein synthesis was higher in the infants than in the adults (3,527 vs 2,685 pmole/hr/mg DNA). The intracellular concentrations of most amino acids were higher than their concentrations in plasma, except for valine and citrulline, which were lower. The "aminograms" in the two pools also were very different. Forty-four percent of the variance (R2) in protein synthesis was accounted for by the intracellular concentrations of leucine, glycine, alanine, and taurine in neonates and 45% by a combination of threonine, valine, methionine, and histidine in adults. The intracellular concentrations of each of these predictor amino acids in adults were, in turn, related to different combinations of the plasma concentrations of threonine, phenylalanine, tryptophan, isoleucine, histidine, citrulline, ornithine, arginine, and glycine. Thus, it is possible to identify sets of intracellular amino acids that predict the level of protein synthesis and to delineate combinations of plasma amino acids whose levels account for a significant portion of the variance in the intracellular predictor amino acids in normal human infants and adults.

Relationship between phenylalanine tolerance and psychological characteristics of phenylketonuric families

The purpose of this study was to find out how genetic and biochemical limitations influence psycho-social performance and to partially test the validity of justification theory. The ability to convert phenylalanine to tyrosine was compared with intellectual and personality characteristics in PKU family members. Each of the tested persons was given an oral dose of phenylalanine, the Shipley-Hartford Intelligence Test, and the Minnesota Multiphasic Personality Inventory (MMPI). Only those persons with reading ability at the sixth grade level or higher were tested. Eighty-six persons were tested: fifteen PKUs, forty-three siblings, and twenty-eight parents. A comparison was made among parents, PKUs, and the siblings. Siblings with the higher 2/3's of P2/T ratios were contrasted with those with the lowest 1/3 of ratios on measures of intelligence and psychopathology. Statistical analyses of the data reflected a trend in support of the justification theory. PKUs had significantly lower intelligence than their sibs and parents. The PKUs' mean IQ was 95 (homozygotes born of heterozygotes), followed by the upper 2/3's sibling mean IQ of 105 (heterozygotes born of nonheterozygote mothers). The lower 1/3 siblings' mean IQ was 107 (nonheterozygotes born from heterozygote mothers), and finally, the parents' mean IQ was 109 (heterozygotes, among them 50% were born from nonheterozygote mothers). The latter three mean IQs are not significantly different from each other. The personality tests revealed a trend toward more abnormality in PKUs than in their heterozygote siblings. The lowest rate of abnormality occurred in the nonheterozygote sibling group; that rate was significantly lower than in all other groups. The parents had the highest absolute rate of personality abnormality, but statistically so compared to the low-ratio siblings.

Analysis of learning in retarded monkeys

Sixteen rhesus monkeys were either subjected to diets high in phenylalanine or parachlorophenyalanine either pre- or postpartum; there were 16 controls. Subjects were tested after removal from the PKU diet on a series of learning tasks. No support was found for suggestions that PKU monkeys a do worse if task difficulty is increased or b have an attentional or short-term memory storage deficit. Equivocal support was found suggesting that c PKU monkeys may have stronger initial biases and d do not attend to the relevant dimension. The most support was found for the hypothesis that e PKU subjects are more emotional which would account for a disruption in performance following negative reinforcement, and difficulty in changing an initial or a learned response pattern. The literature on learning in human PKU and induced PKU in animals is reviewed.

Dietary effects on rhesus social behavior: altered amino acid diets

Twenty-one monkeys continuously fed one of five diets high in tyrosine, histidine, alanine, glycine, or tryptophan between the ages of 1 and 12 months were compared with 20 controls. Social behavior either (a) while on the diet and tested in familiar groups of four or (b) while on a normal diet and paired with unfamiliar monkeys showed no effects of the high amino acid diets, confirming a previous analysis of learning ability.

Plasma phenylalanine levels in phenylketonuric heterozygous and normal adults administered aspartame at 34 mg/kg body weight

Following administration of aspartame (34 mg/kg body wt) in orange juice, plasma concentrations of free amino acids were measured in 12 female subjects known to be heterozygous for phenylketonuria and 22 normal subjects (12 male, 10 female). No change in fasting plasma aspartate concentrations were noted after aspartame loading in either group. In normal male subjects, the mean (+/-S.D.) plasma phenylalanine concentration increased from a fasting value of 5.86 +/- 1.25 mumol/dl. Plasma phenylalanine levels in normal female subjects increased from a mean fasting concentration of 4.83 +/- 0.84 mumol/dl to a men peak value of 8.95 +/- 1.49 mumol/dl suggesting a more rapid absorption, metabolism, and/or clearance of phenylalanine by females. In female heterozygous subjects, the mean peak plasma phenylalanine concentration was significantly higher than in normal females. Plasma phenylalanine values increased from a mean fasting value of 5.92 +/- 1.51 mumol/dl to a mean peak value of 15.1 +/- 4.76 mumol/dl. Similarly, the area under the plasma phenylalanine concentration-time curve was significantly greater in heterozygous female subjects (21.36 +/- 5.10 IU) than in normal female subjects (10.84 +/- 2.32 IU). However, peak plasma phenylalanine levels were well below those associated with toxic effects in all cases.

Cord-blood tyrosine levels in the full-term phenylketonuric fetus and the "justification hypothesis"

The "justification hypothesis" attributes mental retardation in phenylketonuria (PKU) to an inability of the heterozygous mother to deliver an appropriate amount of tyrosine to the PKU fetus who, in turn, is unable to correct for this deficiency because of its genetic constitution. We tested this hypothesis by measuring concentrations of tyrosine and phenylalanine in cord blood obtained at delivery from nine infants with PKU and five infants with persistent (non-PKU) hyperphenylalaninemia (PHP). For each of these specimens there were four control cord-blood specimens from infants born on the same day and, generally, in the same hospital. PKU and PHP groups were similar with respect to cord-blood tyrosine and phenylalanine values. There was no biologically significant deficiency of tyrosine in cord blood of the pooled PKU and PHP deficiency of tyrosine in cord blood of the pooled PKU and PHP groups (54 +/- 10 microM, mean +/- SD) compared with controls (61 +/- 16 microM, P = 0.13). On the other hand, phenylalanine in cord blood of the pooled PKU and PHP groups was significantly increased (144 +/- 30 microM, mean +/- SD) compared with controls (128 +/- 24, P = 0.004). The mangitude of the differences in cord-blood tyrosine and phenylalanine between control and PKU subjects are so small that it is unlikely that they have any consequences for physical and mental development. The justification hypothesis, as it pertains to blood tyrosine at term, is not upheld.