Error Of Amino Acid Metabolism Research Articles

Inborn errors of amino acid metabolism - from underlying pathophysiology to therapeutic advances.

Amino acids are organic molecules that serve as basic substrates for protein synthesis and have additional key roles in a diverse array of cellular functions, including cell signaling, gene expression, energy production and molecular biosynthesis. Genetic defects in the synthesis, catabolism or transport of amino acids underlie a diverse class of diseases known as inborn errors of amino acid metabolism. Individually, these disorders are rare, but collectively, they represent an important group of potentially treatable disorders. In this Clinical Puzzle, we discuss the pathophysiology, clinical features and management of three disorders that showcase the diverse clinical presentations of disorders of amino acid metabolism: phenylketonuria, lysinuric protein intolerance and homocystinuria due to cystathionine β-synthase (CBS) deficiency. Understanding the biochemical perturbations caused by defects in amino acid metabolism will contribute to ongoing development of diagnostic and management strategies aimed at improving the morbidity and mortality associated with this diverse group of disorders.

Evaluation of the risk factors for noncommunicable diseases in patients with inborn errors of amino acid metabolism receiving nutrition therapy.

There is growing concern about the low-protein and high-energy diet therapies used in the treatment of inherited amino acid metabolism disorders. We aimed to identify the risk factors for noncommunicable diseases that may arise from nutritional therapies and suggests approaches that may prevent the development of the noncommunicable diseases. The present study evaluates 112 patients, on long-term nutritional therapy for at least the last 2 years with a diagnosis of an inborn error of the amino acid metabolism, and their 28 healthy siblings. The participants are assessed for the development of overweight and metabolic syndrome based on an analysis of anthropometric parameters, body composition and the results of biochemical tests. Anthropometric measurements including BMI, weight Z-score, waist circumference and fat mass were not significantly different between patients and controls. Height Z-scores were similar in phenylketonuria patients compared to controls, but lower in urea cycle disorders, organic acidemia and maple syrup urine disease groups. No increased risk of development of overweight or metabolic syndrome was detected in the patient group, while there were findings suggesting malnutrition in patients diagnosed with urea cycle disorders. There was a correlation between patients' BMI and C3-carnitine levels in organic acidemia patients and leucine levels in maple syrup urine disease patients. All forms of malnutrition can be prevented in patient groups receiving limited nutrients under a dietary management protocol, based on the findings of anthropometric and biochemical evaluations and analyses of body composition.

An orally administered enzyme therapeutic for homocystinuria that suppresses homocysteine by metabolizing methionine in the gastrointestinal tract.

Classical homocystinuria (HCU) is a rare inborn error of amino acid metabolism characterized by accumulation of homocysteine, an intermediate product of methionine metabolism, leading to significant systemic toxicities, particularly within the vascular, skeletal, and ocular systems. Most patients require lifelong dietary therapy with severe restriction of natural protein to minimize methionine intake, and many patients still struggle to maintain healthy homocysteine levels. Since eliminating methionine from the diet reduces homocysteine levels, we hypothesized that an enzyme that can degrade methionine within the gastrointestinal (GI) tract could help HCU patients maintain healthy levels while easing natural protein restrictions. We describe the preclinical development of CDX-6512, a methionine gamma lyase (MGL) enzyme that was engineered for stability and activity within the GI tract for oral administration to locally degrade methionine. CDX-6512 is stable to low pH and intestinal proteases, enabling it to survive the harsh GI environment without enteric coating and to degrade methionine freed from dietary protein within the small intestine. Administering CDX-6512 to healthy non-human primates following a high protein meal led to a dose-dependent suppression of plasma methionine. In Tg-I278T Cbs-/- mice, an animal model that recapitulates aspects of HCU disease including highly elevated serum homocysteine levels, oral dosing of CDX-6512 after a high protein meal led to suppression in serum levels of both methionine and homocysteine. When animals received a daily dose of CDX-6512 with a high protein meal for two weeks, the Tg-I278T Cbs-/- mice maintained baseline homocysteine levels, whereas homocysteine levels in untreated animals increased by 39%. These preclinical data demonstrate the potential of CDX-6512 as an oral enzyme therapy for HCU.

Glycomacropeptide (GMP) rescued the oxidative and inflammatory activity of free L-AAs in human Caco-2 cells: New insights that support GMP as a valid and health-promoting product for the dietary management of phenylketonuria (PKU) patients

Phenylketonuria represents the most prevalent inborn error of amino acid metabolism. In early diagnosed patients adequate and continued dietary treatment results in a good neurologic outcome. However, due to the natural protein and phenylalanine-restricted diet, oxidative stress represents a concern in phenylketonuric patients. Clear evidences suggest that the pathophysiology of PKU is also dependent by mitochondrial impairment and oxidative stress. In this context due to the tight connection between oxidative and inflammatory stress and noncommunicable diseases (NCDs) development, it is reasonable to hypothesize that PKU patients may present a higher risk to develop NCDs during their life. Currently available protein substitutes on the market include free amino acids (L-AAs), prolonged-release protein substitute and formula containing glycomacropeptide (GMP). Our results suggest that free L-AAs significanlty worsens the intestinal hydrogen peroxide (H2O2) and lipopolysaccharides (LPS)-induced oxidative and inflammatory status in Caco-2 cells, which are significantly restored towards physiological condition by GMP alone and when present in a 1:1 mixture with free L-AAs, providing new preclinical piece of information which can shed a shadow on the mechanism of action of these products on PKU patients and their future management.

Mothers’ lived experience of caring for children with inborn errors of amino acid metabolism

BackgroundInborn errors of amino acid metabolism are chronic conditions that have many sequels. Mothers of these children are facing different challenges which are underdetermined. This study was done to explore lived experience of mothers caring for these children.MethodsThis is an interpretive phenomenology with van Manen’s approach which has 6 steps. Data were gathered by convenience and purposeful sampling. Nine mothers with different experiences were interviewed and the interviews were audiotaped.ResultsSix final themes were revealed from the exploring mothers’ experiences including the future tied to the past, psychosis in the shadow of a lost ideal child, rebellion and blaming, the ways of escaping difficulties, self-forgetting in the shadow of full-time care, passing difficulties in the duality of hope-hopelessness, caring in a continuum of isolation-socialization.ConclusionMothers have multiple challenges in taking care of their children, especially psychologically and financially. So, nurses must plan programs for helping mothers of children with inborn errors of amino acid metabolism to reduce the effects of disease on mothers and consequently the children and the whole family.

Oral Microbiome Traits of Type 1 Diabetes and Phenylketonuria Patients in Latvia.

Some metabolic disorder treatments require patients to follow a specific diet or to consume supplements that, over time, can lead to oral microbiome alterations. Well-known disorders requiring such treatment are phenylketonuria (PKU), an inborn error of amino acid metabolism, and type 1 diabetes (T1D), a metabolic disorder that requires a specific diet regimen. Therefore, the aim of this study was to investigate the oral health and microbiome characteristics that might contribute to caries activity and periodontal disease risk in PKU and T1D patients. In this cross-sectional study, 45 PKU patients, 24 T1D patients, and 61 healthy individuals between the ages of 12 and 53 years were examined. Their anamnestic data and dental status were assessed by one dentist. Microbial communities were detected from saliva-isolated DNA using 16S rRNA gene V3-V4 sequencing on Illumina MiSeq sequencing platform. Results revealed that the PKU patient group displayed the highest number of extracted teeth (on average 1.34), carious teeth (on average 4.95), and carious activity (44.44% of individuals) compared to the T1D and CTRL groups. The lowest numbers of filled teeth (on average 5.33) and extracted teeth (on average 0.63) per individual were observed in T1D patients. Gingivitis appeared more often in the T1D group; however, possible risk of periodontal disease was seen in both the T1D and PKU patient groups. The highest number of differentially abundant genera was detected in the PKU group (n = 20), with enrichment of Actinomyces (padj = 4.17 × 10-22), Capnocytophaga (padj = 8.53 × 10-8), and Porphyromonas (padj = 1.18 × 10-5) compared to the CTRL group. In conclusion, the dental and periodontal health of PKU patients was found to be significantly inferior compared to T1D patients and healthy controls. T1D patients showed early signs of periodontal disease. Several genera that correlate with periodontal disease development were found in both groups, thus suggesting that T1D and PKU patients should seek early and regular dental advice and be educated about proper oral hygiene practices.

The Impact of Nutrition, Physical Activity, Beneficial Microbes, and Fecal Microbiota Transplant for Improving Health.

The recognition that microbes are integral to human life has led to studies on how to manipulate them in favor of health outcomes. To date, there has been no conjoint recommendation for the intake of dietary compounds that can complement the ingested organisms in terms of promoting an improved health outcome. The aim of this review is to discuss how beneficial microbes in the form of probiotics, fermented foods, and donor feces are being used to manage health. In addition, we explore the rationale for selecting beneficial microbial strains and aligning diets to accommodate their propagation in the gut. A pilot clinical trial design is presented to examine the effects of probiotics and exercise in patients with phenylketonuria (PKU); it is the most common inborn error of amino acid metabolism, and it is a complication that requires lifelong dietary intervention. The example design is provided to illustrate the importance of using omics technology to see if the intervention elevates neuroactive biogenic amines in the plasma; increases the abundance of Eubacterium rectale, Coprococcus eutactus, Akkermansia muciniphila, or Butyricicoccus; and increases Escherichia/Shigella in the gut, all as markers of improved health. By emphasizing the combined importance of diet, microbial supplements, and the gut microbiome, we hope that future studies will better align these components, not only to improve outcomes, but also to enhance our understanding of the mechanisms.

Lost in print: difficulty in reading online information pertaining to phenylketonuria.

Phenylketonuria (PKU) is the most prevalent inborn error of amino acid metabolism, necessitating patients to strictly restrict dietary phenylalanine. As this can be a confusing and concerning diagnosis, patients and loved ones will likely be drawn to learn more. A critical factor for citizens to harness the health-related digital information is to ensure that it is easy to read. This study assessed the readability of 100 digital articles related to PKU and explored the effect of the source of online information on the readability of these articles. For each article, 5 readability tests were conducted using on-line readability software. From the 100 assessed websites, 34% were commercially sourced. Of the remaining 66 sites, 40% had.org and 17% had.gov extensions. All 5 tests confirm that a large majority of the sample was written at an unacceptable reading level. In fact, over two-thirds of the sample received a readability score within the difficult range for three of the tests, while the remaining two deemed 49% and 45% of the sample as difficult to read, respectively. Commercially sourced websites were deemed to be more difficult to read than the other sites with respect to the two of the five measures. Despite the large amount of PKU information online, most of it may be incomprehensible to the average person and thus miss the mark in helping patients and caregivers manage their condition. Opportunities exist for authors of digital health promotional information to effectively achieve their goal by using comprehensible, easy-to-read language.

MRNA-based therapy proves superior to the standard of care for treating hereditary tyrosinemia 1 in a mouse model

Hereditary tyrosinemia type 1 is an inborn error of amino acid metabolism characterized by deficiency of fumarylacetoacetate hydrolase (FAH). Only limited treatment options (e.g., oral nitisinone) are available. Patients must adhere to a strict diet and face a life-long risk of complications, including liver cancer and progressive neurocognitive decline. There is a tremendous need for innovative therapies that standardize metabolite levels and promise normal development. Here, we describe an mRNA-based therapeutic approach that rescues Fah-deficient mice, a well-established tyrosinemia model. Repeated intravenous or intramuscular administration of lipid nanoparticle-formulated human FAH mRNA resulted in FAH protein synthesis in deficient mouse livers, stabilized body weight, normalized pathologic increases in metabolites after nitisinone withdrawal, and prevented early death. Dose reduction and extended injection intervals proved therapeutically effective. These results provide proof of concept for an mRNA-based therapeutic approach to treating hereditary tyrosinemia type 1 that is superior to the standard of care.

Designing stimuli-responsive upconversion nanoparticles based on an inner filter effect mimetic immunoassay for phenylketonuria accuracy diagnosis

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism caused by an inherited deficiency in L-phenylalanine-4-hydroxylase (PAH) activity. It is usually controlled by diet and monitored regularly with markers, as PKU is not curable. However, conventional methods for target biomarker analysis are invasive and labor intensive. Here, we report a rapid and sensitive, mimetic immunoassay for detecting phenylpyruvate (PhPY) based on stimuli-responsive upconversion nanoparticles with an inner filter effect (IFE). The strong and specific binding of PhPY and Fe3+ forms a complex with maximum absorption at approximately 640 nm. Upon the addition of LiYF4:Er,Ho@LiYF4 UCNPs (maximum emission at 699 nm), the inner filter effect is triggered along with a concurrent decrease in fluorescence. The proposed method demonstrates ultra sensitivity with a detection limit of 79.63 μg L-1, which is superior to most reported methods, thereby enabling phenylpyruvate assays on human urine.

Toxic Metabolites and Inborn Errors of Amino Acid Metabolism: What One Informs about the Other.

In inborn errors of metabolism, such as amino acid breakdown disorders, loss of function mutations in metabolic enzymes within the catabolism pathway lead to an accumulation of the catabolic intermediate that is the substrate of the mutated enzyme. In patients of such disorders, dietarily restricting the amino acid(s) to prevent the formation of these catabolic intermediates has a therapeutic or even entirely preventative effect. This demonstrates that the pathology is due to a toxic accumulation of enzyme substrates rather than the loss of downstream products. Here, we provide an overview of amino acid metabolic disorders from the perspective of the ‘toxic metabolites’ themselves, including their mechanism of toxicity and whether they are involved in the pathology of other disease contexts as well. In the research literature, there is often evidence that such metabolites play a contributing role in multiple other nonhereditary (and more common) disease conditions, and these studies can provide important mechanistic insights into understanding the metabolite-induced pathology of the inborn disorder. Furthermore, therapeutic strategies developed for the inborn disorder may be applicable to these nonhereditary disease conditions, as they involve the same toxic metabolite. We provide an in-depth illustration of this cross-informing concept in two metabolic disorders, methylmalonic acidemia and hyperammonemia, where the pathological metabolites methylmalonic acid and ammonia are implicated in other disease contexts, such as aging, neurodegeneration, and cancer, and thus there are opportunities to apply mechanistic or therapeutic insights from one disease context towards the other. Additionally, we expand our scope to other metabolic disorders, such as homocystinuria and nonketotic hyperglycinemia, to propose how these concepts can be applied broadly across different inborn errors of metabolism and various nonhereditary disease conditions.

The potential to breed a low‐protein maize for protein‐restricted diets

AbstractInborn errors of amino acid metabolism, such as phenylketonuria (PKU), are human genetic conditions that result in an inability to metabolize certain amino acids, requiring lifelong maintenance of a very restrictive low‐protein diet. In the case of PKU, this diet does not permit consumption of most maize (Zea mays L.) products, yet maize is a major part of the human diet and is especially significant in some cultures. Kernel protein content, and to a lesser extent amino acid composition, is variable in maize. Breeding maize for low kernel protein content or low amounts of specific amino acids such as phenylalanine might expand the restrictive diets and improve quality of life for people with amino acid metabolic disorders. To test the potential of breeding low‐protein‐content maize, diverse samples of hybrid or open‐pollinated cultivars and inbreds were evaluated across two locations in two years for grain protein content and amino acid composition. Consistent with previous reports, kernel protein content varied widely across maize varieties and is highly heritable in the broad sense. We found that phenylalanine content (as a proportion of grain weight) and phenylalanine composition (as a proportion of total protein) are also highly heritable in the broad sense. As previously reported, reasonably accurate estimation of protein content is possible with near‐infrared spectrophotometry, whereas measuring phenylalanine content requires a destructive, costly, and time‐consuming laboratory analysis. Because phenylalanine content was strongly correlated with total kernel protein, breeding primarily aimed at reducing grain protein content should permit efficient selection of maize cultivars sufficiently low in phenylalanine to permit broader inclusion in the PKU diet.

The clinico-radiological findings of MSUD in a group of Egyptian children: Contribution to early diagnosis and outcome.

BackgroundMaple syrup urine disease (MSUD) is an autosomal recessive inborn error of amino acid metabolism, with unique clinico‐radiological findings. This study aims to show the benefit of using the clinico‐radiological findings for early diagnosis of children with MSUD, and confirming this diagnosis using the tandem mass spectrometry (MS/MS), in order to avoid deleterious outcome.MethodsA prospective cohort study conducted in the period from August 2016 to December 2020. Twenty‐one children were included either by selective screening or by high‐risk screening. All children had clinical and neurodevelopmental evaluation, brain magnetic resonance imaging (MRI) assessment, and blood amino acids analysis at diagnosis. Patients were followed clinically.ResultsMost children had acute onsets neuro‐developmental symptoms, with wide range of brain parenchyma involvement on MRI (hyperintensity). Diagnosis of MSUD was confirmed by detecting high serum levels of leucine/isoleucine (mean value 2085.5 μmol/L) in all patients, and elevated levels of serum valine in (81%) of children. In addition, all children showed elevated leucine: alanine ratio, and leucine: phenylalanine ratio.ConclusionsThe characteristic clinico‐radiological features can help in the early diagnosis of MSUD children, thus preventing the delay in laboratory diagnosis and improving their outcomes.

Cardiac Magnetic Resonance Reveals Incipient Cardiomyopathy Traits in Adult Patients With Phenylketonuria.

BackgroundPhenylketonuria is the most common inborn error of amino acid metabolism, where oxidative stress and collateral metabolic abnormalities are likely to cause cardiac structural and functional modifications. We aim herein to characterize the cardiac phenotype of adult subjects with phenylketonuria using advanced cardiac imaging.Methods and ResultsThirty‐nine adult patients with phenylketonuria (age, 30.5±8.7 years; 10‐year mean phenylalanine concentration, 924±330 µmol/L) and 39 age‐ and sex‐matched healthy controls were investigated. Participants underwent a comprehensive cardiac magnetic resonance and echocardiography examination. Ten‐year mean plasma levels of phenylalanine and tyrosine were used to quantify disease activity and adherence to treatment. Patients with phenylketonuria had thinner left ventricular walls (septal end‐diastolic thickness, 7.0±17 versus 8.8±1.7 mm [P<0.001]; lateral thickness, 6.1±1.4 versus 6.8±1.2 mm [P=0.004]), more dilated left ventricular cavity (end‐diastolic volume, 87±14 versus 80±14 mL/m2 [P=0.0178]; end‐systolic volume, 36±9 versus 29±8 mL/m2 [P<0.001]), lower ejection fraction (59±6% versus 64±6% [P<0.001]), reduced systolic deformation (global circumferential strain, −29.9±4.2 % versus −32.2±5.0 % [P=0.027]), and lower left ventricular mass (38.2±7.9 versus 47.8±11.0 g/m2 [P<0.001]). T1 native values were decreased (936±53 versus 996±26 ms [P<0.001]), with particular low values in patients with phenylalanine >1200 µmol/L (909±48 ms). Both mean phenylalanine (P=0.013) and tyrosine (P=0.035) levels were independently correlated with T1; and in a multiple regression model, higher phenylalanine levels and higher left ventricular mass associate with lower T1.ConclusionsCardiac phenotype of adult patients with phenylketonuria reveals some traits of an early‐stage cardiomyopathy. Regular cardiology follow‐up, tighter therapeutic control, and prophylaxis of cardiovascular risk factors, in particular dyslipidemia, are recommended.

Amadori rearrangement products as potential biomarkers for inborn errors of amino-acid metabolism

The identification of disease biomarkers plays a crucial role in developing diagnostic strategies for inborn errors of metabolism and understanding their pathophysiology. A primary metabolite that accumulates in the inborn error phenylketonuria is phenylalanine, however its levels do not always directly correlate with clinical outcomes. Here we combine infrared ion spectroscopy and NMR spectroscopy to identify the Phe-glucose Amadori rearrangement product as a biomarker for phenylketonuria. Additionally, we find analogous amino acid-glucose metabolites formed in the body fluids of patients accumulating methionine, lysine, proline and citrulline. Amadori rearrangement products are well-known intermediates in the formation of advanced glycation end-products and have been associated with the pathophysiology of diabetes mellitus and ageing, but are now shown to also form under conditions of aminoacidemia. They represent a general class of metabolites for inborn errors of amino acid metabolism that show potential as biomarkers and may provide further insight in disease pathophysiology.

Oxidative stress in phenylketonuria—evidence from human studies and animal models, and possible implications for redox signaling

Phenylketonuria (PKU) is one of the commonest inborn error of amino acid metabolism. Before mass neonatal screening was possible, and the success of introducing diet therapy right after birth, the typical clinical finds in patients ranged from intellectual disability, epilepsy, motor deficits to behavioral disturbances and other neurological and psychiatric symptoms. Since early diagnosis and treatment became widespread, usually only those patients who do not strictly follow the diet present psychiatric, less severe symptoms such as anxiety, depression, sleep pattern disturbance, and concentration and memory problems. Despite the success of low protein intake in preventing otherwise severe outcomes, PKU's underlying neuropathophysiology remains to be better elucidated. Oxidative stress has gained acceptance as a disturbance implicated in the pathogenesis of PKU. The conception of oxidative stress has evolved to comprehend how it could interfere and ultimately modulate metabolic pathways regulating cell function. We summarize the evidence of oxidative damage, as well as compromised antioxidant defenses, from patients, animal models of PKU, and in vitro experiments, discussing the possible clinical significance of these findings. There are many studies on oxidative stress and PKU, but only a few went further than showing macromolecular damage and disturbance of antioxidant defenses. In this review, we argue that these few studies may point that oxidative stress may also disturb redox signaling in PKU, an aspect few authors have explored so far. The reported effect of phenylalanine on the expression or activity of enzymes participating in metabolic pathways known to be responsive to redox signaling might be mediated through oxidative stress.

Phenylketonuria in Portugal: Genotype-phenotype correlations using molecular, biochemical, and haplotypic analyses.

BackgroundThe impairment of the hepatic enzyme phenylalanine hydroxylase (PAH) causes elevation of phenylalanine levels in blood and other body fluids resulting in the most common inborn error of amino acid metabolism (phenylketonuria). Persistently high levels of phenylalanine lead to irreversible damage to the nervous system. Therefore, early diagnosis of the affected individuals is important, as it can prevent clinical manifestations of the disease.MethodsIn this report, the biochemical and genetic findings performed in 223 patients diagnosed through the Portuguese Neonatal Screening Program (PNSP) are presented.ResultsOverall, the results show that a high overlap exists between different types of variants and phenylalanine levels. Molecular analyses reveal a wide mutational spectrum in our population with a total of 56 previously reported variants, most of them found in compound heterozygosity (74% of the patients). Intragenic polymorphic markers were used to assess the haplotypic structure of mutated chromosomes for the most frequent variants found in homozygosity in our population (p.Ile65Thr, p.Arg158Gln, p.Leu249Phe, p.Arg261Gln, p.Val388Met, and c.1066‐11G>A).ConclusionOur data reveal high heterogeneity at the biochemical and molecular levels and are expected to provide a better understanding of the molecular basis of this disease and to provide clues to elucidate genotype–phenotype correlations.

Phenylketonuria Diagnosis by Massive Parallel Sequencing and Genotype-Phenotype Association in Brazilian Patients.

Phenylketonuria (PKU) is a common inborn error of amino acid metabolism in which the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine, is functionally impaired due to pathogenic variants in the PAH gene. Thirty-four Brazilian patients with a biochemical diagnosis of PKU, from 33 unrelated families, were analyzed through next-generation sequencing in the Ion Torrent PGM™ platform. Phenotype–genotype correlations were made based on the BioPKU database. Three patients required additional Sanger sequencing analyses. Twenty-six different pathogenic variants were identified. The most frequent variants were c.1315+1G>A (n = 8/66), c.473G>A (n = 6/66), and c.1162G>A (n = 6/66). One novel variant, c.524C>G (p.Pro175Arg), was found in one allele and was predicted as likely pathogenic by the American College of Medical Genetics and Genomics (ACMG) criteria. The molecular modeling of p.Pro175Arg indicated that this substitution can affect monomers binding in the PAH tetramer, which could lead to a change in the stability and activity of this enzyme. Next-generation sequencing was a fast and effective method for diagnosing PKU and is useful for patient phenotype prediction and genetic counseling.

Manipulation of a cation-π sandwich reveals conformational flexibility in phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) is an allosteric enzyme that maintains phenylalanine (Phe) below neurotoxic levels; its failure results in phenylketonuria, an inborn error of amino acid metabolism. Wild type (WT) PAH equilibrates among resting-state (RS-PAH) and activated (A-PAH) conformations, whose equilibrium position depends upon allosteric Phe binding. The RS-PAH conformation of WT rat PAH (rPAH) contains a cation-π sandwich involving Phe80 that cannot exist in the A-PAH conformation. Phe80 variants F80A, F80D, F80L, and F80R were prepared and evaluated using native PAGE, size exclusion chromatography, ion exchange behavior, intrinsic protein fluorescence, enzyme kinetics, and limited proteolysis, each as a function of [Phe]. Like WT rPAH, F80A and F80D show allosteric activation by Phe while F80L and F80R are constitutively active. Maximal activity of all variants suggests relief of a rate-determining conformational change. Limited proteolysis of WT rPAH (minus Phe) reveals facile cleavage within a 4-helix bundle that is buried in the RS-PAH tetramer interface, reflecting dynamic dissociation of that tetramer. This cleavage is not seen for the Phe80 variants, which all show proteolytic hypersensitivity in a linker that repositions during the RS-PAH to A-PAH interchange. Hypersensitivity is corrected by addition of Phe such that all variants become like WT rPAH and achieve the A-PAH conformation. Thus, manipulation of Phe80 perturbs the conformational space sampled by PAH, increasing sampling of on-pathway intermediates in the RS-PAH and A-PAH interchange. The behavior of the Phe80 variants mimics that of disease-associated R68S and suggests a molecular basis for proteolytic susceptibility in PKU-associated human PAH variants.

Minimal Change Disease and Phenylketonuria in an Adult Patient: The Two Sides of Protein Homeostasis.

Phenylketonuria (PKU) is associated with renal impairment so patients with PKU should be periodically screened for proteinuria because podocytopathies, such as minimal change disease (MCD), may be an underlying misdiagnosed condition.MCD in adults demands tight surveillance in case of severe complications including nephrotic syndrome, haematuria and acute kidney injury, and although remission with glucocorticoid therapy is typical, 50-75% of patients will relapse.MCD may be a previously unreported cause of proteinuria and renal involvement in adult patients with PKU, where control of hypertension and moderation of protein overload are essential.