Phenylalanine In Phenylketonuria Research Articles

Dysmyelination and glycolipid interference caused by phenylalanine in phenylketonuria

Phenylketonuria (PKU) is a metabolic disorder connected to an excess of phenylalanine (Phe) in the blood and tissues, with neurological consequences. The disease's molecular bases seem to be related to the accumulation of Phe at the cell membrane surface. Radiological outcomes in the brain demonstrate decreased water diffusivity in white matter, involving axon dysmyelination of not yet understood origin. We used a biophysical approach and model membranes to extend our knowledge of Phe–membrane interaction by clarifying Phe's propensity to affect membrane structure and dynamics based on lipid composition, with emphasis on modulating cholesterol and glycolipid components to mimic raft domains and myelin sheath membranes. Phe showed affinity for the investigated membrane mimics, mainly affecting the Phe-facing membrane leaflet. The surfaces of our neuronal membrane raft mimics were strong anchoring sites for Phe, showing rigidifying effects. From a therapeutic perspective, we further investigated the role of doxycycline, known to disturb Phe packing, unveiling its action as a competitor in Phe interactions with the membrane, suggesting its potential for treatment in the early stages of PKU. Our results suggest how Phe accumulation in extracellular fluids can impede normal growth of myelin sheaths by interfering with membrane slipping and by remodulating free water and myelin-associated water contents.

Dysmyelination and glycolipid interference caused by phenylalanine in phenylketonuria

Dysmyelination and glycolipid interference caused by phenylalanine in phenylketonuria

Study of the l-Phenylalanine Ammonia-Lyase Penetration Kinetics and the Efficacy of Phenylalanine Catabolism Correction Using In Vitro Model Systems.

The kinetics of l-phenylalanine ammonia-lyase (PAL) penetration into the monolayer of liver cells after its release from capsules was studied. The studies showed the absence of the effect of the capsule shell based on plant hydrocolloids on the absorption of l-phenylalanine ammonia-lyase in systems simulating the liver surface. After 120 min of incubation, in all variants of the experiment, from 87.0 to 96.8% of the enzyme penetrates the monolayer of liver cells. The combined analysis of the results concludes that the developed encapsulated form of l-phenylalanine ammonia-lyase is characterized by high efficiency in correcting the disturbed catabolism of phenylalanine in phenylketonuria, which is confirmed by the results of experiments carried out on in vitro model systems. PAL is approved for the treatment of adult patients with phenylketonuria. The encapsulated l-phenylalanine ammonia-lyase form can find therapeutic application in the phenylketonuria treatment after additional in vitro and in vivo studies, in particular, the study of preparation safety indicators. Furthermore, it demonstrated high efficacy in tumor regression and the treatment of tyrosine-related metabolic disorders such as tyrosinemia. Several therapeutically valuable metabolites biosynthesized by PAL via its catalytic action are included in food supplements, antimicrobial peptides, drugs, amino acids, and their derivatives. PAL, with improved pharmacodynamic and pharmacokinetic properties, is a highly effective medical drug.

Structure-based chemical modification strategy for enzyme replacement treatment of phenylketonuria

Structure-based protein engineering coupled with chemical modifications (e.g., pegylation) is a powerful combination to significantly improve the development of proteins as therapeutic agents. As a test case, phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) was selected for enzyme replacement therapy in phenylketonuria [C.R. Scriver, S. Kaufman, Hyperphenylalaninemia:phenylalanine Hydroxylase Deficiency. The Metabolic and Molecular Bases of Inherited Disease, McGraw-Hill, New York, 2001, Chapter 77], an inherited metabolic disorder (OMIM 261600) causing mental retardation due to deficiency of the enzyme l-phenylalanine hydroxylase (EC 1.14.16.1). Previous in vivo studies of recombinant PAL demonstrated a lowering of blood l-phenylalanine levels; yet, the metabolic effect was not sustained due to protein degradation and immunogenicity [C.N. Sarkissian, Z. Shao, F. Blain, R. Peevers, H. Su, R. Heft, T.M. Chang, C.R. Scriver, A different approach to treatment of phenylketonuria:phenylalanine degradation with recombinant phenylalanine ammonia lyase, Proc. Natl. Acad. Sci. USA 96 (1999) 2339; J.A. Hoskins, G. Jack, H.E. Wade, R.J. Peiris, E.C. Wright, D.J. Starr, J. Stern, Enzymatic control of phenylalanine intake in phenylketonuria, Lancet 1 (1980) 392; C.M. Ambrus, S. Anthone, C. Horvath, K. Kalghatgi, A.S. Lele, G. Eapen, J.L. Ambrus, A.J. Ryan, P. Li, Extracorporeal enzyme reactors for depletion of phenylalanine in phenylketonuria, Ann. Intern. Med. 106 (1987) 531]. Here, we report the 1.6 Å three-dimensional structure of Rhodosporidium toruloides PAL, structure-based molecular engineering, pegylation of PAL, as well as in vitro and in vivo PKU mouse model studies on pegylated PAL formulations. Our results show that pegylation of R. toruloides PAL leads to promising therapeutic efficacy after subcutaneous injection by enhancing the in vivo activity, lowering plasma phenylalanine, and leading to reduced immunogenicity. The three-dimensional structure of PAL provides a basis for understanding the properties of pegylated forms of PAL and strategies for structure-based re-engineering of PAL for PKU treatment.

Platelet Na+, K+-ATPase activity as a possible peripheral marker for the neurotoxic effects of phenylalanine in phenylketonuria.

The in vitro effects of phenylalanine or alanine alone or combined on Na+, K+-ATPase activity in membranes from human platelets were investigated. The enzyme activity was assayed in membranes prepared from platelet-rich plasma of healthy donors. Phenylalanine or alanine were added to the assay to final concentrations of 0.3 to 1.2 mM, similar to those found in plasma of phenylketonuric patients. Phenylalanine inhibited Na+, K+-ATPase activity by 20-50% [F(4,25)=11.47 ; p<0.001]. Alanine had no effect on Na+, K+-ATPase activity but when combined with phenylalanine prevented the enzyme inhibition. These results, allied to others previously reported on brain Na+, K+-ATPase activity, may reflect a general inhibitory effect of phenylalanine on this important enzyme activity. Therefore, it is possible that measurement of Na+, K+-ATPase activity in platelets from PKU patients may be a useful peripheral marker for the neurotoxic effects of phenylalanine.

A New Theory of Enterorecirculation of Amino Acids and its use for Depleting Unwanted Amino Acids Using Oral Enzyme-Artificial Cells, as in Removing Phenylalanine in Phenylketonuria

Oral binders remove intestinal bile acid and prevent its reabsorption and recycling thereby lowering systemic cholesterol levels. The results in this paper demonstrate the presence of another extensive enterorecirculation for amino acids. Pancreatic and other glandular secretions into the intestine contain large amounts of proteins, enzymes and polypeptides. Tryptic digestion converts these into amino acids which are then reabsorbed back into the body as they pass down the intestine. This paper shows that this forms a large enterorecirculation of amino acids between the body and intestine. The dietary protein source of amino acids is negligible when compared to the endogenous source, since this paper shows that protein-free diet did not alter the intestinal amino acid concentration. This raises the possibility of using this for the selective depletion of specific body amino acids. In this paper we use a phenylketonuria (PKU) model in rats to test the use of this hypothesis. In PKU rats, artificial cells microencapsulated phenylalanine ammonia lyase (PAL) given orally is more effective than a phenylalanine-free diet. The enzyme artificial cells are more efficient in lowering PHE in the intestine, plasma and cerebrospinal fluid. Compared to PKU on PHE-free diet, this has resulted in better weight gain and general physical condition. Preliminary studies also show that artificial cells microencapsulated asparaginase, glutaminase and tyrosinase given orally can deplete the corresponding amino acid from the intestine.

Prolactin responses to phenylalanine and tyrosine in phenylketonuria

In normal subjects, ingestion of tyrosine or phenylalanine stimulates prolactin (PRL) secretion. In patients with phenylketonuria (PKU), we found normal PRL responses to phenylalanine, demonstrating that conversion of phenylalanine to tyrosine is not necessary for PRL stimulation. PKU patients also showed greater PRL responses to tyrosine during dietary phenylalanine restriction than when consuming an unrestricted diet; this finding is consistent with inhibition by phenylalanine of tyrosine transport across the blood-brain barrier. Such competitive inhibition of a normal brain function may serve as a model for some of the neurotoxic effects of phenylalanine in PKU.

A synopsis of the unconjugated acidic transamination metabolites of phenylalanine in phenylketonuria.

We present blood and urine levels of unconjugated o-hydroxyphenylacetic, phenyllactic and phenylpyruvic acids in 61 children (2 years of age and above) and juveniles with phenylketonuria on or partially off diet. The samples were obtained during 185 scheduled outpatient visits and have been analysed with gas chromatographic methods. The compiled data define reference ranges of phenylalanine transamination capacity and of renal transport of metabolites which may be of value in further studies on the pathogenesis of phenylketonuria.

Metabolism of phenylalanine in phenylketonuria under non‐steady state conditions

Metabolism of phenylalanine in phenylketonuria under non‐steady state conditions

Brain uptake of selenomethionine Se 75. II. Reduced brain uptake of selenomethionine Se 75 in phenylketonuria.

Brain uptake of selenomethionine Se 75 following intravenous injection was studied in nine subjects with phenylketonuria (PKU) and 12 matched mentally retarded controls. Brain content following injection was monitored continuously for 20 minutes using a gamma radiation detector sampling a fixed volume of brain. Uptake was expressed as ratio of head counts per minute divided by dose counts per minute per kilogram of patient. Brain uptake of selenomethionine Se 75 was reduced in the PKU group relative to the control group with a significance of better than 0.001. Reduction of brain uptake of this amino acid analog is compatible with saturation of the blood brain barrier carrier system for selenomethionine Se 75 by the high blood phenylalanine in PKU. Findings derived here by external counting are consistent with previous parallel animal studies in which serum phenylalanine concentrations were elevated by intravenous preloading.

Electroencephalographic effects of L-phenylalanine in the cat.

THE inherited biochemical defect of the inability to oxidize phenylalanine to tyrosine and the consequent elevation of phenylalanine levels in blood and urine has long been known in phenylketonuria. 1 However, the pathogenetic mechanisms of the cerebral abnormality are as yet unclear. The abnormal metabolites of phenylalanine in phenylketonuria have been shown in vitro to inhibit the enzymes dihydroxyphenylalanine (DOPA), 5-hydroxytryptophan, and glutamic acid decarboxylases. 2-5 Christensen 6 suggested that high levels of one or more amino acids have the effect of preventing the cell from capturing other amino acids or utilizing them. Phenylalanine has been demonstrated in vivo to interfere with the intracellular accumulation of tyrosine. 7 Others have described structural defects of myelin in the brains of phenylketonuric patients. 8-11 and the lowering of the cerebroside content of the white matter. 12 The chronic effects of 1-phenylalanine on the central nervous system have been studied experimentally. Retardation

Effects of folic and folinic acids on the metabolism of phenylalanine in phenylketonuria.

Three phenylketonuric children, maintained on a daily regimen containing 65 to 75 Gm. of protein, were given at different times either 10 mg. of folic acid or 9 mg. of folinic acid daily for periods of 5 weeks. No effect was noted on serum phenylalanine with these compounds. Urinary phenylpyruvic and phenyllactic acids were lowered by the folic acid and a mechanism is postulated to account for these changes Three phenylketonuric children, maintained on a daily regimen containing 65 to 75 Gm. of protein, were given at different times either 10 mg. of folic acid or 9 mg. of folinic acid daily for periods of 5 weeks. No effect was noted on serum phenylalanine with these compounds. Urinary phenylpyruvic and phenyllactic acids were lowered by the folic acid and a mechanism is postulated to account for these changes

Decreased 5-Hydroxytryptophan Decarboxylase Activity in Phenylketonuria

One biochemical defect in phenylketonuria is a failure of hydroxylation of phenylalanine to tyrosine. Evidence has also accumulated that there is a disorder of tryptophan metabolism in patients with phenylketonuria. This is demonstrated by the finding of low levels of metabolites of tryptophan in the serum and in the urine. It is postulated that the abnormality in tryptophan metabolism may play some part in the pathogenesis of the mental defect associated with phenylketonuria. The present investigations were designed to determine whether the impaired hydroxylation of tryptophan is due to an inhibition of the decarboxylase involved, by accumulation of metabolites of phenylalanine in patients with phenylketonuria, or to a deficiency of this enzyme. By determining the effects of a low phenylalanine diet or the intravenous administration of 5-hydroxytrytomine, results were obtained which suggest that 5-hydroxytryptophan decarboxylase may be inhibited by abnormal metabolites of phenylalanine in phenylketonuria. It is suggested that mental deficiency in phenylketonuria is associated with this abnormal metabolism of tryptophan rather than with any direct action of the toxic metabolites of phenylalanine. The benefit achieved by a low phenylalanine diet in the treatment of phenylketonuria may be attributed to an increased synthesis of 5-hydroxytryptamine. Prolonged exposure to toxic metabolites of phenylalanine may eventually result in an irreversible inhibition of the decarboxylase. This would account for the better results obtained in the youngest children with phenylketonuria when a low phenylalanine diet is employed in treatment.