Recovery of rat brain from a brief hyperphenylalaninemic insult early in development

The effects of phenylalanine on exercise-induced fat oxidation: a preliminary, double-blind, placebo-controlled, crossover trial

BACKGROUND

When combined with exercise, dietary amino acid (AA) supplementation is an effective method for accelerating fat mobilization. However, the effects of single AAs combined with exercise on fat oxidation remains unclear. We hypothesized that consumption of a specific amino acid, L- phenylalanine, may result in the secretion of glucagon, and when combined with exercise may promote fat oxidation.

METHODS

Six healthy, active male volunteers were randomized in a crossover study to ingest either phenylalanine (3 g/dose) or placebo. Thirty minutes after ingestion each subject performed workload trials on a cycle ergometer for 1 h at 50% of maximal oxygen consumption.

RESULTS

Oral intake of phenylalanine caused a significant increase in the concentrations of plasma glycerol and glucagon during exercise. The respiratory exchange ratio was also decreased significantly following ingestion of phenylalanine.

CONCLUSION

These results suggested that pre-exercise supplementation of phenylalanine may stimulate whole body fat oxidation. No serious or study-related adverse events were observed.

TRIAL REGISTRATION

UMIN000027502 Registered 26 May 2017. Restrospectively registered.

The nature of human hazards associated with excessive intake of amino acids

In recent years there has been a large increase in the consumption of individual amino acids as dietary supplements. This has resulted not only from the use of certain amino acids as flavoring agents, but other amino acids are taken for perceived health benefit, for enhancement of physical performance, as well as for psychological effects. Two reviews of the scientific literature exist that mainly deal with effects in animals, and three major reports consider the safety of amino acids for human consumption. This article is a brief summary of the available evidence regarding the safety of individual amino acids when taken in excess relative to the amounts absorbed from dietary protein.

Egg and breast milk based nitrogen sources compared

A nitrogen source based on egg protein (Vamin 9 glucose) and an alternative with an amino acid profile more similar to breast milk (Vaminolact), were compared in 14 parenterally fed infants. Subjects were randomly allocated to receive one or other amino acid solution, but were otherwise given identical diets. At the start of the study the two groups did not differ significantly in postconceptual age, postnatal age, or weight. Over a six day study period on a stable intake of intravenous nutrients there was no significant difference in growth or nitrogen retention between the two groups. Plasma amino acid profiles in those receiving Vamin 9 glucose, however, were frequently abnormal. Notably, mean concentrations of potentially neurotoxic phenylalanine and tyrosine were significantly higher (140% and 420%, respectively) in patients fed Vamin 9 compared with those given Vaminolact. An amino acid solution based on the composition of breast milk protein therefore brings plasma amino acid profiles during parenteral nutrition closer to those found in breast fed infants, and reduces in particular, the risks of hyperphenylalaninaemia and hypertyrosinaemia.

Progress in experimental phenylketonuria: a critical review

Experimental progress in the development of an accurate and useful model of phenylketonuria (PKU) during the last 15 years is reviewed in detail. From this review it is clear that the recent emergence of models using the combined administration of phenylalanine (phe) and p-chlorophenylalanine (PCPA) constitutes a major success that lays the groundwork for future research into the pathogenesis and treatment of PKU. Biochemical evidence on the pathophysiology of PKU is also briefly reviewed in the context of the behavioral and biochemical adequacy of the models used. It appears that in the past biochemical investigations into PKU have been impaired by use of inadequate models, a situation that should now change if the best of the phe-PCPA models are more widely adopted. New trends in PKU research involve the role of large neutral amino acids other than phe as potential aids in the treatment of PKU and the appearance of a new model based on the use of alpha-methylphenylalanine (AMPhe) combined with phe. It appears that PKU research may be on the brink of a new and productive era as investigations into these promising areas unfold and as new emerge through the full utilization of existing models.

Cerebral ribosomal protein phosphorylation in experimental hyperphenylalaninaemia

Investigations were carried out on the effects of phenylalanine loading on ribosomal protein phosphorylation in cerebral cortices of infant rats. Administration of L-phenylalanine intraperitoneally, in doses of 1 or 2 mg/g body wt., resulted within 30 min in a significant decrease in incorporation of radioactivity from intracisternally administered [32P]Pi into constitutive ribosomal proteins of the cerebral 40S subunit. This phenomenon was not accompanied by significant variations in 32P uptake into the cerebral cytosol. Incorporation of radioactivity into ribosomal proteins of the cerebral 60S subunit exhibited only minor variations under these circumstances. Alterations in the phosphorylation state of cerebral 40S ribosomal proteins induced by phenylalanine loading involved principally the S6 protein, which exists in multiple states of phosphorylation. The proportions of the more highly phosphorylated congeners of this protein were markedly decreased, as detected by two-dimensional electrophoretograms and autoradiographs of the cerebral 40S ribosomal proteins. Phenylalanine loading also altered the relative extent of phosphorylation of the S6 protein in cerebral polyribosomes and monoribosomes. In control animals, the specific radioactivity of 40S proteins in cerebral polyribosomes was five to ten times that of 40S proteins in the monoribosome population. At 1 h after phenylalanine administration, the specific radioactivities of 40S proteins in the two ribosome populations tended to approach equality. These alterations in ribosomal protein phosphorylation were accompanied by a decrease in the proportion of polyribosomes in purified ribosome preparations isolated from cerebral cortices of phenylalanine-treated infant rats. In animals given the higher dose of phenylalanine (2 mg/g body wt.), subsequent administration of a mixture of seven neutral amino acids, which resulted in partial recovery of polyribosomes, also tended to reverse the changes in ribosomal protein phosphorylation. Variations in the activities of ribonuclease enzymes in the cerebral cytosol were also observed under these conditions. Administration of phenylalanine increased the activities of cerebral ribonucleases, whereas subsequent treatment with the amino acid mixture partly reversed this effect. The results suggest that alterations in cerebral ribosomal protein phosphorylation, ribosome aggregation and ribosome function are interrelated in experimental hyperphenylalaninaemia.

Effects of alpha-methylphenylalanine plus phenylalanine treatment during development on myelin in rat brain

The effects of hyperphenylalaninemia induced by treatment with alpha-methylphenylalanine (MPA) plus phenylalanine (PHE) on body and brain weight, on myelin and synaptosome formation, and on the lipids and fatty acids of myelin were studied in rats. The administraton of MPA (2.4 mumol/g body wt) plus PHE (2.6 mumol/g body wt) for 25 and 35 days beginning on the fifth postnatal day did not affect brain development. On doubling the dosage of PHE, body and brain weights and myelin yields were significantly lowered. The lipid composition of myelin from the brains of treated animals was largely unaffected; however, the concentration of sulfatides ws significantly reduced. Unsaturated fatty acid levels in myelin from hyperphenylalaninemic rat brains were reduced while long-chain fatty acids were unaffected. We conclude that as in hyperphenylalaninemia induced by other methods, MPA + PHE treatment impairs body and brain growth, reduces myelin formation, and causes inhibition of fatty acid desaturation in the brain.

Tolerance of protein and lipid synthesis to mild hyperphenylalaninemia in developing rat brain

Chronic injection of low levels (1.5-2.5 mg/g body wt./day for 6-8 days) of L-phenylalanine which did not significantly affect body weight or brain to body weight ratios also did not significantly affect myelin accumulation or synthesis in developing rat brain. A single injection of L-phenylalanine (1 mg/g body wt.) significantly reduced in vivo protein synthesis by 10% in cerebral cortex of 7-day-old rats. Although transient reductions in protein synthesis are produced during low level daily injections, cumulative effects are not expressed in significant reductions in brain to body growth ratios. It is concluded that myelination is significantly inhibited in hyperphenylalaninemia only when other parameters of brain growth and development are affected.