The effects of chronic hyperphenylalaninaemia on mouse brain protein synthesis can be prevented by other amino acids

Abstract

A prolonged elevation in the concentrations of circulating phenylalanine was maintained in newborn mice by daily injections of phenylalanine and a phenylalanine hydroxylase inhibitor, alpha-methylphenylalanine. The result of this chronic hyperphenylalaninaemia was an accumulation of vacant or inactive monoribosomes that persisted for 18 h of each day. An elongation assay in vitro with brain postmitochondrial supernatants demonstrated that, in addition, there was an equally prolonged decrease in the rates of polypeptide-chain elongation by the remaining brain polyribosomes. Analyses of the free amino acid composition in the brains of hyperphenylalaninaemic mice showed a loss of several amino acids from the brain, particularly the large, neutral amino acids, which are co- or counter-transported across plasma membranes with phenylalanine. When a mixture of these amino acids (leucine, isoleucine, valine, threonine, tryptophan, tyrosine, methionine) was injected into hyperphenylalaninaemic mice, there was an immediate cessation of monoribosome accumulation in the brain and there was no inhibition of the rates of polypeptide-chain elongation. Although the concentrations of the large, neutral amino acids in the brain were partially preserved by treatment of hyperphenylalaninaemic mice with the amino acid mixture, the elevated concentrations of phenylalanine remained unaltered. The amino acid mixture had no detectable effect on brain protein synthesis in the absence of the hyperphenylalaninaemic condition.

Cerebral protein synthesis in a genetic mouse model of phenylketonuria

“…The L- [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]leucine method is based on a compartmental model for the behavior of leucine in brain that includes bidirectional carrier-mediated transport of leucine at the bloodbrain barrier, an extracellular and multiple intracellular compartments in brain, and the possibility that unlabeled leucine from the breakdown of tissue protein can be reincorporated into tissue protein. The application of the model together with an appropriate experimental design, i.e., a pulse injection of a tracer dose of L-[1-14 C]leucine followed by a 60-min interval for clearance of the tracer from the blood and the brain, gives rise to Eq.…”

“…The radiolabeled tracer is [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]leucine, which is used in conjunction with quantitative autoradiography to achieve spatial localization of the labeled protein in the brain for the determination of rates of protein synthesis in discrete brain regions (17). The operational equation of the method is:…”

“…Studies of developing mice administered high doses of Phe have shown disaggregation of brain polyribosomes and reduced rates of polypeptide chain elongation on polyribosomes prepared from brain homogenates (9,12); both of these effects were reversed by treatment of the mice with a mixture of neutral amino acids (leucine, isoleucine, valine, threonine, tryptophan, tyrosine, methionine) (13). Direct effects of hyperphenylalaninemia on cerebral protein synthesis in vivo have, however, been more difficult to demonstrate because of the complications of making such measurements in intact animals.…”

Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria

“…Hyperphenylalaninemia led to a loss of several AAs, especially LNAAs, in the brain of newborn mice, with a concomitant decrease in the incorporation of AAs into protein. Interestingly, the coadministration of LNAAs prevented this effect (30). However, the significance of animal models using experimentally induced hyperphenylalaninemia (29,30) remains questionable because of uncontrolled side effects of the PAH inhibitors used, e.g., α-methylphenylalanine.…”

Developmental Regulation of the Rabbit Blood-Brain Barrier LAT1 Large Neutral Amino Acid Transporter mRNA and Protein

“…However, this pathway is inhibited when the supply of brain amino acids is altered by a singular hyperaminoacidemia, like phenylketonuria (PKU) (8 -11). PKU causes a decrease in amino acid supply in brain by competition effects at the BBB, and this decrease in brain protein synthesis is normalized by the administration of amino acids (12). In addition, many pathways of brain neurotransmitter synthesis are also rate-affected by the cerebral availability of precursor large neutral amino acids (6).…”