Inhibition of protein synthesis in Krebs 2 ascites cells and cell-free systems by phenylalanine and its effect on leucine and lysine in the amino acid pool

Inhibition of protein and aminoacyl-tRNA synthesis, and binding and transport sites for aromatic amino acids in the brain in vitro with aromatic acids

The influx of [3H]phenylalanine, [3H]tyrosine and [3H]tryptophan into brain slices and synaptosomes, their binding to synaptic membranes and their incorporation into protein and aminoacyl-tRNA were studied in the presence of an excess of a second aromatic amino acid or some other aromatic acid, viz., phenylpyruvate, phenyllactate, phenylacetate, homogentisate, salicylate or benzoate. The influx into brain slices was strongly inhibited by a second aromatic amino acid and in general also by phenylpyruvate and homogentisate, but the effects of these substances upon the influx into synaptosomes were slight. The binding of phenylalanine and tyrosine to the synaptic membranes was affected mainly by phenylpyruvate and homogentisate, and these were also effective in preventing the formation of aminoacyl-tRNA, and thus apparently inhibited the biosynthesis of proteins and polyphenylalanine. In all cases phenyllactate, phenylacetate salicylate and benzoate had virtually no effect. Phenylalanine seemed to be a noncompetitive, and tyrosine a competitive inhibitor, while tryptophan had both properties, as was also the case with phenylpyruvate and homogentisate. Under phenylketonuric conditions high excesses of phenylalanine and phenylpyruvate, and also certain other aromatic compounds, seemed to occupy the cellular transport sites for amino acids on the cellular membranes and prevent the formation of aminoacyl-tRNAs, thus inhibiting brain protein synthesis. The reduced supply of intracellular amino acids and the inhibition of protein synthesis may constitute one reason for the development of biochemical phenylketonuric abnormalities.

Purine biosynthesis de novo in rat skeletal muscle

Purine biosynthesis by the 'de novo' pathway was demonstrated in isolated rat extensor digitorum longus muscle with [1-14C]glycine, [3-14C]serine and sodium [14C]formate as nucleotide precursors. Evidence is presented which suggests that the source of glycine and serine for purine biosynthesis is extracellular rather than intracellular. The relative incorporation rates of the three precursors were formate greater than glycine greater than serine. Over 85% of the label from formate and glycine was recovered in the adenine nucleotides, principally ATP. Azaserine markedly inhibited purine biosynthesis from both formate and glycine. Cycloserine inhibited synthesis from serine, but not from formate. Adenine, hypoxanthine and adenosine markedly inhibited purine synthesis from sodium [14C]formate.

Prolonged hyperalimentation in catabolic chronic dialysis therapy patients

Plasma concentrations of 25 essential (EAA) and nonessential (NEAA) amino acids were measured pre- and postdialysis in 46 chronic hemodialysis therapy (CDT) patients. Sixteen of these patients with prior weight loss of 14.5 +/- 2.37 pounds in 24 months were administered a GAA solution (EAA + NEAA + glucose) for 20 weeks during each dialysis. Eight of these patients (group 1) responded with improved appetite and weight gain; the remaining eight patients (group 2) with clinically advanced metabolic bone disease continued to lose weight. Five other patients (group 14), biochemically similar to group 1 but with shorter prior dialysis experience, who received EAA (plus glucose) hyperalimentation (including oral I-histidine), experienced weight gain similar to group 1 but displayed significantly different plasma aminograms indicating a deficit of NEAA. When EAA and glucose hyperalimentation was administered without histidine (1 patient) no weight gain occurred and aminograms differed significantly from other groups. Plasma aminograms of 25 weight-stable, nonhyperalimented CDT patients were obtained for comparison. Results indicate GAA hyperalimentation can promote weight gain in catabolic CDT patients with inadequate prior nutritional intake (as in groups 1 and 14) but cannot reverse weight loss when the primary clinical setting is advanced metabolic bone disease and myopathy due to hyperparathyroidism (group 2). Hyperalimentation with glucose and an amino acid solution specifically tailored to the needs of CDT patients may improve results. Plasma phosphoethanolamine levels, normal for weight-stable and elevated in catabolic CDT patients, suggest a possible role for phosphoethanolamine as a marker for catabolism.

Effects of p-chlorophenylalanine and alpha-methylphenylalanine on amino acid uptake and protein synthesis in mouse neuroblastoma cells

The phenylalanine analogues p-chlorophenylalanine and alpha-methylphenylalanine were used to inhibit phenylalanine hydroxylase in animal models for phenylketonuria. The present report examines the affects of these analogues on the metabolism of neuroblastoma cells. p-Chlorophenylalanine inhibited growth and was toxic to neuroblastoma cells. Although in vivo this analogue increased cell monoribosomes by 42%, it did not significantly affect poly(U)-directed protein synthesis in vitro. P-Chlorophenylalanine did not compete with phenylalanine or tyrosine for aminoacylation of tRNA and was therefore not substituted for those amino acids in nascent polypeptides. The initial cellular uptake of various large neutral amino acids was inhibited by this analogue but did not affect the flux of amino acids already in the cell; this suggested that an alteration of the cell's amino acid pools was not responsible for the cytotoxicity of the analogues. In contrast with p-chlorophenylalanine, alpha-methylphenylalanine did not exert these direct toxic effects because the administration of alpha-methylphenylalanine in vivo did not affect brain polyribosomes and a comparable concentration of this analogue was neither growth inhibitory nor cytotoxic to neuroblastoma cells in culture. The suitability of each analogue as an inhibitor of phenylalanine hydroxylase in animal models for phenylketonuria is discussed.

The effects of hyperphenylalaninaemia on the concentrations of aminoacyl-transfer ribonucleic acid in vivo. A mechanism for the inhibition of neural protein synthesis by phenylalanine

An acute administration of phenylalanine to neonatal animals has been reported to result in large decreases in the intracellular concentrations of several essential amino acids in neural tissue, as well as an inhibition of neural protein synthesis. The present report evaluates the effects of the loss of amino acids on the concentrations of aminoacyl-tRNA in vivo, with the view that an alteration in the concentrations of specific aminoacyl-tRNA molecules could be the rate-limiting step in brain protein metabolism during hyperphenylalaninaemia. tRNA was isolated from saline- and phenylalanine-injected mice 30-45 min after injection, by using a procedure designed to maintain the concentrations of aminoacyl-tRNA present in vivo. Periodate oxidation of the non-acylated tRNA and aminoacylation with radioactively labelled amino acids was used to determine the proportion of tRNA that was present in vivo as aminoacyl-tRNA. Although decreases in the intracellular concentrations of alanine, lysine and leucine were observed after phenylalanine administration, the concentrations of alanyl-tRNA, lysyl-tRNA and leucyl-tRNA actually increased by 15%. Although tryptophan has been suggested to be rate-limiting during hyperphenylalaninaemia, the proportion of tryptophan tRNA that was acylated was maximal in both normal and hyperphenylalaninaemic animals. This unexpected increase in aminoacyl-tRNA concentration is discussed as perhaps a secondary effect resulting from the phenylalanine-induced inhibition of protein synthesis. In contrast, the proportion of methionine tRNA that was acylated in vivo after phenylalanine administration was demonstrated to be decreased by approx. 17%. When the isoaccepting species of methionine tRNA were separated by reverse-phase column chromatography, three species were separated, one of which was demonstrated to be the initiator species, tRNAfMet, by the selective aminoacylation and formylation with Escherichia coli enzymes. After the administration of phenylalanine, the acylation of each of the three methionine tRNA species was decreased, with the initiator species being lowered by 10%. This effect on aminoacylation of tRNAfMet may be the primary step by which phenylalanine affects neural protein synthesis, and this is consistent with previous reports that re-initiation may be inhibited during hyperphenylalaninaemia.

Functional homogeneity of leucine pool in retina cells

Data on leucine metabolism in isolated rabbit retina are examined for evidence, for or against, a common intracellular pool of free leucine. Data include values for: concentrations, transport rates, degradative metabolism and protein incorporation of labelled leucine measured over a wide range of concentrations; protein incorporation of labelled threonine, measured simultaneously; and an indirect measurement of protein breakdown. The fall in labelled leucine incorporation into protein, when medium leucine was reduced below 100 microM, corresponded closely with the fall in intracellular specific activity predicted from rate of influx of labelled leucine from medium and rate of release of unlabelled leucine from protein breakdown. Protein incorporation of labelled leucine competed with decarboxylation and outward transport and reduced the free intracellular leucine in about the amounts predicted for a common pool. Implications for measurements using labelled amino acid are discussed.

Neuropathological considerations in cerebro-hepato-renal syndrome (Zellweger's syndrome)

The nervous system in patients with cerebro-hepato-renal syndrome appeared to be affected at various tissue levels. There was evidence of a migrational disorder manifested by polymicrogyria and lack of normal neuronal maturation. There was dysmyelination of the white matter associated with accumulation of neutral fat in astrocytes. Within the peripheral nerves, masses of tangled neurofilaments producing dystrophic axons were demonstrated by electron microscopy. These findings could be explained on the basis of a genetic metabolic defect, one that involved particularly the amino acids. The defect may have interfered with the normal intercellular reaction during embryogenesis resulting in the malformation of multiple organs. The same metabolic abnormality could have caused the hepatic damage and disturbance in normal myelination during the neonatal period.

The mechanism of polyribosome disaggregation in brain tissue by phenylalanine

The injection of neonatal mice with phenylalanine resulted in a rapid decrease in brain polyribosomes and a concomitant increase in monomeric ribosomes. Animals of 1-16 days of age were equally affected by phenylalanine, although the brain polyribosomes of 60-day-old mice were relatively resistant to the effects of phenylalanine. The population of free polyribosomes appeared to be more sensitive to phenylalanine treatment than bound polyribosomes, which were somewhat more resistant to disruption by high concentrations of the amino acid. The effects of phenylalanine were more pronounced with polyribosomes in the cerebral cortex than with those in the cerebellar tissue. The mechanism of polyribosome disruption was shown to be independent of hydrolysis mediated by ribonuclease. Virtually all of the monomeric ribosomes that resulted from phenylalanine treatment were shown to be inactive with regard to endogenous protein synthesis and were present in the cell cytoplasm as vacant couples. These ribosomes were readily dissociated by treatment with 0.5 M-KCl and subsequent ultracentrifugation. These results are discussed in the light of the possibility that high concentrations of phenylalanine disrupt brain protein synthesis by a molecular mechanism that is associated with initiation events.

Incorporation of leucine into human skeletal muscle proteins

The in vitro incorporation of labelled leucine into human skeletal muscle proteins was studied with the aim to elucidate the relationship between the amino acid tissue pools and protein biosynthesis. The distribution volumes of leucine and cycloleucine in skeletal muscle tissue were similar but the equilibration time was shorter for leucine than for cycloleucine. The cellular uptake of leucine and cycloleucine was competitively inhibited by increased concentration of amino acids in the medium indicating an active transport. Optimal stimulation for incorporation of leucine into proteins was obtained at an amino acid concentration in the medium corresponding to 10 times that of normal human plasma. The incorporation of 14C-leucine into skeletal muscle proteins was linear before the total pool of free intracellular 14C-leucine and the incorporation rate of leucine calculated from the specific activity in the medium versus the amino acid concentration in the medium were different in the same experiment indicating a re-utilization of amino acids released at protein degradation. The results are compatible with the hypothesis that the proteolytically released amino acids have a competitive advantage for incorporation as compared with extra- and intracellular free amino acids. It is concluded that the amino acid pool which is in the immediate continuity with the protein biosynthesis sites equilibrates rapidly with the extracellular amino acid pool.

Relationship between intracellular amino acids and protein synthesis in the extensor digitorum longus muscle of rats

1. The incorporation into protein, and the accumulation into the free amino acid pools, of radioactive l-leucine and glycine was studied in rat extensor digitorum longus muscle. 2. The tissue was incubated first with (14)C-labelled and then with (3)H-labelled amino acid. 3. The experimental results were consistent with a model based on the premise that the amino acids in protein were incorporated directly from the extracellular pool.