The effect of morphine tolerance and dependence on cell free protein synthesis

A cell free system consisting of polyribosomes and pH 5 factors of the cytosol was isolated from mouse brain. This system actively promoted the incorporation of radiolabeled amino acids into protein in vitro. Addition of exogenous morphine to a cell free protein synthetic system isolated from chronically morphinized, placebo treated, or naive mouse brain had no effect on the relative synthetic capacity of the system. In addition, morphine did not alter the response to a synthetic mRNA, polyuridylic acid. However, both the polyribosomes and pH 5 factors isolated from chronically morphinized mouse brain were more effective in promoting amino acid incorporation into protein relative to the corresponding fractions from placebo treated mice. Acrylamide gel electrophoresis of the proteins in the incubation mixture showed the increased amino acid incorporation was the result of a general quantitative increase in the specific activity of all of the proteins synthesized by the cell free system.     

Action of light and streptomycin on protein synthesis in cabbage seedlings

The action of light on protein synthesis was examined in the cabbage seedlings, a system extensively used in the studies of anthocyanin synthesis. Continuous red and far red light have no effect on total protein content while they cause a marked decrease in the level of free amino acids in cabbage seedlings. The rate of protein synthesis, measured as incorporation of radioaetively-labelled amino acids into proteins, is clearly stimulated by light. Phytochrome involvement in the light stimulation of the incorporation is also demonstrated by the red-far red reversibility of the response. The relative effectiveness of continuous red and far red light upon the incorporation of amino acids into proteins is affected by the nature of the system used to study the incorporation process. When excised cotyledons and short period of incorporation were used, continuous far red was more effective than red. However, when whole seedlings and long period of incorporation were used, red and far red were equally effective. Streptomycin causes a 10– 15% decrease in the rate of incorporation of amino acids into proteins of all cellular fractions, except the plastid fraction where a much higher inhibition (30%) was observed.     

AXONAL TRANSPORT AND TURNOVER OF PROLINE- AND LEUCINE-LABELED PROTEIN IN THE GOLDFISH VISUAL SYSTEM

Abstract— The suitability of radioactively labeled proline as a marker of axonally transported protein in the goldfish visual system is further investigated and compared with another amino acid, leucine, in double-label experiments. Intraocularly injected proline is incorporated into protein in the eye S times more efficiently than is leucine, while local labeling of brain protein from precursor which has left the eye and entered the blood, (observed in the ipsilateral optic tectum) is five- to eight-fold less from proline than from leucine. The difference is attributed to the superior transport of leucine, an essential amino acid, into the brain from the blood. Once in the brain, the apparent rates of incorporation of the two amino acids are similar. Proline- or leucine-labeled, axonally transported proteins have a longer apparent half-life in the brain than do proteins labeled from intracranial injection of the precursors. By either route, proline-labeled proteins have a longer apparent half-life than leucine-labeled proteins. It is proposed that proline, released from protein breakdown is reutilized to a greater extent than is leucine.     

Compartments of protein metabolism in the developing brain.

We investigated whether the higher rate of amino acid incorporation into immature than into mature brain protein is due to (a) rapid growth, (b) a small rapidly metabolized protein pool, or (c) a higher turnover rate of most of the protein. We measured net growth and the incorporation of [14C]tyrosine or [14C]valine into brain proteins in young rats and mice. The specific activity of the free amino acid pool was kept constant in the tyrosine experiments. Incorporation of tyrosine into protein was continued for up to 30 h by which time the specific activity of protein-bound amino acid reached 1/3 of that of the free (precursor) amino acid. The growth (accretion) of brain proteins was approx. 0.635% per h in mice and rats in the 1-4 day period after birth. In previous studies we found that the turnover rate of the bulk (about 96%) of adult brain proteins is below 0.3% per h. Because of the presence of a small (about 4%) active pool the average turnover rate is 0.6% per h. The present experiments show a degradation rate of 0.7-1.1% per h in the brain proteins of the young. This high metabolic rate is not due to a small rapidly degraded fraction of protein. The very rapid protein fraction previously seen in adult rats is either very small (below 1%) or absent in the young. Thus most of the proteins in the immature brain during the rapid growth phase are formed and broken down at a rate that is approximately three times higher than that of the bulk of proteins in the adult brain. The small active protein pool in the adult on the other hand has a metabolic rate higher than that of the immature brain proteins.     

Biochemical and Autoradiographical Determination of Protein Synthesis in Experimental Brain Tumors of Rats

The rate of leucine incorporation into brain proteins was studied in rats with experimental brain tumors produced by intracerebral transplantation of the glioma clone F98. Incorporation was measured with [14C]leucine using a controlled infusion technique for maintaining constant specific activity of [14C]leucine in plasma, followed by quantitative autoradiography and biochemical tissue analysis. After 45 min the specific activity of free [14C]leucine in plasma was 2.5-3 times higher than in brain and brain tumor, indicating that the precursor pool for protein synthesis was fueled both by exogenous (plasma-derived) and endogenous (proteolysis-derived) amino acids. Endogenous recycling of amino acids amounted to 73% of total free leucine pool in brain tumors and to 60-70% in normal brain. Taking endogenous amino acid recycling into account, leucine incorporation was 78.7 +/- 16.0 nmol/g of tissue/min in brain tumor, and 17.2 +/- 4.2 and 9.7 +/- 3.3 nmol/g/min in normal frontal cortex and striatum, respectively. Leucine incorporation within tumor tissue was markedly heterogeneous, depending on the local pattern of tumor proliferation and necrosis. Our results demonstrate that quantitative measurement of leucine incorporation into brain proteins requires estimation of recycling of amino acids derived from proteolysis and, in consequence, biochemical determination of the free amino acid precursor pool in tissue samples. With the present approach such measurements are possible and provide the quantitative basis for the evaluation of therapeutic interventions.     

A Quantitative Regional Analysis of Amino Acids Involved in Rat Brain Protein Synthesis by High Performance Liquid Chromatography

A biochemical method is described for the simultaneous quantitative estimation of unidirectional blood-brain amino acid influx and protein biosynthesis in individual structures of the rat brain. The method involved a double labeling experiment started by the administration of [14C]carboxyl-labeled amino acids and terminated 2 min after infusion of 3H-labeled amino acids, each at tracer quantities, the total labeling period being 45 min. Specific radioactivities of 14C- or 3H-labeled phenylalanine, tyrosine, leucine, isoleucine, and valine were determined in plasma and in small brain tissue samples for free amino acids, aminoacyl-tRNAs, and proteins. Amino acids were converted to their corresponding 5-dimethylamino-naphthalenesulfonyl (Dns, dansyl) derivatives and separated on HPLC C18 reversed-phase columns isocratically according to a newly developed optimizing procedure. The order of influx values between the neutral amino acids in relation to each other was Leu greater than Tyr greater than Ile greater than Phe greater than Val in every structure examined. Although aminoacylation of tRNAs was found to proceed to a comparable degree for neutral amino acids in all regions investigated, the specific radioactivity of amino acids attached to tRNAs differed substantially from that in the free amino acid pool, especially for leucine and valine. The results indicate the necessity of aminoacyl-tRNA determinations for tracer incorporation studies in protein synthesis analysis. Relative protein synthesis rates in the halothane-anesthetized rat were determined to be 30 and 67-91 pmol total amino acid incorporation/min/mg tissue for white and gray matter, respectively.     

The effect of amino acids on protein metabolism as measured in long-term experiments in immature brain explants

In a study of a system suitable for investigating long-term effects on brain protein metabolism, we measured amino-acid incorpration into isolated immature brain explants incubated under sterile conditions up to ten days. Measurements of changes in total proteins, total DNA, cell number during the experiments, and 14C-thymidine incorporation measurements indicated no significant net growth; new cell formation was below 5% in a 5-day period; therefore, amino-acid incorporation was mainly due to protein turnover. The rate of incorporation in our immature brain preparation was similar to that of the adult brain in vivo: by ten days about one-half of the tissue protein turned over. The label incorporated was released in subsequent incubations with cold amino acids. Such release occurred in all subcellular fractions examined. Incorporation was fairly stable; at temperatures below 30 degrees C it rapidly declined, but it was not affected when phenylalanine or the branched chain amino acids (leucine, isoleucine, valine) were elevated in the incubation medium. Brief exposure to low amino-acid media had no effect; longer exposure resulted in tissue damage. Our model system indicates that overall brain protein turnover is not sensitive to such variations in the level of most amino acids, which may occur under various conditions. Protein metabolism of the nervous system occurs at a high rate. A recent long-term labeling method (Lajtha, Latzkovits, and Toth, 1976) gave a best fit to incorporation curves by assuming two compartments for adult brain proteins, one of which (about 6%) has a half-life of 15 hr and the other (94%) has a half-life of ten days. The disappearance of protein-bound label with time under conditions in which all proteins were previously labeled indicated that most, possibly all, proteins in brain are in a dynamic state (Lajtha and Toth, 1966). Incorporation of amino acids was found in all proteins and structures that have been studied to date; myelin proteins previously thought less active are also metabolized at a significant rate (Sabri, Bone, and Davison, 1974; Lajtha, Toth, Fujimoto, and Agrawal, 1977). We have fairly extensive information available in addition to turnover studies about the mechanisms of protein synthesis in brain (Roberts, 1971); protein breakdown was also studied in some detail (Marks and Lajtha, 1971). In contrast to our knowledge about protein metabolism under physiological equilibrium conditions, our information about alterations during functional demands or pathological conditions is scanty. Although a significant amount of work has been reported, largely because of technical difficulties the results are difficult to interpret unequivocally. The present report represents our effort to address some of the obstacles: to develop a system in which influences on long-term incorporation can be studied...     

Density labeling of proteins with 13C-labeled amino acids: no accumulation of an inactive alpha-amylase precursor in barley aleurone cells.

Gibberellic acid enhances α-amylase (EC 3.2.1.1) production in isolated barley aleurone layers after a lag period of 4 to 8 h, and most of the enzyme is produced after 12 h of hormone treatment. Amino acids necessary for protein synthesis in barley aleurone layers are derived from the degradation of storage proteins in this tissue. Since bromate is an inhibitor of barley protease, in the presence of bromate the production of α-amylase in aleurone layers becomes dependent on exogenous amino acids. We have incubated aleurone layers with bromate plus ¹³C-labeled amino acids and [³H]leucine from 0 to 24, 0 to 12, and 12 to 24 h after the application of gibberellic acid. The chemical quantity of [³H]leucine was negligible in comparison to that of ¹³C-labeled amino acids. Therefore, any density shift of proteins observed must be due to the incorporation of ¹³C-labeled amino acids. The density shift of α-amylase and that of newly synthesized proteins (radioactivity profile) were determined by isopycnic centrifugation in CsCl density gradients. The density shift of α-amylase isolated from aleurone layers incubated with ¹³C-labeled amino acids from 12 to 24 h after the addition of hormone was much larger than that of α-amylase isolated from aleurone layers incubated with ¹³C-labeled amino acids from 0 to 12 h of hormone treatment. By comparing the density shift of α-amylase with that of newly synthesized proteins, it is apparent that essentially all the amylase molecules are de novo synthesized. We can conclude that there is little or no accumulation of an inactive α-amylase precursor in barley aleurone cells between the time of the application of gibberellic acid and the time of the rapid increase in α-amylase activity.     

Serinc, an activity-regulated protein family, incorporates serine into membrane lipid synthesis

Cell membranes contain various transporter proteins, some of which are responsible for transferring amino acids across membrane. In this study, we report another class of carrier proteins, termed Serinc1-5, that incorporates a polar amino acid serine into membranes and facilitates the synthesis of two serine-derived lipids, phosphatidylserine and sphingolipids. Serinc is a unique protein family that shows no amino acid homology to other proteins but is highly conserved among eukaryotes. The members contain 11 transmembrane domains, and rat Serinc1 protein co-localizes with lipid biosynthetic enzymes in endoplasmic reticulum membranes. A Serinc protein forms an intracellular complex with key enzymes involved in serine and sphingolipid biosyntheses, and both functions, serine synthesis and membrane incorporation, are linked to each other. In the rat brain, expression of Serinc1 and Serinc2 mRNA was rapidly up-regulated by kainate-induced seizures in neuronal cell layers of the hippocampus. In contrast, myelin throughout the brain is enriched with Serinc5, which was down-regulated in the hippocampus by seizures. These results indicate a novel mechanism linking neural activity and lipid biosynthesis.     

Properties of RNA fractions from nuclei of brain cells which stimulate incorporation of amino acids by brain ribosomes

Abstract— The properties of RNA fractions from nuclei of brain cells which were capable of stimulating amino acid incorporation into proteins of an homologous ribosomal system were investigated. RNA was routinely prepared from crude nuclear preparations of rat brain by a method which involved treatment with sodium dodecyl sulphate and phenol at 65°. The capacity of this preparation to stimulate incorporation of radioactivity from a mixture of 15 l -[¹⁴C]amino acids was greatly enhanced by preliminary incubation of the ribosomal system from brain for 5–20 min. The response was markedly dependent upon the concentrations of ribosomes and of the pH 5 fraction. The optimal level of Mg²⁺ for basal incorporation of amino acids into protein was 8 mm ; however, incorporation in the presence of nuclear RNA was greater at higher concentrations of Mg²⁺. The response to nuclear RNA was also enhanced as the K⁺ concentration was increased from 25 to 100 mm . The stimulatory effect of nuclear RNA on incorporation of l -[¹²C]eucine was either unaltered or depressed by addition of a mixture of 19 l -[¹²C]amino acids each at concentrations, of 10^?8, 10^?2, or 10^?1 mm . Under appropriate conditions of incubation, basal rates of incorporation and rates of incorporation stimulated by nuclear RNA were linear for 30 min. The response was proportional to the concentration of nuclear RNA between 34 and 136 μg. RNA prepared from ribosomes of rat brain essentially failed to stimulate incorporation of amino acids over this range of concentrations. Fractionation of nuclear RNA by centrifugation in sucrose density gradients revealed that 75 per cent of the stimulatory activity was in the fraction which sedimented below 12 S and contained about 25 per cent of the total RNA. Most of the remaining activity was in the 18 S region. Less than 5 per cent of the RNA in the lightest fraction (< 12 S) exhibited amino acid-acceptor activity, The stimulatory action of nuclear RNA on incorporation of amino acids was readily destroyed by mild treatment with pancreatic ribonuclease, whereas amino acid-acceptor activity was relatively resistant to this treatment. The results suggest that the brain may contain low molecular weight RNA with properties of messenger RNA.     

Incorporation of nonnatural amino acids into proteins by using various four-base codons in an Escherichia coli in vitro translation system

Incorporation of nonnatural amino acids into proteins is a powerful technique in protein research. Amber suppression has been used to this end, but this strategy does not allow multiple incorporation of nonnatural amino acids into single proteins. In this article, we developed an alternative strategy for nonnatural mutagenesis by using four-base codons. The four-base codons AGGU, CGGU, CCCU, CUCU, CUAU, and GGGU were successfully decoded by the nitrophenylalanyl-tRNA containing the complementary four-base anticodons in an Escherichia coli in vitro translation system. The most efficient four-base decoding was observed for the GGGU codon, which yielded 86% of the full-length protein containing nitrophenylalanine relative to the wild-type protein. Moreover, highly efficient incorporation of two different nonnatural amino acids was achieved by using a set of two four-base codons, CGGG and GGGU. This work shows that the four-base codon strategy is more advantageous than the amber suppression strategy in efficiency and versatility.     

Posttranslational Protein Modification by Amino Acid Addition in Intact and Regenerating Axons of the Rat Sciatic Nerve

Abstract: Experiments were performed to determine whether ppsttranslational addition of amino acids to axonal proteins occurs in axons of the rat sciatic nerve. Two ligatures were placed 1 cm apart on sciatic nerves. Six days later, segments proximal to each ligature were removed, homogenized, centrifuged at 150,000 · g , and analyzed for the ability to incorporate ³H-amino acids into proteins. No incorporation of amino acids into proteins was found in the high-speed supernatant, but when the supernatant was passed through a Sephacryl S-200 chromatography column (removing molecules less than 20 kD), [³H]arginine, lysine, leucine and aspartic acid were incorporated into proteins in both proximal and distal nerve segments. Small but consistently greater amounts of radioactivity were incorporated into proteins in proximal segments compared with distal segments, indicating that the components necessary for the reaction are transported axonally. This reaction represents the posttranslational incorporation of a variety of amino acids into proteins of rat sciatic nerve axons. Other experiments showed that the incorporation of amino acids into proteins is by covalent bonding, that the amino acid donor is likely to be tRNA, and that the reaction is inhibited in vivo by a substance whose molecular mass is less than 20 kD. This inhibition is not affected by incubation with physiological concentrations of unlabeled amino acids, by boiling, or by treatment with Proteinase K. When the axonally transported component of the reaction was determined in regenerating nerves, the amount of incorporation of amino acids into protein was 15–150 times that in intact nerves. The results indicate that the components of this reaction are transported axonally in rat sciatic nerves and that the reaction is increased dramatically in growing axons during nerve regeneration.     

Protein composition and synthesis in the adult mouse spinal cord

Propepties of spinal cord proteins were studied in adult mice subjected to unilateral crush or electrical stimulation of sciatic nerve. The protein composition of spinal tissue was determined using SDS-polyacrylamide gel electrophoresis coupled with subcellular fractionation. Comparisons of mouse spinal cord and brain revealed similarities in the types but differences in the concentrations of myelin associated proteins, nuclear histones and other proteins. Comparisons with sciatic nerve proteins demonstrated differences in types of proteins but similarities in the concentration of myelin proteins and nuclear histones. The short term (<2 hrs.) incorporation of radioactive amino acids into spinal cord proteins revealed heterogeneous rates of incorporation. Neither nerve crush six days prior to testing nor sciatic nerve stimulation had a significant effect on the protein composition or amino acid incorporation rates of spinal cord tissue. These observations suggest that known differences in spinal cord function following alterations in nerve input may be dependent upon different mechanisms than have been found in the brain.     

The effects of individual amino acids on the incorporation of labelled amino acids into proteins by brain homogenates

Abstract— The effects of phenylalanine and other amino acids on incorporation of several different ¹⁴C-labelled amino acids into cerebral protein were studied in brain homogenates. Excess of some amino acids had a varied effect with different ¹⁴C-labelled amino acids. Of the unlabelled-labelled amino acid combinations tested the maximal inhibition was obtained with the following: (1) phenylalanine, which inhibited the incorporation of [¹⁴C]tyrosine, and (2) leucine, which inhibited incorporation of [¹⁴C]isoleucine. In both cases the inhibition occurred principally in proteins that were recovered in the 800 g and 13,000 g sediments. Only a small degree of inhibition occurred in proteins that sedimented at 100,000 g, and no inhibition occurred in proteins of the 100,000 g supernatant.     

Introduction of specialty functions by the position-specific incorporation of nonnatural amino acids into proteins through four-base codon/anticodon pairs

Position-specific incorporation of nonnatural amino acids into proteins (nonnatural mutagenesis) via an in vitro protein synthesizing system was applied to incorporate a variety of amino acids carrying specialty side groups. A list of nonnatural amino acids thus far successfully incorporated through in vitro translation systems is presented. The position of nonnatural amino acid incorporation was directed by four-base codon/anticodon pairs such as CGGG/CCCG and AGGU/ACCU. The four-base codon strategy was more efficient than the amber codon strategy and could incorporate multiple nonnatural amino acids into single proteins. This multiple mutagenesis will find wide applications, especially in building paths of electron transfer on proteins. The extension of translation systems by the introduction of nonnatural amino acids, four-base codon/anticodon pairs, orthogonal tRNAs, and artificial aminoacyl tRNA synthetases, is a promising approach towards the creation of "synthetic microorganisms" with specialty functions.     

Free and bound amino acids and proteins in developing grains of rice with enhanced lysine/proteins

 Free amino acids were determined in developing seed of a rice mutant with enhanced grain lysine. This phenotype frequently has enhanced protein. Some free amino acids of developing seed are inversely related to the level of total amino acids in proteins of the mature grain. Amino acids that were enhanced in protein, including aspartic acid, threonine, methionine and lysine, were notably lower in the free amino-acid pool. Our conclusion is that mutant-developing grains process aspartate amino acids more rapidly than the controls. Conversely, arginine, valine and glutamic acid/glutamine accumulate as free amino acids with mutant/control ratios of 1.39, 1.29 and 1.12, respectively. Glutamic acid/glutamine in proteins of mature seeds is lower in the mutant than the control. ³H-lysine incorporation showed enhanced isotope incorporation into at least four proteins. One mutant protein was less actively labelled than analogous controls. The ³Hlysine pattern indicates processing modifications in this useful rice mutant. Received: 14 October 1996/Accepted: 8 November 1996     

Effect of aromatic acids on protein synthesis in subcellular preparations from the rat brain.

The incorporation of [3H]phenylalanine, [3H]tyrosine, and [3H]tryptophan into protein and amino acyl-tRNA was studied in cell-free preparations from rat brain. Tyrosine and tryptophan inhibited the incorporation of phenylalanine into protein, and tyrosine inhibited the incorporation of phenylalanine and tryptophan into amino acyl-tRNAs. In most cases, homogentisate, phenylpyruvate, and phenyllactate inhibited the incorporation of phenylalanine, tyrosine, and tryptophan into protein and amino acyl-tRNAs, and the incorporation of phenylalanine into polyphenylalanine. All other protein amino acids, and phenylacetate, salicylate, and benzoate were wholly ineffectual. The results suggest that the formation of amino acyl-tRNAs may have been the step which was affected most by the inhibitors. The incorporation data at different concentrations of the aromatic amino acids were fitted to the simple Michaelis equation. Homogentisate and phenylpyruvate generally tended to reduce both Km and V in the incorporation of aromatic amino acids into protein and amino acyl-tRNAs, even if V decreased more than Km.     

THE INCORPORATION OF RADIOACTIVE AMINO ACIDS INTO BRAIN SUBCELLULAR PROTEINS DURING TRAINING

—Total proteins, free amino acids, tritiated water and subcellular proteins of mouse brain were examined for changes in radioactivity during operant conditioning after subcutaneous administration of labelled amino acids. The conditioning was based on appetitive learning, using sweetened milk as a reward. During training and incorporation for 20-30 min, both [³H]leucine and [1-¹⁴C]leucine underwent a significant increase in catabolism, resulting in a decreased radioactivity in the free amino acids. [2-²H]Methionine underwent a rapid loss of isotope, so that 90% of the radioactivity was in the form of tritiated water at the end of training, and this phenomenon masked any possible effect of training. The brain uptake of [³⁵S]methionine increased during the training, resulting in an increased radioactivity in the proteins. Uptake of [³H]lysine increased slightly during training only after 1 h incorporation and not after 20 or 30 min, as judged from a time course of radioactivity in the free amino acids. Incorporation into nuclear proteins increased selectively during 20 min, and into nuclear and cytosol proteins after 60 min incorporations. It is concluded that changes in the observed rate of incorporation of a precursor into brain subcellular proteins under the influence of behaviour might be the result of changes in precursor catabolism or uptake, or both, and that each amino acid behaves in a different way. Even the same amino acid gives different results depending on the isotope and its position in the amino acid.     

Residue-specific Incorporation of Noncanonical Amino Acids into Model Proteins Using an Escherichia coli Cell-free Transcription-translation System

The canonical set of amino acids leads to an exceptionally wide range of protein functionality. Nevertheless, the set of residues still imposes limitations on potential protein applications. The incorporation of noncanonical amino acids can enlarge this scope. There are two complementary approaches for the incorporation of noncanonical amino acids. For site-specific incorporation, in addition to the endogenous canonical translational machineries, an orthogonal aminoacyl-tRNA-synthetase-tRNA pair must be provided that does not interact with the canonical ones. Consequently, a codon that is not assigned to a canonical amino acid, usually a stop codon, is also required. This genetic code expansion enables the incorporation of a noncanonical amino acid at a single, given site within the protein. The here presented work describes residue-specific incorporation where the genetic code is reassigned within the endogenous translational system. The translation machinery accepts the noncanonical amino acid as a surrogate to incorporate it at canonically prescribed locations, i.e., all occurrences of a canonical amino acid in the protein are replaced by the noncanonical one. The incorporation of noncanonical amino acids can change the protein structure, causing considerably modified physical and chemical properties. Noncanonical amino acid analogs often act as cell growth inhibitors for expression hosts since they modify endogenous proteins, limiting in vivo protein production. In vivo incorporation of toxic noncanonical amino acids into proteins remains particularly challenging. Here, a cell-free approach for a complete replacement of L-arginine by the noncanonical amino acid L-canavanine is presented. It circumvents the inherent difficulties of in vivo expression. Additionally, a protocol to prepare target proteins for mass spectral analysis is included. It is shown that L-lysine can be replaced by L-hydroxy-lysine, albeit with lower efficiency. In principle, any noncanonical amino acid analog can be incorporated using the presented method as long as the endogenous in vitro translation system recognizes it.     

Effect of phenylalanine on protein synthesis in the developing rat brain

1. Inhibition of the rate of incorporation of [(35)S]methionine into protein by phenylalanine was more effective in 18-day-old than in 8-day-old or adult rat brain. 2. Among the subcellular fractions incorporation of [(35)S]methionine into myelin proteins was most inhibited in 18-day-old rat brain. 3. Transport of [(35)S]methionine and [(14)C]leucine into the brain acid-soluble pool was significantly decreased in 18-day-old rats by phenylalanine (2mg/g body wt.). The decrease of the two amino acids in the acid-soluble pool equalled the inhibition of their rate of incorporation into the protein. 4. Under identical conditions, entry of [(14)C]glycine into the brain acid-soluble pool and incorporation into protein and uptake of [(14)C]acetate into lipid was not affected by phenylalanine. 5. It is proposed that decreased myelin synthesis seen in hyperphenylalaninaemia or phenylketonuria may be due to alteration of the free amino acid pool in the brain during the vulnerable period of brain development. Amyelination may be one of many causes of mental retardation seen in phenylketonuria.