In vitro binding of phenylalanyl-tRNA to neonatal and adult mouse brain ribosomes

Experimental methyl mercury neurotoxicity: locus of mercurial inhibition of brain protein synthesis in vivo and in vitro

Brain cell-free protein synthesis is inhibited by methyl mercury chloride (MeHg) following in vivo or in vitro administration. In this report, we have identified the locus of mercurial inhibition of translation. Intraperitoneal injection of MeHg (40 nmol/g body wt) induced variable inhibition of amino acid incorporation into the post-mitochondrial supernatant (PMS) harvested from the brain of young (10-20-day-old) rats. No mercurial-induced disaggregation of brain polyribosomes nor change in the proportion of 80S monoribosomes was detected on sucrose density gradients. No difference in total RNA was found in the PMS. Initiation complex formation was stimulated by MeHg, as detected by radiolabelled methionine binding to 80S monoribosomes following continuous sucrose density gradient centrifugation. After micrococcal nuclease digestion of endogenous mRNA, both in vivo and in vitro MeHg inhibited polyuridylic acid-directed incorporation of [3H]phenylalanine. However, the in vivo inhibition was no longer observed when [3H]phenylalanyl-tRNAPhe replaced free [3H]phenylalanine in the incorporation assay. The formation of peptidyl[3H]puromycin revealed no difference from controls. There was significant mercurial inhibition of phenylalanyl-tRNA Phe synthetase activity in pH 5 enzyme fractions derived from brain PMS of MeHg-poisoned rats. These experiments revealed that the apparent MeHg inhibition of brain translation in vivo and in vitro is due primarily to perturbation in the aminoacylation of tRNA and is not associated with defective initiation, elongation, or ribosomal function.

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.

Role of elongation factor 1 in the translational control of rodent brain protein synthesis

The translational control of protein synthesis during early postnatal neural development and aging was examined in the mouse and the rat. The activity of brain elongation factor 1 (bEF-1) was found to decrease exponentially with age and to decline parallel to the age-dependent decrease in total protein synthesis in both rodents. This decrement in bEF-1 activity fell within the range of reported age-related decreases in protein synthesis in in vitro systems. The factor was present in multiple forms; the lighter species predominated in older animals, whereas the young light form apparently disappeared with increasing age, and was replaced by other arising from the heavy form. Elongation factor 1 derived from young brains functioned as a rate-limiting component in polypeptide synthesis in previously saturated adult systems. The data suggest that bEF-1 has an important modulatory effect on total brain protein synthesis.

Effect of polyamines on cell-free protein synthesizing systems from rat cerebral cortex, cerebellum and liver

The naturally occurring polyamines spermine and spermidine stimulate in low concentrations the amino acid-incorporating activity of cell-free systems from cerebral cortex, cerebellum and liver from the rat to a significant degree, but in high concentrations they have strong inhibiting effects on these systems. Maximal polyamine stimulation is observed with suboptimal Mg2+ concentrations in the reaction mixtures; increasing Mg2+ to 15 mM results in an inhibition of cell-free protein synthesis by polyamines even at low concentrations. The stimulating effect of spermine and spermidine involves a facilitated binding of template RNA and aminoacylated tRNA to the ribosomes. In contrast, the aminoacylation of tRNA is not influenced. These data indicate that spermine as well as spermidine have regulatory functions during the translational process in cerebral cortex, cerebellum and liver. They seem to interact with ribosomal functions and to cause a shift in the optimal Mg2+ concentration required for the synthesis of brain and liver proteins.

Biochemical studies of chick brain development and maturation: II. Alterations in the mechanisms of cell-free protein synthesis

The molecular mechanisms responsible for the decreased cell-free protein synthetic activity of chicken brain (cerebrum, cerebellum, and optic lobes) from the late embryonic stage to the adult stage were investigated. The changes in polyribosome content closely paralleled changes in cell-free protein synthetic activity; both increased during late embryonic development, reached a maximum around hatching,and thereafter decreased to the level found in the adult. Both cell sap and microsomal or ribosomal fractions from the adult brain tissue were less active in protein synthesis; however, the microsomal or ribosomal fractions contributed more to the decreased protein synthesis than did the cell sap. The lower activity of adult cell sap in protein synthesis was primarily due to a decreased activity in the aminoacylation of tRNA with no apparent change in the ability of the cell sap to catalyze the elongation of polyphenylalanine synthesis. Ribosomal particles (80 S) from adult and embryonic brain tissue had similar biological activities and fidelity in the translation of polyuridylic acid; however, the cell-free protein synthetic activity of the embryonic post-mitochondrial supernatant preparation was more sensitive to inhibitors of the initiation of protein synthesis (aurintricarboxylic acid and polyinosinic acid) than adult post-mitochondrial supernatant, indicating a decreased initiation capacity in adult brain post-mitochondrial supernatant compared to embryonic brain post-mitochondrial supernatant.

Thermostability of mammalian brain ribosomes and the effects of nucleoside triphosphates on their heat-sensitivity

Mammalian brain ribosomes were found to be heat-labile. On preincubation of the ribosomes at 37 degrees C, their ability to participate in polypeptide-synthesis reactions was substantially diminished. Despite the sensitivity of ribosomal protein synthesis to heat-inactivation, preincubation resulted in no significant alterations in ribosomal sedimentation profiles or changes in the integrity of the ribosomal RNA. The thermolability of brain ribosomes was shown to be associated with their inability to bind both template RNA and aminoacyl-tRNA. Similar experiments with brain ribosomal subunits demonstrated that the small (40S) subunit was more sensitive to heat-inactivation than the large (60S) subunit. The presence of ATP (1mm) protected ribosomes from thermal inactivation, although this protection was shown to be temporary. The protection appeared to be specific to nucleoside triphosphates, since GTP and UTP also stabilized ribosomes to thermal denaturation whereas nucleoside diphosphates (ADP) and nucleoside monophosphates (AMP and cyclic AMP) did not alter ribosomal sensitivity to heat. Although 1mm concentrations of nucleoside triphosphates protected ribosomes from heat-inactivation, the presence of higher concentrations resulted in complete inactivation of ribosomal activity.