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

Protein synthesis in rat brain following neonatal exposure to lead

(1) Suckling rats were exposed to lead through the milk of their dams who received a diet of 4% lead carbonate and weanling rats were exposed to 2 injections of 5.0 mg Pb2+/100 g body weight. The brains were used to prepare the following homogenate fractions: postmitochondrial supernatant, postmicrosomal supernatant, ribosomes, initiation factors. (2) The postmitochondrial supernatant fractions were tested in vitro for protein synthesizing activity using the incorporation of labelled phenylalanine, and phenylalanyl-tRNA into peptide. The preparations from the lead-exposed rats had a significant reduction in activity. (3) Peptide formation with the brain ribosomes was not changed in the lead-exposed rats. (4) The aminoacyl-tRNA synthetase reaction was significantly reduced and accounted for most of the reduced peptide formation with brain homogenates from lead-exposed rats. (5) The binding of methionyl-tRNAfMet to ribosomes was increased using initiation factor preparations from the brain of lead-exposed rats.

Speed-accuracy relationships during in vitro and in vivo protein biosynthesis

Poly U-directed incorporation of phenylalanine and leucine into polypeptide has been described in at least 50 papers since 1961. In general, high translation activities are associated with high accuracies, and vice-versa. Moreover, a vast body of independent experimental data (effect of ethanol, temperature, urea, aminoglycosides, etc... on protein synthesis) put together here suggests that, in many circumstances, speed and accuracy of elongation are correlated. This result is to be contrasted with the view that the speed and the fidelity of protein synthesis are two opposing parameters. In this report, recent experimental data on the nature and effect of ribosomal ambiguity (ram) and streptomycin resistance (Strr) mutations are reexamined. Models on the action of streptomycin and other misreading-inducing antibiotics, as well as long-standing ideas on the control of misreading in mammalian systems are critically evaluated. An explanation is provided for the long-befuddling data on the action of gentamicin.

Fidelity of protein synthesis does not decline during aging of cultured human fibroblasts

To ascertain whether the fidelity of protein synthesis declines during cellular aging in vitro, we have developed a cell-free protein synthesizing system from cultured human fibroblasts which actively incorporates phenylalanine into acid-insoluble material upon addition of poly (U). The accuracy of poly (U)-directed protein synthesis was determined by comparing the ratio of leucine to phenylalanine incorporation in extracts of early- and late-passage fibroblasts derived from normal persons and from subjects with two genetic disorders of premature aging, progeria, and Werner syndrome. The results show no decline in translational fidelity at late passage or in prematurely aging cells, and thus fail to support the error catastrophe theory of cellular aging.

Age-dependent changes in brain protein synthesis in the rat

Brain protein synthesis was studied in vivo, in brain slices, and in cell-free systems in rats aged 1, 16, and 24 months. We observed a highly significant reduction in amino acid incorporation with advancing age. This reduction was observed in vivo, in slices, in postmitochondrial supernatant, microsomes, and membrane-bound polysomes. Free heavy polysomes showed no age-dependent decline but formed a smaller proportion of total ribosomes in older animals. These studies suggest that in the rat brain protein synthesis declines before senescence, possibly due to an impairment in the initiation process.

Conservation of ribosomal fidelity during ageing

The function of ribosomes prepared from the liver of young (2--5 months old) and senescent (15--26 months old) mice were compared in vitro. The conclusions that can be drawn from the liver of senescent mice is 10 to 40% lower than that of young mice; (2) the fidelity of translation does not change grossly with age; (3) the thermosensitivity of translational activity of ribosomes and the activity of ribosome-associated ribonuclease do not change with age; (4) there is an age-dependent accumulation of free 40S ribosomal subunits in the cytoplasm of mouse liver. The relation of age-dependent changes in the structure and function of ribosomes is discussed.

Comparison of cell-free protein synthesis by different regions of chicken brain

The cell-free protein synthesis by the postmitochondrial supernatant from chicken cerebrum was twofold greater than protein synthesis by the cerebellum or optic lobes. Ribosomal aggregation of mRNA and ribonuclease activity of the postmitochondrial supernatant from the three brain regions was not statistically different. The higher protein synthetic activity of the cerebral postmitochondrial supernatant was associated with both the postribosomal supernatant (cell sap) and microsomal fractions. Cerebral monomeric ribosomes were more active in polyuridylic acid directed polyphenylalanine synthesis than monomeric ribosomes from either the cerebellum or optic lobes. The ability of cerebral cell sap to support polyuridylic acid directed polyphenylalanine synthesis was 1.6 to 2 times greater than cell sap from the other two regions. Cell sap factors other than tRNAphe or phenylalanyl-tRNA synthetases appear to be responsible for the higher protein synthetic activity of the cbr cell sap.