Deficiency in accessory protein of native 40S ribosomal subunits in the liver of aging rats

The synthesis of amylase in parotid glands of young and old rats

The age-related changes in the rate of synthesis of total and secretory proteins were examined in parotid glands of young (2 months) and old (24 months) rats. The differences in the rate of incorporation of radioactive leucine into acid-insoluble proteins of the gland indicate that the rate of protein synthesis declines with age in this gland. To determine whether the rate of synthesis of secretory proteins changes with age in this gland, the rates of incorporation of [3H]leucine into amylase, a major secretory protein of the gland, were compared by radioactivity determinations. For this comparison, amylase was precipitated with glycogen after incubating the gland slices in the presence of the labeled amino acid. The study shows that rate of synthesis of amylase declines significantly with age in this gland. The possible relationship between the decline in protein synthesis and the reduced level of secretory activity of the gland due to aging is discussed.

Changes in the synthesis of exportable and nonexportable proteins in parotid glands during aging

The age-related differences in the synthesis of exportable and nonexportable proteins of the parotid salivary gland were compared in 2- and 24-months-old rats. Parotid slices from these rats were incubated in the presence of [14C]leucine and the amount of radioactivity incorporated into the water-soluble proteins of the postmicrosomal supernatant was compared. The exportable and nonexportable proteins were identified by electrophoretic separation of these proteins by comparing the banding patterns of the gel preparations from unstimulated glands to those from the glands stimulated to secrete. The radioactivity determination in various protein bands from these rats indicated that the synthesis of exportable secretory proteins declined with age, while that of nonexportable proteins did not appear to change.

Decreased protein synthesis by polysomes, tRNA and aminoacyl-tRNA synthetases isolated from senescent rat liver

A cell-free in vitro protein-synthesizing system isolated from adult(10-13-month) and senescent(24-30-month) female Wistar rat liver is described. Optimal concentrations of polysomes, aminoacyl-tRNA synthetases, tRNA, Mg 2+, K+, and labeled amino acids are reported for this in vitro system. Both are rate and extent of protein synthesis are lower with the system from senescent liver compared to that from adult liver. Senescent polysomes, tRNA synthetases, and tRNA all contribute to this decrease. Free adult or senescent polysomes support protein synthesis to the same extent. However, compared to adult polysomes, senescent membrane-bound polysomes support less protein synthesis and account for an increased proportion of the total polysome fraction. Ribonuclease and protease activities are low in the adult and the senescent in vitro systems and do not account for the decreased protein synthesis with the latter.

Cell-free protein synthesis by kidney from the aging female Fischer F344 rat

This is the first report to describe and characterize a cell-free protein synthesis system derived from kidney tissue. The optimum conditions for [3H]-valine incorporation into protein by the post-mitochondrial supernatant from whole kidneys were found to be: pH 6.9, 7.5 mM MgCl2, 150 mM KCl, 10 mM ATP, and 2 mM GTP. The cell-free protein-synthetising activities of kidneys isolated from 4.5-, 7.5-, 22-, and 31-month-old female Fischer F344 rats were measured using the post-mitochondrial supernatant. A 73-87% decrease in cell-free protein synthesis was observed between 4.5 and 31 months of age. Both the cell sap and microsomal fractions of the kidney post-mitochondrial supernatant from old rats were less active in protein synthesis than these fractions from the kidneys of young rats. No age-related change in the activity of RNAase in the kidney post-mitochondrial supernatant was observed. Kidney ribosomes stripped of endogenous mRNA were found to be active in poly(uridylic acid)-directed polyphenylalanine synthesis. The effect of aging on the fidelity of translation was determined by measuring poly(uridylic acid)-directed [14C]-phenylalanine and [3H]leucine incorporation by kidney ribosomes isolated from rats of various ages. No age-related change in the fidelity of poly(uridylic acid) translation by kidney ribosomes was observed.