Polyamine effects on 14C-leucine transfer to microsomal protein in a rat liver cell free system during ageing

Hepatocyte fine structure during maturation and senescence

Aging is accompanied by a myriad of changes in cell structure, function, and composition. The fact that much of the information concerning age-related alterations in cellular morphology is qualitative precludes meaningful correlations with biochemical changes in order to enhance data interpretation. The mammalian liver has been subjected to both qualitative and quantitative evaluations of hepatocyte structure as a function of aging, i.e., development, maturation, and senescence. Although these data are characterized by considerable variability and, in some instances, blatant contradictions, there exists sufficient agreement in several parameters to permit a consensus in the inbred rat model. Certainly the volume of individual hepatocytes increases with age, and many of the organelle compartments reflect this change. While old rats exhibit a high incidence of polyploidy, there is no definitive evidence to demonstrate a concomitant increase in the binuclear hepatocyte index. Several specific hepatocellular organelles undergo changes in their relative volume or surface area that appear to correlate with functional alterations. The volume density of the lysosomal compartment enlarges significantly during senescence and is accompanied by increased activities of several constituent hydrolases. The hepatic concentration of smooth-surfaced endoplasmic reticulum declines markedly with aging, as does the yield of liver microsomes and the activities of several microsomal enzymes, e.g., mono-oxygenases and glucose-6-phosphatase. However, the responses of the majority of hepatocyte organelles to aging is varied and inconsistent based on the limited data currently available.

Polyamines in mammalian ageing: an oncological problem, too? A review

This review surveys the literature about changes in polyamine contents and levels of activity of the enzymes involved in the polyamine biosynthetic pathway in organs of ageing mammals. The literature about changes in the polyamine levels in physiological fluids in healthy ageing humans is also reviewed. Generally speaking, decreases in the levels of the main polyamines (noticeably putrescine and spermidine) are observed in different mammalian organs with ageing. The polyamine levels in serum and in urine of healthy human beings are also age-related, declining progressively with increasing age. Some major enzymes (i.e., ornithine decarboxylase (EC 4.1.1.17) and S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50) involved in the polyamine biosynthetic pathway show similar trends. Hormonal induction of ornithine decarboxylase activity is strongly reduced in organs of aged animals, as it is in neoplastic organs. There is also some evidence for an age-related decrease in the level of ornithine decarboxylase and its inducibility in mammalian cells cultured in vitro. Some in vitro effects of spermidine and spermine on aged structures or systems are briefly summarized. There is no evidence yet that this generally reduced capacity of mammalian aged organs for polyamine biosynthesis is one of the factors responsible for the well known high incidence of some neoplasias in elderly humans. In view of the typical stimulatory effects of the tumour promoters on polyamine biosynthesis in target tissues and the effects of senescence on the same metabolic pathway, it can be excluded that the ageing process has a tumour promoting effect by itself. However, although the exact mechanism responsible for the increased occurrence of some tumors during mammalian senescence is still obscure, there are enough experimental data (both in humans and in animals) to indicate that the reduced polyamine biosynthetic capacity of aged mammals can account for the slower course of some tumors in elderly patients.

Age-related changes in different steps of protein synthesis of liver and kidney of rats

Protein synthesis in cell-free systems of rat liver and kidney decreases markedly with age. Examination of activity changes of the different steps revealed for both types of organs that reduced binding of aminoacyl-tRNA to ribosomes and reduced peptidyl transfer might be of major importance for the decrease in overall protein synthesis whereas ageing has only little effect on translocation as well as on initiation and termination.

The complement of cytoplasmic tRNAs, including queuosine-containing tRNAs, in adult and senescent Wistar rat liver and their levels of aminoacylation

Our previous studies showed that both total cytoplasmic tRNAs and aminoacyl-tRNA synthetases isolated from senescent (24-30 month) female Wistar rat liver were less capable of supporting cell-free protein synthesis than were the same fractions isolated from adult (10-13 month) rat liver. The present study investigates the molecular basis for this age-related result. No significant age-related differences were found in the extent of aminoacylation of the liver cytoplasmic tRNA population, the total tRNA synthetase activity, the rate of aminoacylation of individual tRNAs, or in the overall complement of tRNA species as detected by two-dimensional gel electrophoresis. In homologous senescent aminoacylation assays, consisting of tRNAs and tRNA synthetases from senescent animals, alanine, arginine and aspartic acid were charged to a greater extent and methionine to a lesser extent compared to homologous adult assays. In heterologous assays, adult synthetases were significantly more active than senescent synthetases when charging isoleucine, methionine, phenylalanine, proline and glutamic acid, and less active when charging alanine, aspartic acid and serine. Also, senescent synthetases charged both adult and senescent tRNAs with methionine to a lesser extent than did adult synthetases. In homologous senescent assays with queuosine-containing tRNAs, asparagine, aspartic acid and histidine were charged to a greater extent and tyrosine to a lesser extent compared to homologous adult assays. Results with queuosine-tRNAs are discussed in terms of their potential ability to lower the efficiency of translation in senescent liver.

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.

A non-steroidal gametic factor linked to DNA modulates delta-aminolevulinic acid synthase inducibility acting on liver transcriptional and translational processes

The effect of an acidic factor of low molecular weight (about 1,000 daltons), extracted from bovine spermatozoan DNA, on the inducibility of delta-aminolevulinic acid synthase by ethanol during aging in rat has been examined. The increased enzyme inducibility in 600-day old rats is supported by stimulation of transcriptional and translational processes; on the contrary, in 30-day old rats, the higher enzyme values induced by ethanol are significantly decreased after factor treatment. The active factor is strongly DNA-bound in the native spermatozoan DNA. This would imply a possible role in regulating gene expression in vivo.

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.

Age-related changes in the cellular level of amylase and protein synthesis in the rat parotid gland

Age-related changes in the cellular content of secretory proteins and protein synthesis were studied in parotid glands of rats of various ages. The secretory protein content (determined by measuring the level of alpha-amylase activity) and the synthesis of proteins (assayed by the rate of incorporation of 3H-leucine into acid-insoluble proteins) decline with increasing age. Morphological and radioautographic studies of the gland indicate that the decline in protein synthesis is due to the reduction in the ability of secretory cells to synthesize proteins.

A comparison of protein synthesis by liver parenchymal cells isolated from Fischer F344 rats of various ages

Rates of protein synthesis by intact liver parenchymal cells isolated from male Fischer F344 rats ranging in age from 2.5 to 30 months were determined by measuring the incorporation of [3H] valine into acid-insoluble material and the specific activity of the extracellular valine. The rate of protein synthesis decreased 44% from 2.5 to 18 months and then increased slightly (18%) from 18 to 30 months. There was no dramatic change in the types of proteins synthesized by isolated liver parenchymal cells isolated from 2- or 18-month-old rats as determined by SDS-polyacrylamide gel electrophoresis. The ribosomal-transit time by liver parenchymal cells isolated from 18-month-old rats was 60% higher than the ribosomal-transit time of liver parenchymal cells isolated from 4-month-old rats. The fidelity of protein synthesis by parenchymal cells isolated from 4- and 18-month old rats was compared by measuring the incorporation of p-fluorophenyl alanine (an analogue of phenylalanine) into acid-insoluble material. Although protein synthesis decreased significantly from 4 to 18 months, the fidelity of protein synthesis remained constant.