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Levels of transfer RNA (tRNA) were determined in liver, kidney, skeletal muscle, heart, and brain of young (35-day), adult (12-month) and old (24-month) female C57BL/6J mice. Kidney and liver showed little change in tRNA levels between young and adult mice, but the levels decreased in old mice. Skeletal muscle tRNA decreased steadily from young to old mice. Heart tRNA increased during maturation to adult organisms and then decreased in old individuals. Brain levels of tRNA increased steadily. No age-related change in the rate of transport of orotic acid into cells was observed. However, all tissues exhibited a decrease in uridine pools between adult and old mice. Most importantly, all tissues of aging mice showed a decrease in the rate of tRNA synthesis.

The effects of age on the initiation and elongation stages of protein synthesis were measured in cell-free preparations from brain, liver, kidney and skeletal muscle of young (3-5 months) and senescent (23-27 months) female C57B1/6J mice. The ability to form initiation complexes from isolated 40 S and 60 S ribosomal subunits decreased only slightly with age. In contrast, the rate of peptide chain elongation decreased by 67% in brain preparations, 80% in liver, 81% in kidney and 85% in skeletal muscle of the senescent mice when compared with the young mice.

The peptide chain elongation stage of protein synthesis in Drosophila melanogaster was found to decrease markedly with age. The decrease paralleled the age-related decrease in overall protein synthesis. In contrast, the termination stage showed little decrease until the organisms were very old. Of the three reactions that comprise peptide chain elongation, the binding of aminoacyl-tRNA to ribosomes decreased greatly with age, and the decrease paralleled that of peptide chain elongation and of overall protein synthesis. The peptidyl transfer reaction decreased moderately, and the translocation reaction exhibited no measurable decrease with age. Thus, decreased binding of aminoacyl-tRNA to ribosomes appeared to be a major contributor to the age-related decreases in peptide chain elongation and overall protein synthesis.

The molecular basis was sought for the previously observed drop in the synthesis of elongation factor one (EF-1) and subsequent decline in overall protein synthesis in aging Drosophila melanogaster . It has been found that translatable poly(A+)RNA for EF-1 disappears at about the same time that EF-1 synthesis decreases. This disappearance is specific for EF-1, since overall poly(A+)RNA levels and their translation to cellular proteins remain constant over the life-span of the organism. The disappearance of translatable RNA is not the result of a specific loss of the polyadenylate segment of poly(A+)RNA, since poly(A−1)RNA exhibits a similar specific loss of translation ability for EF-1.

The rates of accumulation of fluorescent pigments were compared in wild-type Drosophila melanogaster, and in the mutant strains, vestigial, which exhibits a greatly decreased life span, and ebony, which has only a slightly decreased life span. There was no correlation between life span, and rate of fluorescent pigment accumulation. In all three strains, the accumulation rates were essentially the same, indicating that lipid peroxidation wasn ot the primary process in reducing the life spans of the mutants.

Total proteolytic activity and protein synthesis were measured in cell-free extracts of the thymus during its involution in 1–30-week-old C57B1/6J mice, and in 10-, 12-, 14-, and 24-month-old adult mice of the same strain. The mean specific activity of the proteolytic enzymes exhibited no significant change during involution or throughout the mean life-span of the animals. In contrast, the rate of protein synthesis decreased rapidly during involution, and continued to decrease at a slower rate during aging of the mice.

Protein synthetic activity was determined in postmitochondrial preparations from heart, brain, kidney, liver and skeletal muscle of 5-26-month-old female C57B1/6J mice. An age-dependent decrease in the rate of protein synthesis was exhibited by all preparations except heart muscle. A 65% decrease in translational rate was found in liver, with the greatest decrease appearing after 21 months. Translation in the brain preparation declined little during the first 20 months, but dropped 33% between 20 and 26 months. The kidney preparation decreased 30% during the first 16 months and 70% by the end of 26 months of age. Skeletal muscle showed an overall decrease of 85% in translation rate. In contrast, heart muscle decreased no more than 10% over the life-span of the mice. From these results, it appears that aging has a differential effect on protein synthesis in different kinds of cells.

Agarose gel electrophoresis showed that no age-related DNA fragmentation occured in wild-type or vestigial wing Drosophila , or in DNA from mouse brain and heart. DNA from mouse liver possessed a large fragment, estimated at 24 megadaltons, in all ages tested. Phaseolus cotyledon DNA possessed a fragment of approximately 3.5 megadaltons in both imbided seeds and 12-day-old cotyledons. Thus, there appeared to be no age-related increase in the break-down of DNA into large fragments.

Aminoacylation of tRNA was measured in extracts of Drosphila melanogaster of increasing age. During the period when the rate of protein synthesis decreased by 70%, the rate of aminoacylation by a mixture of amino acids decreased no more than 20%. The ability of tRNA to accept amino acids decreased 36%. Aminoacylation of tRNA by each of the 20 amino acids showed age-related decreases in rate of up to 38% and decreases in extent of aminoacylation ranging up to 54%. Aminoacylation enzymes did not exhibit an age-related decrease in activity. However, both the levels of tRNA in the cytosol and the ability of the tRNA to accept amino acids decreased with age, and probably account for the observed decreases in aminoacylation.

Aging of Drosophila melanogaster resulted in a steady increase in the rate of mitochondrial oxidation of pyruvate-malate. At advanced ages, the rate of phosphate incorporation into ATP decreased markedly. This decrease in ATP synthesis was paralleled by a decrease in the activity of mitochondrial ATPase, a probable component of the phosphorylating system. Aging cells steadily accumulated an inhibitor which strongly depressed ATP synthesis when the cytosol of aged cells was incubated with the mitochondria of young cells.

The decrease in the rate of protein synthesis in aging adult Drosophila melanogaster was found previously to be due, to a great extent, to a drop in the rate of peptide chain elongation, and principally to lowered activity of elongation factor one (EF-1). This decrease does not appear to be caused by appearance of an inhibitor of peptide chain elongation. Instead, the synthesis of EF-1 declines markedly early in adult life. This decrease is followed by lowered activity of EF-1 and by a drop in the synthesis of most of the cellular proteins.

The rate of protein synthesis was measured in isolated mitochondria from Drosophila melanogaster and from the livers and kidneys of C57BL/6J mice of increasing ages. Over the life-span of the organisms, the synthesis of mitochondrial proteins decreased to a level which was less than half the original rate. Concomitant with this decrease, the amount of mitochondria which could be isolated from the organisms declined by about 30%. Thus, the separate translational system of mitochondria exhibited an age-related decrease in activity which was in addition to the decrease already observed in the cytoplasmic ribosomal system.

A microsomal preparation from larvae of Drosophila melanogaster readily incorporated a 14C-amino acid mixture into its protein. Amino acid incorporation by microsomes from adult organisms, however, declined markedly with age. The decline in amino acid incorporation did not appear to be due to decreases in ribosomes or messenger RNA, since ribosome levels decreased only 6%, while incorporation decreased more than 70%, and only a 23% decrease in polysome-bound RNA could be detected. However, the ppolyuridylate-promoted incorporation of 14C-phenylalanine into polypeptide exhibited a sharp, age-related decline, indicating that some component of the translation system, other than messenger RNA, is responsible for the observed decline in protein synthesis

The age-related decrease in protein synthesis has been determined in microsomal preparations from head, thorax, and abdomen of Drosophila melanogaster. During 35 days of adult life, protein synthesis by microsomal preparations from whole organisms decreased approximately 75%. However, microsomal preparations from head, thorax, and abdomen exhibited decreases in protein synthesis of 15%, 96%, and 33% respectively, indicating a differential age-related shut-down of protein synthesis in different cells of this organism.