Tissue Protein Turnover Research Articles

Tyrosine oxidation and protein turnover in material tissues and the fetus during pregnancy in rats

Protein metabolism was examined in rats during the anabolic and subsequent catabolic phase of pregnancy. Protein synthesis in maternal muscle and liver, whole body amino acid flux and [ 14C]tyrosine oxidation was measured twice during each phase. Protein fractional synthesis and breakdown rates in the fetus and placenta were determined. 1. Muscle protein synthetic rate increased during the anabolic phase and fell during the catabolic phase. Muscle protein increased during the anabolic phase. 2. Liver protein increased by the end of pregnancy with no change in synthetic rates indicating reduced rates of breakdown. 3. Amino acid flux increased during the catabolic phase of pregnancy indicating increases in whole body protein synthesis. The increases were of sufficient magnitude to permit the deposition of protein occurring in fetuses and placentas during this period. 4. 14CO 2 production from infused [ 14C]tyrosine remained unchanged during the anabolic phase of pregnancy but fell during the catabolic phase due mainly to dilution of label by increased amino acid flux. 5. Maternal protein anabolism is mediated by increased muscle protein synthesis rather than by decreased amino acid catabolism. 6. A 61% fall in protein breakdown rate assists fetal protein deposition during the last 4 days of pregnancy.

Studies on the incorporation of radioactive amino acids into tissue and brush border membrane proteins of mouse jejunum in organ culture

1. 1. The incorporation of radioactive amino acids into brush border membrane proteins of mouse jejunal mucosa has been investigated in organ culture and compared to similar data obtained in intestinal culture of other species. 2. 2. Amino acid precurosor pools for protein synthesis remained almost unchanged during 24 hr culture. 3. 3. By pulse experiments, it has been possible to show a constant rate of protein synthesis up to 24 hr culture. 4. 4. Continuous labelling studies have shown a non-linear incorporation of amino acids into tissue proteins, brush border membrane proteins and total proteins (explants + media) synthesized during the culture. A turnover of tissue proteins has been assumed from the release of labelled proteins into the culture media. The analysis of radioelectrophoregrams of brush border membrane proteins and proteins from the particulate fraction of media showed that the same brush border membrane proteins were synthesized in the tissue and released in the media all along the culture. 5. 6. However, some quantitative differences have been found in the relative labelling between protein bands, suggesting that some proteins would be synthesized or released at different rates depending upon the duration of the culture.

Physiological properties of the innervated and denervated neuromuscular junction of hibernating and nonhibernating ground squirrels

The physiological status of the neuromuscular junction of hibernating and nonhibernating 13-lined ground squirrels was studied to determine whether or not the metabolic changes during hibernation would alter the muscle's response to denervation. It was anticipated that the observations might clarify some aspects of the trophic interrelationship between nerve and muscle. The properties of innervated muscles were not significantly altered after the animals entered hibernation. The strength of contraction, speed of contraction, and resting membrane potential remained unchanged. In addition, extrajunctional sensitivity to acetylcholine did not develop. Because the muscles are inactive during hibernation, we conclude that muscle activity alone does not maintain the physiological properties of muscles. Denervation of muscles from nonhibernating animals resulted in loss of neuromuscular transmission, cessation of miniature end-plate potentials, partial muscle membrane depolarization, and appearance of extrajunctional sensitivity to acetylcholine. In contrast, muscles whose nerves were transected during the hibernating state showed unimpaired neuromuscular transmission and normal miniature end-plate potentials. However, the muscle became partially depolarized, indicating that the regulation of the resting membrane potential is under neurotrophic control and is not influenced solely by the release of acetylcholine (which had remained unchanged). The denervated muscles of hibernating animals did not develop extrajunctional sensitivity to acetylcholine; this probably reflects the low rate of protein turnover in tissues maintained at the low (7°C) body temperature of hibernating animals. Transection of the sciatic nerve of hibernating animals produced histologically demonstrable retrograde changes in the motor neurons of the lumbar spinal cord. It thus appears that hibernation does not adversely affect certain fundamental functions of the nervous system, such as transmission of nerve impulses, anterograde transmission of neurotrophic influences, and the retrograde transmission of signals which initiate the cell body's reaction to injury.

Tissue enrichments and protein turnover measured with 15N-glycine.

VALUES for total body protein turnover (TBPT) measured in man using 15N-glycine1–3 are comparable with those obtained using 14C-amino acids4. Because 14C is radioactive it is of limited clinical use. The stable isotope 15N has a potentially wider application, although its use has been restricted for technical and conceptual reasons4. When a tracer labelled with 15N is used to measure TBPT the results are based on the enrichment of urea. This assumes that both urea and newly synthesised protein are derived from the same precursor N pool. Because urea is synthesised in the liver the enrichment of the precursor N pool for protein synthesis in the liver may determine the enrichment of urea. Thus the measurement of TBPT using 15N-glycine may be weighted heavily by the rate of hepatic protein turnover4,5. Similar problems may arise when urinary ammonia is the end product (M.H.N.G. and J. C. Waterlow, to be published). The extent of labelling of the precursor N pool is affected by at least two factors. First, after infusion of a 14C-amino acid, its specific radioactivity at plateau is lower in tissues than in plasma6–9, due to dilution with unlabelled amino acid derived from intracellular protein breakdown. This is termed internal recycling and also applies when 15N-glycine is infused. Second, when 15N-glycine is infused, extensive rapid intracellular transamination occurs especially in liver and kidney, so that the 15N is distributed to other amino acids, which will have a higher enrichment within the cell than in the plasma. Thus, depending on the balance between internal recycling and transamination, the intracellular enrichment of amino acids may be greater or less than the plasma enrichment. To investigate this problem we have infused 15N-glycine, and measured the enrichment of α-amino-N urea-N and ammonia-N in various tissues. The TBPT calculated from the liver urea-N enrichment gives a value which we consider representative and which agrees with published values based on 14C.

Effects of intermittent hyperbaric oxygen on guinea pig lung elastin and collagen

The effect of high oxygen pressure on collagen and elastin turnover in lung parenchyma tissue was studied in guinea pigs. Three groups of animals was used. One group (I) was given a clinically excessive exposure of 3 h daily for 9 days of 3 ATA oxygen. A second group (II) was given 3 ATA compressed air on the same schedule, and a third (III), a control, was given the same handling conditions only. Collagen and elastin which amounted to 10.45 and 4.62 mg, respectively, per 100 mg dry defatted parenchymal tissue was labeled with [14C]proline injected ip. Connective tissue protein turnover was estimated by labeling half of each group before exposure (a); effects on biosynthesis were determined by labeling the other half postexposure (b). No differences in hydroxyproline specific activity of collagen fractions or elastin were found with respect to either biosynthesis or turnover. Alveolar wall length-tension measurements were not changed. Since exposures were in excess of those used for treatment of gas gangrene, ischemic burns or wound healing, these results suggest that the metabolism of lung connective tissue is unaffected by the short-term exposure to hyperbaric oxygen.

Protein-energy interrelationships during dietary restriction: effects on tissue nitrogen and protein turnover.

Young adult female rats were fed diets containing either 50 or 0% lactalbumin at levels of 2, 4 or 6 g diet/day for 2, 8, and 16 days. There was no other protein in the diet. Tissue nitrogen and loss of radioactivity from tissues labeled with 14C- and 3H-glutamate were measured. In a second study, similar rats were fed graded levels of lactalbumin at food intake levels of 3, 5, or 7 g/day. Change in tissue nitrogen varied with the tissue, the time of observation, and the severity of the food restriction. After 8 days, animals fed high levels of protein at the most severe food restriction showed increases in gastrocnemius nitrogen and losses in liver nitrogen, while after 16 days both tissues had marked nitrogen losses. Nitrogen losses at the lowest level of food intake increased with the dietary protein level, whereas dietary protein was protective of tissue nitrogen at higher food intakes. Severity of energy restriction had no effect on loss of tissue radioactivity and the apparently longer tissue protein half-lives from animals fed protein-free diets are attributed to increased amino acid recycling. Such results indicate that short-term studies and overall nitrogen balance experiments will fail to identify changes occurring in different tissues and may yield misleading results.

The Effect of Age on Tissue Protein Synthesis

Currently available data on the effect of age in tissue protein synthesis and turnover are reviewed and experimental evidence is presented on age changes in amino acid incorporation by rat liver purified microsomes. It has been observed that the purification of ‘crude’ microsomal preparations from contaminating lysosomes by density gradient centrifugation is accompanied by an increase in amino acid incorporatmg activity of the microsomal preparations, no matter whether these are prepared from young or from old animals, and that the amino acid incorporating activity of rat liver purified microsomes prepared from old animals (19–25 months) is lower than that prepared from adult animals (7–11 months) which in turn is lower than that from young animals (2–3 months), no matter whether the activity is expressed per mg of microsomal protein or per mg of microsomal RNA. Also, experimental evidence is presented which is consistent with the presence in liver purified microsomes of heat-labile inhibitor(s) of amino acid incorporation, and with their accumulation (or increase in effect) in the microsomes with the age of the animal. Several major features of the decline with age in tissue protein synthesis are outlined and a number of possible factors or causes, beside the microsomal inhibitor(s), contributing to the decline with age in tissue protein synthesis, are discussed.