Protein synthesis rates in rat muscle and skin based on Lysyl-tRNA radioactivity

Protein turnover via autophagy: implications for metabolism

Autophagy is a process of cellular "self-eating" in which portions of cytoplasm are sequestered within double-membrane cytosolic vesicles termed autophagosomes. The autophagosome cargo is delivered to the lysosome, broken down, and the resulting amino acids recycled after release back into the cytosol. Autophagy occurs in all eukaryotes and can be up-regulated in response to various nutrient limitations. Under these conditions, autophagy may become essential for viability. In addition, autophagy plays a role in certain diseases, acting to prevent some types of neurodegeneration and cancer, and in the elimination of invading pathogens. We review the current information on the mechanism of autophagy, with a focus on its role in protein metabolism and intracellular homeostasis.

The pleiotropic role of autophagy: from protein metabolism to bactericide

Autophagy is in principle a nonselective, bulk degradation system within cells, with a contribution to intracellular protein degradation estimated to be as large as that of the ubiquitin--proteasome system. The primary roles of autophagy are baseline turnover of intracellular proteins and organelles, production of amino acids in nutrient emergency, and regression of retired tissues. These functions guarantee rejuvenation and adaptation to adverse conditions, and even underlie dynamic processes such as development/metamorphosis. In addition, several other roles for autophagy have recently been discovered, such as presentation of endogenous antigens and degradation of invasive bacteria. This review will discuss the biological significance of autophagy from yeast to higher eukaryotes.

A model of whole-body protein turnover based on leucine kinetics in rodents

The measurement of fractional synthesis rate is based on the following assumptions: amino acids for protein synthesis are supplied by an intracellular pool; amino acids from protein degradation are not recycled preferentially to protein synthesis; and proteins turn over at a homogeneous rate. To test these assumptions, a mechanistic, theoretical model of protein turnover for a nongrowing 26-g mouse was developed on the basis of data from the literature. The model consisted of three protein pools turning over at fast (102 micromol Leu, t1/2= 11.5 h), medium (212 micromol Leu, t1/2 = 16.6 h) or slow (536 micromol Leu, t1/2 = 71.5 h) rates and extracellular (1.69 micromol Leu), leucyl-tRNA (0.0226 micromol Leu) and intracellular (5.72 micromol Leu) amino acid pools that exchanged amino acids. The flow of amino acids from the protein pools to the leucyl-tRNA pool determined the amount of recycling. The flow of amino acids from the extracellular pool to aminoacyl tRNA determined the amount of channeling. Two flooding dose data sets were used to evaluate specific radioactivity changes predicted by the model. Predictions of specific radioactivities using flooding dose, pulse dose or continuous infusion methods indicated that the model can be a useful tool in estimating the rates of channeling and recycling. However, it was found that use of data from flooding dose experiments might cause inaccurate predictions of certain fluxes.

Measurement of human tissue protein synthesis: an optimal approach

This paper reviews the evidence for and against the adoption of methods for the measurement of human tissue protein synthesis based upon the incorporation of stable isotopically labeled amino acids administered either as a continuous infusion or as a flooding dose. The practical advantages of the flooding dose method are the relative ease of application of the tracer and the ability to make a repeat measurement within approximately 2 h. For the method depending upon continuous infusion of labeled amino acid, the advantages include the use of labeled amino acids at true tracer doses (i.e., with no disturbance of metabolism) and the ability to make simultaneous measurements of whole body turnover and limb or organ turnover (given appropriate sampling techniques). The crucial question concerning the accuracy of the two methods (e.g., the 2-fold difference in the rate of skeletal muscle protein synthesis) remains unresolved, but in our opinion more evidence exists in favor of the values obtained from the continuous infusion method. Furthermore, as techniques for measurement of stable isotopically labelled amino acids improve, the length of time necessary for tracer infusion will fall, and the practical advantages of the flooding dose protocol will lessen in comparison.

Stimulation of protein, RNA, and nucleotide synthesis in lymphocytes after abdominal surgery is not affected by postoperative amino acid supply

The effects of abdominal surgery on protein, RNA, and de novo purine nucleotide synthesis in lymphocytes, and modification of these changes by postoperative amino acid supply, were investigated in 24 patients undergoing cholecystectomy (n = 12) or removal of gastric cancer (n = 12). Mono-nuclear cells were isolated from the peripheral venous blood and incubated with radioactive tracers in vitro. Protein and RNA synthesis, as measured using [14C] glycine and [3H]uridine, respectively, increased postoperatively. Nucleotide synthesis determined by the incorporation of radioactivity from [14C] glycine into nucleotides increased simultaneously. The concentration of 5-phosphoribosyl 1-pyrophosphate (PRPP) estimated by the incorporation of [14C]adenine into nucleotides also increased. These changes were greater and of longer duration in patients with cancer operation than in those with cholecystectomy. In neither case were they affected by the amount of amino acid intake, or increases in energy intake. These results suggest that abdominal surgery stimulates protein and ribonucleic acid (RNA) synthesis in lymphocytes. Increased RNA synthesis may be ensured by increased synthesis of nucleotides, and increased PRPP concentrations appear to regulate the rate of nucleotide synthesis. The responses are apparently dependent upon the severity of surgery, but unrelated to the amount of amino acid supplied postoperatively.

Isolation of aminoacyl-tRNA and its labeling with stable-isotope tracers: Use in studies of human tissue protein synthesis

We isolated aminoacyl-tRNA (60-70% yield) from human and rat tissues and measured, by GC/MS, its labeling in vivo by [15N]- and [13C]leucine. Tracer dilution artifacts seemed unlikely since, after infusion of L-[1-13C,15N]leucine into rats, (i) muscle leucyl-tRNA labeling exceeded tissue free leucine labeling, (ii) values were largely unaffected by storing over 5 min at 22 degrees C, and (iii) L-[2,4,5-methyl-13C]leucine was not incorporated into leucyl-tRNA during homogenization. Leucyl-tRNA labeling in liver and muscle suggested charging from extra- and intracellular pools: e.g., after infusing L-[1-13C,15N]leucine, rat muscle tissue free leucine 13C labeling (8.97 +/- 0.30 atom % excess) exceeded that by 15N (3.37 +/- 0.33 atom % excess), and both were significantly lower (P less than 0.02) than venous plasma (13C, 12.1 +/- 1.8; 15N, 5.54 +/- 0.6 atom % excess) indicating tracer dilution by transamination and by proteolysis; however, leucyl-tRNA labeling by either isotope (13C, 10.26 +/- 0.50; 15N, 4.72 +/- 0.72 atom % excess) was significantly above mixed tissue free leucine (P less than 0.05). Labeling of leucyl-tRNA in human erector spinae muscle (obtained after preoperative L-[1-13C]leucine infusion) was, at 4.98 +/- 0.43 atom % excess, lower (27%) than venous plasma leucine (P less than 0.05) and intermediate between muscle free leucine (9% lower; P less than 0.01) and venous alpha-ketoisocaproate (11% higher; P less than 0.02). Human placental leucyl-tRNA labeling (after predelivery tracer infusion) was 37% lower (P less than 0.05) than maternal uterine vein labeling but not significantly different from placental free leucine or umbilical arterial leucine.

In vivo cerebral protein synthesis rates with leucyl-transfer RNA used as a precursor pool: determination of biochemical parameters to structure tracer kinetic models for positron emission tomography

Leucine oxidation and incorporation into proteins were examined in the in vivo rat brain to determine rates and compartmentation of these processes for the purpose of structuring mathematical compartmental models for the noninvasive estimation of in vivo human cerebral protein synthesis rates (CPSR) using positron emission tomography (PET). Leucine specific activity (SA) in arterial plasma and intracellular free amino acids, leucyl-tRNA, alpha-ketoisocaproic acid (KIC), and protein were determined in whole brain of the adult rat during the first 35 min after intravenous bolus injection of L-[1-14C]leucine. Incorporation of leucine into proteins accounted for 90% of total brain radioactivity at 35 min. The lack of [14C]KIC buildup indicates that leucine oxidation in brain is transaminase limited. Characteristic specific activities were maximal between 0 to 2 min after bolus injection with subsequent decline following the pattern: plasma leucine greater than or equal to leucyl-tRNA approximately KIC greater than intracellular leucine. The time integral of leucine SA in plasma was about four times that of tissue leucine and twice those of leucyl-tRNA and KIC, indicating the existence of free leucine, leucyl-tRNA, and KIC tissue compartments, communicating directly with plasma, and separate secondary free leucine, leucyl-tRNA, and KIC tissue compartments originating in unlabeled leucine from proteolysis. Therefore, a relatively simple model configuration based on the key assumptions that (a) protein incorporation and catabolism proceed from a precursor pool communicating with the plasma space, and (b) leucine catabolism is transaminase limited is justified for the in vivo assessment of CPSR from exogenous leucine sources using PET in humans.

Stimulation of de novo biosynthesis of purine and pyrimidine nucleotides in the liver of rats following burn injury

Third-degree burn injury covering 25% of the body surface was imposed on rats. The de novo biosynthesis of purine and pyrimidine nucleotides in the liver of these rats was measured by the incorporation of labeled glycine and bicarbonate into the respective bases. They were increased one day after injury (day 2) and returned to the control values three days after injury (day 4). As expected, the metabolic flow through 5-phosphoribosyl l-pyrophosphate (PPRibP), estimated using [14C]ribose as a tracer, varied in a similar manner. The activities of glutamine phosphoribosylpyrophosphate amidotransferase and carbamoyl-phosphate synthetase II on day 2 did not change significantly. The nucleotide concentrations, effectors of the enzymes, also did not change significantly. The concentrations of PPRibP on days 2 and 4 were 85% higher (P less than .02) and similar to that of controls, respectively, and the elevated concentration was a major factor responsible for the increased nucleotide biosynthesis. Increased synthesis caused the elevation of PPRibP concentrations. There were, however, no significant changes in the factors so far known to regulate PPRibP synthesis.

Methods for studying protein synthesis and degradation in liver and skeletal muscle

Different methods used for measuring protein turnover in liver and skeletal muscle are described, with special emphasis on technical and practical aspects and the advantages and limitations of different techniques. In the first part of the review, the concept of precursor specific radioactivity and its importance for accurate determination of protein synthesis rate is discussed. In the second part, different in vivo techniques for protein turnover measurements are reviewed, including continuous administration of tracer amino acid, flooding dose technique, indirect measurement of protein synthesis, and estimation of protein degradation in vivo. In the third part of the report, in vitro techniques are described, including measurement of protein turnover in incubated liver slices, perfused liver, isolated hepatocytes, incubated isolated muscles or muscle biopsies, and perfused rat hemicorpus. In vivo techniques are preferred when accurate absolute values of protein turnover rates are desired. In vitro techniques offer the advantage of standardized conditions, maintaining strict control of substrate and hormone concentrations, and eliminating complicating interactions with other tissues. For several in vitro techniques, a good correlation has been demonstrated between relative changes in protein turnover in vitro and in vivo in different conditions.

Protein synthesis and degradation in biopsies of rat skeletal muscle

The use of 20- to 40-mg biopsies of rat skeletal muscle to measure protein synthesis and degradation rates in vitro was investigated and compared to that of the intact extensor digitorum longus (EDL) and soleus muscles. During incubations in oxygenated Krebs-Ringer bicarbonate buffer with glucose, insulin, 23 amino acids at 10 times rat plasma levels, and [14C]tyrosine, the specific activity of intracellular tyrosine approximated that of the incubation medium and was constant in the biopsy, the EDL, and the soleus. The rate of incorporation of tyrosine into the protein of the biopsy was constant for 3 hr and was 39 and 32% of the rates of the EDL and soleus, respectively. The rate of release of tyrosine from protein in the biopsy during incubations in buffer with glucose and cycloheximide was constant for 3 hr and was intermediate between the rates of the EDL and soleus. The effects of starvation on the in vitro protein metabolism of the biopsy were the same as on the intact muscles. The 42% decrease in synthesis and the 53% increase in degradation in the biopsy were intermediate between the changes measured in the EDL and soleus muscles. The ability of this technique to identify proportional changes in the in vitro protein synthesis and degradation rates makes this a valid technique suitable for the measurement of changes of in vitro protein metabolism using serial biopsies from larger animals, including man.

The effect of acute and chronic glucocorticoid excess on leucine kinetics and protein turnover in vivo

The present studies were undertaken to assess the effects of excess cortisol on amino acid exchange in the conscious dog. Three groups of 18-hr fasted dogs with catheters chronically implanted in the femoral artery were studied: Group I (n = 6) received saline; Groups II and III (n = 5, each) received ACTH intravenously (1 U/min) for 7 hr; in addition, Group III received ACTH, 500 U/day intramuscularly for 4 days. Leucine rates of appearance (Ra) and clearance were measured using a constant infusion of L-4,5-[3H]leucine. ACTH treatment resulted in a 9-fold increase in plasma cortisol in Groups II and III (from 2 +/- 1 to 18 +/- 1 and 17 +/- 2 micrograms/dl, in II and III, respectively P less than 0.001), with no effect on either plasma insulin or glucagon. Plasma leucine (mmole/liter) increased from 118 +/- 6 (I) to 153 +/- 6 (II, P less than 0.005) to 275 +/- 35 (III, P less than 0.001). Leucine Ra (micromoles/kilogram/minute) did not change in II, but rose by 39% (P less than 0.005) in III. Clearance (milliliters/kilogram/minute) dropped from 25 +/- 2 (I) to 18 +/- 2 (II, P less than 0.005), to 15 +/- 2 (III, P less than 0.001). It is concluded that acute elevations of cortisol increased plasma leucine only by inhibiting its rate of disposal, whereas chronic elevations had a dual effect; they inhibited leucine disposal and increased its entry into the plasma compartment, suggesting an inhibition of protein synthesis and stimulation of protein breakdown.