Sampling of the leucine pool from the growing peptide chain: difference in leucine specific activity of peptidyl-transfer RNA from free and membrane-bound polysomes

Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Defining the physiological mechanisms that set metabolic rates and the`cost of living' is important for understanding the energy costs of development. Embryos and larvae of the sea urchin Lytechinus pictus(Verrill) were used to test hypotheses regarding differential costs of protein synthesis in animals differing in size, rates of protein synthesis, and physiological feeding states. For embryos, the rate of protein synthesis was 0.22±0.014 ng protein embryo-1 h-1 (mean ±s.e.m.) and decreased in unfed larvae to an average rate of 0.05±0.001 ng protein larva-1 h-1. Fed larvae had rates of synthesis that were up to 194 times faster than unfed larvae (9.7±0.81 ng protein larva-1 h-1). There was no significant difference, however, in the cost of protein synthesis between these larvae with very different physiological states. Furthermore, the cost of synthesis in the larval stages was also similar to costs measured for blastula and gastrula embryos of 8.4±0.99 J mg-1 protein synthesized. The cost of protein synthesis was obtained using both direct (`inhibitor') and indirect (`correlative') measurements; both methods gave essentially identical results. Protein synthesis accounted for up to 54±8% of metabolic rate in embryos. Percent of metabolism accounted for by protein synthesis in larvae was dependent on their physiological feeding state, with protein synthesis accounting for 16±4% in unfed larvae and 75±11% in fed larvae. This regulation of metabolic rate was due to differential rates of synthesis for a fixed energy cost per unit mass of protein synthesized. The cost of synthesizing a unit of protein did not change with increasing rates of protein synthesis. We conclude that the cost of protein synthesis is independent of the rate of synthesis, developmental stage, size and physiological feeding state during sea urchin development.

Effect of feed intake on ovine hindlimb protein metabolism based on thirteen amino acids and arterio-venous techniques

It has been suggested that protein synthesis in peripheral tissues: (1) responds in a curvilinear manner to increasing feed intake over a wide range of feeding levels; and (2) has a greater sensitivity to intake than protein breakdown. The aim of the present experiment was to test these hypotheses across the ovine hindlimb. Six growing sheep (6-8 months, 30-35 kg), with catheters in the aorta (two), posterior vena cava and jugular vein, received each of four intakes of dried grass pellets (0.5, 1.0, 1.5 and 2.5 x maintenance energy; M) for a minimum of 7 d. A U-13C-labelled algal hydrolysate was infused intravenously for 10 h and from 3-9 h para-aminohippuric acid was infused to measure plasma flow. Arterial and venous plasma were obtained over the last 4 h and the concentrations and enrichments of thirteen (13)C-labelled amino acids (AA) were determined by GC-MS. As intake increased, a positive linear response was found for plasma flow, arterial concentrations of the aromatic and branched-chain AA, total flow of all AA into the hindquarters and net mass balance across the hindquarters (except glycine and alanine). Based on two separate statistical analyses, the data for protein synthesis showed a significant linear effect with intake (except for phenylalanine, glycine and alanine). No significant curvilinear effect was found, which tends not to support hypothesis 1. Nonetheless, protein synthesis was not significantly different between 0.5, 1.0 and 1.5 x M and thus the 2.5 x M intake level was largely responsible for the linear relationship found. There was no significant response in protein breakdown to intake, which supports hypothesis 2.

Measuring protein synthesis by mass isotopomer distribution analysis (MIDA)

The measurement of protein kinetics by isotopic techniques has been hindered by the long-standing difficulty of accurately measuring the isotope content of the biosynthetic precursor pool (aminoacyl-tRNA in tissues). Mass isotopomer distribution analysis (MIDA) is a stable isotope-mass spectrometric (MS) technique for measuring biosynthetic precursor enrichments from measurements on a polymeric product, based on combinatorial probabilities of labeled and unlabeled monomeric subunits. Proteins contain complex isotopomer patterns as a result of their relatively high molecular mass, however, so that resolution of the individual mass isotopomers in the polymeric product (an analytic requirement for MIDA) is technically difficult. An approach for measuring protein synthesis by MIDA is described and tested here: First, in vitro, using a synthetic peptide present in human serum albumin; and then, in vivo, for albumin synthetic rate in rats. A peptide contained in human serum albumin (SVVLLLR) and theoretically recoverable from trypsin/chymotrypsin proteolysis was synthesized by solid-phase peptide synthesis using known mixtures of natural abundance and [5,5, 5-2H3]leucine. Additionally, enriched and natural abundance peptides were mixed in vitro to simulate in vivo biosynthesis and to address problems of instrument accuracy, precision, and data management. The mass isotopomer patterns of the synthetic peptides were analyzed using electrospray ionization (ESI) with both magnetic sector and quadrupole mass analyzers. The resolution of the magnetic sector was superior to that of the quadrupole instrument, but accuracy and precision in the measurement of mass isotopomer abundances and kinetic parameters were comparable and both gave values close to those predicted. Next, rats were infused with [5,5,5-2H3]leucine intravenously, and a leucine-rich peptide was isolated and purified from trypsin-digested rat serum albumin (RHPDYSVSLLLR, 1456 Da) and then analyzed by ESI-MS using a magnetic sector instrument. Precursor pool enrichments and fractional synthetic rates (0.45 +/- 0.03 day-1, t1/2 = 1.53 +/- 0.09 days) were calculated. Biosynthetic rates of rat serum albumin were congruent with previously published values. In summary, measurement of protein synthesis and precursor pool enrichments by MIDA is technically feasible and practical in vivo using proteolytically derived peptides and ESI-MS analysis.

Precursor pools of protein synthesis: a stable isotope study in a swine model

The accuracy of using other free pools in lieu of tRNA for calculation of tissue protein synthesis in liver (L), skeletal muscle (SM), and heart (H) was assessed in six adult miniature swine using L-[1-13C]leucine and L-[ring-2H5]phenylalanine as tracers. L leucyl-tRNA enrichment was higher than arterial plasma leucine and ketoisocaproate (KIC) enrichments, and L phenylalanyl-tRNA enrichment was higher than arterial phenylalanine enrichment (P < 0.05). No such differences were noted in SM and H. Leucyl- and phenylalanyl-tRNA enrichments in L were best predicted by the respective amino acid enrichments in tissue fluid [TF; Leu: slope (m) = 0.954 +/- 0.035; Phe: m = 1.011 +/- 0.032] using linear regression analysis to determine the accuracy of the prediction, whereas plasma phenylalanine reasonably predicted phenylalanyl-tRNA (artery: m = 0.821 +/- 0.032; vein: m = 0.947 +/- 0.135). In SM, plasma KIC (artery: m = 0.846 +/- 0.046; vein: m = 0.881 +/- 0.043) and TF leucine (m = 0.788 +/- 0.034) predicted leucyl-tRNA with high accuracy. In H tissue, TF (m = 0.991 +/- 0.044) was the best predictor of leucyl-tRNA enrichment, whereas arterial phenylalanine (m = 0.912 +/- 0.015) was the most reliable predictor of phenylalanyl-tRNA enrichment. The relationships between aminoacyl-tRNA and other free pools in the same species under the same study conditions differ in different tissues. Use of KIC in lieu of leucyl-tRNA for calculating muscle protein synthesis is supported by this study.

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.

Age-related changes in collagen synthesis and degradation in rat tissues. Importance of degradation of newly synthesized collagen in regulating collagen production

During developmental growth, collagens are believed to be continuously deposited into an extracellular matrix which is increasingly stabilized by the formation of covalent cross-links throughout life. However, the age-related changes in rates of synthetic and degradative processes are less well understood. In the present study we measured rates of collagen synthesis in vivo using a flooding dose of unlabelled proline given with [14C]proline and determining production of hydroxy[14C]proline. Degradation of newly synthesized collagen was estimated from the amount of free hydroxy [14C]proline in tissues 30 min after injection. Collagen fractional synthesis rates ranged from about 5%/day in skeletal muscle to 20%/day in hearts of rats aged 1 month. At 15 months of age, collagen fractional synthesis rates had decreased markedly in lung and skin, but in skeletal muscle and heart, rates were unchanged. At 24 months of age, synthesis rates had decreased by at least 10-fold in all tissues, compared with rates at 1 month. The proportion of newly synthesized collagen degraded ranged from 6.4 +/- 0.4% in skin to 61.6 +/- 5.0% in heart at 1 month of age. During aging the proportion degraded increased in all tissues to maximal values at 15 months, ranging from 56 +/- 7% in skin to 96 +/- 1% in heart. These data suggest that there are marked age-related changes in rates of collagen metabolism. They also indicate that synthesis is active even in old animals, where the bulk of collagens produced are destined to be degraded.

Rates of protein synthesis--a review

The rates of protein synthesis can be measured by a variety of methods including pulse labeling, massive precursor administration, Scornik method, continuous feeding of labeled precursor, infusion, and pellet implantation. Each technique has some advantages and disadvantages. Massive precursor administration and infusion are the most widely used. The advantage of massive precursor administration is its simplicity, however, the amino acid concentration used is much higher than physiological levels. Infusion, however, is much more complicated as a technique and requires complicated calculations. The synthesis rates can also be calculated from degradation curves. Some of the above techniques can be used both in vivo and in vitro, and also for different organs (Shahbazian et al. (1987), Int. J. Dev. Neurosci., 5: 39-42). The brain has rapid rates of protein synthesis both in vivo and in vitro, the latter being much lower for adults.

Effect of selected denervations on glycoconjugate composition and tissue morphology during the initiation phase of limb regeneration in adult Ambystoma

This study was undertaken to assess the effects of various quantities of neural tissue on the temporal relationship of matrix glycoconjugates to the regeneration morphology. 1) Denervation before amputation revealed that a threshold level of nervous tissue was necessary to activate a regeneration response from the tissue, i.e., appearance of regeneration-specific morphologies and glycoconjugates. 2) Denervation after amputation demonstrated that the level of neural tissue necessary to maintain these responses was below the level necessary to activate the regeneration response. If neural tissue was completely removed there was a concomitant loss of regenerate morphologies and glycoconjugates. 3) Bilateral amputation of a neurogenically intact limb and its completely denervated contralateral limb revealed that the regeneration response was a localized phenomenon during the first 30 days after amputation. After 30 days the regeneration response appeared within the previously degenerated denervating limb. The results suggest that the factors controlling the regenerative response in adult Ambystoma are large diffusible substances that can be transported by the circulation and can affect the regenerative response in remote, previously activated, tissues.

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.

Compartmentalization of proline pools and apparent rates of collagen and non-collagen protein synthesis in arterial smooth muscle cells in culture

Rates of collagen and non-collagen protein synthesis in rabbit arterial smooth muscle cells (SMC) were determined by using the specific (radio)activity of [3H]proline in the extracellular, intracellular, and prolyl-tRNA pools. The intracellular free proline specific activity was only 25% of the extracellular value in cultures incubated for 12 h in 0.25 mM-proline. The specific activity of prolyl-tRNA was less than 10% of the extracellular specific activity. Increasing the extracellular proline concentration 10-fold (to 2.5 mM), while keeping the extracellular specific activity of proline constant, resulted in equilibration of the specific activities of intracellular and extracellular free proline, but the specific activity of prolyl-tRNA remained at less than 10% of the extracellular specific activity. Therefore, calculated rates of collagen and non-collagen protein synthesis were greatly underestimated using the intracellular or extracellular specific activity of proline. SMC were also incubated with 0.1 mM-[14C]ornithine in 0.25 nM or 2.5 mM non-labelled proline to examine synthesis de novo of proline and prolyl-tRNA from ornithine. In SMC cultures containing 0.25 mM unlabelled proline, the specific activity of intracellular ornithine was approx. 45% of the extracellular specific activity, due to the production of unlabelled ornithine. The specific activity of ornithine-derived intracellular free proline in SMC incubated with 2.5 mM-proline was significantly lower than in SMC incubated in 0.25 mM-proline, due to the influx of unlabelled proline. However, a corresponding difference in the specific activity of [14C]prolyl-tRNA between SMC in 0.25 mM- or 2.5 mM-proline was not observed. Ornithine-derived [14C]proline was incorporated into proteins in a manner different from that of exogenously added radiolabelled proline. A much higher proportion of the proline synthesized de novo was channelled into collagen synthesis relative to total protein synthesis. Together, these results show that intracellular proline pools are highly compartmentalized in arterial SMC. They also suggest that proline synthesized from ornithine may enter a prolyl-tRNA pool separate from that of proline entering from the extracellular medium.

Higher eukaryotic aminoacyl-tRNA synthetases in physiologic and pathologic states

The aminoacyl-tRNA synthetases play a dual role in cell metabolism by synthesizing aminoacyl-tRNAs and an odd dinucleotide diadenosine-5', 5''-P1, P4-tetraphosphate which appears to be involved in DNA replication and the control of cell proliferation. This review is a synthesis of recent results on the structure, genetics, cell biology, physiology, role in neoplasia, and role in autoimmune myositis of the higher eukaryotic aminoacyl-tRNA synthetases.

Histological analysis of limb regeneration in postmetamorphic adult Ambystoma

Previous investigation into the regenerative ability of postmetamorphic adult land phase Ambystoma has revealed that these species have the capacity to completely regenerate a limb, given optimal environmental conditions, and the gross morphological characteristics of limb regeneration in these species compared favorably with the external regeneration morphology of aquatic phase forms. The present study concerns a histological and histochemical examination of the regenerating limb tissues and their respective extracellular and intracellular tissue matrices. Postmetamorphic adult Ambystoma were amputated through the forearm, placed within optimal environmental conditions, and allowed to regenerate. The tissues were harvested at designated intervals after amputation and prepared for light microscopic examination. The limb tissues were assayed histologically for similarities to and differences from previously established regeneration morphologies. It was noted that specific correlations (i.e., apical epidermal cap formation, but outgrowth and elongation, palette formation, and digit formation) existed between regeneration histologies in these species and those previously reported for the aquatic urodeles, newt, axolotl, and larval salamander. By utilizing the histological and histochemical characteristics of the tissue, the regenerate limb was divided into five tissue units: epidermal, blastemal, soft, hard, and neuro/vascular. Based on the unique morphology of their extracellular matrices and respective histochemical staining patterns, four distinct blastemal regions were delineated within the blastemal units: subregenerate epidermal blastema, soft-tissue blastema, hard-tissue blastema, and core blastema. Histochemically, changing patterns of highly sulfated, weakly sulfated, and carboxylated polysaccharides and glycosylated compounds were located within both the extra- and intracellular stump and regenerate tissue matrices during regeneration. In addition, these patterns of intra- and extracellular macromolecular material correlated to previous reports of similar-type compounds assayed during regeneration in aquatic urodeles. With this in mind, the adult land phase Ambystoma can be considered an appropriate model system for studies concerning normal limb regeneration.

Protein synthesis in skeletal muscle of the perfused rat hemicorpus compared with rates in the intact animal

The rate of protein synthesis was measured in muscles of the perfused rat hemicorpus, and values were compared with rates obtained in whole animals. In gastrocnemius muscle of fed rats the rate of synthesis measured in the hemicorpus was the same as that in the whole animal. However, in plantaris, quadriceps and soleus muscles rates were higher in the hemicorpus than those in vivo. In the hemicorpus, starvation for 1 day decreased the rate of protein synthesis in gastrocnemius and plantaris muscles, in parallel with decreases in the RNA content, but the soleus remained unaffected. Similar effects of starvation were observed in vivo, so that the relationships between rates in vivo and in the hemicorpus were the same as those in fed rats. Proteins of quadriceps and plantaris muscles were separated into sarcoplasmic and myofibrillar fractions. The rate of synthesis in the sarcoplasmic fraction of the hemicorpus from fed rats was similar to that in vivo, but synthesis in the myofibrillar fraction was greater. In the plantaris of starved rats the rates of synthesis in both fractions were lower, but the relationships between rates measured in vivo and in the perfused hemicorpus were similar to those seen in fed rats. The addition of insulin to the perfusate of the hemicorpus prepared from 1-day-starved animals increased the rates of protein synthesis per unit of RNA in gastrocnemius and plantaris muscles to values above those seen in fed animals when measured in vivo or in the hemicorpus. Insulin had no effect on the soleus. Overall, the rates of protein synthesis in the hemicorpus differed from those in vivo. However, the effect of starvation when measured in the whole animal was very similar to that measured in the isolated rat hemicorpus when insulin was omitted from the perfusate.

Preferential utilization of exogenously supplied leucine for protein synthesis in estradiol-induced and uninduced cockerel liver explants

A cockerel liver explant system has been used to study protein synthesis and ribosome transit times. After a 2-hr preincubation of explant tissue in the presence of a large concentration of nonradioactive leucine, a small quantity of [3H]leucine was added and the kinetics of uptake of [3H]leucine into the intracellular acid-soluble leucine pool was compared to the incorporation of [3H]leucine into protein. Incorporation of [3H]leucine into protein reaches a linear rate almost immediately after addition of label, whereas the acid-soluble pool does not reach constant specific activity until much later. The length of time needed to reach a linear rate of incorporation of [3H]leucine into protein is approximately equal to the length of time needed to equilibrate nascent polypeptide chains with labeled precursor--that is, one average ribosome transit time. Therefore, it seems that the immediate precursor pool for protein synthesis reaches constant specific activity almost instantly after addition of [3H]leucine. The results indicate that at least part of the supply of leucine for protein synthesis is derived directly from the exogenous incubation medium and not from the intracellular acid-soluble amino acid pool.

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

Measurement of protein synthesis in individual organs is important in understanding metabolic changes in injury, sepsis, or starvation. Methods, mostly isotopic, for measuring synthesis are plagued by problems of experimental design and interpretation. Thus it is desirable to use a variety of methods based on different assumptions. The present study is the first to isolate radioactive aminoacyl-tRNA in the study of protein synthesis in muscle and skin. Male rats, 200-300 g, trained to eat chow for 4 hr/day were studied at 2 hr (absorptive) or 16 hr (postabsorptive) after a meal. Under ether anesthesia, a tracer dose of L-[4-5-3H(N)]-lysine was infused at a constant rate. At 20, 30, or 40 min 1 ml of arterial blood was withdrawn and 2-g samples of skin and thigh muscle were quickly excised and frozen. Samples were pooled from 4 to 7 rats for each infusion period. Concentrations and specific activities were determined for plasma lysine, and for free, tRNA, and protein-bound lysine in muscle and skin. Protein renewal rates in absorptive and postabsorptive periods averaged 6 and 9% per day in muscle, and 20 and 35% in skin. The data for muscle confirms results of other methods and suggests little contribution of rapidly turning over protein. The contribution of skin to whole body protein synthesis, about 500 mg . 100 g-1 . day-1, is similar in magnitude to the contributions of muscle, liver, or intestine.

Protein synthesis in germinating pollen of Petunia: role of proline

Pollen of Petunia hybrida was germinated in artificial medium. At the beginning of the incubation, a large amount of proline, which comprises about half of the total free amino acid pool, was released into the medium. Part of this proline is reutilized by the pollen. Uptake of radioactive amino acids and their incorporation into proteins were studied. The highest rate of protein synthesis was found directly after the onset of germination. The endogenous free proline pool was found to be compartmentalized; one of the compartments is the protein precursor pool; its size is probably much less than 50% of the total free proline in the pollen.

Effects of postovulatory aging on leucine uptake and incorporation in mouse eggs. A brief note

In order to delineate biochemical alterations during gamete senescence leucine uptake and incorporation in aging mouse eggs were investigated. One hour after ovulation eggs were removed from the oviducts and labeled (unaged) or incubated for 24 hours before labeling (in vitro aged) with [3H]leucine. In vivo aged eggs were removed from the oviducts 25 hours after ovulation and incubated in medium containing [3H]leucine. Aging of eggs for 24 hours resulted in a significant increase in the accumulation of [3H]leucine. The observed increase was larger in ova aged in vitro than in vivo, indicating that culture conditions affect leucine accumulation. Comparison of in vitro aged and unaged eggs revealed no significant difference in net leucine incorporation in to acid-insoluble material. In addition to previously documented morphological alterations, the data presented herein describe some of the biochemical changes that accompany aging of the mouse ovum. These results are discussed in light of theories of aging proposed for somatic cells.

Protein synthesis in pulmonary alveolar macrophages. Source of amino acids for leucyl-tRNA

Extracellular, intracellular and tRNA-bound leucine pools of the adherent pulmonary alveolar macrophage were examined to determine the relationships between them and the precursor for protein synthesis. When cells were cultured in media of various leucine concentrations, the patterns of isotope distribution in intracellular and extracellular leucine did not correlate with the patterns seen in protein-bound leucine. hence, the free leucine pools cannot be used reliably as precursors for calculating rates of protein synthesis. tRNA-bound leucine, however, behaved isotopically as if it were the precursor. Constant synthetic rates were calculated using the tRNA specific activity over a wide range of leucine concentrations. In addition, by measuring the tRNA-bound specific activities of three different amino acids, leucine, valine and phenylalanine, and their respective specific activities in protein, we were able to calculate independently three separate but identical synthetic rates. At physiological amino acid concentrations, the macrophage intracellular leucine pool and the tRNA-bound leucine pool received less than half of their amino acids from extracellular sources. At 5 mM external leucine, the intracellular specific activity was indistinguishable from that of the medium leucine, but the specific activity of the tRNA-bound leucine pool remained only about 50% that of the extracellular value. The most straightforward interpretation of why the tRNA-bound leucine did not flood with external label under conditions where the intracellular pool has reached equilibrium is to propose that some portion of the leucine for protein synthesis is derived directly from protein turnover before the degradation products have mixed with the common amino acid pool.

Relative synthesis of cardiac contractile proteins. Evidence for synthesis from the same precursor pool

The relative molar synthesis of cardiac contractile proteins has been measured in the perfused heart under control haemodynamic conditions. This synthesis, of myosin heavy chains, individual light chains (1 and 2), actin and tropomyosin, was determined from isolated guinea-pig hearts perfused for 3h simultaneously with constant specific radioactivities and concentrations of [3H]lysine and [3H]phenylalanine. The data strongly suggest that all of the proteins studied were synthesized from the same precursor pools of lysine and phenylalanine, since the ratio of the specific activities of the two labels was the same in all of the proteins. Measurement of molar synthesis of each contractile protein was the same with either labelled amino acid. Under control haemodynamic-perfusion conditions, the relative molar synthesis of the contractile proteins was actin greater than heavy chains greater than light chain 2 greater than light chain 1 greater than tropomyosin.

Synthesis and catabolism of rabbit alpha 1-antitrypsins F and S

The metabolic relationship between the two major forms of rabbit alpha 1-antitrypsin, F and S, was investigated by using labeling techniques in vivo and in vitro. After the injection of [14C]leucine, the S/F specific-radioactivity ratio showed characteristic changes with time: at 1 h, the ratio was high (1.2-1.4), but by later times (5-7h) it decreased to a value of approx. 1.1. Two different techniques were used to purify alpha 1-antitrypsin for labelling with iodine. The half-lives of the differentially labelled and simultaneously injected F- and S-forms were 68.1 (+/- 7.6 S.D) and 55.3 (+/- 8.1 S.D)h respectively. Combined electrophoretic and gamma-spectrometric studies provided no evidence for metabolic interconversion of the alpha 1-antitrypsin forms in the circulation. These observations suggest that rabbit alpha 1-antitrypsins F and S are, despite their close chemical composition and immunological identity, metabolically independent proteins. Therefore the possibility is raised that alpha 1-antitrypsin synthesis in rabbits is controlled by two autosomal genes or two sets of such genes.

Whole body and tissue protein synthesis in cattle

1. The daily rates of synthesis of protein by the whole body and by the individual tissues were determined in two Hereford x Friesian heifers (236 kg and 263 kg live weight), and a dry Friesian cow (628 kg live weight). 2. The rate of whole-body protein synthesis (g protein/d) was estimated from the total flux through the blood of [3H]leucine and [3H]tyrosine following infusion at a constant rate for 8 h. 3. The fractional rates of protein synthesis (ks) in the tissues (g synthesized/d per g tissue protein) were obtained after slaughter of the animals at the end of the infusion period. The fractional rate of protein synthesis was calculated assuming that the specific radioactivity of free tyrosine in either the blood (to give ks, b) or the tissue homogenate (to give ks, h) defined closely the specific radioactivity of the amino acid precursor for protein synthesis. total protein synthesis (As, b or As, h; g/d) in an individual tissue was calculated as the product of ks, b (or ks, h) x protein content. 4. Based on the total leucine flux, i.e. without correction for oxidation, 1.6 kg protein were synthesized daily in the heifers; for the cow this value was 2.0 kg/d. 5. The sum of the daily total synthesis in the major tissues (muscle+bone+brain, gastrointestinal tract (GIT), liver, hide) gave values of 1.4-1.9 kg/d based on As, b, and 2.2-3.0 kg/d based on As, h. 6. percentage contributions of the individual tissues to the total protein synthesis were similar in all three animals, for example based on As, h muscle was 12-16; carcass (muscle+bone+brain) 32-33; GIT 38-46; liver 7-8; skin 14.21%. 7. The contribution of muscle to total synthesis estimated from the leucine flux was 19-22%; this value is in agreement with those calculated on the same basis for other species. 8. The energy cost of protein synthesis was estimated to account for a maximum of 30% of heat production.

Uncoupling of amino acid turnover on transfer RNA from protein synthesis in HeLa cells

The aminoacylation of tRNA has been investigated in relation to protein aynthesis in living HeLa cells. In cells growing normally, the rates of tRNA charing are compatible with the observed entry of amino acids into protein. In contrast, when protein synthesis is inhibited 95--98% by either reduced temperature or cycloheximide, aminoacylation of tRNA is relatively unaffected. We conclude that, under these conditions, the aminoacylation of tRNA is uncoupled from subsequent steps in protein synthesis. These results provide for the first time a possible biological role for the observed aminoacyl-tRNA hydrolase activities of the tRNA synthetases.

The effect of dietary protein deficiency on albumin synthesis and on the concentration of active albumin messenger ribonucleic acid in rat liver

In rats fed on a protein-deficient diet, albumin synthesis as a percentage of total liver protein synthesis falls from the normal value of approx. 15% to about 8%. We have extracted total cytoplasmic RNA from individual rat livers and measured the concentration of active albumin mRNA by translation in a reticulocyte lysate system from which the endogenous mRNA had been removed [Pelham & Jackson (1976) Eur. J. Biochem. 67, 247-256]. In this messenger-dependent system it is possible to measure the synthesis of albumin as a proportion of the overall protein synthesis promoted by the addition of the hepatic RNA. The results show that the concentration of translatable albumin mRNA in samples of total cytoplasmic RNA from livers of protein-deficient rats is decreased markedly. These findings suggest that dietary protein supply affects selectively the synthesis and/or functional stability of albumin mRNA in rat liver.

Use of the constant infusion technique for measuring rates of protein synthesis in the New Zealand White rabbit

1. To study the potential of the constant-infusion technique for measuring rates of protein synthesis in New Zealand White rabbits, animals were infused for up to 6 h with radioactively-labelled tyrosine. 2. Labelled tyrosine from plasma and tissues was isolate from labelled metabolites by ion-exchange chromatography. 3. Analysis of serial blood and muscle biopsy samples removed under anaesthesia showed that the specific radioactivity (SR) of the free tyrosine pools reached an approximately constant value within 2 h. 4. Certain commercial preparations of L-[side-chain 2,3-3H]tyrosine were contaminated with 300 mg radioactive D-tyrosine/g. The D-isomer appeared to enter the muscle intracellular pool. 5. In constant-infusion experiments L-[3H]tyrosine could replace the uniformly-14C-labelled L-isomer for the determination of rates of protein synthesis in muscle. L-[side-chain 2,3-3H]tyrosine may not be suitable for use as a precursor for measuring rates of liver protein synthesis. 6. Evidence is presented that the precursor of liver protein synthesis may not be well defined by the SR for free tyrosine of the homogenate. 7. The technique was used to measure the rates of protein synthesis in adult rabbits. The rates of protein synthesis in liver and muscle were measured and from measurements of tyrosine flux the mean rate of whole-body protein synthesis was calculated as 13-8 g/kg per d.

Amino acid transport in the rat exocrine pancreas. I. Transport of neutral amino acids and their utilization in protein synthesis

The transport and utilization of three neutral amino acids in protein synthesis was studied using isolated pancreatic lobules in vitro. The significance of the extracellular and intracellular amino acid pool in this process was analyzed removing the extracellular pool (labeled by inulin) by a cold-wash procedure. This was especially useful in short-term experiments. Double-labeling experiments indicated a predominant utilization of the intracellular amino acid pool during protein synthesis. The advantage of isolated pancreatic lobule preparations compared to tissue fragments or slices was emphasized by fine structural studies. Using freeze-etching techniques on the same preparations, differences in the distribution of membrane particles between luminal and lateral plasma membranes described earlier were confirmed, as well as the abundant occurrence of gap junctions on both membrane faces.

Compartmentation of free amino acids for protein biosynthesis. Influence of diurnal changes in hepatic amino acid concentrations of the composition of the precursor pool charging aminoacyl-transfer ribonucleic acid

To investigate further the mechanisms by which amino acids are segregated for protein biosynthesis, the distribution of a pulse of [3H]valine was monitored in hepatic amino acid pools at seven intervals in the diurnal cycle of meal-fed rats. Although each condition was characterized by a unique balance between intracellular and extracellular valine, in every case the specific radioactivity of valyl-tRNA at steady state was higher that that of intracellular valine but below the extracellular value. Further, the specific radioactivity of the valyl-tRNA could be accurately predicted if extracellular and intracellular valine were combined in proportions specified by the transmembrane concentration gradient. These observations not only substantiate our earlier conclusions that the amino acids used for protein synthesis do not originate exclusively from either the intracellular or extracellular pools, but also strengthen our theory that the membrane transport system is the physical basis for such compartmentation. On the basis of these data we present a method for measuring the specific radioactivity of the precursor pool for protein biosynthesis in cases where the actual isolation of the aminoacyl-tRNA is not technically feasible, and also suggest a theoretical basis for interpreting the unequal distribution of both total and [3H]valine between intracellular and extracellular fluids.