Effects of Insulin, Glucose, and Amino Acids on Protein Turnover in Rat Diaphragm*

SUMMARY A simple method is described for measuring rates of protein synthesis and degradation in isolated rat diaphragm. Muscles incubated in Krebs-Ringer bicarbonate buffer showed a linear rate of synthesis for 3 hours. At the same time, the muscle released tyrosine and ninhydrin-positive material, primarily amino acids, at a linear rate. This release was not a nonspecific leakage of material from the intracellular pools, but reflected net protein degradation. Tyrosine was chosen for studies of protein turnover, since it rapidly equilibrates between intracellular pools and the medium, it can be measured fluorometrically, and it is neither synthesized nor degraded by this tissue. To follow protein degradation independently of synthesis, muscles were incubated in the presence of cycloheximide. Under these conditions, the amount of tyrosine in the intracellular pools was constant, while the muscle released tyrosine at a linear rate. This tyrosine release was used as a measure of degradation. This preparation was used to study the influence of various factors known to be important for muscle growth on protein synthesis and degradation. Similar effects were obtained with diaphragms of normal and fasted rats

From the Department of Physiology, Harvard Medical School, Boston, Massachusetts 02115 SUMMARY A simple method is described for measuring rates of protein synthesis and degradation in isolated rat diaphragm.

Muscles incubated
in Krebs-Ringer bicarbonate buffer showed a linear rate of synthesis for 3 hours. At the same time, the muscle released tyrosine and ninhydrin-positive material, primarily amino acids, at a linear rate. This release was not a nonspecific leakage of material from the intracellular pools, but reflected net protein degradation. Tyrosine was chosen for studies of protein turnover, since it rapidly equilibrates between intracellular pools and the medium, it can be measured fluorometrically, and it is neither synthesized nor degraded by this tissue.
To follow protein degradation independently of synthesis, muscles were incubated in the presence of cycloheximide.
Under these conditions, the amount of tyrosine in the intracellular pools was constant, while the muscle released tyrosine at a linear rate. This tyrosine release was used as a measure of degradation. This preparation was used to study the influence of various factors known to be important for muscle growth on protein synthesis and degradation. Similar effects were obtained with diaphragms of normal and fasted rats although the latter showed decreased synthesis and increased protein degrabtion.
Insulin by itself not only stimulated synthesis but also inhibited protein degradation (even in the presence of cycloheximide).
These two effects served to reduce the net release of tyrosine from muscle protein to comparable extents.
Effects of insulin on synthesis and degradation were greater when glucose was also present in the medium. Glucose by itself inhibited protein degradation but in the absence of insulin glucose had no significant effect on synthesis.
Nevertheless, glucose stimulated incorporation of radioactive tyrosine into protein, but this effect was due to an increased intracellular specific activity. Unlike glucose neither /3-hydroxybutyrate or octanoic acid had any demonstrable effects on protein degradation. Five times normal plasma concentrations of the amino acids had larger effects. The three branched chain amino acids together stimulated synthesis and reduced degradation, while the remaining plasma amino acids did not affect either process significantly.
Thus leucine, isoleucine, and valine appear responsible for the effects of plasma amino acids on protein turnover in the muscle. Leucine by itself or isoleucine and valine together, also were able to inhibit protein degradation and promote synthesis.
The protein content of a cell or tissue is determined by the balance between the rates of protein synthesis and degradation. Most investigations of mammalian growth mechanisms have concentrated on the control of protein synthesis. 111 muscle, for example, over-all rates of synthesis are influenced by a variety of hormones (l), nutritional status (2)) and the level of muscular activity (3). However, alterations in rates of protein breakdown may also contribute to the growth of animal and bacterial cells (4). The mean rate of degradation of muscle protein appears to change during compensatory growth (5), denervation atrophy (6), food deprivation (7), muscular dystrophy (8), and hormone treatment (6,9). The cellular mechanism responsible for these changes in degradative rates or their relationship to the concomitant alterations in rates of protein synthesis are presently unknown.
Investigations of protein degradation have been limited by methodological difficulties. Such studies have generally utilized intact animals and followed the disappearance of radioactively labeled proteins.
These experiments are prone to a variety of difficulties and possible artifacts (10, 35). For example, the reported average half-lives of muscle protein from such studies vary over a several fold range (8, 11)) presumably because of methodological problems.
In this paper we describe a simple method for study of over-all rates of protein synthesis and degradation in isolated skeletal muscle.
It has been used here to investigate the influence of insulin, glucose, and amino acids in rat diaphragm.
It has long been known that insulin is essential for normal growth of muscle (1,3). Insulin can stimulate amino acid transport (12) and protein synthesis (1) in isolated muscle.
In addition, this hormone appears to reduce protein degradation in liver 290 by guest on March 24, 2020 http://www.jbc.org/ Downloaded from (13)) adipose tissue (14), heart (9), and fibroblasts (15). The present studies have investigated the effects of insulin and glucose on protein degradation in isolated muscle. Glucose by itself has been reported to promote protein synthesis in isolated rat diaphragm (16), although this finding has been questioned.
In addition, administration of glucose to a fasted organism (2) sharply reduces the utilization of body protein reserves. Another factor essential for normal growth is an adequate supply of required amino acids.
In bacteria (17), cultured animal cells (18), and in perfused liver (19) or heart (20), supply of amino acids can promote protein synthesis or inhibit degradation. Therefore the effects of serum amino acids on protein turnover in muscle were also investigated. They weighed between 45 and 80 g at killing.

MATERIALS AND METHODS
In any experiment, groups of rats of similar weights (within 5 g) were used.
Fasted animals were deprived of food for about 40 hours prior to the experiment.
During this time, they lost approximately 25yo of their body weight.
The rats were killed by cervical dislocation. The diaphragms were then removed and placed in Krebs-Ringer bicarbonate (KRB) buffer (al), pH 7.4, containing chloramphenicol (0.3 mg per liter) and saturated with a 95% 0,-5% CO, gas mixture. The hemidiaphragms were dissected away from the ribs and cut into two pieces.
Each quarter diaphragm was rinsed, blotted, weighed (approximately 30 mg), and placed into a flask containing 3 ml of KRBi at 37". The flasks were stoppered, re-equilibrated with the gas mixture, and then preincubated for 30 min at 37" with shaking. Then the muscles were transferred to flasks containing 3 ml of fresh medium, and incubated for 1 to 3 hours. At the end of incubation 2-ml aliquots of the medium were combined with 0.5 ml of cold 5Oyo trichloroacetic acid, mixed, and centrifuged. be diaphragms were blotted, and homogenized in 1 ml of cold 0.01 M potassium phosphate buffer (pH 7.4); the homogenizing tubes were rinsed with an additional 1.0 ml of the buffer.
The homogenate and wash were combined with 0.5 ml of 50'% trichloroacetic acid, mixed, and centrifuged. The acidsoluble supernatants were then decanted to obtain a sample of the muscles' amino acid pools. For measurements of radioactivity in protein, the acid-precipitated material was washed twice with 5yo trichloroacetic acid and once with ethanol-ether Studies presented elsewhere indicate that this approach provides a valid measure of the amount of protein synthesis (26). In this paper, "muscle pools" refers to the total amount of NPM or tyrosine in the muscle that is acid-soluble (i.e. muscle pools denote the material found in both the intracellular space and the inulin space).
Data on the effects of insulin or nutrients on breakdown are expressed as differences between the control and treated pieces of the same diaphragm at the end of the incubation.
The combined amount of NPM or tyrosine in the muscle pools and the medium from the control piece was subtracted from the combined amount from the treated piece to give the net change in amount released from protein.  1) and the specific activity of the medium was not lowered significantly by the small amount of unlabeled tyrosine released by the tissue.
Previous experiments in this laboratory have indicated that tyrosine equilibrates rapidly between the medium and the intracellular fluid and that the free intracellular tyrosine is a valid measure of the precursor pool for protein synthesis (26). Therefore the rate of protein synthesis (i.e. the nanomoles of tyrosine incorporated) was linear after the preincubation ( Fig. 1).
Although protein synthesis occurred at a linear rate, more ninhydrin-positive material (NPM) and tyrosine were present in the combined medium and muscle pools at the end of incubation than at the beginning ( Fig. 2). Most of the NPM released into the medium represented amino acids found in muscle protein, and the remainder was ammonia and taurine ( Table 1). The composition of the NPM released into the medium differed from that in the intracellular pools; for example, taurine comprised Thus the release of amino acids by the muscle was selective and not a nonspecific leakage from the intracellular pools. Some of the NPM that appeared in the medium was attributable to a reduction in the NPM within the muscle (Fig. 2). However, the loss of material from intracellular pools could not account for all of the NPM that appeared in the medium.
In fact, tyrosine was released into the medium, while no change was observed in the muscle's content of tyrosine ( Fig. 2). Furthermore the amount of tyrosine released by the muscle in 3 hours was several times larger than the amount of free tyrosine originally present in the tissue (Fig. 2).
Rat diaphragm has been reported neither to synthesize nor degrade tyrosine (27). To verify this point, muscles were incubated in the presence of ['YJtyrosine and then samples of the muscle pools and medium were fractionated by thin layer chromatography (butanol-l-glacial acetic acid-HtO; 60: 15:25). The only measurable band of radioactivity had an RF characteristic of L-tyrosine.
In addition, when the muscles were incubated with and tyrosine in the muscle pools and in the medium were measured at each time point. The NPM and tyrosine in the muscle pools (intracellular and extracellular combined) and that released from protein (i.e. net protein breakdown) are plotted as mean f S.E. of tissues from five fed rats. muscle protein occurred at a linear rate throughout the a-hour mcubation (Fig. 2). During incubation in unsupplemented Krebs-Ringer buffer, the net rate of protein degradation, estimated from the net release of tyrosine, was 2 to 2.5 times greater than the rate of protein synthesis, calculated from the amount of [i4C]tyrosine incorporated into protein. Changes in the release of amino acids from cell protein could be due to changes in rates of protein synthesis or breakdown. To facilitate the study of degradation independently of possible changes in protein synthesis, synthesis was blocked by addition of cycloheximide.
At a concentration of 0.5 mM, this inhibitor caused a 95% reduction in the incorporation of [i4C]tyrosine into protein.
This degree of inhibition was also observed when insulin or glucose were added in the presence of cycloheximide. When tissues were incubated with cycloheximide, increased amounts of NPM and tyrosine were recovered in the medium, presumably because amino acids destined for protein synthesis were released into the medium.
In most experiments, addition of cycloheximide did not alter the amount of NPM or tyrosine recovered in the muscle.
In the presence of cycloheximide, the release of tyrosine occurred at a linear rate for 3 hours (Fig. 3), although the net release of NPM from protein was not linear for unknown reasons.
Thus, despite the presence of cycloheximide, protein degradation continued at a linear rate. Effects of Insulin, Glucose, and Amino Acids-Incubation of the diaphragm in the presence of insulin and glucose was found to inhibit the release of amino acids, including tyrosine, from the muscle (Table I).
At the same time, the total amount of amino  Hemidiaphragms of eight fasted rats were preincubated for 30 min and then transferred to fresh medium for a a-hour incubation. Control tissues were incubated in KRB buffer; others in KRB supplemented with glucose, 11 mM, and insulin, 0.1 unit per ml. At the end of the incubation, proteins were removed from muscle homogenates and medium with trichloroacetic acid. The acidsoluble fractions from eight muscles were combined, as were those from the media, and amino acid analyses performed.
Net change is the difference between the amino acid content of the muscle and medium of tissues treated with insulin and glucose and the control tissues. acids within the tissue also decreased (Table 1) . Thus addition of insulin and glucose reduced the net output of all amino acids, with the exception of alanine. The magnitude of this reduction varied for the different amino acids, presumably because of differences in the amino acid composition of proteins (30) and in the metabolism of the various amino acids. The anomalous increase in alanine in response to insulin and glucose is in accord with previous reports that muscle synthesizes large amounts of this amino acid from glucose (31, 32). These findings are consistent with an inhibition of net protein breakdown in the muscle and with the well documented effects of insulin on protein balance in muscle in viva (1).
The inhibition of net amino acid release from protein by insulin and glucose could represent a direct effect of insulin or could be secondary to insulin's promotion of glucose transport (12). To resolve this question, the actions of insulin and glucose were examined separately. Addition of insulin alone inhibited net proteolysis, as indicated by reduced amounts of ninhydrin-positive material lost from protein (Table II).
This decreased release of amino acids could have resulted from the well known (1) stimulation of protein synthesis by insulin (see below). However, in preliminary studies (3), the stimulation of tyrosine incorporation by insulin appeared insufficient to account for the net reduction in tyrosine release from tissue protein.
This find- Incubation of pieces of diaphragm was carried out as described in Fig. 2  ing suggested that insulin also inhibited protein degradation in the muscle.
To test this possibility, muscle was incubated in the presence of cycloheximide with or without insulin. Although protein synthesis was blocked almost completely, insulin still reduced the amounts of NPM and tyrosine released from muscle protein (Tables II and III).
This small, but significant, change indicates that insulin by itself can inhibit protein degradation independently of any effect on synthesis. Addition of glucose (10 to 22 mM) by itself to the incubation medium also reduced the amount of NPM and tyrosine released from muscle protein both in the presence and absence of cycloheximide (Tables II and III). Thus glucose, like insulin, can inhibit protein degradation.
Similar effects were observed in tissues from fed and fasted animals (Table III).
In the latter, the average rate of proteolysis was significantly enhanced over the rate in tissues from fed rats. Insulin, by itself, consistently stimulated protein synthesis (Table  III). Similar effects were found in diaphragms from fasted rats, in which the basal rate of synthesis was significantly less than in tissues from fed animals (Table III).
By contrast, in most experiments, glucose seemed to stimulate protein synthesis slightly (Table III), but we were never able to demonstrate a significant effect of glucose on synthesis with tissues from fed or fasted rats (p > 0.05).
When insulin and glucose were present in the medium at the same time, they caused larger decreases in the release of NPM (Table II) and tyrosine (Tables II and III) from protein than did either agent alone (p < 0.01).
Since these results were obtained in both the presence and absence of cycloheximide, insulin and glucose appeared to have additive effects on protein degradation. No evidence for synergistic effects was obtained.
Interestingly, addition of glucose to the medium in the presence of insulin consistently augmented tyrosine incorporation (Table III), although glucose by itself had no significant effect on protein synthesis. The absence of any effect on synthesis appears to contradict previous observations (16)) where increased incorporation of labeled amino acid was reported upon incubation with glucose. In our experiments, addition of glucose also stimulated the incorporation of radioactivity (counts per min) into protein, particularly when low concentrations of tyrosine were prbsent in the medium (Table IV). At the same time, however, glucose increased the intracellular specific activity of [%]tyrosine to an extent that can account for the enhanced incorporation of radioactivity.
When these changes in specific activity were taken into account, no effect of glucose on protein synthesis was evident.
The increased intracellular specific activity must have resulted from the decreased release of tyrosine from protein (Tables II and III), since in related studies, no evidence for the alternative explanation, an effect of glucose on [j4C]tyrosine transport, was obtained.
When pieces of the same diaphragm were incubated with and without glucose in the presence of higher concentrations of [14C]tyrosine (40 C(M), the alterations in protein breakdown appeared insufficient to alter intracellular and extracellular specific activity.
Under these latter conditions, glucose did not increase incorporation of radioactivity into protein (Table IV) However, neither P-hydroxybutyrate nor octanoate inhibited the release of tyrosine from protein, while glucose did (Table V).
In other experiments neither @-hydroxybutyrate nor pyruvate influenced the rate of protein synthesis or release of NPM from protein.
These results suggest that glucose does not affect proteolysis by simply supplying energy to the muscle.
The supply of amino acids has been shown to promote protein synthesis and to inhibit protein degradation in both microbial cells (17,33) and in certain mammalian tissues (18,19). To determine whether amino acids also affect protein turnover in muscles, amino acids were added to the medium at the concentrations normally found in plasma (34) or 5 times this concentration.
However, to measure tyrosine release, tyrosine was added in much lower amounts (0.01 mM) than the other amino Quarter diaphragms were preincubated for 30 min in the pres-resulted from an inability to determine accurately the extraence or absence of nonradioactive tyrosine and then transferred cellular specific activity (26). The extracellular specific activity to the same media for 3 hours incubation.
[r4C]Tyrosine (0.015 was assumed to equal that in the medium, which is probably in-&i per ml) was added to all flasks.
Glucose was used at 22 mM. correct under these conditions (26). If instead, the extracellular The nanomoles of tyrosine incorporated into protein were calcu-specific activity was assumed to resemble the intracellular one, lated for each piece of muscle in the usual fashion.
Average value for the control tissue is given along with the average per cent change in treated pieces from the same diaphragm.
Significance was determined by paired analysis. acids. These amino acid mixtures promoted the incorporation of tyrosine into protein (Table VI) ; they also reduced the amount of tyrosine released from protein, when protein synthesis was blocked with cycloheximide (Table VI). When the two concentrations of amino acids were compared, 5 times plasma concentrations had significantly greater effects on both synthesis and degradation (p < 0.01) than plasma levels. Similar results were obtained with tissues from fed and fasted rats (Table VI).
Finally experiments were undertaken to see if any one group of amino acids might be particularly important in regulating protein turnover. Table VII shows that addition of the branched chain amino acids (leucine, isoleucine, and valine) alone decreased protein catabolism, when measured in the presence of cycloheximide.
The remaining amino acids (i.e. all but the branched chain) did not alter the rate of protein breakdown significantly under the same conditions.
The branched chain amino acids were also capable of stimulating protein synthesis to the same extent as the complete mixture of amino acids (Table VII). Again, no effect was seen with the remaining amino acids. In other experiments, the effect of branched chain amino acids (at A 41% decrease in the net release of tyrosine from diaphragm protein was observed. This change is approximately the sum of the decrease in protein breakdown (-26%) and the increase in protein synthesis (+23%) observed in Table VII. Under the same conditions, the remaining plasma amino acids (also at 5 times normal concentrations) did not reduce tyrosine release from protein significantly.
These experiments thus suggest a crucial role of leucine, isoleucine, and valine in regulating net protein balance in muscle.
Table VII further shows that these three amino acids need not all be present together to influence protein turnover. Upon incubation of the diaphragm with leucine alone or with isoleucine and valine together, protein degradation decreased and protein synthesis increased significantly.
The effects of leucine, isoleucine, and valine on synthesis (but not catabolism) appeared approximately additive.

DISCUSSION
These experiments with skeletal muscle indicate that supply of nutrients and hormones can rapidly alter rates of protein degradation independently of any effect they may have on protein synthesis (Tables II through VII).
These studies utilized relatively simple procedures that are applicable to other incubated tissues and that offer significant advantages for investigations of average rates of protein turnover.
Most studies in this area have followed the disappearance of labeled proteins in tivo; this latter approach is more expensive than that employed here and is subject to potential artifacts (10, 11, 35). The present experiments used diaphragms from young (50 to 80 g) rats, because the muscles are thin and permit rapid diffusion of nutrients.
In such growing animals, the diaphragm must be accumulating protein, but upon incubation in KRB, the muscle released amino acids into the medium.
These effects could not be accounted for simply by the leakage of materials from the intracellular pools (Table I), but instead reflected net protein catabolism.
During the incubation, degradation of muscle proteins occurred at 2 to 2.5 times the rate of synthesis (Figs. 1 and  2, Table III). Net protein breakdown also occurs in muscle in wivo (e.g. in fasting or disuse atrophy) and has also been observed upon incubation of slices of other organs in tit~o.~ Presumably the lack of glucose, serum amino acids, insulin, and possibly other factors caused the net catabolism in the incubated diaphragm. Our initial studies estimated net protein catabolism from the release of ninhydrin-positive material (i.e. the release of all amino acids) from protein.
However, data on total production of NPM are a qualitative rather than a quantitative measure of proteolysis.
For example, muscle (31, 32) synthesizes alanine and releases it into the circulation in a manner that does not correlate with protein breakdown (Table  I, 32). Furthermore, estimation of protein degradation by the measurement of NPM is complicated (36) by the presence in muscle of high concentrations of taurine, creatine, and ammonia which react with ninhydrin but are not directly related to protein turnover.
Therefore, for most studies, we estimated protein breakdown from the release of tyrosine, which is neither synthesized nor degraded in muscle.
Release of tyrosine from protein (unlike NPM) occurred at a linear rate under all conditions, while the intracellular concentration of this amino acid (but not NPM) remained constant (Fig. 2). Thus tyrosine offers similar advantages for studies of protein turnover in muscle as does phenylalanine in cardiac muscle (20) and valine in perfused liver (13).
A major difficulty in evaluating degradative rates has been the reincorporation of amino acids released from proteins (35). Conditions that influence rates of incorporation can thus give the impression of altering rates of degradation. The present studies avoided this problem by following degradation in the presence of cycloheximide.
In addition to its simplicity, this approach yielded data that were far more reproducible than those obtained by the alternative, more laborious procedure of measuring net tyrosine release in the absence of cycloheximide and adding the rate of protein synthesis measured simultaneously. One potential difficulty with this approach is that under certain conditions, inhibitors of protein synthesis can reduce intracellular protein degradation (18,19,37) probably through some indirect feedback mechanism (17). In fact, in related studies, we have also found that cycloheximide can reduce proteolysis in the dia- mean half-life calculated from these data and from the measured tyrosine content of diaphragm proteins is approximately 6 days. This figure is indeed similar to the value of 6 to 8 days measured by Millward (11) in intact rats. In addition, the factors found to reduce proteolysis in the presence of cycloheximide had similar effects on net protein degradation measured without this drug. Similarly our unpublished studies of muscles from fasted or dystrophic animals showed changes in protein degradation that correlate with data obtained by isotopic methods in intact animals.* Since the incubated muscles studied here were in negative nitrogen balance, the factors found to influence protein turnover may not do so in the intact organism.
However, the factors that promoted synthesis and inhibited degradation in the incubated diaphragm have long been recognized as important for muscle growth in uiuo. Insulin is essential for normal growth of muscle, and in fasting or diabetic organisms (1,31), where muscle wasting occurs, administration of insulin decreases the net release of amino acids from muscle (38). This effect is similar to that seen with incubated diaphragms and has generally been believed to result from a stimulation of protein synthesis (1) and amino acids transport (1,38). In addition, insulin can inhibit the degradation of protein in muscle (Tables IV to VI), as it also does in liver (13), heart (20), adipose tissue (14), and mouse fibroblasts (15). These various actions of insulin should be additive in promoting the accumulation of protein in muscle.
In the incubated diaphragm, the percentage change in degradation (15 to 20%) was actually severalfold smaller than the percentage stimulation of synthesis (30 to 50%) ; however, since degradation occurred 2 to 3 times more rapidly than synthesis in the untreated tissues, these two actions of insulin had comparable effects on net amino acid release from the tissue.
Since the inhibition of degradation, like the stimulation of synthesis, was demonstrable in unsupplemented KRB buffer, these effects of insulin also can not be secondary to the hormone's stimulation of glucose or amino acid transport. Presumably the various effects of insulin are dependent on some single common intracellular regulatory factor. It has been suggested that insulin may exert its "pleiotypic effects" by lowering intracellular cyclic AMP; however, related experiments with the incubated diaphragm have failed to show a stimulation of proteolysis by cyclic AMP or dibutyryl cyclic AMP. In the fasting organism, supply of carbohydrates spares the utilization of body nitrogen reserves, which are primarily in muscle protein (2). These effects have been attributed to the release of insulin in response to increased glucose levels; however, the present experiments indicate that glucose supply may also act on muscle directly to inhibit the mobilization of tissue proteins for gluconeogenesis.
Glucose by itself reduced protein degradation in diaphragms of fed and fasted rats. Since /3-hydroxybutyrate and octanoate, which can also serve as major energy substrates for the diaphragm, did not reduce protein degradation (Table V), glucose is probably not serving simply as a source of ATP.
In repeated experiments, glucose by itself was not found to stimulate protein synthesis in muscles of fed or fasted rats, although glucose did so when added in the presence of insulin.
Thus glucose and insulin had synergistic actions in stimulating synthesis and additive effects in reducing proteolysis (Table 111).
The experiments with glucose (Table IV) documented possible dangers inherent in using data on amino acid incorporation as a measure of protein synthesis.
By inhibiting degradation, glucose increased intracellular specific activity of labeled precursors and thus caused increased incorporation of radioactivity into protein.
This effect, without corrections for precursors' specific activity, would give the impression of enhanced protein synthesis. Such complications can be minimized, if the labeled precursors are present in the incubation medium in high concentrations (at least plasma levels), such that release of nonradioactive amino acids from tissue proteins has less influence on intracellular and extracellular specific activities. Interestingly, previous studies on the rat diaphragm have disagreed as to whether glucose can promote amino acid incorporation (1,16). Those studies reporting increased incorporation of labeled amino acids (16) used only trace amounts of precursor and thus their findings probably resulted from the reduced degradation.
Although the addition of serum amino acids to the incubation medium increased rates of protein synthesis and decreased degradation in a dose-dependent fashion (Table VI), the branched chain amino acids alone appeared responsible for these effects ( Table VII).
Supply of all other amino acids together was ineffective against either process.
The circulating levels of leucine, isoleucine, and valine vary under different physiological conditions (e.g. fasting) and thus may influence protein turnover in muscle in wiuo. It is interesting in this regard that the branched chain amino acids have also been shown to increase production of alanine in muscle and thereby influence the amount of gluconeogenic precursors reaching the liver (32).
Unlike other required amino acids, leucine, isoleucine, and valine are degraded in skeletal muscle at rates comparable to their rates of incorporation into protein (29, 39). It is thus possible that the rapid oxidation of these compounds, which increases in fasting (4) and diabetes (32), may limit the supply of precursors for protein synthesis.
In the perfused liver, plasma amino acids have also been found to influence net protein turnover, although the specific amino acids responsible for these effects are not leutine, isoleucine, or valine. Tryptophan appears especially important in controlling hepatic protein synthesis, while proline, methionine, tryptophan, and phenylalanine appear critical in retarding protein degradation (19). These amino acids are all rapidly metabolized in liver, but not muscle, and liver does not degrade leucine, isoleucine, or valine to a significant extent. Thus those amino acids capable of regulating protein turnover in different organs vary in a manner that may be related to their rates of degradation.
It is presently unclear whether the branched chain amino acids themselves or some metabolite is involved in regulating the degradative process in muscle. In bacteria, lack of a required amino acid also leads to increased protein degradation, although this effect appears to result from lack of the corresponding amino acyl-tRNA rather than lack of the amino acid (17). Insufficient supply of any specific charged tRNA prevents synthesis, increases degradation, and causes a variety of other growth-inhibitory changes, such as decreased RNA synthesis (4). Possibly analogous control mechanisms regulate protein catabolism in muscle and other mammalian tissues.