Myofibrillar protein turnover and urinary N-tau-methylhistidine output. Response to dietary supply of protein and energy

Urinary excretion of 3-methylhistidine: an assessment of muscle protein catabolism in adult normal subjects and during malnutrition, sepsis, and skeletal trauma

The urinary excretion of 3-methylhistidine (3 MEH) has been shown to be a reliable index of muscle protein breakdown. It is decreased in protein-calorie malnutrition and increased during the hypercatabolic phase of sepsis and thermal trauma. Losses of 3 MEH after moderate to severe skeletal trauma in man and animals are reported as increased or unchanged. To clarify this response, 24 male and 6 female skeletal trauma patients were evaluated for 24 hr urinary losses of 3 MEH, nitrogen and creatinine. Eight of the 24 males also received a catabolic steroid for treatment of a head injury. In addition, 3 male and 1 female septic patients were similarly evaluated. Controls consisted of 10 volunteers on a meat free diet for 4 days and of 8 volunteers who were given only intravenous 5% dextrose in water for 3 days. The 3 MEH excretion for all control males was 3.6 mumole/Kg/day and for females was 2.8 Skeletal trauma produced a 280% increase for the males and a 225% increase for the females. Trauma with steroids caused a 325% increase. Sepsis induced a 227% increase in 3 MEH losses for males and 292% for females during the febrile episode. Creatinine excretion also increased significantly in response to trauma and sepsis but the magnitude of the increase was less than for 3 MEH. This was reflected in the 3 MEH to creatinine molar ratio increase from 0.018 for controls to 0.030-0.040 in sepsis and trauma. Patients with extensive body weight loss showed decreases in 3 MEH and creatinine excretion and a molar ratio similar to controls. The calculated contribution of muscle protein to whole body protein breakdown in the trauma and septic groups showed a twofold increase compared to the control group. The data indicate that the increased muscle protein catabolic response following stress of skeletal trauma and sepsis provides an insight on the origin of the large urinary nitrogen losses following such insults.

Interaction of thyroid status and diet on muscle protein breakdown in the rat, as measured by N tau-methylhistidine excretion

The effects of thyroid status and of dietary protein and energy content on muscle protein breakdown have been compared in young rats, using urinary N tau-methylhistidine (3-methylhistidine; 3-Mehis) output as a measure of myofibrillar protein breakdown. Recently, thyroidectomized young rats receiving an adequate diet grew slowly and showed a reduction in 3-Mehis output, relative to the weight of their leg muscles. When the thyroidectomized rats were given 2-5 microgram thyroxine (T4) per 100 g body weight, output of 3-Mehis increased, even when growth was prevented by feeding a diet low in protein. This demonstrates that thyroid-status affects muscle protein breakdown through a mechanism other than via a change in growth rate. In confirmation of previous studies, administration of a diet low in protein to young rats prevented growth and reduced output of 3-Mehis relative to leg muscle weight. The reduction in protein breakdown is not likely to be due to reduced thyroid function, since the diet low in protein increased plasma total triiodothyronine (T3) levels. Furthermore, thyroidectomized rats fed the low-protein diet also underwent a decrease in output of 3-Mehis. In contrast to protein deficiency, a moderate dietary energy deficiency allowed some growth to occur, and output of 3-Mehis, relative to muscle mass, was not depressed. Nevertheless, total T3 concentration in the plasma of these animals was considerably reduced, indicating once more that total circulating thyroid hormone levels are not the only determinants of muscle protein breakdown. Finally, reduced plasma total T3 levels occurred in energy-deficient diets even in thyroidectomized rats receiving exogenous T4. This suggests that diet affects metabolism of the hormone independently of changes in secretory output from the thyroid gland.

Effect of hyperthyroidism on the contractile and histochemical properties of fast and slow twitch skeletal muscle in the rat

Male Wistar rats were made hyperthyroid by intraperitoneal injections on alternate days for 1-6 weeks, of 200 micrograms/kg triiodothyronine (T3). The effects of this treatment on the contractile properties of the soleus, a slow twitch and the extensor digitorum longus (EDL), a fast twitch skeletal muscle, were studied in vivo in the anaesthetized animal. Post mortem, serial frozen sections of both muscles were stained histochemically for myosin ATPase, succinic dehydrogenase and phosphorylase. Muscle fibres were classified as either slow twitch (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG). Elevation of plasma T3 levels is associated with progressive alterations in the muscle fibre populations of both muscles. In the soleus there is conversion of SO to FOG fibres while in the EDL the main effect is FG to FOG conversion. There are also marked changes, mainly confined to the soleus muscle, in contractile properties; progressive increases in isometric twitch and tetanic tension and in the rates of contraction and relaxation during both twitch and tetanus. The effect of T3 on slow muscle contractility may be related to its effect on muscle phenotype. However, changes in the former precede detectable fibre population changes. T3 may influence properties such as the CA2+ binding activity of sarcoplasmic reticulum of existing slow twitch fibres before the later changes associated with the interconversion of fibre types occur.

The effect of protein depletion and repletion on muscle-protein turnover in the chick

Rates of growth and protein turnover in the breast muscle of young chicks were measured in order to assess the roles of protein synthesis and degradation in the regulation of muscle mass. Rates of protein synthesis were measured in vivo by injecting a massive dose of L-[1-14C]valine, and rates of protein degradation were estimated as the difference between the synthesis rate and the growth rate of muscle protein. In chicks fed on a control diet for up to 7 weeks of age, the fractional rate of synthesis decreased from 1 to 2 weeks of age and then changed insignificantly from 2 to 7 weeks of age, whereas DNA activity was constant for 1 to 7 weeks. When 4-week-old chicks were fed on a protein-free diet for 17 days, the total amount of breast-muscle protein synthesized and degraded per day and the amount of protein synthesized per unit of DNA decreased. Protein was lost owing to a greater decrease in the rate of protein synthesis, as a result of the loss of RNA and a lowered RNA activity. When depleted chicks were re-fed the control diet, rapid growth was achieved by a doubling of the fractional synthesis rate by 2 days. Initially, this was a result of increased RNA activity; by 5 days, the RNA/DNA ratio also increased. There was no evidence of a decrease in the fractional degradation rate during re-feeding. These results indicate that dietary-protein depletion and repletion cause changes in breast-muscle protein mass primarily through changes in the rate of protein synthesis.

The urinary excretion of N tau-methyl histidine by cattle: validation as an index of muscle protein breakdown

1. The recoveries of radioactivity in cattle urine following the intravenous administration of N tau-[14CH3]methyl histidine were essentially quantitative in 5--7 d in non-lactating cows, bulls and steers and did not change with age. 2. The N tau-methyl histidine was excreted unchanged in urine. 3. N tau-methyl histidine occurred in muscle extracts both in the free form and as a perchloric acid-soluble, acid-labile form which accounted for approximately 85% of the total non-bound N tau-methyl histidine in muscle and appeared identical to a similar component identified in muscle extracts of sheep and pigs. 4. There was probably an age-related decrease in the concentration of the acid-labile component in muscle but which did not produce a measurable change in recovery of radioactivity in urine. 5. The daily excretion of N tau-methyl histidine (E, mumol) by male cattle was highly correlated with live weight (W, kg) by the equation: E = 50 . 4 + 3 . 536 (+/- 0 . 044)W (r 0. 997). The excretions progressively decreased from 4 . 04 mumol/d per kg at 100 kg weight to 3 . 62 mumol/d per kg at 600 kg. 6. By the criterion of the rate of clearance of labelled N tau-methyl histidine from the body, the excretion of N tau-methyl histidine in urine appears to be a valid index of muscle protein breakdown in cattle.

Clinical usefulness of urinary 3-methylhistidine excretion in indicating muscle protein breakdown

Urinary excretion of the post-translationally modified amino-acid 3-methylhistidine, derived from the contractile proteins actin and myosin, was measured in patients with conditions associated with nitrogen loss. The ratio of 3-methylhistidine:creatinine excretion, a measure of the fractional catabolic rate of myofibrillar protein was increased in severe injury, thyrotoxicosis, neoplastic disease, prednisolone administration, and sometimes Duchenne muscular dystrophy. In myxoedema, osteomalacia, and hypothermia the ratio was decreased; and starvation, elective operations, and rheumatoid arthritis had little effect. Provided that the diet is meat free, measurement of urinary 3-methylhistidine may provide useful information on the cause of protein loss.

Urinary excretion of 3-methylhistidine as an index of muscle protein catabolism in postoperative trauma: the effect of parenteral nutrition

The effect of different intravenous nutritional regimens on nitrogen balance and 3-methylhistidine (3-MeHIS) excretion were studied during a 6-day period following major elective surgery in 28 patients. All patients were kept on a synthetic diet 4 days prior to surgery and were given 0.1 g amino acid N and 120 kJ/kg . day. Postoperatively, all patients received parenteral nutrition with approximately 170 kJ/kg . day. Postoperatively, all patients received parenteral nutrition with approximately 170 kJ/kg . day. Three groups of patients were given varying amounts and proportions of amino acids while in one group no amino acids were administered. Preoperatively, urinary 3-MeHIS excretion (determined by a newly developed automatic analyzer) was 240.3 mumole/day +/- 9.2, nitrogen balance was -1.8 g N +/- 0.19. Postoperatively, nitrogen balance was less negative when amino acids were given. The degree of improvement depended on the amount, but not on the composition of nitrogen administered. In all four groups, 3-MeHIS outputs were elevated when compared with preoperative excretion. The 3-MeHIS excretion (mumole/day) was increased more in patients on high amino acid supply than in patients with low or no nitrogen supply. In each of the groups the 3-MeHIS excretion was negatively correlated to the nitrogen balance. Regression analyses suggest that postoperative muscle protein breakdown occurs in relation to the body protein loss. Amino acid administration seems not to decrease muscle protein breakdown, but rather, appears to stimulate protein synthesis, resulting in less net protein loss. The mean rate of muscle protein breakdown in the postoperative state was estimated to be 80 g/day, assuming 4.2 mumole 3-MeHIS per g mixed human muscle protein. This exceeded the mean preoperative breakdown by about 23 g muscle protein per day. This increase might be due to the metabolic response to the trauma and also in part to tissue damage by the surgical procedure.

Muscle protein catabolism in diabetes: 3-methylhistidine excretion in the spontaneously diabetic "BB" rat

The muscle protein lost in uncontrolled diabetes may be due to decreased synthesis, increased catabolism, or to any combination of alteration in these rates that results in net loss. Differing methods of examining these rates in vivo and in vitro have given conflicting results. We assessed the rate of catabolism of proteins containing 3-methylhistidine (3-MH) by measurement of its urinary excretion in spontaneously diabetic "BB" Wistar rats. Prior to overt diabetes, rates of excretion were appropriate to the age of the rats (1.46 +/- 0.15 mumole/day), with 34%-47% as the nonacetylated form. Accompanying diabetes there was an increase in urine urea nitrogen of two to threefold over 4-14 days, and an increase in ammonium nitrogen of sixfold. 3-MH excretion doubled by 4 days, and 81%-96% was excreted as the nonacetylated form. Subcutaneous insulin in doses sufficient to improve glycosuria and hyperglycemia was associated with normalized total 3-MH excretion (N-acetyl 3-MH plus 3-MH) but a greater proportion than normal appeared in the nonacetylated form. These results suggest that muscle protein catabolism increased with insulin deficiency and that this defect can be corrected by therapy. Both untreated and treated diabetic rats appear to have a limited capacity for acetylation of 3-MH prior to its excretion.

Dietary protein intake and 3-methylhistidine excretion in the rat

1. Rats were fed for 14 d on diets containing 50, 150 or 250 g/kg casein as the protein source. The daily excretion of Nt-methylhistidine (His(tau Me)), a non-re-utilized amino acid, was determined. 2. His(tau Me) excretion/100 g body-weight appeared to be unaffected by increasing the concentration of dietary protein from 150 to 250 g/kg. Assuming no change in the proportion of muscle in the animals these results are indicative of no change in myofibrillar protein catabolic rate. The excretion rate/100 g body-weight of the animal given 50 g/kg casein was lower than the other two treatments, especially towards the end of the 14 d treatment period. Thus at this time the myofibrillar protein catabolic rate was lower than in the animals fed on the higher protein diet. 3. In the animals fed on the high protein diet there was a tendency for this (tau Me) excretion rate/100 g body-weight to increase with age. 4. Nitrogen balance and creatinine excretion results are also presented.

Diurnal response in endogenous amino acid oxidation of meal-fed rats

A model has been developed to measure the effects of dietary protein on daily fluctuations in the rate of endogenous amino acid oxidation in meal-fed and starved rats. In addition, N tau-methylhistidine and hydroxyproline were utilized to determine changes in the rate of degradation of myofibrillar and collagen proteins. In rats meal-fed a normal diet of 18% (w/w) casein, a diurnal response was observed in rate of oxidation of radioactive amino acids contained in endogenous labelled body protein, with a nadir 16--20 h and maximum 4--8 h after beginning the feeding. This observation in part may be related to alterations in flux of amino acids from non-hepatic tissues to site of oxidation in liver, as well as alterations in rates of amino acid oxidation after a protein meal. When meal-fed a 70% protein diet, the maximal rates of endogenous amino acid oxidation were significantly increased by 4--8 h after meal-feeding, with no change in fractional rates of degradation of myofibrillar- or collagen-protein breakdown. This could suggest increases in activities of enzymes involved in amino acid oxidation, in rats meal-fed 70% compared with 18% dietary protein. In contrast, meal-feeding of a protein-free diet muted the diurnal response in the rate of oxidation of endogenously labelled amino acids, which correlated with a decrease in the fractional rate of degradation of myofibrillar or collagen protein. Thus dietary protein is apparently responsible for the observed diurnal rhythm rhythms in the rate of amino acid oxidation, whereas carbohydrates tend to mute the response.

Hyperinsulin hypoglycemia in the neonate: therapeutic choices

Lack of elevated serum insulin in 3 patients with nesidioblastosis following 95% pancreatectomy leaves one with the theoretical possibility that the recurrent hypoglycemia is not due to excess insulin secretion, but rather an iatrogenic hypoglucagon state. If this explanation is correct, progression to a "total" pancreatectomy may not be the appropriate form of therapy. It seems to us that further evaluation of other hormones secreted by the endocrine portion of the pancreas must be clarified before continuing extensive or total pancreatic resections for nesidioblastosis in infancy.

Quantitative importance of non-skeletal-muscle sources of N tau-methylhistidine in urine

Direct measurement of N tau-methylhistidine turnover in skeletal muscle, skin and gastrointestinal muscle indicates that these three tissues contribute only 24.9, 6.8 and 9.8% of the total urinary excretion. Measurement of the decay rate of radioactively labelled N tau-methylhistidine in urine indicates that skeletal muscle accounts for 74.5% of the urinary excretion and this is probably an overestimate. These results suggest that the common assumption, that N tau-methylhistidine in urine originates almost entirely from skeletal muscle, may be wrong.

Free amino acid excretion in tetraplegic patients

Free amino acid chromatographic fingerprints obtained in automated analysers in 24-hour urines may help to quantitate muscle and collagen changes occurring after cerival spinal cord injury. The technique was used to compare the urine of six healthy and ten tetraplegic men, 18 to 25 years old, three of whom became paralysed in diving, one in a mountain fall, one one in the fall of a pipe, four in car accidents, and one in a football game. Nine weeks to 14 years after injury, the tetraplegics excreted less total free amino acids, lysine, and 3-methylhistidine than the controls, whereas hydroxyproline and hydroxylysine were similar in both groups. Values of urine 3-methylhistidine, measured 9, 13, 19 and 20 weeks post onset in one patient, and 9, 12, 16, 18, 23, and 26 weeks post onset in another, were related to the duration of paralysis. These results indicate that collagen and muscle metabolism may not be quantitated by measuring urine free amino acids.

Effects of two energy: nitrogen ratios in patients with gastroenterological disease and malnutrition

In 10 patients with active gastroenterological disease and protein-malnutrition (weight: 77.3 +/- 2.6 (mean +/- SEM) percent of ideal body weight, serum-albumin levels: 2.59 +/- 0.17 mg/100 ml) a randomized crossover study was performed to assess the effects of two energy:nitrogen ratios on body cell replenishment. After at least 3 days for equilibration, the total parenteral nutrition (TPN) study carried out with 354 +/- 5 mg of casein hydrolysate-nitrogen/kg/day, divided in two 7-day periods during which two nonprotein calorie supplies of 47 +/- 1 kcal/kg/day and 81 +/- 4 kcal/kg/day were given. The same 50 +/- 5% dextrose and fat emulsion energy sources were used in the two periods. Nitrogen (Kjeldahl method) and potassium retention, and weight and serum albumin concentration gains were all significantly better (Student t test) during the hypercaloric regimen than during the normocaloric regimen. In the 10 patients, the protein-sparing effect of nonprotein calories "added" during the hypercaloric regimen was demonstrated and represented 17% of the constant infused nitrogen. The more catabolic patient was prior to TPN, the more energy-dependent was the protein-sparing effect observed (r = +0.638). Preliminary data obtained with 3-methylhistidine urine determination suggests that the protein-sparing effect of "added" calories was due to an increased protein synthesis. Finally, body cell replenishment was better with the higher 230 +/- 6 energy:nitrogen ratio than with the lower 132 +/- 4 energy:nitrogen ratio, which suggests that the hypercaloric TPN regimen was useful in such patients.

Urinary 3-methylhistidine:creatinine ratio in patients on long-term parenteral nutrition

The urinary 3-methylhistidine (3-MH) and creatine excretion were measured serially in two patients on total parenteral nutrition for 201 and 225 days, respectively. Variations in excretion were related to clinical events, such as sepsis; 3-MH excretion and the 3-MH:creatinine ratio were raised in association with some episodes of infection, but not all. It is concluded that, although infection is often associated with increased myofibrillar protein breakdown, this is not always the case. It is suggested that in susceptible patients a high 3-MH:creatine ratio may indicate occult infection not detectable by other means.

Adequate function of the cell: interactions between the needs of the cell and the needs of the organism

The continuous maintenance of cell adequate function (cellular stress) is considered to be the main feature which characterizes the activity of a live cell. In accordance with the concept of cellular stress, the transition of the cell to the state of adequate functioning represents a dual commitment. The cell must function adequately to respond to the demands of the organism and to provide for its own needs. Thus, it is proposed that two systems exist for the regulation of the adequate function of the cell: one enabling the cell to respond to the demands of the organism, and the other providing for the needs of the cell itself. The hormonal system of the organism represents the first of these mechanisms, and I believe that another system exists for regulation of intracellular needs. Adequate function of the cell is determined by the regulative, controlling and defense mechanisms. The balance between the needs of the cell and of the organism should be a concern of physician, since it provides for the optimal function and health of the organism as a whole.

N tau-methylhistidine content of organs and tissues of cattle and an attempt to estimate fractional catabolic and synthetic rates of myofibrillar proteins of skeletal muscle during growth by measuring urinary output of N tau-methylhistidine

1. Distribution ofNτ-methylhistidine (3-methylhistidine; Me-His) among organs and tissues in cattle was determined. From the amount of Me-His in skeletal muscle protein and daily urinary output of Me-His, fractional catabolic and synthetic rates of myofibrillar proteins of skeletal muscle during growth were calculated.

2. More than 93.4% of the total Me-His (35.6 mg/kg body-weight) in the analysed cattle tissues occurred in skeletal muscle protein. The amount of Me-His in tissues other than skeletal muscle was relatively small. Daily urinary excretion of Me-His in the cattle which were fed on hay and concentrate was 135 mg at a growing stage of 217 kg body-weight and 145 mg at a stage of 312 kg. The Me-His content of foodstuffs was also carefully checked in the present investigation. Assuming that absorbed dietary Me-His in quantitatively excreted in the urine without delay, the contribution of Me-His in foodstuffs was calculated to be approximately 30 % of the urinary Me-His. Rumen protozoa and bacteria contained little Me-His.

3. From these results, fractional catabolic and synthetic rates of myofibrillar proteins of the cattle at a growing stage of 217 kg body-weight were calculated to be 1.22%/d (half-life 56 d) and 2.73%/d, while these rates at a stage of 312 kg body-weight were 1.02%/d (half-life 67 d) and 1.51%/d respectively. These values were calculated on the same assumptions as those for the rat (Younget al.1972; Funabikiet al.1976). Of the Me-His present in foodstuffs 94% was tentatively assumed to be excreted into urine.

Effect of glucocorticoid administration on the rate of muscle protein breakdown in vivo in rats, as measured by urinary excretion of N tau-methylhistidine

The role of glucocorticoids in regulating the rate of muscle protein breakdown was evaluated by measuring excretion of N(tau)-methylhistidine during administration of various doses of corticosterone to adrenalectomized rats. Groups of rats received daily subcutaneous injections of 0, 0.2, 0.5, 1.0, 5.0 or 10.0mg of corticosterone/day per 100g body wt. for 7 days, followed by 3 days without hormone treatment, after which they were killed. A group with intact adrenal glands served as an additional control. All animals were pair-fed with the untreated adrenalectomized group. No significant differences were noted in growth rate or N(tau)-methylhistidine excretion between the intact or adrenalectomized control groups, or those given 0.2, 0.5 and 1.0mg of corticosterone, whereas growth ceased and N(tau)-methylhistidine excretion rose markedly in the groups receiving 5 and 10mg of corticosterone. After these two high doses of corticosterone, but not after lower doses, there was a loss of weight of the gastrocnemius muscle per 100g of final body wt., but not of the soleus and extensor digitorum longus muscles. The two highest doses of corticosterone also resulted in an increase in liver weight per 100g of final body wt. Lower doses of corticosterone did not cause these changes. Plasma corticosterone concentrations, measured on the final day of injection and again at the time of killing, were decreased to near zero by adrenalectomy and were little raised by doses of 0.2 and 0.5mg daily, but were increased to within the normal range by the 1mg dose. At 5 and 10mg doses, plasma corticosterone concentrations were sustained at 2-3 times those of intact rats, and thus in the range reported for rats exposed to severe stress. Rats given 5 and 10mg doses of corticosterone had glycosuria, and showed considerably elevated concentrations of insulin in the plasma. It is concluded that plasma concentrations of glucocorticoids within the normal range do not regulate the rate of muscle protein breakdown, whereas excessive plasma concentrations of corticosteroids, equivalent to those observed in severe stress, can accelerate muscle protein breakdown.

Ntau-methylhistidine in human skeletal and smooth muscle proteins

1. Fifteen pieces of rectus abdominus muscle and fifteen pieces of taenia muscle were removed from patients undergoing various surgical procedures. 2. The muscles were extracted, hydrolysed and the content of Ntau-methylhistidine was measured. 3. The rectus muscles contained 3.13 +/- 0.7 mumols Ntau-methylhistidine/g fat-free dry solid and the taenia muscles 2.4 +/- 0.6 mumols/g fat-free dry solid. There was a statistically significant difference (P less than 0.05) between these values using the Student's unpaired tau test, although this could have been due to contamination of the taenia muscles with connective tissue. 4. The muscle content of Ntau-methylhistidine was at least 40% higher than the only other reported value and therefore the method of calculating muscle protein breakdown based on Ntau-methylhistidine excretion requires revision.

Effect of dietary protein on urinary excretion of 3-methylhistidine in rat

The excretion of the amino acid 3-methylhistidine in urine has been shown to be correlated with protein catabolism in skeletal muscle. In rats, 3-methylhistidine is partly acetylated (N-acetyl-3-methylhistidine) and it has been proposed that the relative amounts of 3-methylhistidine and N-acetyl-3-methylhistidine in urine is age dependent. In this experiment the effect of dietary protein quality on urine excretion of 3-methylhistidine and N-acetyl-3-methylhistidine was studied. Six groups of rats (mean weight 80 g) were fed diets containing 10% protein of different quality, the net protein utilization ranging from 76.7 for egg albumin to 28.9 for wheat gluten. The excretion of non-acetylated 3-methylhistidine was not dependent on the diet. There was, however, a good correlation between protein quality and total urine 3-methylhistidine (3-methylhistidine plus N-acetyl-3-methylhistidine), the higher the protein quality, the greater being the excretion of total 3-methylhistidine. The relative amounts of 3-methylhistidine and N-acetyl-3-methylhistidine correlated with the mean body weight, but not the age, of the animals. This study therefore demonstrates that a relationship exists between the nutritive quality of the dietary protein and the urinary excretion of 3-methylhistidine in rats.

The age dependence of intracellular proteolysis: changes of the substrate proteins

Liver cytosol proteins of young (4--6 months) and old (18--27 months) rats were degraded in vitro by papain, pronase, trypsin, pepsin, cathepsin D from rat liver and a soluble lysosomal enzyme mixture from rat liver. We could demonstrate the capability of the latter enzyme mixture to degrade proteolytically the cytosol proteins of young animals about 20% faster than those of the older animal group. Digesting radioactive labelled "young" cytosol in the presence of unlabelled "old" cytosol the possibility could be excluded, that this effect was due to an inhibitor of macromolecular size present in the "old" cytosol.

The effect of surgical trauma on muscle protein turnover in rats

The rate of synthesis and catabolism of sarcoplasmic- and myofibrillar-muscle protein was measured in operated, sham-operated and food-restricted rats by using Na2 14CO3. The food-restricted group underwent sham operations and were limited to the food intake of the operated animals. Protein synthesis and catabolism were increased in the sarcoplasmic-muscle fraction in operated rats compared with that in sham-operated or food-restricted rats. The rate of synthesis of the myofibrillar protein decreased in operated animals, but the rate of catabolism was not altered in the myofibrillar-muscle fraction of the operated animals compared with that in food-restricted and sham-operated animals. In the operated animals, there was a net loss of protein from the muscle. Thus the rats that underwent surgery lost muscle protein, primarily as a result of a decrease in synthesis of myofibrillar protein. The changes in protein turnover in operated animals were not due to decreases in food intake, since protein turnover in sham-operated animals that were restricted to the food intake of the operated rats was not different from that in sham-operated rats fed ad libitum.

Energy and protein intakes as determinants of nitrogen balance

Changes in caloric intake below or above energy needs affect nitrogen balance, so that any change in caloric intake results in a corresponding alteration in nitrogen balance. The improvement in nitrogen balance caused by an increase in energy intake, however, can be frustrated if intake of protein is inadequate; conversely, the beneficial effects of an increase in protein intake can be inhibited by an inadequate energy intake. These observations mean that, in experimental studies, nitrogen balance is the result of levels of both energy and protein; in consequence, protein requirements can be interpreted only from such studies, where energy intake is also defined under the experimental conditions. In addition to the effect of dietary energy supply, dietary carbohydrate has a short-term specific effect on protein metabolism not shared by fat, in which plasma amino acids are diverted into muscle protein through the action of insulin released by the dietary carbohydrate. This metabolic interaction also occurs between carbohydrate and amino acids absorbed from the same meal.

Protein degradation in muscle: response to feeding and fasting in growing rats

Degradation rates of muscle proteins were determined in young rats allowed access to standard rat chow 12 h/day. degradation was assessed by determination of 3-methylhistidine (3MH) excretion rates. 3MH is a nonreutilized amino acid produced almost exclusively within the actin and myosin of skeletal and cardiac muscle. Because plasma levels of 3MH are low and renal clearance is high, excretion reflects myofibrillar degradative rates. Excretion of 3MH was determined for 4-h periods beginning 12 and 20 h after initiation of feeding and after 24-and 48-h fasts. Excretion of 3MH per 4-h period increased with time after the last feeding. Because creatinine excretion is a function of muscle mass dividing 3MH excretion by creatinine excretion represents myofibrillar degradation per unit muscle mass, the fractional degradative rate. Degradation rates rose from 4.6 to 14.5%/day between 12 and 16 and 60 and 64 h after the beginning of the last meal. These results support the presence of a diurnal pattern of protein degradation as well as increased muscle degradation during starvation.

Urinary excretion of methylarginine in human disease

Methylated amino acids are excreted in urine upon degradation of some tissue proteins. The urinary excretion ratios of NG,N'G-dimethylarginine (syn-DMA) and NG,NG-dimethylarginine (unsym-DMA) were studied in healthy adults and in patients with various diseases. The normal ratio of sym- to unsym-DMA in urine was 0.98 and ranged from 0.71 to 1.33; ratios were not significantly different in multiple sclerosis, cerebrovascular accident, cancer, and systemic lupus erythematosus. However, patients with liver, disease, including chronic active hepatitis, were found on average to have a significantly altered ratio of 0.79, range 0.49-1.30, owing to an increase in the excretion of unsym-DMA. Hence measurements of the urinary excretion of dimethylarginine could become a useful aid in assessing recovery of liver cells in patients with chronic liver disease.

Labile protein reserves and protein turnover

The concept of a labile protein reserve is based on the relatively slow establishment of a new equilibrium in the rate of nitrogen excretion after an abrupt change in dietary supply. The evidence reviewed shows that a majority of this nitrogen is derived from or deposited in skeletal muscle proteins. The rates of synthesis and degradation of total body protein are rapid in large animals (man and swine) and are correlated with heat production. The rate of protein synthesis in skeletal muscle greatly exceeds the rate of growth and is sensitive to nutritional status. The rate exceeds the rate of degradation in response to the ingestion of an adequate diet so that tissue proteins are accumulated, but it decreases below the rate of degradation under conditions of deprivation. In this latter state, proteins of skeletal muscle supply amino acids for energy or for the synthesis of other more essential proteins, e.g., milk proteins during lactation. Thus, we conclude that the labile protein reserve is a product of the normal, dynamic metabolism of protein.

The role of enzyme degradation in enzyme turnover during tissue differentiation

1. The role of enzyme degradation in enzyme turnover during tissue differentiation has been investigated by the use of equations based on the model of Berlin and Schimke (Mol. Pharmacol. 1 (1965) 149-156). 2. A digital computer has been used to calculate the change in enzyme amounts which would result from different changes in the rates of synthesis and degradation of an enzyme during cell differentiation. The energetics of these alternative processes have been compared. 3. The results demonstrate that changes in the rates of synthesis and degradation of an enzyme after a differentiation stimulus enhance the flexibility and rapidity of changes in enzyme amount during enzyme accumulation. Decreases in the rate of degradation of an enzyme during enzyme accumulation lead to a considerable saving of energy. 4. Hypotheses are proposed to account for the different modes of turnover of protein subgroups during cytodifferentiation in terms of specific changes in the rates of synthesis and degradation of the subgroups.

Quantitative aspect of the myofibrillar protein turnover in transient state on dietary protein depletion and repletion revealed by urinary excretion of N7-methylhistid;ne

The fractional rates of synthesis and breakdown of myosin and actin in skeletal muscle of young adult male rats were measured during 2 weeks of ad libitum feeding of a protein-free diet, and 8 days of refeeding with an adequate protein diet. Daily urinary excretion of NT-methylhistidine (3-methylhistidine) by the NT-methylhistidine pool of the body gave the fractional breakdown rate of the myosin-actin pool. The fractional synthesis rate of the myosin-actin pool was calculated from the fractional breakdown rate and the size of NT-methylhistidine pool in the body. The feeding of the protein-free diet resulted in a decrease in body weight and a decrease in daily urinary excretion of NT-methylhistidine. Refeeding caused an increase in body weight and a progressive increase in daily urinary excretion of NT-methylhistidine. At the start of the experiment, the fractional breakdown rate of the myosin-actin pool was 4% per day and with prolonged protein depletion, the rate decreased to 1.25% per day. The fractional synthesis rate also decreased more rapidly than the breakdown rate. On refeeding for one day with an adequate protein diet, the fractional synthesis rate increased from 0.75 to 5.75% per day. Accumulation of skeletal muscle protein by refeeding was accompanied by a difference between the faster rate of synthesis and slower rate of breakdown even though the fractional breakdown rate increased during the rehabilitation period.

Role of protein degradation in the growth of livers after a nutritional shift

Fractional rates of synthesis and degradation of liver porteins were estimated during the rapid restoration of liver mass observed in protein-depleted mice when they are fed with an adequate diet. 1. Net protein gain was fastest 12h after the nutritional shift, when it reached a rate of 48% per day. 2. The RNA/protein ratio in livers of protein-depleted animals was essentially the same as in normal livers; it increased by a maximum of 13% 12h after the nutritional shift. 3. Rates of protein synthesis in vivo were measured by the incorporation into liver protein of massive amounts of L-[1-14C]leucine. In protein-depleted animals, the rate of synthesis per mg of RNA was 72% of that in normal livers. Normal rates were recovered within 12h of the nutritional shift. 4. The fraction of newly synthesized protein retained by the liver was studied after they were pulse-labelled by the intravenous injection of radioactive leucine, and, 5 min later, pactamycin (an inhibitor of the initiation of protein synthesis); 3h later the livers in both experimental situations retained 58% of the newly synthesized protein. 5. Fractional rates of protein degradation were estimated either from the difference between the synthesis of stable liver proteins and the net protein increase, or by the disappearance of radioactivity from the liver protein previously labelled by the administration to the mice of NaH14CO3. Both procedures demonstrated a large decrease in the rate of protein degradation during liver growth.

The relative importance of muscle protein synthesis and breakdown in the regulation of muscle mass

The effects of growth-suppressing and muscle-wasting treatments on muscle protein turnover and amino acid concentrations were determined in vivo. All treatments depressed protein synthesis and some treatments depressed protein breakdown. Only prolonged starvation increased protein breakdown. Muscle protein mass is regulated primarily through alterations in protein synthesis in all except emergency conditions. The increased concentrations of the branched-chain amino acids indicate that they are unlikely to be involved in this regulation.