Adaptation of the rat to a low-protein diet: the effect of a reduced protein intake on the pattern of incorporation of L-[14C] lysine

The single-biopsy approach in determining protein synthesis in human slow-turning-over tissue: use of flood-primed, continuous infusion of amino acid tracers

Muscle protein synthesis (MPS) rate is determined conventionally by obtaining two or more tissue biopsies during a primed, continuous infusion of a stable isotopically labeled amino acid. The purpose of the present study was to test whether tracer priming given as a flooding dose, thereby securing an instantaneous labeling of the tissue pools of free tracee amino acids, followed by a continuous infusion of the same tracer to maintain tracer isotopic steady state, could be used to determine the MPS rate over a prolonged period of time by obtaining only a single tissue biopsy. We showed that the tracer from the flood prime appeared immediately in the muscle free pool of amino acids and that this abundance could be kept constant by a subsequent continuous infusion of the tracer. When using phenylalanine as tracer, the flood-primed, continuous infusion protocol does not stimulate the MPS rate per se. In conclusion, the flood-primed, continuous infusion protocol using phenylalanine as tracer can validly be used to measure the protein synthesis rate in human in vivo experiments by obtaining only a single tissue biopsy after a prolonged infusion period.

Measuring synthesis rates of nitrogen-containing polymers by using stable isotope tracers1,2

The major N-containing polymer compounds in the body include protein, RNA, and DNA. The endogenous gastrointestinal secretions as well as the portal-drained visceral and peripheral immune responses are basic physiological functions. Elevated endogenous secretions and immune activities, as affected by developmental stages, diets, and management factors, decrease the availability of dietary nutrients for peripheral muscle synthesis and deposition. Measurements of in vivo protein, RNA, and DNA synthesis rates associated with the viscera, peripheral immune cells, and skeletal muscles should, in principle, be the sensitive biochemical and cellular endpoints for studying factors affecting nonruminant nutrition, metabolism, and growth. The selection of stable isotope tracers for precursors, routes of tracer delivery, and mass spectrometric analyses of tracer enrichments are the major methodological considerations. To measure in vivo protein, RNA, and DNA synthesis rates, oral feeding with heavy water (2H2O), and continuous infusion of [U-13C]glucose and [15N]Gly intravenously for labeling the sugar moieties ribose and deoxyribose and de novo purine base synthesis have been established. Flooding doses of tracer Phe, for example, L-[ring-2H5]Phe, via the i.p. route are reliable and cost-effective for measuring in vivo protein synthesis rates, especially for the viscera in small nonruminants. Therefore, measurements of the major N-containing polymer synthesis rates in the viscera, the peripheral immune cells, and muscles through oral feeding with 2H2O and/or i.p. flooding doses of Phe tracers are the emerging tools for studying nonruminant nutrition, metabolism, and growth under research and field test conditions.

Three weeks of caloric restriction alters protein metabolism in normal-weight, young men

The effects of prolonged caloric restriction (CR) on protein kinetics in lean subjects has not been investigated previously. The purpose of this study was to test the hypotheses that 21 days of CR in lean subjects would 1) result in significant losses of lean mass despite a suppression in leucine turnover and oxidation and 2) negatively impact exercise performance. Nine young, normal-weight men [23 +/- 5 y, 78.6 +/- 5.7 kg, peak oxygen consumption (Vo2 peak) 45.2 +/- 7.3 ml.kg(-1).min(-1), mean +/- SD] were underfed by 40% of the calories required to maintain body weight for 21 days and lost 3.8 +/- 0.3 kg body wt and 2.0 +/- 0.4 kg lean mass. Protein intake was kept at 1.2 g.kg(-1).day(-1). Leucine kinetics were measured using alpha-ketoisocaproic acid reciprocal pool model in the postabsorptive state during rest and 50 min of exercise (EX) at 50% of Vo2 peak). Body composition, basal metabolic rate (BMR), and exercise performance were measured throughout the intervention. At rest, leucine flux (approximately 131 micromol.kg(-1).h(-1)) and oxidation (R(ox); approximately 19 micromol.kg(-1).h(-1)) did not differ pre- and post-CR. During EX, leucine flux (129 +/- 6 vs. 121 +/- 6) and R(ox) (54 +/- 6 vs. 46 +/- 8) were lower after CR than they were pre-CR. Nitrogen balance was negative throughout the intervention ( approximately 3.0 g N/day), and BMR declined from 1,898 +/- 262 to 1,670 +/- 203 kcal/day. Aerobic performance (Vo2 peak, endurance cycling) was not impacted by CR, but arm flexion endurance decreased by 20%. In conclusion, 3 wk of caloric restriction reduced leucine flux and R(ox) during exercise in normal-weight young men. However, despite negative nitrogen balance and loss of lean mass, whole body exercise performance was well maintained in response to CR.

Effects of protein restriction and acute refeeding on leucine and lysine kinetics in young men

To explore the effects of altered protein intake on the uptake and utilization of dietary amino acids in human subjects, six healthy male Massachusetts Institute of Technology students were studied during three dietary periods each of 9 days, with tracer infusion studies conducted on days 8 and 9. During one period the diet provided a generous protein intake (1.5 g.kg-1.day-1) and during the other two a low-protein intake (0.1 g.kg-1.day-1). Tracer infusions (fed state) were given while subjects received either a liquid formula, supplying the prior protein intake or (on day 9) a generous intake. Combinations of [2H3]Leu and [13C]Leu and of [15N]Lys and [2H4]Lys were given via continuous intravenous or intragastric infusion for 4 h to estimate leucine kinetics and the first-pass splanchnic uptake of tracer. Dietary-induced changes in leucine flux and oxidation were similar irrespective of the tracer and/or route of administration and sampled pool (leucine or KIC) used for calculation. Estimates of absolute changes in the rates of leucine disappearance via nonoxidative metabolism (protein synthesis) and of appearance via protein breakdown depended on the route of [13C]Leu administration. However, it appears that both altered rates of protein synthesis and breakdown contribute to the adaptations required in the body nitrogen economy when protein intakes are restricted and subsequently resupplied. From the intragastric [13C]Leu protocol, the first-pass splanchnic uptake of tracer was the same (approximately 30%) for generous, low-protein, and refed conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Early protein undernutrition and subsequent realimentation in turkeys. 2. Effect on weights and proportions of organs and tissues

Male and female Large White Nicholas turkeys were fed corn and soybean meal diets for 24 and 20 wk, respectively. The same series was fed with a low protein modification (75% of control in protein and 66% of control in methionine and lysine per Mcal ME) during 0 to 6 wk of age. For the study of organ and tissue growth, six male turkeys per treatment were sampled at 2, 4, 6, 8, 12, 16, 20, and 24 wk of age. Six females per treatment were also sampled at 2, 4, 6, 8, 11, 14, 17, and 20 wk of age. Weights of eviscerated carcass, breast, thigh, drumstick, wing, back, neck, feathers, liver, heart, pancreas, alimentary tract less gizzard, and gizzard were depressed by early protein undernutrition to 6 wk of age. Compared with values for controls, undernutrition increased proportions of the liver, alimentary tract less gizzard, gizzard, and heart at 6 wk of age; proportions of the back, neck, feathers, and pancreas were not affected, but breast and thigh proportions were reduced. Compensatory growth during realimentation eliminated all differences between undernourished and control turkeys in weight and proportions of organs and tissues by 20 wk of age.

Synthesis and degradation of collagens in skin of healthy and protein-malnourished rats in vivo, studied by 18O2 labelling

To explore the effects of growth retardation, caused by restricted protein intake, on collagen turnover in the whole skin, Sprague-Dawley rats (n = 20) were labelled with 18O2 and fed on either an adequate (18%) or a low (3%) lactalbumin diet. Skin biopsies were obtained at intervals during the following 6 months. Independent groups of animals (n = 186) were used to determine the size of the 0.5 M-acetic acid-soluble and -insoluble collagen pools in the entire skin of healthy and malnourished rats. Collagen was estimated by measurement of hydroxyproline. Soluble-collagen synthesis rates were equivalent to 99 +/- 8 mumol of hydroxyproline/day in healthy animals and 11 +/- 2 mumol/day in malnourished rats. Insoluble-collagen synthesis rates were 32 and 5 mumol of hydroxyproline/day in the healthy and protein-depleted rats respectively. The degradation of soluble collagen amounted to 37 +/- 8 and 6 +/- 2 mumol of hydroxyproline/day in the healthy and malnourished groups respectively. Efflux of collagen from the soluble collagen, defined as the sum of the rate of soluble collagen that is degraded plus that which matures into insoluble collagen, was 70 +/- 8 and 11 +/- 2 mumol of hydroxyproline/day in the healthy and malnourished groups respectively. Insoluble collagen was not degraded in either group. The fraction of soluble collagen leaving the pool that was converted into insoluble collagen was 0.46 in both diet groups. It is concluded that the turnover of soluble collagen is markedly decreased with malnutrition, but degradation and conversion into insoluble collagen account for the same proportions of efflux from the soluble-collagen pool as in rapidly growing rats.

Whole-body protein turnover in chicks fed control, histidine, or methionine plus cystine-free diets

Whole-body protein turnover rates were measured in chicks fed control, histidine-free (-His), and methionine plus cystine-free (-Met, Cys) diets. After chicks were fed the experimental diets ad libitum for 7 days, they were injected with a massive dose of L-[4-3H]-phenylalanine (40 microCi/100 g body weight) and rate of protein synthesis was estimated from the incorporation of phenylalanine into protein. The rate of protein degradation was estimated as the difference between the synthesis and growth rates of protein. In chicks fed the amino acid-devoid diets, the fractional synthesis rates of protein (FSR, percent per day) were significantly lower than those in control chicks, whereas fractional degradation rates of protein (percent per day) were constant for all dietary treatments. The FSR in -Met, Cys chicks was lower than that in -His chicks and the lower FSR in -Met, Cys chicks seemed to result in more body protein loss. The RNA/protein ratio of chicks was almost the same in all treatments, but the protein synthesized per unit RNA varied depending on the dietary treatments. The lower amount of protein synthesized per unit RNA in -Met, Cys chicks was postulated to result from inhibition of protein synthesis by a shortage of available methionine associated with a higher degradation rate of methionine itself. These results indicated that different growth responses between -His and -Met, Cys chicks were primarily caused by the difference in the rates of protein synthesis.

[Utilization of 15N-labeled urea in laying hens. 7. 15N-incorporation into the amino acids of different muscle types]

3 colostomized laying hybrids received 1% 15N labelled urea with 96.06 atom-% 15N excess (15N') with a commercial ration over a period of 6 days. After the application of the same ration with unlabelled urea on the following 2 days the animals were butchered. In the muscles of the breast, the leg and the heart, the labelling of total nitrogen and the incorporation of urea 15N' into 15 amino acids of the 3 different kinds of muscles were ascertained. On average, significant differences could be ascertained between the atom-% 15N of the muscles of the skeleton and those of the heart. The 15N' of the breast and leg muscles was 0.25 and 0.34 atom-% resp.; that of the cardial proteins 0.71 atom-% 15N'. The incorporation of urea 15N into the basic amino acids is low and varies both between the kinds of muscles and between the amino acids. On average the highest level of labelling was found among the essential amino acids valine, isoleucine and leucine; the average atom-% 15N' for the muscles of the breast is 0.13, of the leg 0.17, and of the heart 0.27; the 15N' quota of branched chain amino acids in the total 15N' of the respective muscle is accordingly 6.0%, 5.0% and 4.5%. The non-essential amino acids, particularly glutamic acid, are more highly labelled in the muscles than the essential ones. A 15N' for glutamic acid of 0.24 atom-% in the breast muscles, of 0.27 atom-% in those of the legs and of 0.64 atom-% in the heart muscle could be detected. The average quota of the 15N' of these acid amino acids in the 15N' for breast, leg and heart muscles is 7.4, 6.2 and 6.7 resp. The quota of the 15N' in the 6 non-essential amino acids in the total 15N' in all 3 kinds of muscles is approximately two thirds and in the 9 essential ones one third of the total 15N'. Although the results show that there is a certain incorporation of 15N' from urea into the amino acids of the muscle proteins, their contribution to meeting the demands is to be considered irrelevant.

The effects of 6 hours of hypoxia on protein synthesis in rat tissues in vivo and in vitro

Rates of protein synthesis were measured in vivo in several tissues (heart, skeletal muscles, liver, tibia, skin, brain, kidney, lung) of fed rats exposed to O2/N2 (1:9) for 6 h starting at 08:00-11:00 h. Protein synthesis rates were depressed by 15-35% compared with normoxic controls in all of the tissues studied. The decreases were greatest in the brain and the skin. Although hypoxia inhibited gastric emptying, its effects on protein synthesis could probably not be attributed to its induction of a starved state, because protein-synthesis rates in brain and skin were not decreased by a 15-18 h period of starvation initiated at 23:00 h. Furthermore, we showed that protein synthesis was inhibited by hypoxia in the rat heart perfused in vitro, suggesting a direct effect. The role of hypoxia in perturbing tissue nitrogen balance in various physiological and pathological states is discussed.

Physiological model for the pharmacokinetics of cis-dichlorodiammineplatinum (II) (DDP) in the tumored rat

A physiological model has been developed to describe the disposition of cis-dichlorodiammine-platinum(II) (DDP) following i.v. dosing in the female rat bearing the Walker 256 carcinoma. The model simulates concentrations of DDP and its mobile and fixed metabolites in plasma, liver, gut, skin, muscle, tumor, carcass, and kidney, and DDP and mobile metabolite excretion following a 4 mg/kg dose. In the kinetic model, DDP binds irreversibly to low MW nucleophiles and macromolecules (largely proteins) within the plasma and tissue compartments to form mobile and fixed metabolites, respectively. Reaction rates for the formation of each metabolite are tissue/organ specific. The rate constant for the biotransformation of DDP to fixed metabolite in plasma (k2P = 0.0082 min-1) was determined from in vitro incubation studies. This rate was used as the basis for estimating the biotransformation rate constants for DDP to fixed and mobile metabolites in other compartments. Both DDP and mobile metabolite are assumed to follow flow-limited transport, to freely traverse compartmental barriers, and to partition equally in all compartments. Both are excreted in the urine, the major route of Pt elimination. Urinary excretion is modeled as a linear process involving filtration only; an assumption based on a calculated renal clearance of 1.1 ml/min, a value very similar to the estimated GFR. Biliary excretion is a minor route of mobile metabolite elimination and is modeled as a linear process occurring in the liver. Four hours after dosing, approximately 60% of the administered Pt remains in the tissues and plasma. Of this, over 75% of the plasma Pt and 90% of the metal ion in every other compartment is fixed (protein bound). Fixed Pt can be eliminated from a compartment only after its biotransformation to mobile metabolite. In most compartments this rate of elimination corresponds closely to the average rate of protein turnover in that compartment.

Rate of endogenous methionine oxidation in rats at different levels of methionine intake

The fractional rates of loss of 14C and body-weight were measured in growing female rats after a single dose of [l-14C]methionine. After the dose, the rats were maintained for 30 d on diets containing graded amounts of methionine and killed at 30 d. By 20 d after the dose the level of labelling of body proteins was fairly uniform. From 20 to 30 d, 14CO2 was collected at intervals. The rats were killed at 30 d and the total radioactivity remaining in the body was measured. The fractional loss of 14CO2 between days 20 and 30 was constant regardless of methionine intake except at the highest intake level. After correcting the fractional loss of 14CO2 for urinary 14C loss, a value of 35 mg/kg body-weight (20 mg/kg body-weight 0.75) was obtained for the rate of endogenous methionine oxidation.

Protein synthesis in skin and bone of the young rat

Fractional rates of protein synthesis in tissues of young growing rats were estimated by injection of flooding amounts (1.5 mmol/kg body-weight) of [3H]phenylalanine. Rates of 63.6%/d and 90.4%/d respectively were obtained in the skin and bone (tibia) of fed animals. These rates were comparable to that in liver (86.3%/d) but considerably higher than in muscle (16.9%/d). Absolute amounts of protein synthesized in tissues of fed rats were estimated. Together the skin and bones accounted for 25% of whole-body synthesis, a value similar to the contribution of liver (15%) and muscle (25%). In fed rats the ratio, RNA:protein in skin and bone was lower than in liver, but much higher than in muscle. However, the amounts of protein synthesized per unit RNA in skin and bone were higher than in both liver and muscle. After 2 d of starvation the fractional rates of protein synthesis in skin and bone fell by 26% and 31% respectively. This was greater than the fall in liver (17%) but less than in muscle (66%). In bone the fall in synthesis was accompanied by decreases in both RNA:protein and synthesis per unit RNA, but in skin there was a fall in RNA:protein which was partially countered by an increase in the rate of synthesis per unit RNA.

Influence of low-energy diets on whole-body protein turnover in obese subjects

Rates of whole-body protein synthesis and breakdown in obese subjects have been measured by three methods: constant intravenous infusion of [1-14C]leucine, repeated oral doses of [15N]glycine, and a single oral dose of [15N]glycine. The three techniques gave similar rates of synthesis and breakdown when the subjects received a normal diet containing 8.0 MJ and 70 g protein. After 3 wk on a low-energy diet (2.1 MJ), repeat measurements were made. When the low-energy diet contained protein (50 g), rates of protein synthesis and breakdown were little different from those with the normal diet. When the low-energy diet contained no protein, there was a 40% fall in whole-body protein synthesis and a smaller fall in breakdown. Excretion of 3-methylhistidine in the urine did not change with either low-energy diet. We conclude that the decrease in dietary energy from 8.0 to 2.1 MJ did not influence protein turnover, but that dietary protein was necessary if rates of whole-body protein synthesis and breakdown were to be maintained.

The effects of food deprivation on protein turnover and nucleic acid concentrations of active and immobilized extensor digitorum longus muscles of the rat

Deprivation of food caused significant changes in the weight, protein content, protein turnover and RNA concentrations of the extensor digitorum longus muscle. Simultaneous immobilization to render the muscle inactive did not make the tissue any more susceptible to the effects of starvation. In contrast, immobilization in a stretched state resulted in less muscle wasting after deprivation of food.

The effects of dietary nitrogen level on the collagen of rat skin

1. Male rats of approximately 120 g body-weight were maintained on a commercial stock diet containing 204 g crude protein (nitrogen x 6.25)/kg, a hydroxyproline-free high-protein (HP) diet containing 200 g casein/kg as the only protein source, or a low-protein (LP) diet containing 40 g casein/kg. After 6 weeks on these diets half of each group was transferred to a non-protein (NP) diet and the experiment was continued for a further 6 weeks. Animals from each group were killed at 4 d, 3 weeks and 6 weeks after the transfer to the NP diet. 2. Throughout the experiment the urinary excretion of N, hydroxyproline and creatinine, and the content and solubility of the skin collagen were determined. 3. When compared with a control group killed at the beginning of the experiment the rats maintained on the LP diet showed an increase of 25% in total N content of the skin but collagen content increased by 100%. Rats transferred from the HP to the NP diet lost both N and collagen from the skin, but those transferred from the LP to the NP diet lost N but increased the collagen content by 42%. 4. Protein deprivation brought about marked changes in the solubility of the skin collagen, suggesting an increase in the rate of maturation of skin collagen.

Changes in protein synthesis after trauma: importance of nutrition

The effect of trauma (femur fracture) on protein synthesis in rats given four different parenterally administered nutrient formulations ("diets") was investigated. Diet I was a maintenance formulation. It contained sufficient amino acids and glucose to keep the rats in nitrogen balance but was inadequate to support growth. Diet II was amino acids only, diet III was severely hypocaloric glucose, and diet IV was diet I minus the amino acids. The rats received parenterally all nutrients for the duration of the 4-5 day study. Twenty-four hours posttrauma, the fractional synthesis rates of liver, lung, heart, kidney, and muscle were estimated by infusing [15N]glycine for 17.5-19 h. (T.P. Stein, J.C. Oram-Smith, M.J. Leskiw, H.W. Wallace, L.C. Long, and J.M. Leonard. The effects of protein and calorie restriction on protein synthesis in the rat. Am. J. Physiol. 230: 1321-1326, 1976.) Two trends were found: i) on the amino acid-containing diets (I and II), most protein synthesis rates were increased posttrauma. Lung was the exception: no change was found; ii) rats on the amino acid deficient diets (III and IV) showed a decrease in the fractional muscle and lung synthesis rates posttrauma. We concluded that in this rat trauma model the requirement is mostly for amino acid nitrogen posttrauma.

[Nitrogen metabolism of broilers fed 15-N labeled wheat. 4. 15-N concentration in muscles and nitrogen utilization]

Broilers were fed 15N labelled wheat and the concentration of the labelled isotope in muscular tissue was determined. A negative correlation was found to exist between the percentage proportion of N in the muscle fractions and the level of 15N frequency established 12 hrs after administration of the labelled ration. Higher rates of tracer incorporation were observed in the muscles of legs and other muscles (predominatly red muscles) than in pectoral muscles (white muscles). The heart was shown to exhibit the lowest N content and the highest level of 15N frequency of all muscle fractions. In view of the fact that some specific difficulties will arise when certain factors (trace elements, vitamins, ergotropic substances, genetic factors) which influence the N metabolism of fattening poultry, are estimated on the basis of the conventional N balance trials the present tracer technique is recommended for use as an alternative method in studies of the kind just described. In the present trial 20% of the wheat N were utilized for the production of meat N in muscular tissue.

[Nitrogen metabolism of broilers fed N-15 labeled wheat. 3. Blood, liver, spleen, thymus, kidney, brain, pancreas, labile N reservoir]

Studies on the N metabolism of 4-week-old male broilers fed 15N Labelled wheat revealed a close correlation existing between the levels of 15N concentrations in the small intestine and in the plasma NPN fraction of the blood. The highest level of excess 15Natom0/0 in plasma proteins was noted as late as 20-30 hrs after administration of the last tracer dose. The mean life time of plasma proteins in broilers is less than 2.0 days. The rapid and statistically significant decrease of 15N frequency in the liver, spleen, thymus, and kidneys in the period from 12 hrs after withdrawal of the last 15N dose until the end of the trial reflects the intensity of N turnover in these organs. In the period from 6 hrs to 12 hrs after the last N intake the mean quantities of N in the liver, small intestine and kidneys decreased by 18%, 29%, 26% and 16%. The quantity of N accumulated in these organs after food intake is gradually released into the organism.

The effect of protein deprivation and starvation on the rate of protein synthesis in tissues of the rat

1. The fractional rate of protein synthesis was measured in tissues of rats in vivo by continuous infusion of [14C]tyrosine. In growing animals proteins of liver and kidney were renewed at a rate greater than 50% per day, those in skeletal muscle, brain and heart at a rate between 13 and 23% per day. 2. Protein synthesis was also measured in liver, kidney, heart, brain and skeletal muscle of rats either given a protein-free diet for 21 days or starved for 2 days. During the first 2 days no clear differences between the effects of these two regimes could be detected. 3. Gastrocnemius muscle did not lose tissue protein till after 9 days without protein in the diet. The rate of protein synthesis was halved after 1 day and halved again after 21 days without protein. It was deduced that the rate of protein breakdown in muscle had declined also. 4. In liver the loss of protein was immediate without any apparent change in the fractional rate of protein synthesis. Between 2 and 21 days of dietary protein deprivation the liver lost protein slowly but the fractional rate of protein synthesis was increased. It is proposed that lack of protein in the diet also causes an increase in the rate of liver protein breakdown.

Critical evaluation of a method for estimating amino acid requirements for maintenance in the rat by measurement of the rats of 14C-labelled amino acid oxidation in vivo

1. The object of the experiments was to estimate the maintenance requirements for lysine and leucine by a radioactive method. Rats were given a single dose of14C-labelled lysine or leucine and groups of animals were killed after 15, 20 and 30 d.

2. After 20 d the specific radioactivity (SR) of protein was approximately the same in liver, muscle and viscera; it was somewhat lower in skin. Once uniform SR is achieved, the rate of loss of radioactivity is a measure of the rate of endogenous loss of the amino acid.

3. The rate of loss between 20 and 30 d was measured in two ways: from the daily output of expired14CO2, and from the decrease, over the 10 d interval, of the total amount of radioactivity retained in the body.

4. For the first 15 d after administration of the labelled amino acid, all rats were given a low-protein or low-amino acid diet on which body-weight was maintained constant. For the second 15 d period some rats were kept on this diet; others were transferred either to a protein-free diet or to a diet lacking the specific amino acid (lysine or leucine) which had been administered in the labelled form.

5. The fractional rate of amino acid loss in the different experiments ranged from 1.5 to 3.5%/d, being greatest with the protein-free diet. The absolute rates of loss were calculated from measurements of the total lysine and leucine content of rats.

6. The best estimates of the rate of endogenous amino acid loss obtained in this way, expressed as mg/kg0.75per d were: lysine 136, leucine 80. These estimates are higher than most estimates of maintenance requirements obtained by growth or nitrogen balance methods and possible reasons for these discrepancies are discussed.

Concentration of elongation factor 2 in rat skeletal muscle during protein depletion and re-feeding

Rat skeletal-muscle elongation factor 2 was assayed by causing it to react with NAD(+) by using fragment A of diphtheria toxin as the catalyst. Dietary protein restriction decreased the concentration of elongation factor 2 in homogenates of whole muscle. These decreases paralleled a decline in muscle RNA so that the number of molecules of elongation factor 2 per ribosome appeared to be independent of the diet. We conclude that elongation factor 2 is probably not the factor limiting the rate of muscle protein synthesis and is not responsible for the fall in the protein-synthetic rate in vivo observed in the muscles of animals whose dietary protein intake is inadequate.

Effect of an adequate-protein diet after a low-protein diet on protein catabolism in growing rats

1. Metabolic alterations related to protein catabolism were studied in rats in transitional states induced by changing from a low-protein (LP) (50 g casein/kg) diet to a high-protein (HP) (250 g casein/kg) diet.

2. Twenty-four hours after the diet was changed, the rats showed a more rapid increase in live-weight gain than controls that had been fed on the HP diet throughout. On the 5th day after the diet change, their increase in body-weight had returned almost to the control rate. Food and therefore nitrogen intakes on the 1st and 5th days after the change in diet were the same as those of the controls. It seems likely therefore that the initial high rate of live-weight gain is an indication of a metabolic adaptation which occurred on the LP diet and which did not fully return to normal until the 5th day after the change of diet.

3. N balance was higher 24 h after the change in diet than in the controls, owing to a reduction in total urinary N and in urea excretion, but when measured on the 5th day it was similar in both groups.

4. Carcass N determination showed that, after 7 d on the LP diet total-, trichloroacetic acid (TCA)-soluble- and TCA-insoluble-N contents (in terms of mg/g rat) were all slightly lower than control values but they had almost returned to normal 5 d after the diet change. There was a significant increase in the TCA-soluble-N content after 24 h on the HP diet to a value greater than the control value.

5. Proteolysis was measured in vitro by incubation of liver slices and diaphragms under anaerobic conditions. With liver slices it was significantly lower 24 h after the diet change than in control rats. On the 5th day it was significantly higher than 24 h after the diet change but had not quite reached the control level. In the diaphragm, proteolysis was also lower 1 d after the diet change, and had not increased at all by the 5th day.

6. Ureogenesis in the liver was reduced significantly 24 h after the diet change and it had almost returned to the control level on the 5th day. On the other hand, arginase (L-arginine amidinohydrolase;EC3·5·3·1) activity was significantly lower 24 h after the diet change and did return completely to the control level on the 5th day.

7. These results show that the initial increased N balance and reduced N excretion were due to enzymic adaptation to the LP diet, the reduced N excretion being attributable to reduction in hepatic urea production.

Dietary protein intake and skeletal-muscle protein metabolism in rats. Studies with salt-washed ribosomes and transfer factors

1. Aspects of skeletal muscle protein synthesis in vitro were studied in young rats given a low-protein diet for up to 10 days and during re-feeding with an adequate diet. 2. Partially purified muscle transfer factors (transferases I and II), crude and purified (NH(4)Cl-washed) ribosomes and a pH5 enzyme fraction were prepared for this purpose. 3. A marked decrease in the capacity of crude ribosomes to carry out cell-free polypeptide synthesis occurred within 4 days of feeding the low-protein diet. 4. The capacity of salt-washed ribosomes to promote amino acid polymerization, in the presence of added transfer factors and aminoacyl-tRNA, was only slightly decreased by the dietary treatment. 5. However, the capacity of salt-washed ribosomes to bind (14)C-labelled aminoacyl-tRNA was decreased by feeding the low-protein diet. 6. The capacity of the pH5 enzyme fraction to promote amino acid incorporation in a complete cell-free system was decreased within 2 days of feeding the low-protein diet. There is no evidence that the change is associated with aminoacyl-tRNA synthetase or binding enzyme activities of the pH5 fractions. 7. These changes are discussed in relation to the diminished rate of protein synthesis in the intact muscle cell when rats are given a low-protein diet.

The effects of different dietary contents of protein on amino acid and glucose production and on the contribution of amino acids to gluconeogensis in sheep

1. Free amino acid entry rates, rates of portal uptake of amino acids and rates of glucose synthesis from amino acids have been measured during the continuous intravenous infusion of [U-14C]-labelled mixed amino acids in six sheep receiving diets that supplied different amounts of protein.

2. In four of these sheep and in one other, total rates of glucose production have been measured using continuous intravenous infusions of [U-14C]-labelled glucose.

3. A signiiicant correlation was found between total amino acid entry rate ( Y mg/min.kg) and daily protein intake ( X g/kg):Y= 2·14+ 1·38X (r = 0·878, 0·02 > P > 0·01).

4. A significant correlation was found between the rate of absorption of amino acids into the portal system ( Y mg/min.kg) and the daily protein intake ( X g/kg):Y= 0·58 + 0·58X (r = 0·884, 0·02 > P > 0·01).

5. A highly significant correlation was found between total glucose production rates (Ymglmin. kg) and daily protein intake ( X g/kg): and daily protein intake ( X g/kg): Y = 2.14+ 1.38X (r = 0·878, 0·02 > P > 0·01).Y= 0·375f0·702X (r = 0·866, 0·005 > P > 0·001).

6. The mean proportional contribution to total amino acid entry made by portal absorption was 33·5 f 1·8 yo (six animals).

7. A significant correlation was found between the rate of glucose production from amino acids (Ymg/min.kg) and the rate of entry of amino acids ( X mg/min.kg):Y= 0-189X-0·414 ( r = 0·84, 0·01 > P > 0·005).

8. The best approximation of the proportion of glucose derived from amino acids was 28·16 f 5·1 % (six animals).

9. The specific radioactivities of amino acids in liver, kidney and muscle did not approach those found in plasma during infusions of [U-14C]-labelled mixed amino acids of up to 6·75 h.

Relationship between body collagen and urinary hydroxyproline excretion in young rats fed on a low-protein or low-calorie diet

1. Hydroxyproline was determined in urine, carcass, and skin of young rats fed on low-protein and low-calorie diets.

2. Protein and calorie deprivation in young rats resulted in a marked fall in the soluble as well as insoluble collagen content of the skin.

3. The collagen content of the carcass (excluding skin) was slightly greater in the deficient rats than in the weanling rats, but very much lower than in the well-fed rats.

4. In rats subjected to protein and calorie deficiency, urinary hydroxyproline was markedly reduced. In these rats, most of the reduction in urinary hydroxyproline was observed within 1 week of feeding the deficient diet. Protein deficiency had affected urinary excretion of hydroxyproline more severely than calorie deficiency.

5. The reduction of urinary hydroxyproline in the deficient animals can be attributed to decreased turnover rate of their body collagen. This apparent decrease in turnover rate is explained as being due to an increase in the proportion of collagen of slow turnover in these animals.

6. Protein and calorie deficiency had also resulted in the depletion of non-collagen nitrogen of both carcass and skin.

Amino acid balance across tissues of the forearm in postabsorptive man. Effects of insulin at two dose levels

Amino acid balance across skeletal muscle and across subcutaneous adipose tissue plus skin of the forearm has been quantified in postabsorptive man before and after insulin infusion into the brachial artery. Skeletal muscle released significant amounts of alpha amino nitrogen after an overnight fast. Most individual amino acids were released. Alanine output was by far the greatest. The pattern of release probably reflects both the composition of muscle protein undergoing degradation and de novo synthesis of alanine by transamination. A significant correlation was observed between the extent of release of each amino acid and its ambient arterial concentration. Elevation of forearm insulin in eight subjects from postabsorptive (12 muU/ml) to high physiologic levels (157 muU/ml) in addition to stimulating muscle glucose uptake blocked muscle alpha amino nitrogen release by 74%. Consistent declines in output were seen for leucine, isoleucine, tyrosine, phenylalanine, threonine, glycine, and alpha-aminobutyric acid. Alanine output was insignificantly affected. Doubling forearm insulin levels (from 10 to 20 muU/ml) in eight subjects increased muscle glucose uptake in three and blocked alpha amino nitrogen output in two although both effects were seen concurrently in only one subject. Changes in net amino acid balance after insulin could be accounted for by increased transport of amino acids into muscle cells or retardation of their exit. It is likely that ambient arterial amino acid concentrations are established and maintained primarily by the extent of muscle amino acid release. The individual amino acids whose outputs from forearm muscle decline after forearm insulinization correspond well with those whose levels fall systematically after systemic insulinization. This suggests that declines in amino acid levels after systemic insulinization are due to inhibition of muscle release. Doubling basal insulin approaches the threshold both for blockade of muscle amino acid output and stimulation of glucose uptake, effects which appear to occur independently.

Adaptation of mammalian protein metabolism to amino acid supply

The metabolism of the animal is equipped to adapt to changes in both the internal and the external environment. Among internal factors are activity versus rest and sleep, and the menstrual cycle in the case of the female. Metabolism must also respond to variations in the external environment, such as heat and cold, and notably the availability of food. Metabolic adaptation to nutrient supply is of two kinds. First, there are transient physiological adaptations to the intermittent intake of nutritionally adequate meals. These short-lived adaptations account for a large part of the diurnal variations that have been observed in the protein metabolism of mammals (Wurtman, 1969). Secondly, long-term adaptive reactions occur when there is a decrease in availability of an essential nutrient in the diet. Under such circumstances, tissue constituents are lost to varying degrees from different parts of the body. It is proposed to discuss here mainly short-term physiological adaptations to variations in amino acid supply and their relevance to the needs of the body for dietary protein. Adaptive changes resulting from long-term protein deficiency are considered in the paper by Waterlow & Stephen (1969).

Metabolism of amino acids in protein-calorie-deficient rats

The overall oxidative degradation of leucine and phenylalanine, measured in vivo in rats fed on a 2%-casein diet for 8 weeks, is markedly decreased as compared with controls, whereas that of glutamate and alanine is apparently unaffected. The decrease in leucine degradation is due, at least in part, to a block before the formation of 3-methylbutyryl-CoA (isovaleryl-CoA) in the catabolic pathway. This phenomenon is accompanied by increased incorporation of [(14)C]leucine into liver proteins, decreased urinary excretion of leucine and increased urinary excretion of 4-methyl-2-oxopentanoate (alpha-oxoisocaproate) by protein-depleted animals. The results suggest the existence of adaptive mechanisms that function to conserve an indispensable carbon skeleton.