Interference between leucine, isoleucine and valine during intestinal absorption

1. The reciprocal interference between l-leucine, l-isoleucine and l-valine during absorption was studied in rats both in vivo and with an everted-sac preparation in vitro. 2. After feeding with the amino acids alone there was a considerable increase in their concentration in the intestinal lumen followed by a rapid disappearance, indicating efficient absorption. Absorption was reflected by a high concentration of the respective amino acids in the portal plasma. Isoleucine and valine inhibited the absorption of leucine, and leucine inhibited the absorption of isoleucine and valine. Inhibition of absorption by the interfering amino acid was generally partly overcome after 30–60min., probably through the absorption of the interfering amino acid. At that time the rise in the concentration of the amino acid in portal plasma began. 3. These results were confirmed by experiments in vitro: isoleucine and valine inhibited the absorption rate of leucine, and leucine that of isoleucine and valine. 4. Active absorption of amino acids was rapid at low concentrations and depressed at higher concentrations.     

Acetyl Groups in Native Glucomannan from Easter Lily Bulbs

The in vitro digestibility of rice glutelin and wheat glutenin was investigated with a view to assessing their nutritional qualities, using casein and bovine serum albumin (BSA) as references. The following hydrolytic processes were adopted: pepsin-pancreation digestion (a model system before intestinal absorption) and aminopeptidase-prolidase hydrolysis [a model system for the intestinal mucosa (membrane digestion) and after intestinal absorption (intracellular hydrolysis)]. The pepsin-pancreatin digests were first examined. The degree of amino acid released from the proteins was 30% (glutelin), 23% (glutenin), 24% (casein) and 30% (BSA). A similar release pattern of individual amino acids was observed for all the proteins. The amounts of large peptide fractions increased in the order: glutelin < glutenin < casein < BSA. Glutelin was highly digestible. Apart from containing high amounts of glutamic acid (glutamine), cystine and proline, the large peptide fractions of glutelin were also rich in threonine, glycine and isoleucine while those of glutenin were only rich in glycine. The aminopeptidase-prolidase digests were examined next. Glutelin was almost completely hydrolyzed to amino acid, except for a low release of cystine, suggesting that the amino acid residues constituting glutelin could be easily utilized as nutrients in the living tissues. The degree of amino acid released from the proteins was 97% (glutelin), 93% (glutenin), 90% (casein) and 79% (BSA).

The convenient application of these model systems for the assessment of the in vitro digestibility of food proteins have been discussed.     

中华绒螯蟹幼蟹对亮氨酸和异亮氨酸的需要量

以初始体重为(0.900.02) g的中华绒螯蟹幼蟹为试验对象, 采用酪蛋白、明胶、进口鱼粉和晶体氨基酸为蛋白源, 配制成12组试验饲料, 研究中华绒螯蟹幼蟹对亮氨酸(Leu)和异亮氨酸(Ile)的需要量。饲料中Leu水平为0.87%、1.26%、1.64%、2.03%、2.39%和2.81% (分别记为Leu-1Leu-6组), Ile水平为0.69%、1.21%、1.70%、2.19%、2.70%和3.21% (分别记为Ile-1Ile-6组)。试验周期为60d。结果表明: (1)饲料中Leu含量为2.39%时, 幼蟹特定生长率和全蟹粗蛋白含量达到最大值, 而各组之间的成活率和全蟹水分、粗脂肪以及灰分无显著差异(P 0.05)。当饲料Leu含量大于2.39%时, 河蟹肌肉Leu含量、肌肉必需氨基酸总量和肌肉氨基酸总量均显著高于其他各试验组(P 0.05)。(2)饲料中Ile含量为2.19%时, 幼蟹特定生长率和全蟹粗蛋白含量达到最大值, 而各试验组之间的成活率和全蟹水分、粗脂肪以及灰分差异不显著(P 0.05)。河蟹肌肉Ile含量、必需氨基酸总量和氨基酸总量随饲料中Ile含量增加呈先升高后降低的趋势, 最大值均出现在Ile-5组(2.70% Ile), 但Ile-4组(2.19% Ile)与Ile-5组差异不显著(P 0.05)。根据特定生长率与饲料Leu或Ile水平的折线模型, 确定中华绒螯蟹幼蟹饲料亮氨酸和异亮氨酸的适宜需要量分别为饲料干物质的2.36%和2.25%, 即饲料蛋白的5.88%和5.72%。       

家蚕体内因缺乏维生素B6而引起的若干代谢变动

采用不含桑叶粉末、以去维生素牛乳酪蛋白为蛋白源的准合成饲料饲育家蚕Bombyx mori 5龄幼虫,探讨了缺乏维生素B₆（VB₆）对蚕体氨基酸代谢、脂肪酸代谢以及转氨酶活力的影响。缺乏VB₆引起支链氨基酸分解代谢受阻,幼虫体液中大量积累亮氨酸、缬氨酸和异亮氨酸。同时因绢丝腺发育停滞,丝氨酸也在体液中积累。另一方面,缺乏VB₆幼虫体液中赖氨酸、脯氨酸、精氨酸、甲硫氨酸和谷氨酸含量减少,其中赖氨酸尤为突出。推测缺乏VB₆引起赖氨酸分解代谢亢进。结果还表明,缺乏VB₆幼虫体内脂肪酸代谢异常,谷丙转氨酶活力显著低下。     

Probiotic Administration Increases Amino Acid Absorption from Plant Protein: a Placebo-Controlled,Randomized, Double-Blind,Multicenter, Crossover Study

The fate of dietary protein in the gut is determined by microbial and host digestion and utilization. Fermentation of proteins generates bioactive molecules that have wide-ranging health effects on the host. The type of protein can affect amino acid absorption, with animal proteins generally being more efficiently absorbed compared with plant proteins. In contrast to animal proteins, most plant proteins, such as pea protein, are incomplete proteins. Pea protein is low in methionine and contains lower amounts of branched-chain amino acids (BCAAs), which play a crucial role in muscle health. We hypothesized that probiotic supplementation results in favorable changes in the gut microbiota, aiding the absorption of amino acids from plant proteins by the host. Fifteen physically active men (24.2 ± 5.0 years; 85.3 ± 12.9 kg; 178.0 ± 7.6 cm; 16.7 ± 5.8% body fat) co-ingested 20 g of pea protein with either AminoAlta™, a multi-strain probiotic (5 billion CFU L. paracasei LP-DG® (CNCM I-1572) plus 5 billion CFU L. paracasei LPC-S01 (DSM 26760), SOFAR S.p.A., Italy) or a placebo for 2 weeks in a randomized, double-blind, crossover design, separated by a 4-week washout period. Blood samples were taken at baseline and at 30-, 60-, 120-, and 180-min post-ingestion and analyzed for amino acid content. Probiotic administration significantly increased methionine, histidine, valine, leucine, isoleucine, tyrosine, total BCAA, and total EAA maximum concentrations (Cmax) and AUC without significantly changing the time to reach maximum concentrations. Probiotic supplementation can be an important nutritional strategy to improve post-prandial changes in blood amino acids and to overcome compositional shortcomings of plant proteins. ClinicalTrials.gov Identifier: ISRCTN38903788

     

N-ε-fructosyllysine and N-ε-carboxymethyllysine,but not lysinoalanine,are available for absorption after simulated gastrointestinal digestion

Food processing leads to a variety of chemical modifications of amino acids in food proteins. Recent studies have shown that some modified amino acids resulting from glycation reactions can pass the intestinal barrier when they are bound in dipeptides. In this study, we investigated as to what extent modified amino acids are released from post-translationally modified casein during simulated gastrointestinal digestion. Casein was enriched with N-ε-fructoselysine, N-ε-carboxymethyllysine, and lysinoalanine, in different degrees of modification. The casein samples were subjected to a two-step proteolysis procedure, simulating gastrointestinal digestion. The digestibility of modified casein as measured by analytical size-exclusion chromatography (SEC) decreased with increasing degree of modification especially after enrichment of fructoselysine and lysinoalanine. Semi-preparative SEC of digested casein samples revealed that fructoselysine and carboxymethyllysine are released bound in peptides smaller than 1,000 Da, which is comparable to native amino acids. The glycation compounds should, therefore, be available for absorption. Lysinoalanine as a crosslinking amino acid, however, is mostly released into longer peptides of at least 30–40 amino acids which should strongly impair its absorption availability.     

Distinct anabolic signalling responses to amino acids in C2C12 skeletal muscle cells

The essential amino acids (EAA) activate anabolic signalling through mechanisms, which are unclear in detail but include increased signalling through the mammalian target of rapamycin complex 1 (mTORC1). Of all the EAA, the branched chain amino acid (BCAA) leucine has been suggested as the most potent in stimulating protein synthesis, although there have been no studies investigating the effects of each EAA on anabolic signalling pathways. We therefore undertook a systematic analysis of the effect of each EAA on mTORC1 signalling in C2C12 myotubes whereby cells were serum (4 h) and amino acid (1 h) starved before stimulation with 2 mM of each amino acid. Immunoblotting was used to detect phosphorylated forms of protein kinase B (Akt)/mTORC1 signalling enzymes. The phosphorylation of Akt was unchanged by incubation with EAA. Phosphorylation of mTOR and 4E binding protein-1 (4EBP1) were increased 1.67 ± 0.1-fold and 2.5 ± 0.1-fold, respectively, in response to leucine stimulation but not in response to any other EAA. The phosphorylation of ribosomal s6 kinase (p70S6K1) was increased by stimulation with all EAA with the exceptions of isoleucine and valine. However, the increase with leucine was significantly greater, 5.9 ± 0.3-fold compared to 1.6–2.0-fold for the non-BCAA EAA. This pattern of activation was identical in ribosomal protein s6 (RPS6) with the additional effect of leucine being 3.8 ± 0.3-fold versus 1.5–2.0-fold. Phosphorylation of eukaryotic initiation/elongation factors eIF2α and eEF2 were unaffected by EAA. We conclude that leucine is unique amongst the amino acids in its capacity to stimulate both mTOR and 4EBP1 phosphorylation and to enhance p70S6K1 signalling.     

Proteolytic processing of the alpha-chain of the lysosomal enzyme, beta-hexosaminidase, in normal human fibroblasts

The two subunits of beta-hexosaminidase undergo many post-translational modifications characteristic of lysosomal proteins, including limited proteolysis. To identify proteolytic cleavage sites in the alpha-chain, we have biosynthetically radiolabeled the transient forms, isolated these by immunoprecipitation, gel electrophoresis, and electroelution, and subjected them to automated Edman degradation. The position of the NH2-terminal amino acid was inferred from the elution cycle of the radioactive amino acid and the primary sequence encoded in the alpha-chain cDNA. The amino terminus of the precursor obtained by in vitro translation of SP6 alpha-chain mRNA in the presence of microsomes was leucine 23. The same amino terminus was found in precursor alpha-chain synthesized by normal human fibroblasts (IMR90) in a 1- or 3-h pulse or secreted by these cells in the presence of NH4Cl. The alpha-chain isolated after a 3-h pulse followed by a 5-h chase (intermediate form) included a mixture of molecular species of which the amino terminus was arginine 87 (most abundant), histidine 88, or leucine 90. After a 20-h chase (mature form) the latter species predominated. This mature form of the alpha-chain remained fully reactive with antibody raised against the carboxyl-terminal 15 amino acids, indicating little if any proteolysis at the carboxyl terminus. Thus synthesis and maturation of the alpha-chain of beta-hexosaminidase includes two major proteolytic cleavages: the first, between alanine 22 and leucine 23, removes the signal peptide to generate the precursor form, whereas the second occurs between the dibasic amino acids, lysine 86 and arginine 87. The second cleavage is followed by trimming of 3 additional amino acids to give the mature form of the alpha-chain.     

Hepatic and portal-drained viscera balances of amino acids, insulin, glucagon and gastrin in the pig after ingestion of casein or rapeseed proteins

Hepatic and intestinal balances of amino acids, insulin, glucagon and gastrin were studied in 6 non-anaesthetized Large White pigs (mean body weight 64 +/- 4.8 kg) after ingestion of casein or rapeseed proteins. The animals were fitted with permanent catheters in the portal vein, the brachiocephalic artery and the right hepatic vein. In addition, 2 electromagnetic flow probes were implanted, one around the portal vein and the other around the hepatic artery. After a preliminary adaptation to each diet the animals received at 1-wk intervals and according to a double latin square design, 3 test meals of 800 g each, one containing 23.2% of rapeseed concentrate (diet RA 12) and the others 13.9 or 27.8% of hydrochloric casein (diets CA 12 and CA 24). Each observation period lasted 12 h. Amino acids from all diets were very well absorbed. In 12 h, the absorption of total amino acids as a percentage of the ingested quantities was 99% for CA 12, 102% for CA 24 and 104% for RA 12. Hepatic uptake of total amino acids in 12 h expressed as a percentage of the absorbed quantities was 13% for CA 12, 66% for CA 24 and 25% for RA 12. Differences in the hepatic extraction rate of essential amino acids appeared between the 2 levels of casein ingestion and for Arg between the 2 protein sources. Whatever the nature of the ingested protein or the level of casein, the liver showed a net production of Asp and Glu. The production and hepatic balance of insulin were the lowest after ingestion of RA 12. No differences were noted in the same parameters for glucagon and gastrin. Independently of the nutritional situation, the hepatic extraction rate of insulin appeared to be higher than those of glucagon and gastrin. Our results showed that the nature as well as the level of dietary proteins have large effects on the sequence and volume of absorptive phenomena, the hepatic metabolism of nutrients, the production of gastrointestinal hormones and the non-hepatic tissue disposal of absorbed nutrients.     

Metabolism of branched-chain amino acids in starved rats: the role of hepatic tissue

Parameters of branched-chain amino acids (BCAA; leucine, isoleucine and valine) and protein metabolism were evaluated using L-[1-(14)C]leucine and alpha-keto[1-(14)C]isocaproate (KIC) in the whole body and in isolated perfused liver (IPL) of rats fed ad libitum or starved for 3 days. Starvation caused a significant increase in plasma BCAA levels and a decrease in leucine appearance from proteolysis, leucine incorporation into body proteins, leucine oxidation, leucine-oxidized fraction, and leucine clearance. Protein synthesis decreased significantly in skeletal muscle and the liver. There were no significant differences in leucine and KIC oxidation by IPL. In starved animals, a significant increase in net release of BCAA and tyrosine by IPL was observed, while the effect on other amino acids was non-significant. We conclude that the protein-sparing phase of uncomplicated starvation is associated with decreased whole-body proteolysis, protein synthesis, branched-chain amino acid (BCAA) oxidation, and BCAA clearance. The increase in plasma BCAA levels in starved animals results in part from decreased BCAA catabolism, particularly in heart and skeletal muscles, and from a net release of BCAA by the hepatic tissue.     

Branched-chain amino acids and arginine suppress MaFbx/atrogin-1 mRNA expression via mTOR pathway in C2C12 cell line

The effect of amino acid on muscle protein degradation remains unclear. Recent studies have elucidated that proteolysis in catabolic conditions occurs through ubiquitin-proteasome proteolysis pathway and that muscle-specific ubiquitin ligases (atrogin-1 and MuRF1) play an important role in protein degradation. In the present study, we examined the direct effect of 5 mM amino acids (leucine, isoleucine, valine, glutamine and arginine) on atrogin-1 and MuRF1 levels in C2C12 muscle cells and the involved intracellular signal transduction pathway. Leucine, isoleucine and valine suppressed atrogin-1 and MuRF1 mRNA levels (approximately equal to 50%) at 6 and 24 h stimulations. Arginine showed a similar effect except at 24 h-treatment for atrogin-1 mRNA. However, glutamine failed to reduce atrogin-1 and MuRF1 mRNA levels. The inhibitory effect of leucine, isoleucine or arginine on atrogin-1 mRNA level was reversed by rapamycin, although wortmannin did not reverse the effect. PD98059 and HA89 reduced basal atrogin-1 level without influencing the inhibitory effects of those amino acids. The inhibitory effect of leucine, isoleucine or arginine on MuRF1 mRNA levels was not reversed by rapamycin. Taken together, these findings indicated that leucine, isoleucine and arginine decreased atrogin-1 mRNA levels via mTOR and that different pathways were involved in the effect of those amino acids on MuRF1 mRNA levels.     

Kinetics of L-[1-(13)C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein

In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1-(13)C]leucine was examined when ingested either together with a mixture of free amino acids simulating casein (extrinsically labeled; condition A), along with the intact casein (extrinsically labeled; condition B), or bound to casein (intrinsically labeled; condition C). Fed state leucine oxidation (Ox), nonoxidative leucine disposal (NOLD), protein breakdown, and splanchnic uptake have been compared using an 8-h oral [1-(13)C]leucine and intravenous [(2)H(3)]leucine tracer protocol while giving eight equal hourly mixed meals. Lower leucine Ox, increased NOLD, and net protein synthesis were found with condition C compared with condition A (19.3 vs. 24.9; 77 vs. 55.8; 18.9 vs. 12.3 micromol. kg(-1). 30 min(-1); P < 0.05). Ox and NOLD did not differ between conditions B and C. Splanchnic leucine uptake calculated from [1-(13)C]- and [(2)H(3)]leucine plasma enrichments was between 24 and 35%. These findings indicate that the form in which leucine is consumed affects its immediate metabolic fate and retention by the body; the implications of these findings for the tracer balance technique and estimation of amino acid requirements are discussed.     

Studies concerning the specificity of the effect of leucine on the turnover of proteins in muscles of control and diabetic rats.

The protein anabolic effect of branched chain amino acids was studied in isolated quarter diaphragms of rats. Protein synthesis was estimated by measuring tyrosine incorporation into muscle proteins in vitro. Tyrosine release during incubation with cycloheximide served as an index of protein degradation. In muscles from normal rats the addition of 0.5 mM leucine stimulated protein synthesis 36--38% (P less than 0.01), while equimolar isoleucine or valine, singly or in combination were ineffective. The three branched chain amino acids together stimulated no more than leucine alone. The product of leucine transamination, alpha-keto-isocaproate, did not stmino norborane-2-carboxylic acid (a leucine analogue) were ineffective. Leucine and isoleucine stimulated protein synthesis in muscles from diabetic rats.Leucine, isoleucine, valine and the norbornane amino acid but not alpha-ketoisocaproate or beta-hydroxybutyrate decreased the concentration of free tyrosine in tissues during incubation with cycloheximide; tyrosine release into the medium did not decrease significantly. Leucine caused a small decrease in total tyrosine release, (measured as the sum of free tyrosine in tissues and media), suggesting inhibition of protein degradation. The data suggest that leucine may be rate limiting for protein synthesis in muscles. The branched chain amino acids may exert a restraining effect on muscle protein catabolism during prolonged fasting and diabetes.     

The digestion rate of protein is an independent regulating factor of postprandial protein retention

To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of identical amino acid composition and nitrogen contents but of different digestion rates. Four groups of five or six young men received an L-[1-13C]leucine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking casein composition (AA), a single meal of rapidly digested whey proteins (WP), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rate. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) meals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). The fast meals induced a strong, rapid, and transient increase of aminoacidemia, leucine flux, and oxidation. After slow meals, these parameters increased moderately but durably. Postprandial leucine balance over 7 h was higher after the slow than after the fast meals (CAS: 38 +/- 13 vs. AA: -12 +/- 11, P < 0.01; RPT-WP: 87 +/- 25 vs. WP: 6 +/- 19 micromol/kg, P < 0.05). Protein digestion rate is an independent factor modulating postprandial protein deposition.     

Net appearance of amino acids in portal blood during the digestion of casein or rapeseed proteins in the pig

The kinetics of appearance of amino acids (AA) in portal blood following the ingestion of casein or rapeseed protein were compared. Six pigs, fitted with permanent catheters in the portal vein and in the carotid artery, as well as with an electromagnetic flow probe around the portal vein, received three 800 g test meals, one containing 12% rapeseed proteins (RA12) and the others containing 12% and 24% casein (CA12 and CA24), at 1-week intervals and according to a double Latin square design. Portal and arterial blood samples were collected and portal blood flow rate was recorded for 8 h after the test meals. At the end of measurement, an average of 76.1 +/- 5.6% (mean +/- SEM) of total AA from the CA24 diet had appeared in portal blood, compared with 94.3 +/- 10.4% for the CA12 diet and 103.5 +/- 12.6% for the RA12 diet. Similar results were obtained for essential AA. Differences were found in the kinetics of appearance of individual AA. Eight hours after the meal, 79% of lysine, 84% of methionine, and 73% of valine from the CA24 diet had appeared in portal blood compared, respectively, with 100, 89, and 83% from the CA12 diet and 99, 86, and 106% from the RA12 diet. Arginine from rapeseed had a net appearance level lower (82%) than the overall mixture of essential AA. With casein diets, the net appearance of arginine reached 97% (CA12) and 82% (CA24). Following the ingestion of rapeseed proteins, there seemed to be a significant appearance of endogenous AA in portal blood.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of gastric digestive products from casein on CCK release by intestinal cells in rat

We investigated the ability of gastric digestive products from casein to stimulate cholecystokinin release by intestinal cells using the isolated vascularly perfused rat duodenojejunum. Casein digests were prepared with an in vitro system simulating gastric digestion and emptying.

The luminal infusion of the digesta emptied from the artificial stomach for the first 10 minutes produced a sharp rise of portal cholecystokinin-like immunoreactivity to 300% of basal, followed by a well-sustained plateau secretion until the end of the infusion. The residual casein fraction of this digest brought about a modest cholecystokinin secretion, while the peptide component was as strong a stimulant as total digest. The peptide responsible for this effect was the glycomacropeptide that is a glycosylated fragment (106–169) of κ-casein. Only the slightly glycosylated forms of the peptide originating from variant A of κ-casein were active. The carbohydrate-free peptide did not alter basal cholecystokinin. The highly glycosylated forms of the peptide and the slightly glycosylated peptide from κ-casein variant B induced only a transient and low rise of portal cholecystokinin. The removal of N-acetylneuraminic acid from the active peptide suppressed its effect, while the infusion of an N-acetylneuraminic acid solution induced only a very low response.

It is concluded that the glycomacropeptide released from dietary casein during gastric digestion can stimulate cholecystokinin release by intestinal cells in the rat. A well-defined structure is required for the peptide activity. A part of the peptide chain and some glycosidic chains containing N-acetylneuraminic acid, especially those bound to the amino acid residue threonyl 31 of caseinomacropeptide variant A, would be involved in this structure.     

Acute regulation of IGF-I by alterations in post-exercise macronutrients

This investigation sought to examine the contributions of exercise and nutrient replenishment on in vivo regulation of the insulin-like growth factor-I (IGF-I) axis components. Eight college-aged males completed three high-intensity interval training (HIIT) protocols followed by three post-exercise nutritional protocols: (1) placebo (EX); (2) carbohydrate only (CHO); and (3) essential amino acid/carbohydrate (EAA/CHO). Samples were analyzed for growth hormone (GH), free IGF-I, IGFBP-1, IGFBP-2, insulin, hematocrit, hemoglobin, serum leucine, matrix metalloproteinase-9 (MMP-9) proteolytic activity, and presence of IGFBP-3 protease activity. No evidence for IGFBP-3 proteolysis was observed. Significant increases in [free IGF-I] and [leucine] were observed in the EAA/CHO group only. Significant differences were noted in [IGFBP-1] and [IGFBP-2] across conditions. Significant increases in [GH] and MMP-9 activity were observed in all groups. These results indicate that post-exercise macronutrient ratio is a determinant of [free IGF-I], [IGFBP-1 and -2] and may play a role in modulating the IGF-I axis in vivo.     

Non-essential amino acids play an important role in adaptation of the rat exocrine pancreas to high nitrogen feeding

We have previously demonstrated that feeding a diet with a high amino acid (60% AA diet) content, as a mixture simulating casein, induced pancreatic growth and pancreatic protease production in rats. In the present study, we examined the effects of an increasing dietary content of essential amino acids (EAA, x1 - x3 in exp. 1 and x1 - x3.3 in exp. 2) and non-essential amino acids (NEAA, x1 - x3 in exp. 1 and x1 - x5.2 in exp. 2) on pancreatic growth, amylase and protease adaptation using casein-type amino acid mixtures (exp. 1, basal diet; 20% AA diet) and egg white-type amino acid mixtures (exp. 2, basal diet; 12% AA diet). Pancreatic growth and trypsin activity were induced as the dietary content of NEAA was increased in experiments 1 and 2. Amylase activity in the pancreas was also induced as the dietary content of NEAA was increased, even with the decrease in dietary carbohydrate in experiment 2. The values of all pancreatic variables decreased with the increase in dietary EAA (x2 and x3) without an increase in NEAA. The changes in the pancreas were coincident with increases in plasma arginine and lysine concentrations and a decrease in the plasma alanine concentration. In rats fed a 60% AA diet (EAA and NEAA x3), in the case of which the EAA content was balanced with the NEAA content, pancreatic growth and protease production increased and reached maximum levels as the plasma amino acid concentrations decreased, except for alanine. These results show that NEAA, not EAA, are associated with induction of pancreatic growth and protease production upon feeding a diet with a high AA content, and that some metabolites may be involved in the induction process. The suppression of pancreatic growth and protease production in rats fed the high EAA diets without balanced NEAA may be associated with impairment of amino acid metabolism rather than the increments in the concentration of one or more essential amino acids. Our results also suggest that there is an unknown mechanism or unknown factors involved in regulating pancreatic amylase.     

Effect of alanyl-glutamine on leucine and protein metabolism in irradiated rats

Summary. The mechanism by which glutamine produces a favorable effect in the treatment of sepsis, injury, burns and abdominal irradiation is not completely understood. The main aim of this study was to evaluate the effect of alanyl-glutamine (AlaGln) administration on the metabolism of proteins in irradiated rats. The rats were exposed to whole-body irradiation (8 Gy) and then fed intragastrically with a mixture of glucose and amino acids either with AlaGln or without AlaGln. At 48 hours after irradiation, parameters of whole-body protein metabolism and DNA synthesis in intestinal mucosa were investigated using a primed, continuous infusion of [1-¹⁴C]leucine and [³H]thymidine. In addition, we evaluated the effect of irradiation and AlaGln on gut morphology, blood count and amino acid concentrations in blood plasma and skeletal muscle. Control rats were not irradiated but were given identical treatment. An increase in whole-body leucine oxidation, and insignificant changes in whole-body proteolysis and in protein synthesis were observed after irradiation. In irradiated rats we observed a decrease in muscle glutamine concentration, a decrease in protein synthesis in jejunum, colon and heart, and an increase in synthesis of proteins of blood plasma and spleen. Morphological examination and measurement of DNA synthesis failed to demonstrate any favorable effect of AlaGln supplementation on irradiated gut. However, administration of AlaGln resulted in a decrease in whole-body proteolysis and leucine oxidation which caused an increase in the fraction of leucine incorporated into the pool of body proteins. We conclude that the data obtained demonstrate that irradiation induces metabolic derangement associated with increased oxidation of essential branched-chain amino acids (valine, leucine and isoleucine) and that these disturbances can be ameliorated by administration of AlaGln. Received February 14, 2000 Accepted July 12, 2000     

Effect of high levels of selected dietary essential amino acids on hypercholesterolemia and down-regulation of hepatic LDL receptors in rabbits.

Earlier studies showed that the elevation of serum total and low density lipoprotein (LDL) cholesterol levels produced in rabbits by feeding high levels of a casein amino acid mixture in a cholesterol-free, semipurified diet was due primarily to the essential amino acids (EAA) in the mixture. Replacing all of the non-essential amino acids in the mixture by glutamic acid (45% EAA+Glu) had little effect on the hypercholesterolemia produced by the EAA. Experiments designed to identify the hypercholesterolemic EAA showed that (i) feeding high levels of ketogenic EAA only (45% EketoAA) gave a substantial but variable elevation of serum total and LDL cholesterol and (ii) feeding high levels of all EAA except arginine (45% EAA-Arg) gave a particularly strong hypercholesterolemic response. In rabbits fed the 45% EAA-Arg diet and to a lesser extent, in those fed the 45% EAA+Glu diet, EDTA-sensitive binding of 125I-LDL to hepatic membranes in vitro was reduced compared to a control, low-cholesterolemic group fed all essential and non-essential amino acids at a level corresponding to 14.7% casein, indicating that the hypercholesterolemia was associated with down-regulation of hepatic LDL receptors.