Free amino acids in post-mortem cerebral cortices from patients with Alzheimer-type dementia

Concentrations of free amino acids were measured in the cerebral cortices of post-mortem brains from 5 histologically verified cases of Alzheimer-type dementia (ATD) and 8 histologically normal controls. The concentration of glutamate in the ATD brains was significantly lower in the superior frontal, orbital, cingulate and inferior temporal cortices when compared with the control brains. The concentrations of taurine and gamma-aminobutyric acid in the ATD brains were significantly lower in the inferior temporal cortex. These findings suggest that amino acid neurons could be involved in ATD.     

Voltammetrically monitored brain ascorbate as an index of excitatory amino acid release in the unrestrained rat

To discover the significance of changes in the extracellular concentration of brain ascorbate, we used linear sweep voltammetry to monitor the ascorbate signal. Recordings were made with carbon paste electrodes implanted in the striatum and hippocampus of anaesthetised and unanaesthetised rats under a variety of conditions. Intraperitoneal administration of excitatory amino acid transmitters, but not tyrosine or glycine, increased extracellular striatal ascorbate; similarly, microinfusion of L-glutamate beside striatal electrodes enhanced the ascorbate signal. Electrical stimulation of the perforant path increased the extracellular concentration of dentate ascorbate in the unanaesthetised, but not in the anaesthetised, rat. These results support our hypothesis that changes in the extracellular concentration of brain ascorbate monitored by voltammetry reflect the release of excitatory amino acids.     

Influence of dietary protein level on protein self-selection and plasma and brain amino acid concentrations

Control of protein intake was studied in young rats that were allowed to choose between either protein-free and 55% casein diets or 15% and 55% casein diets. Animals on the protein-free vs. 55% casein regimen exhibited a lower weight gain, a lower cumulative energy intake and a greater cumulative total protein intake during the 13-day study compared to rats selecting between 15% and 55% casein. The daily average proportion of total food selected as casein by animals choosing between protein-free and 55% casein diets increased from 15% to 38% during the course of the study. In contrast, rats choosing between 15% and 55% casein chose 18-22% of total food as protein throughout the entire study. Long-term protein intake or protein selection did not correlate significantly with whole-brain contents of 5-HT or 5-HIAA. Our results suggest that protein intake is not regulated at a constant proportion of total calories, but is controlled between a minimum level that will support rapid growth and a maximum that, if exceeded, would require the animal to undergo substantial metabolic adaptation. The mechanism controlling protein selection may involve diet-induced changes in the brain content of total free indispensable amino acids.     

Protein turnover,amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (¹³C-lysine, ¹⁵N-glycine, ²H₅-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg⁻¹·day⁻¹ compared to 0.8 g·kg⁻¹·day⁻¹ in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.     

Subcellular distribution of the immunoreactive peptide histidine isoleucine in the rat brain and release from synaptosomes in vitro

Subcellular fractionation studies of rat brain homogenate revealed that peptide histidine isoleucine-like immunoreactivity (PHI-LI) was enriched in synaptosomal preparations. PHI-LI was released by high potassium concentrations from synaptosomal pellets incubated in vitro. These results suggest a possible physiological role of PHI in the central nervous system as a neurotransmitter or modulator of synaptic function.     

Formation of the amino acid pool of the brain in rats differing in preference for ethanol

Institute of Biochemistry, Academy of Sciences of the Belorussian SSR, Grodno. ALKO Research Laboratories, Helsinki, Finland. (Presented by Academician of the Academy of Medical Sciences of the USSR G. V. Morozov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 107, No. 4, pp. 471–473, April, 1989.     

Effect of branched-chain amino acid supplementation on the exercise-induced change in aromatic amino acid concentration in human muscle

A mixture of the three branched-chain amino acids (BCAAs) was supplied to subjects during two types of sustained intense exercise, a 30 km cross-country race and a full marathon, and the effect on plasma and muscle concentrations of aromatic and BCCAs was studied. When BCAAs (7.5–12 g) were taken during exercise, the plasma and muscle (vastus lateralis) concentration of these amino acids increased, while in the placebo groups the concentration of BCAAs decreased in the plasma and remained unchanged in the muscle. In the placebo group, both types of exercise caused a 20–40% increase in the muscle concentration of the aromatic amino acids, tyrosine and phenylalanine, and the plasma concentration of these amino acids was increased after the marathon. Since tyrosine and phenylalanine are neither taken up nor metabolized by skeletal muscle, the increases in their concentrations in muscle might indicate net protein degradation during exercise. However, when the subjects were supplied with BCAAs during exercise, the increases in tyrosine and phenylalanine concentrations in both muscle and plasma were prevented. These results suggest that an intake of BCAAs during exercise can prevent or decrease the net rate of protein degradation caused by heavy exercise.     

Control of immune response by amino acid metabolism

Summary: The interaction between pathogenic microorganisms and their hosts is regulated by reciprocal survival strategies, including competition for essential nutrients. Though paradoxical, mammalian hosts have learned to take advantage of amino acid catabolism for controlling pathogen invasion and, at the same time, regulating their own immune responses. In this way, ancient catabolic enzymes have acquired novel functions and evolved into new structures with highly specialized functions, which go beyond the struggle for survival. In this review, we analyze the evidence supporting a critical role for the metabolism of various amino acids in regulating different steps of both innate and adaptive immunity.     

Dose-response effects of free fatty acids on amino acid metabolism and ureagenesis

Aim: Free fatty acids (FFAs) are important fuels and have vital protein‐sparing effects, particularly during conditions of metabolic stress and fasting. However, it is uncertain whether these beneficial effects are evident throughout the physiological range or only occur at very high FFA concentrations. It is also unclear whether secondary alterations in hormone levels and ketogenesis play a role. We therefore aimed at describing dose–response relationships between amino acid metabolism and circulating FFA concentrations at clamped hormone levels. Methods: Eight healthy men were studied on four occasions (6 h basal, 2 h glucose clamp). Endogenous lipolysis was blocked with acipimox and Intralipid was infused at varying rates (0, 3, 6 or 12 μL kg^?1 min^?1) to obtain four different levels of circulating FFAs. Endogenous growth hormone, insulin and glucagon secretion was blocked by somatostatin (300 μg h^?1) and replaced exogenously. 15N‐phenylalanine, 2H4‐tyrosine and 13C‐urea were infused continuously to assess protein turnover and ureagenesis. Results: We obtained four distinct levels of FFA concentrations ranging from 0.03 to 2.1 mmol L^?1 and 3‐hydroxybutyrate concentrations from 10 to 360 μmol L^?1. Whole‐body phenylalanine turnover and phenylalanine‐to‐tyrosine degradation decreased with increasing FFA levels as did insulin‐stimulated forearm fluxes of phenylalanine. Phenylalanine, tyrosine and urea concentrations also decreased progressively, whereas urea turnover was unperturbed. Conclusion: Circulating FFAs decrease amino acid concentrations and inhibit whole‐body phenylalanine fluxes and phenylalanine‐to‐tyrosine conversion. Our data cover FFA concentrations from 0 to 2 mmol L^?1 and indicate that FFAs exert their protein conserving effects in the upper physiological range (>1.5 mmol L^?1).     

Subcutaneous release of amino acid loads on food and water intakes in the rat

A cellophane sac containing a mixture of amino acids was implanted under the skin of the freely fed and watered rat. After its dissolution in body fluid, the load diffused out of the sac according to an exponential function. Water intake increased maximally during the period of most rapid release of amino acids into the body. Food intake was depressed most effectively somewhat later. The anorexigenic effect of a mixture consisting solely of essential amino acids was greater than that of a mixture containing a proportion of dispensable amino acids. Effects of amino acids which facilitate the urea cycle indicated that ammonia toxicity contributed to the suppression of intake by the highest dose of essential amino acids (90 mmoles/kg). The moderate suppression of feeding by a mixture containing both essential and dispensable amino acids was potentiated by inclusion of glucose. This suggested that metabolic effects of the amino acids, other than ammonia production, contributed to the observed inhibition of feeding.     

Effect of branched-chain amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects

Five male endurance-trained subjects performed exhaustive exercise on a cycle ergometer at a work rate corresponding to 75% of their Vo2max after reduction of their muscle glycogen stores. During exercise the subjects were given in random order a 6% carbohydrate solution contining 7 g L^-1 of branched-chain amino acids (BCAA), a 6% CHO solution and flavoured water. The physical performance was lowered in four of the five subjects when they were given flavoured water during exercise as compared with the two conditions when CHO was supplied. No difference in performance was found when the subjects were given CHO+BCAA or only CHO during exercise. When CHO+BCAA was supplied the plasma and muscle (vastus lateralis) concentrations of BCAA increased during exercise by 120 and 35%, respectively. In the other conditions there was no change or a slight decrease in the plama concentrations of BCAA, but the muscle concentrations of BCAA were decreased after exercise. The plasma concentration of glutamine over the whole exercise period and 5 min after exercise was higher when CHO+BCAA were supplied during exercise compared with a supply of CHO alone or water. However, exercise caused no change in the muscle concentration of glutamine, whereas that of glutamate decreased in all three conditions. A supply of CHO+BCAA or CHO alone did not affect the exercise-induced increase in the plasma and muscle concentration of aromatic amino acids, indicating that neither BCAA nor CHO influenced the net protein degradation during exercise.     

Differential sensitivity of selected brain areas to excitatory amino acids

The 22Na+ efflux stimulated by selected agonists of the 4 excitatory amino acid receptors, previously detected in the striatum, has been studied on 22Na+-preloaded slices prepared from 10 major areas of the rat brain. All brain areas were found to be sensitive, albeit to varying extents, to excitatory amino acids. The cerebellum was exceptional in its high sensitivity to kainate and quisqualate and in the absence of effect of N-methyl-D-aspartate. These results support the suggestion that excitatory amino acids interact with heteregenous receptors which differ from each other not only in their pharmacological properties but also in their regional distribution.     

Effects of low and high doses of fenofibrate on protein,amino acid,and energy metabolism in rat

A feared adverse effect of dyslipidaemia therapy by fibrates is myopathy. We examined the effect of fenofibrate (FF) on protein and amino acid metabolism. Rats received a low (50 mg/kg, LFFD) or high (300 mg/kg, HFFD) dose of FF or vehicle daily by oral gavage. Blood plasma, liver, and soleus and extensor digitorum longus muscles were analysed after 10 days. The FF‐treated rats developed hepatomegaly associated with increased hepatic carnitine and ATP and AMP concentrations, decreased protein breakdown, and decreased concentrations of DNA and triglycerides. HFFD increased plasma ALT and AST activities. The weight and protein content of muscles in the HFFD group were lower compared with controls. In muscles of the LFFD group there were increased ATP and decreased AMP concentrations; in the HFFD group AMP was increased. In both FF‐treated groups there were increased glycine, phenylalanine, and citrulline and decreased arginine and branched‐chain keto acids (BCKA) in blood plasma. After HFFD there were decreased levels of branched‐chain amino acids (BCAA; valine, leucine and isoleucine), methionine, and lysine and increased homocysteine. Decreased arginine and increased glycine concentrations were found in both muscles in FF‐treated animals; in HFFD‐treated animals lysine, methionine, and BCAA were decreased. We conclude that FF exerts protein‐anabolic effects on the liver and catabolic effects on muscles. HFFD causes signs of hepatotoxicity, impairs energy and protein balance in muscles, and decreases BCAA, methionine, and lysine. It is suggested that increased glycine and decreased lysine and methionine levels are due to activated carnitine synthesis; decreased BCAA and BCKA levels are due to increased BCAA oxidation.     

Transport of branched-chain amino acids in brain slices of developing and adult rats

The accumulation of labelled leucine, isoleucine and valine by cerebral slices of developing and adult rats was studied. The accumulation increased with age by 15–25%. It was strongly (from 52 to 86%) inhibited by a 100-fold excess of phenylalanine, tryptophan and another branched-chain amino acid, but moderately activated by GABA and glutamate. The inhibitions evoked by leucine and isoleucine were slightly stronger in young than in adult rats. The corresponding 2-oxoacid analogs of leucine, isoleucine and valine were also inhibitory but less effective. The 30-min accumulation of ³H-labelled branched-chain amino acids was ostensibly higher than the increase in their total concentrations in incubated slices, which apparently bespeaks lively homoexchange of endogenous intracellular and labelled extracellular amino acids.     

Modulation of human lymphocyte proliferation by amino acids.

The amino acids required for phytohaemagglutinin (PHA) induced lymphocyte proliferation were determined by the 3H-thymidine incorporation in amino acid-deficient media. Results indicate that the PHA-stimulated lymphocytes require alanine and serine in addition to 13 other amino acids present in Eagle's minimal essential medium (arginine, cysteine, glutamine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, tyrosine, and valine). The omission of any one of the 13 amino acids would stop almost completely the proliferation of PHA-stimulated lymphocytes. The omission of serine from RPMI 1640 medium caused a mean reduction of 64% of cell proliferation, while the addition of alanine to PRMI 1640 culture medium caused a mean increment of 52%. The lymphocyte proliferation appears to be modulated by amino acids in the culture medium, and for optimal growth of lymphocytes, all these 15 amino acids are essential.     

Mouse embryo cleavage, metabolism and viability: role of medium composition

The cleavage, metabolism and viability of mouse zygotes wereassessed after culture in media of different ionic and metabolitecomposition. Medium with a high potassium concentration, characteristicof mammalian oviduct fluid, inhibited cleavage and blastocystformation (P < 0.01). This inhibition was partially alleviatedby the removal of phosphate, and subsequently abolished by supplementationwith amino acids, vitamins, insulin, epidermal growth factorand transferrin (AVIET). Glucose uptake by cultured blastocysts,measured fluorimetrically, was not affected by the ionic ormetabolite composition of the medium, but was significantlyreduced by the inclusion of AVIET (P < 0.01). Lactate productionwas also significantly reduced in the presence of AVIET (P <0.01). Calculations of metabolic activity revealed that embryoscultured in the presence of AVIET had a glycolytic activitysimilar to embryos developed in vivo. In contrast, embryos culturedin conventional embryo culture media exhibited an elevated glycolyticactivity. Culturing embryos for 4 days in a reduced lactateconcentration (4.79 mM), significantly increased fetal developmentafter transfer, compared with embryos cultured in the concentrationof lactate present in conventional embryo culture media (23.3mM; P < 0.01). In contrast, when embryos were transferredon day 3 of culture, significantly more fetuses were obtainedfrom embryos cultured in high levels of lactate (P < 0.01).Supplementation of medium with AVIET significantly increasedresultant fetal weights after transfer (P < 0.05). This studydemonstrates that different media are required to maintain embryoviability on successive days of culture, and highlights thepotential limitations of employing simple salt solutions forthe culture of preimplantation mammalian embryos.     

Identification of quinolinic acid in rat and human brain tissue

An analytical technique for the determination of the excitotoxic compound quinolinic acid (2,3-pyridine dicarboxylic acid) in brain tissue has been developed. Following sample prepurification by ion exchange and high pressure liquid chromatography, quinolinic acid is converted to the dihexafluoroisopropyl ester and the derivative is analyzed by mass fragmentography. Using the present technique quinolinic acid has been identified in both rat and human brain tissue.     

The use of antioxidants to prevent glutamate-induced derangement of calcium ion metabolism in rat cerebral cortex synaptosomes

Glutamate is shown to induce increases in intracellular Ca²⁺ concentrations ([Ca²⁺]_i), increases in⁴⁵Ca²⁺ influx, decreases in the activity of Na⁺, K⁺,-ATPase activity, and activation of the Na⁺/Ca²⁺ exchanger in rat cerebral cortex synaptosomes. NMDA receptor antagonists virtually prevented these effects. Preincubation of synaptosomes with α-tocopherol, superoxide dismutase, and ganglioside GM₁ normalized [Ca²⁺]_i,⁴⁵Ca²⁺, influx, and Na⁺, K⁺-ATPase activity in rat cerebral cortex synaptosomes exposed to glutamate. Glutamate and GM₁ activated the Na⁺/K⁺ exchanger, and their effects were additive. Calcium ions entering cerebral cortex nerve cells via NMDA receptors during exposure to high glutamate concentrations appeared to be only the trigger for the processes activating free-radical reactions. Activation of these reactions led to increases in Ca²⁺ influx into cells, decreases in Na⁺, K⁺-ATPase activity, and significant increases in [Ca²⁺]_i, though this could be prevented by antioxidants and gangliosides. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 85, No. 4, pp. 488–496, April. 1999.