Concentrations Of Large Neutral Amino Acids Research Articles

Correlations of blood and brain biochemistry in phenylketonuria: Results from the Pah-enu2 PKU mouse

BackgroundIn phenylketonuria (PKU), treatment monitoring is based on frequent blood phenylalanine (Phe) measurements, as this is the predictor of neurocognitive and behavioural outcome by reflecting brain Phe concentrations and brain biochemical changes. Despite clinical studies describing the relevance of blood Phe to outcome in PKU patients, blood Phe does not explain the variance in neurocognitive and behavioural outcome completely. MethodsIn a PKU mouse model we investigated 1) the relationship between plasma Phe and brain biochemistry (Brain Phe and monoaminergic neurotransmitter concentrations), and 2) whether blood non-Phe Large Neutral Amino Acids (LNAA) would be of additional value to blood Phe concentrations to explain brain biochemistry. To this purpose, we assessed blood amino acid concentrations and brain Phe as well as monoaminergic neurotransmitter levels in in 114 Pah-Enu2 mice on both B6 and BTBR backgrounds using (multiple) linear regression analyses. ResultsPlasma Phe concentrations were strongly correlated to brain Phe concentrations, significantly negatively correlated to brain serotonin and norepinephrine concentrations and only weakly correlated to brain dopamine concentrations. From all blood markers, Phe showed the strongest correlation to brain biochemistry in PKU mice. Including non-Phe LNAA concentrations to the multiple regression model, in addition to plasma Phe, did not help explain brain biochemistry. ConclusionThis study showed that blood Phe is still the best amino acid predictor of brain biochemistry in PKU. Nevertheless, neurocognitive and behavioural outcome cannot fully be explained by blood or brain Phe concentrations, necessitating a search for other additional parameters. Take-home messageBlood Phe is still the best amino acid predictor of brain biochemistry in PKU. Nevertheless, neurocognitive and behavioural outcome cannot fully be explained by blood or brain Phe concentrations, necessitating a search for other additional parameters.

Association Between Macronutrient Intakes and Sleep Quality in Middle-Aged and Older Population in Singapore

Abstract Objectives Quality sleep is essential to health and poor sleep may increase the risk of obesity and type 2 diabetes. Previous studies have reported that macronutrient intakes, such as dietary protein, may be associated with sleep quality. However research, on this relationship in older adults who often exhibit poorer sleep quality is still limited. Therefore, the objective of this research is to assess the association between macronutrient intakes and sleep quality in middle-aged and older population in Singapore. Methods In this cross-sectional study, 104 men and women (59 ± 6 years) were recruited within Singapore. Their food intakes and sleep quality were assessed using 3-day food records and Pittsburg Sleep Quality Index questionnaire, respectively. In addition, plasma amino acid concentration was measured from the collected blood samples. The relationship between diet, plasma amino acids and sleep quality were evaluated using multiple linear regression and adjusted for age, BMI and gender. Results From this study, no association were observed between total energy intake (%E) from protein and tryptophan (Trp) intakes and sleep quality parameters. However, %E from fat was positively associated with sleep efficiency (SE%) (β-coefficient = 19.6, P = 0.02). A similar positive association with SE% was also observed for mono- and poly-unsaturated fatty acids. In contrast, %E from carbohydrate was inversely associated with SE% (β-coefficient = −21.2, P = 0.03). Sugar intake was also inversely associated with sleep quality parameters. Although plasma Trp concentration was not associated with sleep quality parameters, Trp and large neutral amino acids concentration ratio was inversely associated with sleep duration (β-coefficient = −19.2, P = 0.02). Conclusions In conclusion, consuming a diet with a higher energy from fat and lower energy from carbohydrate may improve sleep quality in Singapore middle-aged and older adults. The type of fat and carbohydrate consumed may also affect sleep quality. Funding Sources This research study is funded by the National University of Singapore (Grant number: R-143–000-A03–133).

Large neutral amino acid status in association with P:T ratio and diet in adult and pediatric patients with phenylketonuria.

BackgroundIntake of large neutral amino acids (LNAA) may inhibit phenylalanine (PHE) transport across the blood brain barrier and assist with blood PHE control in patients with phenylketonuria (PKU). We evaluated the interrelationship between LNAA in plasma and diet on Phe:Tyr (P:T) ratio in patients with PKU and the influence of dietary factors on plasma LNAA markers.MethodsPlasma amino acid values and 3‐day food record analysis from two studies (34 male/30 female; age 4.6‐47 years) were examined. For pediatrics (<18 years) and adults (≥18 years) the relationship between P:T ratio, plasma LNAA, and dietary intake patterns were investigated.ResultsDietary factors influencing P:T ratio included intake of total protein (g/kg), medical food (MF) protein (g/kg, % below Rx), and LNAA (g) in the full cohort (P < .05). Associations were found between plasma valine and other dietary and plasma LNAA in pediatrics (P < .05) and plasma LNAA with dietary LNAA intake in adults (P = .019). Plasma P:T ratio was inversely associated with plasma LNAA concentrations in both age groups (P < .05). Aside from histidine in pediatrics (P = .024), plasma LNAA did not differ by having plasma PHE levels within or above the therapeutic range (120‐360 μmol/L). Plasma LNAA in both age groups was similar to reported healthy control values.ConclusionP:T ratio is significantly tied to dietary LNAA, adherence to MF Rx, and plasma LNAA concentrations. Additionally, P:T ratio and valine may be effective clinical proxies for determining LNAA metabolic balance and LNAA quality of the diet in patients with PKU.

Large neutral amino acid supplementation as an alternative to the phenylalanine-restricted diet in adults with phenylketonuria: evidence from adult Pah-enu2 mice

Phenylketonuria treatment mainly consists of a phenylalanine-restricted diet but still results in suboptimal neuropsychological outcome, which is at least partly based on cerebral monoamine deficiencies, while, after childhood, treatment compliance decreases. Supplementation of large neutral amino acids (LNAAs) was previously demonstrated in young phenylketonuria mice to target all three biochemical disturbances underlying brain dysfunction in phenylketonuria. However, both its potential in adult phenylketonuria and the comparison with the phenylalanine-restricted diet remain to be established. To this purpose, several LNAA supplements were compared with a severe phenylalanine-restricted diet with respect to brain monoamine and amino acid concentrations in adult C57Bl/6 Pah-enu2 mice. Adult phenylketonuria mice received a phenylalanine-restricted diet, unrestricted diet supplemented with several combinations of LNAAs or AIN-93M control diet for 6 weeks. In addition, adult wild-type mice on AIN-93M diet served as controls. The severe phenylalanine-restricted diet in adult phenylketonuria mice significantly reduced plasma and brain phenylalanine and restored brain monoamine concentrations, while brain concentrations of most nonphenylalanine LNAAs remained subnormal. Supplementation of eight LNAAs was similarly effective as the severe phenylalanine-restricted diet to restore brain monoamines, while brain and plasma phenylalanine concentrations remained markedly elevated. These results provide biochemical support for the effectiveness of the severe phenylalanine-restricted diet and showed the possibilities of LNAA supplementation being equally effective to restore brain monoamines in adult phenylketonuria mice. Therefore, LNAA supplementation is a promising alternative treatment to phenylalanine restriction in adult phenylketonuria patients to further optimize neuropsychological functioning.

Presumptive brain influx of large neutral amino acids and the effect of phenylalanine supplementation in patients with Tyrosinemia type 1.

IntroductionHereditary Tyrosinemia type 1 (HT1) is a rare metabolic disease caused by a defect in the tyrosine degradation pathway. Current treatment consists of 2-(2-nitro-4-trifluoromethylbenoyl)-1,3-cyclohexanedione (NTBC) and a tyrosine and phenylalanine restricted diet. Recently, neuropsychological deficits have been seen in HT1 patients. These deficits are possibly associated with low blood phenylalanine concentrations and/or high blood tyrosine concentrations. Therefore, the aim of the present study was threefold. Firstly, we aimed to calculate how the plasma amino acid profile in HT1 patients may influence the presumptive brain influx of all large neutral amino acids (LNAA). Secondly, we aimed to investigate the effect of phenylalanine supplementation on presumptive brain phenylalanine and tyrosine influx. Thirdly, we aimed to theoretically determine minimal target plasma phenylalanine concentrations in HT1 patient to ensure adequate presumptive brain phenylalanine influx.MethodsData of plasma LNAA concentrations were obtained. In total, 239 samples of 9 HT1 children, treated with NTBC, diet, and partly with phenylalanine supplementation were collected together with 596 samples of independent control children. Presumptive brain influx of all LNAA was calculated, using Michaelis-Menten parameters (Km) and Vmax-values obtained from earlier articles.ResultsIn HT1 patients, plasma concentrations and presumptive brain influx of tyrosine were higher. However, plasma and especially brain influx of phenylalanine were lower in HT1 patients. Phenylalanine supplementation did not only tend to increase plasma phenylalanine concentrations, but also presumptive brain phenylalanine influx, despite increased plasma tyrosine concentrations. However, to ensure sufficient brain phenylalanine influx in HT1 patients, minimal plasma phenylalanine concentrations may need to be higher than considered thus far.ConclusionThis study clearly suggests a role for disturbed brain LNAA biochemistry, which is not well reflected by plasma LNAA concentrations. This could play a role in the pathophysiology of the neuropsychological impairments in HT1 patients and may have therapeutic implications.

A reassessment of the blood–brain barrier transport of large neutral amino acids during acute systemic inflammation in humans

We reassessed data from a previous study on the transcerebral net exchange of large neutral amino acids (LNAAs) using a novel mathematical model of blood-brain barrier (BBB) transport. The study included twelve healthy volunteers who received a 4-h intravenous lipopolysaccharide (LPS) infusion (total dose: 0·3ng/kg), a human experimental model of the systemic inflammatory response during the early stages of sepsis. Cerebral blood flow and arterial-to-jugular venous LNAA concentrations were measured prior to and after LPS, and the BBB transport and brain extracellular concentrations of LNAAs were calculated. The arterial concentration and unidirectional cerebral influx of phenylalanine increased after LPS. The BBB transport of tyrosine was unaffected, while its concentration in the brain extracellular fluid increased. These findings suggest that LPS infusion leads to an increased cerebral uptake of phenylalanine, which is then metabolized to tyrosine. This may reflect a neuroprotective mechanism that 'detoxifies' excess intracerebral phenylalanine in the clinical setting of sepsis.

Large Neutral Amino Acid Supplementation Exerts Its Effect through Three Synergistic Mechanisms: Proof of Principle in Phenylketonuria Mice.

BackgroundPhenylketonuria (PKU) was the first disorder in which severe neurocognitive dysfunction could be prevented by dietary treatment. However, despite this effect, neuropsychological outcome in PKU still remains suboptimal and the phenylalanine-restricted diet is very demanding. To improve neuropsychological outcome and relieve the dietary restrictions for PKU patients, supplementation of large neutral amino acids (LNAA) is suggested as alternative treatment strategy that might correct all brain biochemical disturbances caused by high blood phenylalanine, and thereby improve neurocognitive functioning.ObjectiveAs a proof-of-principle, this study aimed to investigate all hypothesized biochemical treatment objectives of LNAA supplementation (normalizing brain phenylalanine, non-phenylalanine LNAA, and monoaminergic neurotransmitter concentrations) in PKU mice.MethodsC57Bl/6 Pah-enu2 (PKU) mice and wild-type mice received a LNAA supplemented diet, an isonitrogenic/isocaloric high-protein control diet, or normal chow. After six weeks of dietary treatment, blood and brain amino acid and monoaminergic neurotransmitter concentrations were assessed.ResultsIn PKU mice, the investigated LNAA supplementation regimen significantly reduced blood and brain phenylalanine concentrations by 33% and 26%, respectively, compared to normal chow (p<0.01), while alleviating brain deficiencies of some but not all supplemented LNAA. Moreover, LNAA supplementation in PKU mice significantly increased brain serotonin and norepinephrine concentrations from 35% to 71% and from 57% to 86% of wild-type concentrations (p<0.01), respectively, but not brain dopamine concentrations (p = 0.307).ConclusionsThis study shows that LNAA supplementation without dietary phenylalanine restriction in PKU mice improves brain biochemistry through all three hypothesized biochemical mechanisms. Thereby, these data provide proof-of-concept for LNAA supplementation as a valuable alternative dietary treatment strategy in PKU. Based on these results, LNAA treatment should be further optimized for clinical application with regard to the composition and dose of the LNAA supplement, taking into account all three working mechanisms of LNAA treatment.

Dietary ʟ-Tryptophan Supplementation with Reduced Large Neutral Amino Acids Enhances Feed Efficiency and Decreases Stress Hormone Secretion in Nursery Pigs under Social-Mixing Stress1–3

Tryptophan (Trp), the rate-limiting substrate of serotonin [5-hydroxytryptoamine (5-HT)] synthesis in the brain, competes with large neutral amino acids (LNAA) to cross the blood-brain barrier. This study was designed to evaluate the effect of ʟ-Trp supplementation on nursery pigs experiencing social-mixing stress and fed diets varying in LNAA concentrations. Forty-eight individually housed barrows at 6 wk of age were randomly allotted to 4 dietary treatments based on a 2 × 2 factorial arrangement, with ʟ-Trp supplementation (0 or 0.6%) and LNAA concentrations (4.5 or 3.8%) as the 2 main factors. Pigs were fed the diets for 7 d. On d 4, pigs within a treatment were paired in a new pen to create social-mixing stress and behavior was recorded for 24 h. Body weight was measured on d 0, 4, 5, and 7. Saliva and blood were collected on d 4 and 7. On d 7, pigs were killed to obtain hypothalami. During the entire period excluding the mixing day (d 5), ʟ-Trp supplementation improved (P < 0.01) feed efficiency of pigs and lowering the LNAA further enhanced (P < 0.05) the effects of ʟ-Trp. Supplementation of 0.6% ʟ-Trp increased (P < 0.001) hypothalamic 5-HT and 5-hydroxyindoleacetic acid. The salivary cortisol concentration was reduced (P < 0.05) by lowering the LNAA. Collectively, lowering the LNAA further enhanced the improvement of feed efficiency by ʟ-Trp supplementation of nursery pigs under social-mixing stress in association with reduced stress hormones, indicating that reducing LNAA in the diet can facilitate the effect of ʟ-Trp on the stress response of pigs.

Amitriptyline and clomipramine increase the concentration of administered L-tryptophan in the rat brain.

The tricyclic antidepressant amitriptyline has been shown to reduce concentrations of large neutral amino acids (LNAA) in rat plasma. Compounds with that property might interact with such amino acids used as therapeutic agents with a central site of action by causing a change in the relationship between the administered LNAA and its endogenous LNAA competitors for carrier-mediated transport through the blood-brain barrier into the brain. This study was performed to investigate if the antidepressant agents amitriptyline and clomipramine could, by such a mechanism, increase brain concentrations of administered tryptophan. Intraperitoneal administration of L-tryptophan alone (100 mg kg-1) resulted in an increase in the concentration of tryptophan in the rat brain from 14 +/- 0.7 to 100 +/- 4.3 nmol g-1 compared with rats given saline only. When rats were given tryptophan with amitriptyline (25 mg kg-1, i.p.) or clomipramine (25 mg kg-1, i.p.) brain concentrations of tryptophan were increased even further, to 150 +/- 4.5 and 157 +/- 10.2 nmol g-1, respectively. Administration of L-tryptophan alone resulted in an increase in the rat plasma tryptophan ratio [(concentration of tryptophan)/(total concentration of LNAAs)] from 0.14 +/- 0.003 to 0.42 +/- 0.011 compared with rats given saline only. When rats were given tryptophan with amitriptyline or clomipramine the plasma tryptophan ratios were increased even further to 0.52 +/- 0.017 and 0.54 +/- 0.025, respectively. All these effects were statistically significant (P < 0.001). These findings support the hypothesis that tricyclic antidepressants could interact with administered tryptophan by changing the relationship in plasma between tryptophan and its endogenous LNAA competitors for transport into the brain, resulting in higher concentrations of tryptophan in the brain. It is possible that this could be the mechanism of the previously reported finding that clomipramine and tryptophan potentiate each other in the treatment of depression.

High-glycaemic index and -glycaemic load meals increase the availability of tryptophan in healthy volunteers

The purpose of the present study was to determine the influence of the glycaemic index (GI) and glycaemic load (GL) on the ratio of tryptophan (TRP) relative to other large neutral amino acids (LNAA). Ten healthy men (age 22·9 (sd 3·4) years; BMI 23·5 (sd 1·6) kg/m2) underwent standard GI testing, and later consumed each of a mixed-macronutrient (1915 kJ; 66·5 % carbohydrate (CHO), 17 % protein and 16·5 % fat) high-GI (MHGI), an isoenergetic, mixed-macronutrient low-GI (MLGI) and a CHO-only (3212 kJ; 90 % CHO, 8 % protein, 2 % fat) high-GI (CHGI) meal on separate days. The GI, GL and insulin index values (e.g. area under the curve) were largest after the CHGI meal (117, 200, 158), followed by the MHGI (79, 59, 82) and MLGI (51, 38, 56) meals, respectively (all values were significantly different, P < 0·05). After the MHGI and MLGI meals but not after the CHGI meal, TRP was elevated at 120 and 180 min (P < 0·05). After the CHGI, LNAA was lower compared with the MLGI (P < 0·05); also the rate of decline in LNAA was higher after CHGI compared with MHGI and MLGI (both comparisons P < 0·05). The percentage increase from baseline in TRP:LNAA after CHGI (23 %) was only marginally higher than after the MHGI meal (17 %; P = 0·38), but it was threefold and nearly significantly greater than MLGI (8 %; P = 0·05). The present study demonstrates that the postprandial rise in TRP:LNAA was increased by additional CHO ingestion and higher GI. Therefore, the meal GL appears to be an important factor influencing the postprandial TRP:LNAA concentration.

Plasma tryptophan and tyrosine levels are independent risk factors for delirium in critically ill patients

The pathophysiology of delirium remains elusive though neurotransmitters and their precursor large neutral amino acids (LNAAs) may play a role. This pilot study investigated whether alterations of tryptophan (Trp), phenylalanine (Phe), and tyrosine (Tyr) plasma levels were associated with a higher risk of transitioning to delirium in critically ill patients. Plasma LNAA concentrations were determined on days 1 and 3 in mechanically ventilated (MV) patients from the MENDS randomized controlled trial (dexmedetomidine vs. lorazepam sedation). Three independent variables were calculated by dividing plasma concentrations of Trp, Phe, and Tyr by the sum of all other LNAA concentrations. Delirium was assessed daily using the confusion assessment method for the intensive care unit (CAM-ICU). Markov regression models were used to analyze independent associations between plasma LNAA ratios and transition to delirium after adjusting for covariates. The 97 patients included in the analysis had a high severity of illness (median APACHE II, 28; IQR, 24-32). After adjusting for confounders, only high or very low tryptophan/LNAA ratios (p = 0.0003), and tyrosine/LNAA ratios (p = 0.02) were associated with increased risk of transitioning to delirium, while phenylalanine levels were not (p = 0.27). Older age, higher APACHE II scores and increasing fentanyl exposure were also associated with higher probabilities of transitioning to delirium. In this pilot study, plasma tryptophan/LNAA and tyrosine/LNAA ratios were associated with transition to delirium in MV patients, suggesting that alterations of amino acids may be important in the pathogenesis of ICU delirium. Future studies evaluating the role of amino acid precursors of neurotransmitters are warranted in critically ill patients.

Cerebrospinal fluid concentrations of large neutral and basic amino acids in Macaca mulatta: diurnal variations and responses to chronic changes in dietary protein intake

In rats, dietary protein intake influences brain concentrations of tryptophan, tyrosine, and other large neutral amino acids (LNAAs) and the neurotransmitters to which they are linked. Few experiments have examined these dietary protein–amino acid relationships in nonhuman primates, in relation to time of day or dietary protein content. We therefore examined the effect in monkeys of changes in chronic protein intake on 24-hour plasma and cerebrospinal fluid (CSF) concentrations of LNAAs (tyrosine, phenylalanine, branched-chain amino acids) and basic amino acids. Juvenile male monkeys ( Macaca mulatta) consumed for sequential 4-week periods diets differing in protein content (∼23% → ∼16% → ∼10% → ∼6% protein [percentage of energy]). The daily ration was presented as a morning meal of fruit and an afternoon meal of fruit and a commercial diet to mimic feeding patterns in the wild. During week 4 on each diet, blood and CSF were sampled repeatedly over a 48-hour period via indwelling catheters. Plasma and CSF LNAA concentrations varied markedly with time of day and dietary protein content, showing up to 4-fold variations. Diurnal variations in plasma and CSF basic amino acids were smaller in magnitude and generally not strongly linked to dietary protein content. A measure of the competitive transport of LNAAs across the blood-brain barrier, calculated using plasma concentrations of the LNAAs and their blood-brain barrier kinetic constants, predicted the observed CSF concentration of each LNAA examined remarkably well, except for phenylalanine. Based on observations in rats, the variations in the CSF concentrations of the LNAAs in monkeys may be large enough to influence metabolic and signaling pathways in brain to which they have been linked.

Changes in Plasma Amino Acid Levels Do Not Predict Satiety and Weight Loss on Diets with Modified Macronutrient Composition

Objective: Serotonin mediates satiety in the central nervous system. Brain serotonin content depends on the plasma ratio of tryptophan (Trp) to large neutral amino acids (LNAA) and may be affected by diet composition. We examined whether high-carbohydrate or high-protein diets induce satiety and weight loss by altering plasma concentrations of these amino acids. Methods: In study 1 (n = 16, BMI = 27.0 ± 2.3), we compared plasma Trp and LNAA concentrations averaged over 24 h after 2 weeks of consuming isocaloric diets containing either 45 or 65% of total energy as carbohydrate. In study 2 (n = 19, BMI = 26.2 ± 2.1), we made the same measurements following diets containing either 15 or 30% of total energy as protein. To assess satiety in both studies, we recorded caloric intake and weight changes during a subsequent 12-week period of ad libitum consumption of the experimental diets. Results: Ad libitum caloric intake fell by 222 ± 81 kcal/day with a 3.7 ± 0.6 kg weight loss at 12 weeks in study 1. Ad libitum caloric intake fell by 441 ± 63 kcal/ day with a 4.9 ± 0.5 kg weight loss at 12 weeks in study 2. The 24-hour averaged plasma concentration of Trp and the Trp:LNAA ratio were unaffected by the isocaloric increase in carbohydrate or protein consumption that preceded the ad libitum administration of the 2 diets. Conclusion: An increase in either carbohydrate or protein intake increases satiety and leads to significant weight loss, however, these effects are not mediated by an increase in plasma concentration of Trp or the Trp:LNAA ratio.

Requirement of tryptophan in relation to the supply of large neutral amino acids in laying hens

The present study was undertaken to find out whether the tryptophan requirement of laying hens is influenced by the supply of large neutral amino acids (LNAA). A factorial experiment was performed in which the dietary tryptophan concentration was varied at six different levels (1.0, 1.25, 1.5, 1.75, 2.0, and 2.5 g tryptophan/kg diet). As the second factor, the dietary concentrations of LNAA (isoleucine, valine, leucine, phenylalanine, and tyrosine) were varied at two levels. The first level provided an adequate supply of these amino acids; at the second level the concentrations of these amino acids were 40% higher than at the first level. The tryptophan requirement was estimated by a broken-line model and an exponential model of regression analysis. The tryptophan intake required for optimum (100% of maximum in the broken-line model, 95% of the maximum in the exponential model) egg production and daily egg mass was lower in hens fed the diets with high LNAA concentrations (145 and 155 mg/hen per day, respectively, in average of both models) than in hens fed the diets with adequate concentrations of LNAA (184 and 198 mg/hen per day, respectively, in average of both models). In contrast, the tryptophan requirement for optimum BW gain was lower in hens fed the diets with adequate LNAA concentrations (178 mg tryptophan per day) than in hens fed the diets with a high concentration of LNAA (212 mg tryptophan per day). In conclusion, the study suggests that an interaction between dietary LNAA and tryptophan exists in laying hens.

Large neutral amino acids auto exchange when infused by microdialysis into the rat brain: implication for maple syrup urine disease and phenylketonuria

Maple syrup urine disease (MSUD) and phenylketonuria (PKU) are associated with accumulation of large neutral amino acids (LNAA) in blood and tissues and a decrease of other LNAA not directly related to the enzyme defects. One characteristic shared by both the elevated and decreased amino acids is that all are substrates for transport via the large neutral amino acid transporter. In this study, the blood brain barrier was effectively bypassed using microdialysis to determine the immediate effect of infused phenylalanine, tyrosine, 2-amino-2-norborane-carboxylic acid (BCH), and leucine and α-ketoisocaproate on extracellular levels of LNAA. The concentration of non-infused LNAA increased in the interstitial fluid, presumably due to trans-stimulated exchange of these LNAA from intracellular pools as the infused LNAA entered the cells. Such trans-stimulated exchange can potentially deplete cells of multiple essential LNAA. It is proposed that brain cells in disorders such as MSUD and PKU may be subject to two mechanisms that limit the availability of a full complement of these amino acids: competition for transport of LNAAs at the blood brain barrier and trans-stimulated exchange out of neuronal cells for subsequent metabolism or sequestration in the periphery.

β-Adrenergics enhance brain extraction of levodopa.

We sought to determine whether beta-adrenergic agonists enhance the brain extraction of L-dopa and L-leucine. Systemic administration of beta-adrenergic agonists increase brain concentrations of L-dopa and other large neutral amino acids (LNAA) in rats and monkeys and may improve symptoms and reduce daily L-dopa requirement in patients with Parkinson's disease. Cerebral blood flow (CBF) using [3H]nicotine and the extraction fraction of 14C-labeled L-dopa or L-leucine were measured simultaneously in various brain regions of conscious rats using the dual-isotope indicator fractionation technique after intraperitoneal administration of isoproterenol (a peripheral nonselective beta-adrenergic agonist), or clenbuterol (a beta2-adrenergic agonist that crosses the blood-brain barrier), or beta-adrenergic agonist preceded by nadolol (a peripheral nonselective beta-adrenergic antagonist), or saline vehicle. Both beta-adrenergic agonists increased regional brain extraction fraction of L-dopa and L-leucine tracers by 35-45%, without altering regional CBF. These changes were accompanied by about a 30% decrease in plasma branched chain LNAA concentrations. Nadolol blocked all these effects. beta-Adrenergic agonists increase the brain extraction of L-dopa and leucine, mainly by peripheral mechanisms that reduce the levels of other competing plasma LNAAs for transport. Thus, beta-adrenergic agonists might be useful in the treatment of patients with Parkinson's disease by enhancing delivery of L-dopa to the brain.

Distribution volume of 3-O-methyl-6.

The distribution volume (DV) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DV as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DV for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DV = 1.5-0.00094*[LNAA], R--2 = 0.79) resulted in an 8% DV decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 +/- 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 +/- 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Reduction of large neutral amino acid concentrations in plasma and CSF of patients with maple syrup urine disease during crises.

Neurological dysfunction is common in patients with maple syrup urine disease (MSUD). However, the mechanisms underlying the neuropathology of this disorder are poorly understood. We determined the concentrations of all amino acids in plasma of patients with MSUD during crises (with severe CNS symptoms) and after recovery in the hope of detecting possible alterations of these levels during metabolic decompensation. Blood samples obtained from 11 children with MSUD aged 1 month to 7 years and from 10 age-matched controls (5 months to 6 years) with no evidence of metabolic disease were examined for their amino acid content by high-performance liquid chromatography. We observed that leucine, isoleucine and valine concentrations were respectively 30, 9 and 3 times higher than normal values, whereas the concentrations of the large neutral amino acids (LNAA) phenylalanine, tyrosine, tryptophan and methionine were significantly lower during metabolic decompensation as compared to the controls. In addition, concentrations of leucine, but not of valine or isoleucine, were inversely related to the LNAA concentrations in plasma. The concentrations of these amino acids in plasma returned to normal values when patients were clinically well. CSF amino acid concentrations also showed decreased amounts of LNAA and increased concentrations of branched-chain amino acids. It is possible that the decrease in plasma concentrations of LNAA may lead to a deficit of these essential amino acids in the brain as well as of their products such as proteins and neurotransmitters, a fact that might be related to the neurological dysfunction of MSUD.

Large neutral amino acid changes and delirium in febrile elderly medical patients.

A hypothesized but unexplored mechanism for delirium in older persons is that changes in plasma large neutral amino acid (LNAA) concentrations alter brain serotonin levels, result in neurotoxicity, or both. Therefore we performed a prospective study of 21 acutely febrile long-term-care residents to study the relationship between LNAA changes and delirium. Plasma LNAA concentrations were evaluated during illness and 1 month later. Delirium was diagnosed by using the Confusion Assessment Method. Other data included age, body mass index, cognitive impairment, comorbidity, gender, maximum temperature, and medication use. Seven subjects (33%) were delirious during febrile illness. Although the phenylalanine (PHE)/LNAA ratio was higher during illness in both delirious and nondelirious groups, a two-sample t test demonstrated that delirium was associated with a higher illness PHE/LNAA ratio (p = .03). The amount of change in PHE/LNAA from illness to recovery was not different between the delirious and nondelirious groups. Tryptophan/LNAA was not associated with delirium during illness or at recovery. These findings identify another potentially fruitful area of investigation for the prevention and treatment of delirium in older persons.

Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria.

Large neutral amino acids (LNAAs), including phenylalanine (Phe), compete for transport across the blood-brain barrier (BBB) via the L-type amino acid carrier. Accordingly, elevated plasma Phe impairs brain uptake of other LNAAs in patients with phenylketonuria (PKU). Direct effects of elevated brain Phe and depleted LNAAs are probably major causes for disturbed brain development and function in PKU. Competition for the carrier might conversely be put to use to lower Phe influx when the plasma concentrations of all other LNAAs are increased. This hypothesis was tested by measuring brain Phe in patients with PKU by quantitative 1H magnetic resonance spectroscopy during an oral Phe challenge with and without additional supplementation with all other LNAAs. Baseline plasma Phe was approximately 1,000 micromol/l and brain Phe was approximately 250 micromol/l in both series. Without LNAA supplementation, brain Phe increased to approximately 400 micromol/l after the oral Phe load. Electroencephalogram (EEG) spectral analysis revealed acutely disturbed brain activity. With concurrent LNAA supplementation, Phe influx was completely blocked and there was no slowing of EEG activity. These results are relevant for further characterization of the LNAA carrier and of the pathophysiology underlying brain dysfunction in PKU and for treatment of patients with PKU, as brain function might be improved by continued LNAA supplementation.