Postprandially, amino acids and di/tripeptides are thought to be primarily absorbed in the proximal small intestine. However, there have been no in vivo demonstrations of regional differences in amino acid transport dynamics between the proximal and distal small intestines. We monitored plasma amino acid responses in the jejunal and ileal mesenteric veins of rats after oral administration of a diet or an amino acid mixture (Expt 1) and in the portal vein after direct administration of the amino acid mixture into the lumen of the jejunum or ileum (Expt 2). In Expt 1, the total and some amino acid concentrations in the jejunal mesenteric vein were slightly higher than those in the ileal mesenteric vein after oral administration of the amino acid mixture, suggesting that the ileum actively transports luminal amino acids to the basolateral side, comparable to the jejunum. In Expt 2, portal amino acid concentrations were elevated to a greater extent after direct administration of the amino acid mixture into the ileal lumen than into the jejunal lumen. These results demonstrate regional differences in amino acid transport dynamics in vivo and suggest that the ileum has a higher capacity for transporting amino acids than the jejunum. Our findings highlight the importance of the ileum in postprandial amino acid absorption and metabolism.

An isotope dilution model for partitioning phenylalanine and tyrosine uptake by the liver of the lactating dairy cow is constructed and solved in the steady state. An original ten-pool model is adopted and solved by cleaving it into two five-pool sub-models, one representing phenylalanine and the other tyrosine. If assumptions are made, model solution permits calculation of the rate of phenylalanine and tyrosine uptake from portal vein and hepatic arterial blood supply, hydroxylation, and synthesis and degradation of constitutive protein. The model requires the measurement of plasma flow rate through the liver in combination with amino acid concentrations and plateau isotopic enrichments in arterial and portal and hepatic vein plasma during a constant infusion of [1-13C]phenylalanine and [2,3,5,6-2H]tyrosine tracers. It also requires estimates of the rate of oxidation and protein export secretion. Analysis of measurement errors in experimental enrichments and infusion rates on model solutions indicated that accurate values of the intracellular and extracellular enrichments are central to minimising errors in the calculated flows. Solving the model by cleaving into two five-pool schemes rather than solving the ten-pool scheme directly is preferred as there appears to be less compounding of errors and the results consistently appear to be more biologically feasible. The model provides a means for assessing the impact of hepatic metabolism on amino acid availability to peripheral tissues such as the mammary gland.

Phenylalanine (PHE) and to a lesser extent TYR are two commonly used amino acid tracers for measuring protein metabolism in a variety of species and tissues. The model examined in this paper was developed to resolve trans-organ and stable isotope dilution data collected from experiments with lactating dairy cows using these tracers. Two methods of solving the model, i.e. as two four-pool submodels, one representing PHE and the other TYR, or as an integrated eight-pool model, are investigated and the alternative solutions are contrasted. Solving the model as the two four-pool submodels rather than the integrated 8-pool model is preferred as the equations are slightly simpler and their application less susceptible to any compounding of measurement errors. The data used to illustrate the model were taken from experiments conducted to investigate the effects of high and low protein diets on the partitioning of PHE and TYR between milk protein synthesis and other metabolic fates by the mammary gland.

Ca2+-sensing receptor (CaSR) represents a potential therapeutic target for inflammatory bowel diseases and strongly prefers aromatic amino acid ligands. We investigated the regulatory effects of dietary supplementation with aromatic amino acids – tryptophan, phenylalanine and tyrosine (TPT) – on the CaSR signalling pathway and intestinal inflammatory response. The in vivo study was conducted with weanling piglets using a 2 × 2 factorial arrangement in a randomised complete block design. Piglets were fed a basal diet or a basal diet supplemented with TPT and with or without inflammatory challenge. The in vitro study was performed in porcine intestinal epithelial cell line to investigate the effects of TPT on inflammatory response using NPS-2143 to inhibit CaSR. Dietary supplementation of TPT alleviated histopathological injury and decreased myeloperoxidase activity in intestine challenged with lipopolysaccharide. Dietary supplementation of TPT decreased serum concentration of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, IL-12, granulocyte-macrophage colony-stimulating factor, TNF-α), as well as the mRNA abundances of pro-inflammatory cytokines in intestine but enhanced anti-inflammatory cytokines IL-4 and transforming growth factor-β mRNA levels compared with pigs fed control diet and infected by lipopolysaccharide. Supplementation of TPT increased CaSR and phospholipase Cβ2 protein levels, but decreased inhibitor of NF-κB kinase α/β and inhibitor of NF-κB (IκB) protein levels in the lipopolysaccharide-challenged piglets. When the CaSR signalling pathway was blocked by NPS-2143, supplementation of TPT decreased the CaSR protein level, but enhanced phosphorylated NF-κB and IκB levels in IPEC-J2 cells. To conclude, supplementation of aromatic amino acids alleviated intestinal inflammation as mediated through the CaSR signalling pathway.

It has been nearly 70 years since the discovery that strict adherence to a diet low in phenylalanine prevents severe neurological sequelae in patients with phenylalanine hydroxylase deficiency (phenylketonuria; PKU). Today, dietary treatment with restricted phenylalanine intake supplemented with non-phenylalanine amino acids to support growth and maintain a healthy body composition remains the mainstay of therapy. However, a better understanding is needed of the factors that influence N balance in the context of amino acid supplementation. The aim of the present paper is to summarise considerations for improving N balance in patients with PKU, with a focus on gaining greater understanding of amino acid absorption, disposition and utilisation. In addition, the impact of phenylalanine-free amino acids on 24 h blood phenylalanine/tyrosine circadian rhythm is evaluated. We compare the effects of administering intact protein v. free amino acid on protein metabolism and discuss the possibility of improving outcomes by administering amino acid mixtures so that their absorption profile mimics that of intact protein. Protein substitutes with the ability to delay absorption of phenylalanine and tyrosine, mimicking physiological absorption kinetics, are expected to improve the rate of assimilation into protein and minimise fluctuations in quantitative plasma amino acid levels. They may also help maintain normal glycaemia and satiety sensation. This is likely to play an important role in improving the management of patients with PKU.

In a longitudinal cohort study of young Australian adults, we reported that for women higher baseline levels of fish consumption were associated with reduced incidence of new depressive episodes during the 5-year follow-up. Fish are high in both n-3 fatty acids and tyrosine. In this study, we seek to determine whether n-3 fatty acids or tyrosine explain the observed association. During 2004–2006, a FFQ (nine fish items) was used to estimate weekly fish consumption among 546 women aged 26–36 years. A fasting blood sample was taken and high-throughput NMR spectroscopy was used to measure 233 metabolites, including serum n-3 fatty acids and tyrosine. During 2009–2011, new episodes of depression since baseline were identified using the lifetime version of the Composite International Diagnostic Interview. Relative risks were calculated using log-binomial regression and indirect effects estimated using the STATA binary_mediation command. Potential mediators were added to separate models, and mediation was quantified as the proportion of the total effect due to the mediator. The n-3 DHA mediated 25·3 % of the association between fish consumption and depression when fish consumption was analysed as a continuous variable and 16·6 % when dichotomised (reference group: <2 serves/week). Tyrosine did not mediate the association (<0·1 %). Components in fish other than n-3 fatty acids and tyrosine might be beneficial for women’s mental health.

The benzoylcyclohexane-1,3-diones, the triketones, are potent bleaching herbicides whose structure-activity relationships and physical properties are substantially different from classical bleaching herbicides, which affect phytoene desaturase. The first clue to their unique mechanism of action was the discovery that rats treated with a triketone were found to be tyrosinemic. Additionally, examination of the rat urine revealed the accumulation of p-hydroxyphenylpyruvate (HPP) and p-hydroxyphenyllactate. These results suggested that this chemically induced tyrosinemia was the result of the inhibition of p-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27), and this suggestion was confirmed when a triketone was shown to be a potent inhibitor of rat liver HPPD. In plants, HPPD is a component of the biosynthetic pathway to plastoquinone (PQ), which in turn is a key cofactor of phytoene desaturase. The expectation that triketone-treated plants should accumulate tyrosine while having reduced PQ levels was dramatically demonstrated in the meristematic tissue of ivyleaf morningglory. Plant HPPD, like the mammalian enzyme, was inhibited in vitro by triketones. These biochemical effects provide evidence that the triketone herbicidal mechanism of action is HPPD inhibition leading to a deficiency of PQ, a key cofactor for carotenoid biosynthesis. Other chemical classes of bleaching herbicides were also examined for their ability to elevate tyrosine and deplete PQ as a definitive means of establishing their mode of action and for delineating the structural and physical chemical requirements for an HPPD herbicide. Evidence is provided to support the claim that a 2-benzoylethen-1-ol substructure is the minimum substructure required for a potent HPPD inhibitor.

When glyphosate [N-(phosphonomethyl)glycine] was fed through the transpiration stream of excised bean (Phaseolus vulgaris L. ‘Red Kidney’) shoots, it was readily absorbed and transported to the leaves where it inhibited transpiration. Maximum inhibition (40 to 50%) resulted from tissue concentrations of 50 to 3000 nmoles glyphosate/g fresh weight, while transpiration was significantly inhibited (20%) by as little as 15 nmoles/g fresh weight. A combination of 5 X 10-4M tyrosine (Tyr) and 5 x 10-4M phenylalanine (Phe) supplied continuously through the transpiration stream prevented the inhibition of transpiration by glyphosate during the course of 8-h experiments. Tyrosine (10-3M) alone delayed the transpiration response to glyphosate by 2 to 3 h but 10-3M Phe alone did not affect the inhibition by glyphosate. Levels of endogenous Tyr and Phe were about 50% lower in glyphosate-treated leaves than in the controls 6 h after treatment.

Recently, it has been found that the gut microbiota influences functions of the host brain by affecting monoamine metabolism. The present study focused on the relationship between the gut microbiota and the brain amino acids. Specific pathogen-free (SPF) and germ-free (GF) mice were used as experimental models. Plasma and brain regions were sampled from mice at 7 and 16 weeks of age, and analysed for free d- and l-amino acids, which are believed to affect many physiological functions. At 7 weeks of age, plasma concentrations of d-aspartic acid (d-Asp), l-alanine (l-Ala), l-glutamine (l-Gln) and taurine were higher in SPF mice than in GF mice, but no differences were found at 16 weeks of age. Similar patterns were observed for the concentrations of l-Asp in striatum, cerebral cortex and hippocampus, and l-arginine (l-Arg), l-Ala and l-valine (l-Val) in striatum. In addition, the concentrations of l-Asp, d-Ala, l-histidine, l-isoleucine (l-Ile), l-leucine (l-Leu), l-phenylalanine and l-Val were significantly higher in plasma of SPF mice when compared with those of GF mice. The concentrations of l-Arg, l-Gln, l-Ile and l-Leu were significantly higher in SPF than in GF mice, but those of d-Asp, d-serine and l-serine were higher in some brain regions of GF mice than in those of SPF mice. In conclusion, the concentration of amino acids in the host brain seems to be dependent on presence of the gut microbiota. Amino acid metabolism in the host brain may be modified by manipulating microbiota communities.

The appearance of a red hue to the hair in black coated cats and dogs has previously been reported as a “red hair syndrome”. Such changes in hair colour are related to an alteration in the proportions of two types of pigments produced by melanocytes; black eumelanin and brown pheomelanin. In black cats, it has been demonstrated that higher levels of phenylalanine + tyrosine (Phe+Tyr) than those recommended for growth are required to support eumelanin synthesis. The purpose of this study was to evaluate if a similar observation could be made in dogs. Twelve black coated puppies (Black Labrador retrievers and Newfoundlands) were divided into 3 groups of 4 and fed 3 diets with increasing concentrations of Phe+Tyr (A: 4 g/Mcal; B: 5.8 g/Mcal; C: 7 g/Mcal) for a period of 6 months. Quantification of plasma amino acids (Phe, Tyr, Cys) and spectrocolourimetry of hair samples from the Labrador retrievers (as the a* dimension of CIE Lab system) were performed at the beginning, during and at the end of the study. There was a significant negative linear relationship between plasma Tyr levels and a* values of hair in Labrador dogs on diets A and B, suggesting that a diet with total Phe+Tyr content of 6 g/Mcal (higher than the growth recommended allowance) was necessary to ensure an optimal black coat colour in these puppies and that levels up to 7 g/Mcal can lead to a more intense black coat colour. Moreover, similar to what was found in kittens, plasma levels of Tyr up to 54 μmol/l did not guarantee an optimal black colour coat and led to the “reddish hair” appearance in black coated puppies.

Reducing the CP level of the diet allows for a reduction in N excretion without limiting performance as long as the amino acid (AA) requirements are covered. The availability of crystalline AA has permitted for a considerable reduction in the CP level of diets, practically used in pig nutrition. The adoption of low CP diets and the extent to which the CP content can be reduced further depends on the knowledge of the minimum levels of indispensable AA that maximize growth. The standardized ileal digestible (SID) Phe : Lys and Tyr : Lys requirements and the possibility to substitute Tyr by Phe have never been studied in piglets. The objectives of this study were to estimate these requirements in 10 to 20 kg pig as well as to determine the extent to which Phe can be used to cover the Tyr requirement. In three dose–response studies, six pigs within each of 14 blocks were assigned to six low CP diets (14.5% CP) sub-limiting in Lys at 1.00% SID. In experiment 1, the SID Phe : Lys requirement estimate was assessed by supplementing a Phe-deficient diet with different levels of l-Phe to attain 33%, 39%, 46%, 52%, 58%, and 65% SID Phe : Lys. Because Phe can be used for Tyr synthesis, the diets provided a sufficient Tyr supply. A similar approach was used in experiment 2 with six levels of l-Tyr supplementation to attain 21%, 27%, 33%, 39%, 45% and 52% SID Tyr : Lys. Phenylalanine was supplied at a level sufficient to sustain maximum growth (estimated in experiment 1). The SID Phe : Lys and SID Tyr : Lys requirements for maximizing daily gain were 54% and 40% using a curvilinear-plateau model, respectively. A 10% deficiency in Phe and Tyr reduced daily gain by 3.0% and 0.7%, respectively. In experiment 3, the effect of the equimolar substitution of dietary SID Tyr by Phe to obtain 50%, 57%, and 64% SID Phe : (Phe+Tyr) was studied at two limiting levels of Phe+Tyr. From 57% to 64% SID Phe : (Phe+Tyr), performance was slightly reduced. In conclusion, it is recommended not to use a Phe+Tyr requirement in the ideal AA profile but rather use a SID Phe : Lys of 54% and a SID Tyr : Lys of 40% to support maximal growth.

Experimental Camembert cheeses were made to investigate the effects on browning of the following factors: inoculation with Yarrowia lipolytica, the use of Penicillium candidum strains with different proteolytic activity, the addition of tyrosine, and the addition of Mn2+, thus leading to 16 different variants of cheese. Two physical colour parameters were used to describe browning, depending on the location in the cheeses: a whiteness index for the outside browning (mould mycelium), and a brownness index for the inside browning (surface of the cheese body). Mn2+ promoted a significant increase of browning at both locations, whereas Yar. lipolytica had the opposite effect. Outside browning was significantly more intense when using the Pen. candidum strain with higher proteolytic activity. A significant interaction was found between Yar. lipolytica and Pen. candidum. The yeast had no effect in combination with a low proteolytic strain of Pen. candidum, but significantly reduced proteolysis and browning in combination with a high proteolytic strain of Pen. candidum. We further confirmed that both strains of Pen. candidum were able to produce brown pigments from tyrosine, and thus both are presumably responsible for the browning activity in this type of cheese.

Using a cross-linking approach, we recently demonstrated that radiolabeled peptides or misfolded proteins specifically interact in vitro with two luminal proteins in crude extracts from pancreas microsomes. The proteins were the folding catalysts protein disulfide isomerase (PDI) and PDIp, a glycosylated, PDI-related protein, expressed exclusively in the pancreas. In this study, we explore the specificity of these proteins in binding peptides and related ligands and show that tyrosine and tryptophan residues in peptides are the recognition motifs for their binding by PDIp. This peptide-binding specificity may reflect the selectivity of PDIp in binding regions of unfolded polypeptide during catalysis of protein folding.

N5-(l-1-carboxyethyl)-l-ornithine synthase [E.C. 1.5.1.24] (CEOS) from Lactococcus lactis has been cloned, expressed, and purified from Escherichia coli in quantities sufficient for characterization by biophysical methods. The NADPH-dependent enzyme is a homotetramer (Mr ≅ 140,000) and in the native state is stabilized by noncovalent interactions between the monomers. The far-ultraviolet circular dichroism spectrum shows that the folding pattern of the enzyme is typical of the α,β family of proteins. CEOS contains one tryptophan (Trp) and 19 tyrosines (Tyr) per monomer, and the fluorescence spectrum of the protein shows emission from both Trp and Tyr residues. Relative to N-acetyltyrosinamide, the Tyr quantum yield of the native enzyme is about 0.5. All 19 Tyr residues are titratable and, of these, two exhibit the uncommonly low pKa of ∼8.5, 11 have pKa ∼ 10.75, and the remaining six titrate with pKa ∼ 11.3. The two residues with pKa ∼ 8.5 contribute approximately 40% of the total tyrosine emission, implying a relative quantum yield >1, probably indicating Tyr-Tyr energy transfer. In the presence of NADPH, Tyr fluorescence is reduced by 40%, and Trp fluorescence is quenched completely. The latter result suggests that the single Trp residue is either at the active site, or in proximity to the sequence GSGNVA, that constitutes the βαβ fold of the nucleotide-binding domain. Chymotrypsin specifically cleaves native CEOS after Phe255. Although inactivated by this single-site cleavage of the subunit, the enzyme retains the capacity to bind NADPH and tetramer stability is maintained. Possible roles in catalysis for the chymotrypsin sensitive loop and for the low pKa Tyr residues are discussed.

The present paper offers a dual 14CO2 breath test approach to study the metabolic utilization of free amino acids in the body. Using the carboxyl-[14C]isotopomer of an amino acid as the test substrate the percentage recovery of the isotope as 14CO2 reflects which part of the labelled amino acid flux has been decarboxylated. The residual C fragments may flow to total oxidation at least to the level recovered for the universal [14C]isotopomer. In the case that recovery for total oxidation is less than for decarboxylation, part of the [14C]fragments are retained in the body by either exchange or non-oxidative pathways. Utilization of tyrosine and leucine was measured in the post-absorptive phase in adult rats conditioned on isoenergetic diets containing 210, 75 or 0 g protein/kg. It was shown that the level of dietary protein exerts an influence on both decarboxylation and total oxidation. Although the responses of leucine and tyrosine were not different for total oxidation, there was a difference between the amino acids in their relative rate of decarboxylation. That this dual 14CO2 breath test approach can be used as a tool to evaluate whether the protein and amino acid supply has been adequate to support actual requirements is discussed.

Amino acid utilization: Amino acid requirements: Leucine: Tyrosine