Tryptophan Biochemistry: Structural, Nutritional, Metabolic, and Medical Aspects in Humans

L-Tryptophan is the unique protein amino acid (AA) bearing an indole ring: its biotransformation in living organisms contributes either to keeping this chemical group in cells and tissues or to breaking it, by generating in both cases a variety of bioactivemolecules. Investigations on the biology of Trp highlight the pleiotropic effects of its small derivatives on homeostasis processes. In addition to protein turn-over, in humans the pathways of Trp indole derivatives cover the synthesis of the neurotransmitter/hormone serotonin (5-HT), the pineal gland melatonin (MLT), and the trace amine tryptamine. The breakdown of the Trp indole ring defines instead the “kynurenine shunt” which produces cell-response adapters as L-kynurenine, kynurenic and quinolinic acids, or the coenzyme nicotinamide adenine dinucleotide (NAD+). This reviewaims therefore at tracing a “map” of themainmolecular effectors in human tryptophan (Trp) research, starting from the chemistry of this AA, dealing then with its biosphere distribution and nutritional value for humans, also focusing on some proteins responsible for its tissue-dependent uptake and biotransformation.We will thus underscore the role of Trp biochemistry in the pathogenesis of human complex diseases/syndromes primarily involving the gut, neuroimmunoendocrine/stress responses, and the CNS, supporting the use of -Omics approaches in this field

Changes in peripheral benzodiazepine receptors in patients with bipolar disorder

Peripheral benzodiazepine (BDZ) receptors were investigated by means of the binding of the specific ligand 3H-PK 11195 to platelet membranes in patients suffering from bipolar disorder and in healthy controls. The results showed that the density (Bmax) of peripheral BDZ receptors was significantly higher in patients than in control subjects, with no change in the dissociation constant. No correlation with demographic or clinical features was observed. These findings would suggest that alterations of peripheral BDZ receptors are present in patients suffering from bipolar disorder, but it is premature to conclude whether they may be related to the pathophysiology of the disorder, or are secondary to changes occurring in other systems, such as those regulating the stress response

Brain-Derived Neurotrophic Factor (BDNF) and Serotonin Transporter (SERT) in Platelets of Patients with Mild Huntington’s Disease: Relationships with Social Cognition Symptoms

Peripheral biological correlates of early-stage Huntington’s disease (HD) are currently attracting much interest given their possible use as prognostic predictors of later neurodegeneration. Since deficits in social-cognition processing are present among the initial disease symptoms, aim of this work was to appraise, in blood platelets, Brain-Derived Neurotrophic Factor (BDNF) and serotonin (5-HT) transporter (SERT), two proteins involved in human adaptive behavior as potential biochemical correlates of such disabilities in mild-HD. Thirteen gene positive and symptomatic patients (9M/4W, HD-stage II, age> 40y) together 11 gender/age matched controls without a concurrent diagnosis of psychiatric disorders, underwent a blood test to determine BDNF storage and membrane-bound SERT in platelets by ELISA immune-enzyme and [3H]-paroxetine ([3H]-PAR) binding assays, respectively. Concomitantly, all subjects were examined through a battery of socio-cognitive and emotion recognition questionnaires. Results showed moderately increased intra-platelet BDNF amounts (+20-22%) in patients versus controls, whereas [3H]-PAR binding parameters, maximum density (Bmax) and dissociation constant (KD), did not appreciably vary between the two groups. While patients displaying significantly reduced cognitive/emotion abilities, biochemical parameters and clinical features or psychosocial scores did not correlate each other, except for platelet BDNF and the illness duration, positively correlated, or for SERT KDs and angry voice recognition ability, negatively correlated in both controls and patients. Therefore, in this pilot investigation, platelet BDNF and SERT did not specifically underlie psychosocial deficits in stage II-HD. Higher platelet BDNF storage in patients showing lasting-mild symptoms would derive from compensatory mechanisms. Thus, supplementary investigations are warranted by also comparing patients in other illness’s phases

Melatonin and pro-hypnotic effectiveness of the antidepressant Trazodone: A preliminary evaluation in insomniac mood-disorder patients

Objective To preliminary investigate the link between the darkness hormone melatonin (MLT) and the pro-hypnotic effectiveness of the atypical antidepressant Trazodone (TRZ) in a group of mood disorder patients suffering of insomnia. Design and methods The study's design comprised: i) the enrolment of insomniac outpatients, ii) baseline (t0) psychiatric and biochemical examinations; iii) the subsequent patients' introduction into a treatment with TRZ for 3–4 weeks, followed by post-therapy re-evaluations (t1). The MLT function was investigated by t0/t1 ELISA determinations of 6-hydroxy-MLT sulfate (6-OH-MLTs) levels in early-morning urines and HPLC analysis of morning MLT serum amount. Concomitantly, TRZ and its metabolite m-chloro-phenylpiperazine (m-CPP) were measured by HPLC in serum to monitor patients' compliance/metabolism. Results Seventeen insomniac outpatients, displaying mild symptoms of depression/anxiety resistant to antidepressants, completed TRZ therapy (dose:10–20 mg/day, bedtime). Serum TRZ levels (127 ± 57 ng ml− 1, mean ± SD) confirmed patients' compliance, while the anxiogenic metabolite m-CPP resulting almost undetectable. Moreover, the 6-OH-MLTs output was found increased at t1 vs. baseline values (t1: 58.4 ± 45.02 ng ml− 1; t0: 28.6 ± 15.8 ng ml− 1; mean ± SD, P < 0.05) in 9 patients who recovered both insomnia and depression/anxiety (P < 0.01). Unresponsive subjects showed instead no post-therapy 6-OH-MLTs variation (t1: 48.53 ± 50.70 ng ml− 1; t0: 49.80 ± 66.53 ng ml− 1). Morning MLT in serum slightly diminished at t1 without reaching the statistical significance, not allowing therefore to define the patients' outcome. Conclusions This initial investigation encourages to explore MLT networks as possible correlates of TRZ pro-hypnotic responses

Alteration of serotonin transporter density and activity in fibromyalgia

The aim of the study was to evaluate the kinetic parameters of a specific serotonin transporter (SERT) and serotonin uptake in a mentally healthy subset of patients with fibromyalgia. Platelets were obtained from 40 patients and 38 healthy controls. SERT expression and functionality were evaluated through the measurement of [(3)H]paroxetine binding and the [(3)H]serotonin uptake itself. The values of maximal membrane binding capacity (B(max)) were statistically lower in the patients than in the healthy volunteers, whereas the dissociation constant (K(d)) did not show any statistically significant variations. Moreover, a decrease in the maximal uptake rate of SERT (V(max)) was demonstrated in the platelets of patients, whereas the Michaelis constant (K(m)) did not show any statistically significant variations. Symptom severity score (tiredness, tender points index and Fibromyalgia Impact Questionnaire) were negatively correlated with B(max )and with V(max), and positively correlated with K(m). A change in SERT seems to occur in fibromyalgic patients, and it seems to be related to the severity of fibromyalgic symptoms

Palmitate-induced lipotoxicity alters acetylation of multiple proteins in clonal β cells and human pancreatic islets

Type 2 diabetes is characterized by progressive β cell dysfunction, with lipotoxicity playing a possible pathogenetic role. Palmitate is often used to examine the direct effects of lipotoxicity and it may cause mitochondrial alterations by activating protein acetylation. However, it is unknown whether palmitate influences protein acetylation in β cells. We investigated lysine acetylation in mitochondrial proteins from INS-1E β cells (INS-1E) and in proteins from human pancreatic islets (HPI) after 24 h palmitate exposure. First, we confirmed that palmitate damages β cells and demonstrated that chemical inhibition of deacetylation also impairs INS-1E function and survival. Then, by 2-D gel electrophoresis, Western Blot and Liquid Chromatography-Mass Spectrometry we evaluated the effects of palmitate on protein acetylation. In mitochondrial preparations from palmitate-treated INS-1E, 32 acetylated spots were detected, with 13 proteins resulting over-acetylated. In HPI, 136 acetylated proteins were found, of which 11 were over-acetylated upon culture with palmitate. Interestingly, three proteins, glutamate dehydrogenase, mitochondrial superoxide dismutase, and SREBP-1, were over-acetylated in both INS-1E and HPI. Therefore, prolonged exposure to palmitate induces changes in β cell protein lysine acetylation and this modification could play a role in causing β cell damage. Dysregulated acetylation may be a target to counteract palmitate-induced β cell lipotoxicity

A relationship between oxytocin and anxiety of romantic attachment

The formation of social bonding is fundamental for several animals, including humans, for its relevant and obvious impact upon reproduction and, thus, survival of the species. Recent data would suggest that oxytocin might be one of the mediators of this process. Given the paucity of data on the possible involvement of oxytocin in human attachment, the present study was aimed to explore the possible relationships between the plasma levels of this neuropeptide and romantic attachment in healthy subjects. Forty-five healthy subjects who volunteered for the study, were included in the study. The romantic attachment was assessed using the Italian version of the so-called "Experiences in Close Relationships" (ECR), a self-report questionnaire for measuring this parameter in adults. The results showed that attachment anxiety and oxytocin are positively linked in romantic attachment to a statistically significant degree (r = 0.30, p = 0.04), that is, the higher the oxytocin levels the higher the score on the anxiety scale of the ECR. The authors suggest the hypothesis that this link represents one of the biological processes resulting in those rewarding emotions related to romantic attachment

The expression of platelet serotonin transporter (SERT) in human obesity

Serotonin (5-HT) is a well-known modulator of eating behavior. However, the molecular mechanisms linking its action to body weight balance have been only partially elucidated. Since platelets are a suitable peripheral model to study 5-HT transport, metabolism and release, we herein evaluated the expression of the platelet 5-HT re-uptake system (SERT) by [3H]-paroxetine binding assay. A cohort of 114 unrelated individuals (34 males, 80 females; age, mean +/- SD: 38.57 +/- 12.47 years) without major psychiatric disorders, was recruited following a naturalistic design regarding age or gender and classified accordingly to their body mass index (BMI). Subjects were divided into 5 groups: normal-weight (NW), overweight (OW) and grade I-III obese (OB) individuals. For gender analyses, data were transformed into [3H]-paroxetine density (Bmax)/BMI ratios to overcome both the disparity of women vs. men number and anthropometric differences between sexes.[3H]-paroxetine Bmax (SERT density, fmol/mg proteins) was reduced in platelet membranes of grade II (p &lt; 0.01) and III (p &lt; 0.001) obese subjects vs. controls and in overweight subjects (p &lt; 0.05) vs. grade III obese individuals. Considering all patients together, a strong negative correlation between Bmax and BMI (r = -0.449; P &lt; 0.0001) was demonstrated. Conversely, [3H]-paroxetine KD (dissociation constant, nM) did not differ among groups. No gender-related variation concerning Bmax/BMI ratios was observed in this cohort of subjects.The down-regulation of SERT in platelet membranes of severe human obesity (BMI &gt; 35 Kg/m2) confirms the involvement of 5-HT system in body weight gain. Moreover, this findings may help to elucidate those monoamine-endocrine networks acting on fat storage, adipocyte signaling and energy balance. Targeting 5-HT/5-HT-related markers will possibly uncover the existence of human obesity subtypes

Serotonin transporter (SERT) and translocator protein (TSPO) expression in the obese ob/ob mouse

Background: An ever growing body of evidences is emerging concerning metabolism hormones, neurotransmitters or stress-related biomarkers as effective modulators of eating behavior and body weight in mammals. The present study sought at examining the density and affinity of two proteins related to neurotransmission and cell metabolism, the serotonin transporter SERT and the cholesterol import-benzodiazepine site TSPO (translocator protein), in a rodent leptin-lacking mutant, the obese ob/ob mouse. Binding studies were thus carried out in brain or peripheral tissues, blood platelets (SERT) and kidneys (TSPO), of ob/ob and WT mice supplied with a standard diet, using the selective radiochemical ligands [(3)H]-paroxetine and [(3)H]-PK11195. Results: We observed comparable SERT number or affinity in brain and platelets of ob/ob and WT mice, whilst a significantly higher [(3)H]-PK11195 density was reported in the brain of ob/ob animals. TSPO binding parameters were similar in the kidneys of all tested mice. By [(3)H]-PK11195 autoradiography of coronal hypothalamic-hippocampal sections, an increased TSPO signal was detected in the dentate gyrus (hippocampus) and choroids plexus of ob/ob mice, without appreciable changes in the cortex or hypothalamic-thalamic regions. Conclusions: These findings show that TSPO expression is up-regulated in cerebral regions of ob/ob leptin-deficient mice, suggesting a role of the translocator protein in leptin-dependent CNS trophism and metabolism. Unchanged SERT in mutant mice is discussed herein in the context of previous literature as the forerunner to a deeper biochemical investigation