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<p>In plants, S-adenosyl-L-methionine-dependent phospho-base <em>N</em>-methyl transferases catalyze the three sequential methylations of phosphoethanolamine to phosphocholine, the precursor for choline and the major membrane phospholipid phosphatidylcholine. The enzyme phosphoethanolamine <em>N</em>-methyltransferase (PEAMT) catalyzes the first and committing step in choline synthesis, a step for which no known by-pass has been found. In <em>Arabidopsis thaliana</em> there are two loci annotated as encoding PEAMT and a putative PEAMT, At3g18000 (<em>NMT1</em>) and<em> </em>At1g73600 (<em>NMT3</em>), respectively. A related gene product that catalyzes the last two methylations is encoded by locus At1g48600 (<em>NMT2</em>). The objective of this study was to investigate the role of <em>NMT3 </em>in <em>Arabidopsis</em>. Three SALK lines carrying independent T-DNA insertions in At1g73600 were used: SALK_062703, SALK_016929c and SALK_120703c.</p> <p>Genomic DNA was used for PCR and sequence analysis of the products established the insertion of T-DNA in the protein coding region of At1g73600 for all three lines. Gene expression was analyzed by q-PCR. Primer design was particularly important for <em>NMT3 </em>transcript quantification by q-PCR. In SALK_062703 <em>nmt3 </em>mutants, the T-DNA is in exon 8 and in the SALK_120703c line it is in intron 6. In both cases, no <em>NMT3 </em>transcripts were detected using primers that annealed to sites 3’ to the position of the T-DNA in the gene. However, low levels of transcripts were detected using primers that annealed at positions 5’ to the site of T-DNA insertion. In the SALK_016929c line the position of the T-DNA insertion was in exon 2 and primers annealing near the site of the T-DNA insertion showed no <em>NMT3 </em>expression in this mutant but amplifying the mid portion of the gene showed WT levels of <em>NMT3 </em>transcripts. Thus all the mutants produce truncated <em>NMT3 </em>transcripts and by avoiding areas that overlap truncated transcript regions we could differentiate between <em>NMT3</em> knock-out or knock-down expression.</p> <p>Wild-type (<em>NMT3</em>) and <em>nmt3 </em>seedlings from the three lines grown on defined media plates showed no difference with respect to primary root length, number or density of lateral roots, and total root length. Exposing seedlings to salt (50 or 75 mM NaCl) led to reductions in root growth but again, roots of wild-type plants were indistinguishable from the mutant seedlings. One anomaly relates to the <em>nmt3</em> SALK_120703c<em> </em>line which showed two root phenotypes. On saline media most of the seedlings had longer roots that resembled the wild-type and other mutant lines and about a third had shortened roots. Whether the seedlings had long or short roots on salt, they all lacked <em>NMT3 </em>transcripts. This line is likely carrying another insertion that yields a salt-sensitive root phenotype. Mutant plants at four-weeks looked like wild-type plants and time of flowering was not reproducibly delayed or accelerated in mutant plants relative to wild-type.</p> <p>In wild-type seedlings the relative expression level of the three <em>NMT </em>genes is similar at day or night with transcript abundance ranked in the order <em>NMT3</em> > <em>NMT2 </em>> <em>NMT1. nmt3 </em>seedlings harvested midday showed no detectable <em>NMT3</em> expression but the abundance of <em>NMT1 </em>transcripts was 6.2-fold and 1.7-fold higher relative to wild-type in shoots and roots, respectively. At night, <em>NMT1 </em>expression in shoots of<em> nmt3 </em>seedlings decreased 4.8-fold relative to the level of <em>NMT1 </em>expression at midday while transcripts detected in roots increased slightly (1.3-fold). Using SALK_036291 <em>nmt1 </em>seedlings we found that <em>NMT3 </em>expression in shoots and roots was modestly up-regulated in the absence of <em>NMT1 </em>expression and the expression of <em>NMT3 </em>is lower at night than during the day. Also, regardless of the genotype or time of day, <em>NMT2 </em>expression remained constant even when <em>NMT1 </em>and <em>NMT3 </em>transcripts underwent several-fold changes in abundance. Interestingly, four-week old <em>nmt3 </em>plants of the SALK_062703 line showed that <em>NMT3 </em>expression is knocked-out in leaves but only knocked-down in roots.</p> <p><em> NMT3 </em>was the most highly expressed of the three <em>NMT </em>genes monitored by q-PCR. Nonetheless, three independent T-DNA insertion lines defective for <em>NMT3</em> expression were wild-type by appearance and as such, offer compelling evidence that NMT3 is not required by <em>Arabidopsis. </em>The increased expression of <em>NMT1 </em>in <em>nmt3 </em>plants and <em>NMT3</em> in <em>nmt1 </em>plants strongly suggests that plants compensate for the loss of one gene by up-regulating, to varying extents, the expression of the remaining <em>NMT </em>gene. If this is the case, a reasonable prediction made for a cross between <em>nmt1 </em>and <em>nmt3 </em>plants is that it would be lethal unless plants have yet another way to circumvent the loss of an essential enzyme for this committing metabolic bottleneck in choline synthesis.</p> / Master of Science (MSc)

Type II Diabetes Mellitus (T2D) and cardiovascular diseases (CVD) are non-communicable chronic diseases that involves impairments in glucose metabolism and vascular function. Multiple factors may increase the risk for T2D, including but not limited to genetics, obesity and lifestyle, such as physical inactivity and diet. The gut microbiota, the human's largest population of microorganisms, plays an essential role in health and disease. The physiology and function of the gastrointestinal tract can be influenced by the diet. Phosphatidylcholine (PC), a source of choline in the diet, is rich in Western-type diets. Gut microbiota metabolize choline to trimethylamine (TMA) which circulates and is oxidized in the liver to form trimethylamine N-oxide (TMAO). As a result, ingestion of PC or choline could increase levels of TMAO. Preclinical studies indicate a role of TMAO in the development of atherosclerosis. Likewise, multiple observations support a potential role of TMAO in the development of insulin resistance and T2D. Much of the research has been conducted on rodent models, while others are observational human studies. Whether acute and short-term increases in TMAO contribute to impairments in insulin sensitivity in humans remains unknown. To address this, we performed two studies utilizing a double-blind, placebo controlled, crossover design. Eligible participants consumed a 1000mg/day dose of choline bitartrate and placebo (maltodextrin) the night before each testing session (for the acute choline study) or for 4 weeks (for the short-term choline ingestion study). Oral glucose tolerance test, continuous glucose monitoring, flow-mediated dilation, and applanation tonometry was performed the day after the acute choline load and before and after the short-term choline ingestion period. We hypothesized that gut microbiota-generated increase in TMAO will impair insulin sensitivity, glucose tolerance, endothelial function and arterial stiffness in healthy sedentary humans. Following acute choline ingestion, significant increases in plasma TMAO (p = 0.013) and choline (p = 0.003) were evident. There was no statistically significant difference in insulin sensitivity, glucose tolerance or in any of the endothelial function and arterial stiffness measurements. Four weeks of 1000mg choline ingestion per day, significantly increased plasma (p = 0.042) and urine (p = 0.008) TMAO concentrations compared to the placebo. However, no significant differences were observed for any other measurements of insulin sensitivity, glucose tolerance, glycemic variability, endothelial function, and arterial stiffness. More research is needed to elucidate the mechanisms behind the mechanistic observations between elevated TMAO concentrations and T2D and CVD. / Doctor of Philosophy / Type 2 diabetes mellitus (T2D) and cardiovascular diseases (CVD) increase the risk of all-cause mortality. Choline is a nutrient that can be found in foods such as red meat, dairy, fish, and eggs. Choline is metabolized from bacteria in our gut and a metabolite called trimethylamine (TMA) is formed. TMA is then oxidized in the liver and trimethylamine-N-oxide (TMAO) is produced. A Western-type diet is rich in red meat, dairy, fish, and eggs and has been shown to increase production of the compound TMAO. Preclinical studies have suggested a causal role of TMAO in atherosclerosis and T2D and elevated plasma TMAO concentrations have been associated with an increased risk for CVD and T2D in observational studies. However, the causal nature of this relationship in humans is unknown. The studies described herein aimed to investigate the effects of increases in TMAO on insulin sensitivity and vascular function in healthy adults. The first study tested the effect of increasing TMAO on insulin sensitivity, glucose tolerance, and vascular function following an acute choline load (1000mg) and placebo (carbohydrate) the night before each testing session. In the second study, we examined the effect of increasing TMAO on insulin sensitivity, glucose tolerance, and vascular function in healthy adults, following a short-term choline load (1000mg/day) and placebo (carbohydrate) for 4 weeks. Acute and short-term choline ingestion significantly increased plasma TMAO concentrations. No significant differences were observed following acute or short-term choline ingestion for any measurement of insulin sensitivity, glucose tolerance 24-hout glycemic variability, vascular function., and arterial stiffness.

There is an association between the human microbiome and disease. Gut microbes metabolize dietary sources to release trimethylamine (TMA). TMA is absorbed and then oxidized by flavin monooxygenase 3 (FMO3) to form trimethylamine N-oxide (TMAO). Elevated TMAO is associated with increased risk of cardiovascular disease and type 2 diabetes; however, the causal nature is unclear. There is also limited evidence supporting the efficacy of strategies to reduce accumulation of TMAO. Therefore, the purpose of these studies is to determine the effects of increases in TMAO on cardiometabolic health. In study 1, healthy sedentary and endurance trained males consumed a high fat diet. Blood samples were obtained in a fasted state and every hour during a 4-hour high fat challenge. We hypothesized sedentary individuals would produce higher TMAO concentrations. In study 2, healthy sedentary individuals consumed an acute 1000 mg dose of choline (CHOL) and placebo (PLC). Fasted blood samples were collected, flow-mediated dilation (FMD) and oral glucose tolerance (OGT) were measured. In study 3, healthy sedentary individuals consumed 4-wks of CHOL and PLC. Fasted blood samples were collected, FMD and OGT were measured. We hypothesized acute and 4-wk choline supplementation would impair FMD and OGT. In study 1, neither fasting (1.49± 1.2 µM vs. 2.25 ± 1.4 µM, p>0.05) or postprandial TMAO changed significantly with the HFD in sedentary or endurance trained individuals even with the endurance group consuming more TMA dietary precursors. Study 2 found increased plasma TMAO concentrations after choline supplementation on day 1(PLC; 4.14 ± 2.6 μM vs. CHOL; 23.6 ± 33.8 μM, p=0.018) and day 2 (PLC; 5.13±4.9 μM vs. CHOL; 32.6±37.5 μM, p=0.082) however, there were no differences in OGT or FMD. Study 3 found no differences in FMD or OGT following 4-wks of choline consumption. In summary, there were no differences between sedentary and endurance trained individuals fasting or post-prandial TMAO. There was also no effect on acute or 4-wk supplementation of choline on FMD and OGT. More research is needed to understand effects of elevated TMAO on cardiometabolic health. / Doctor of Philosophy / For years, research has been performed to identify the health effects of eating large amounts of red meat on cardiovascular disease (CVD). Consuming red meat, fish, poultry and eggs increases a substance created during digestion and metabolism, called trimethylamine N-oxide (TMAO). Elevated TMAO has been associated with increased risk of CVD and type 2 diabetes but the direct causes are unknown. The purpose of these studies is to determine the effects of increases in TMAO on health in humans. Study 1 included healthy, sedentary and endurance trained males who consumed a high fat diet. Blood samples were collected to measure TMAO before and after a high fat meal. Study 2 included healthy, sedentary males and females who consumed 2 days of 1000 mg of choline, which is commonly found in red meat fish and eggs, and a placebo (carbohydrate) after subjects completed a series of tests to evaluate health. Study three included healthy, sedentary males and females who consumed 4-weeks of 1000 mg of choline per day and a placebo (carbohydrate). Following supplementation subjects underwent a series of tests to assess health. Overall, there were no differences found between sedentary and endurance trained individuals. Acute and 4-week supplementation of choline did not affect measures of blood sugar or blood vessel function.

Streptococcus pneumoniae (pneumococcus), a gram-positive bacterium, is commensal to the human nasopharynx. It is also a common cause of respiratory tract infections and multiple invasive diseases worldwide. Pneumococci attach to the nasopharynx, lung, and vascular endothelial cells, which contributes to colonization as well as to the development of pneumonia, bacteremia, and meningitis. Choline binding proteins (CBPs) are a unique set of cell wall proteins conserved within pneumococci. CBPs bind noncovalently to the phosphocholine of the cell wall through choline binding domains. The choline binding domains of the CBPs are highly conserved; however, they are diverse in their affinities and functions due to differences in their functional domains. Several of the CBPs are predicted to play a role in adherence and colonization, though direct evidence of binding to epithelial receptors is lacking. This project focuses on the ability to express and purify some of the lesser characterized CBPs which are predicted to serve as adhesins and to identify their cognate ligand proteins on host cells with the hope of identifying novel bacterial-host interactions that contribute to colonization.

La reconnaissance moléculaire est un phénomène omniprésent dans les systèmes vivants et intervient dans de nombreux processus biologiques comme la reconnaissance cellulaire ou encore la transmission de signaux par les neurotransmetteurs. L’élaboration de molécules synthétiques capables de mimer l’action des récepteurs naturels en complexant sélectivement un substrat cible est, à l’heure actuelle, très recherchée pour la détection ou le diagnostic en biologie et médecine. Parmi l’ensemble des récepteurs synthétiques, les hémicryptophanes sont des molécules cages composées d’un cyclotribenzylène connecté à une autre unité moléculaire par trois bras espaceurs. Les travaux de cette thèse reposent sur l’élaboration de nouveaux hémicryptophanes et l’étude de leurs propriétés de complexation vis-à-vis de molécules d’intérêt biologique. Dans un premier temps, la chiralité de ces récepteurs a été utilisée pour étudier leurs propriétés de reconnaissance stéréosélective face à différents sucres et analogues de neurotransmetteurs. De bonnes diastéréosélectivités et énantiosélectivités ont ainsi pu être observées en milieu organique pour les substrats étudiés. En parallèle, plusieurs hémicryptophanes hydrosolubles ont été synthétisés et ont permis de reconnaitre sélectivement des neurotransmetteurs comme la choline dans l’eau. Enfin, une dernière partie de cette thèse à été consacrée à la mise en place d’une voie de synthèse pour rendre ces récepteurs fluorescents, dans le but d’élaborer par la suite des sondes moléculaires capables de détecter et de suivre spatio-temporellement des molécules d’intérêt biologique dans les systèmes vivants par fluorescence. / Molecular recognition is a very important phenomenon for living systems as it occurs in many biological processes like cell-cell recognition or transmission of signals by neurotransmitters. Nowadays, the design of synthetic host molecules able to mimic natural receptors by complexing selectively a target substrate, is much sought-after for detection or diagnostic in biology and medicine. Among all the different synthetic receptors, hemicryptophanes are cage-shape molecules which are composed of a cyclotribenzylene moiety connected to another molecular unit by three spacer arms. This thesis is about the synthesis of new hemicryptophanes and the study of their complexation properties toward biologically important molecules. First, the stereoselective recognition of carbohydrates and neurotransmitter analogues by these chiral receptors was investigated in organic solvents and revealed good enantioselectivities and diastereoselectivities. In parallel, several water-soluble hemicryptophanes were synthesized and showed an aptitude for recognizing selectively ammonium substrates like choline in water. The last part was devoted to the development of a synthetic pathway to make hemicryptophanes fluorescent, in order to design molecular probes able to track biologically important molecules in living systems by fluorescence.

Bile is a biofluid synthesized by liver and concentrated in the gallbladder. Interference with the bile flow may cause cholestasis. Primary sclerosing cholangitis (PSC) is an inflammatory cholestatic disorder which eventually may result in liver cirrhosis and failure. The management of PSC is controversial. The only effective treatment for end stage disease is orthotopic liver transplantation (OLT). However, cholangiocarcinoma (CC), which is the major complication of this long-lasting disease, is an absolute contraindication for the surgery. Therefore, early diagnosis of the disease can not only improve the outcome of PSC, but also facilitate the allocation of donated livers to those who can benefit from transplantation. Unfortunately, the diagnosis of CC is challenging. Endoscopic retrograde cholangiopancreatography (ERCP), the gold standard technique, is highly invasive. Non-invasive alternatives such as magnetic resonance cholangiopancreatography (MRCP) have lower accuracy. Therefore, it is essential to develop more accurate and less invasive diagnostic techniques. Magnetic resonance spectroscopy (MRS) is an evolving technique with potential to detect disease-related metabolic changes. In vitro studies have proven the capacity of MRS in the early detection of hepatopancreatobiliary (HPB) disorders based on the metabolic analysis of bile obtained invasively. An in vivo alternative has been attempted by others on human bile within the gallbladder. However, due to the poor quality of the acquired spectra, quantification of most major bile metabolites was not possible, except for choline-containing phospholipids (chol-PLs). In the current study, the quality of the in vivo 1D spectra has been greatly improved, and we have obtained the first 2D L-COSY spectra from bile within the gallbladder. Spectral data from healthy controls and PSC patients were compared. Statistically significant differences in the concentrations of chol-PLs, and glycine- and taurine-conjugated bile acids were revealed in the 1D analysis. Our 2D spectra also demonstrated potential for the detection of metabolic differences between the two groups. The success of these studies indicates a strong potential of in vivo bile MRS techniques to characterize and diagnose a wide variety of HPB disorders. / May 2014

La stéatohépatite non alcoolique (NASH) est une pathologie émergente dans nos pays industrialisés du fait de l'obésité et de l'insulino-résistance. A ce jour, la pathogenèse de la NASH est mal connue et il n'existe pas de traitement prévenant son évolution cirrhogène chez certains patients. Le but de ce travail consistait à étudier les modifications bioénergétiques et métaboliques de mitochondries hépatiques et d'hépatocytes isolés de rats atteints de NASH.<br /> Dans un premier temps, nous avons élaboré un régime alimentaire enrichi en acides gras saturés afin d'induire une NASH. Au terme de 14 semaines de régime, nous n'avons pas mis en évidence de surcharge lipidique au niveau du foie des animaux recevant ce régime enrichi. Les mesures effectuées avec des mitochondries isolées de foies et avec des hépatocytes isolés n'ont montré aucune différence au niveau de la bioénergétique ou du métabolisme hépatique entre les différents groupes de rats. En revanche, une modification de la répartition tissulaire a été observée, avec une augmentation des masses des tissus adipeux blanc et brun. Dans cette étude, l'absence de stéatose hépatique et de NASH semble due à une augmentation du stockage des lipides au niveau du tissu adipeux blanc, et à une augmentation de leur oxydation par un processus thermogène au niveau du tissu adipeux brun, permettant ainsi de « brûler » l'excès calorique.<br />La deuxième partie du travail concernait l'étude des modifications bioénergétiques et métaboliques induites par un régime carencé en choline et méthionine connu pour induire une NASH chez le rat. Nous avons montré que les mitochondries de foie et les hépatocytes isolés de rats traités, avaient une respiration augmentée. Cette stimulation de la respiration était due à un découplage de la chaîne respiratoire par un mécanisme de « proton leak » ET de « redox slipping » au niveau de la cytochrome c oxydase. Ce découplage avait pour conséquence de stimuler l'utilisation de substrats lipidiques et de diminuer la production de radicaux libres de l'oxygène. En parallèle, l'étude avec des hépatocytes isolés nous montre une augmentation de l'oxydation lipidique et de la néoglucogenèse. <br />Ces résultats suggèrent des adaptations des fonctions mitochondriales et métaboliques des foies de rats atteints de NASH qui permettraient de limiter la surcharge lipidique et le stress oxydant

Très récemment, les solvants de type « mélanges eutectiques profonds (MEP)» ont été décrits comme une alternative sérieuse et économiquement plus réaliste aux liquides ioniques. En effet, ces solvants qui consistent en un mélange d'un sel organique (ammonium ou phosphonium) et d'un donneur de liaison hydrogène peuvent également être liquides à température ambiante, non volatils et présentant une excellente stabilité thermique. De plus, contrairement aux liquides ioniques, ces nouveaux solvants sont très facilement préparés et leur innocuité ainsi que leur bonne biodégradabilité sont sensiblement améliorées. Dans le domaine des procédés enzymatiques, si la biocatalyse en milieu liquide ionique est très documentée, il n'existe que très peu de publications décrivant des réactions de biotransformation en MEP. Concernant les lipases en particulier, outre leurs applications dans le biofaçonnement des corps gras, ces enzymes sont également utilisées dans des réactions dites de lipophilisation pour la synthèse de nouvelles molécules à haute valeur ajoutée (tensioactifs, antioxydant lipophilisés). Au travers cette étude nous nous sommes investis à tester le potentiel des MEP en tant que nouveaux milieux réactionnels « verts » pour la synthèse lipasique. Ce travail n'a pas eu comme objectif de faire l'éloge de ces solvants pour leur utilisation dans le domaine de la biocatalyse, mais surtout d'évaluer leur capacité à favoriser ou non des synthèses lipasiques. Par ailleurs, nous nous sommes engagés à essayer de comprendre, d'un point de vue fondamental, l'organisation supramoléculaire de ce type de milieux pour déterminer les paramètres qui influencent le plus la réactivité et la stabilité enzymatique dans ce type d'environnement. Les variations des conditions réactionnelles (solvants et biocatalyseurs) ont permis de mettre en évidence la très nette supériorité de deux MEP (Chlorure de cholinium:Urée et Chlorure de cholinium:glycérol) pour la réalisation de réactions d'alcoolyses biocatalysées par la lipase B de Candida antarctica. Toutefois, les résultats ont montré que les réactions de biotransformations de composés phénoliques dans ces MEP sont extrêmement difficiles à réaliser sans l'addition d'eau. De profondes études (pH, activité thermodynamique de l'eau, activité et stabilité de la lipase, composition du solvant, etc.) réalisées sur des mélanges du type MEP-eau ont permis de finement adapter les conditions de réaction pour optimiser la catalyse enzymatique dans ce type de solvant. Compte tenu des difficultés rencontrées pour la lipophilisation de composés phénoliques, nous sommes toutefois parvenus à synthétiser toute une gamme de dérivés lipophiles d'acides férulique et coumarique de C4 à C16 (chaîne aliphatique) avec des rendements élevés. / With the emergence of the green chemistry concept in the 90s, many studies have been dedicated to the discovery of new reactions media both suitable and efficient for chemical/enzyme catalysis. Up to now, the main efforts have focused on the development of ionic liquids. However, recently a novel class of solvent called "deep eutectic mixtures (DES)", have been described as a serious alternative and economically stronger than ionic liquids. Such solvents are formed by mixing an organic salt (ammonium or phosphonium) with a hydrogen-bond donor. Just like ionic liquid, DES may also be liquid at room temperature, non-volatile and have excellent thermal stability. However, unlike most ionic liquids, these new solvents are biodegradable, inexpensive, and very easy to prepare. In the field of biocatalysis, whereas the studies in ionic liquid are deeply documented, the published papers describing biotransformation reactions in DES are very low, especially in lipase-catalyzed processing, where these enzymes may be used in so-called "lipophilisation reactions", for the synthesis of new molecules with high added value (surfactants or lipophilized antioxidants).The main objective of this work was to assess and test the potential of DES as new "green" reaction media for lipase-catalyzed synthesis. On a fundamental point of view, this study provides valuable information to understand how the different components involved in these mixtures could contribute to their functional properties in order to enhance their use in various applications. Changes in reaction conditions (solvents and biocatalysts) allowed us to highlight the clear superiority of two DES (chloride cholinium:Urea and chloride cholinium:glycerol) to carry out lipase-catalyzed reactions using the lipase B from Candida antarctica as biocatalyst. However, our results showed that the biotransformations of dissolved substrates (such as phenolic compounds) in DES are extremely difficult to achieve without the addition of water. Studying DES-water mixtures (pH, thermodynamic activity of water, activity and stability of lipase, mixtures composition, etc ...) we were able to fine-tune the reaction conditions to optimize the performance of the lipasic catalysis. Thus, given the difficulties encountered when performing lipase-catalyzed reactions with substrates of two different polarities, it was still possible to synthesize high yields of a full range of lipophilic derivatives of ferulic and coumaric acids from C4 to C16 (aliphatic chain).

A colina é um nutriente essencial nas dietas de frangos de corte. Os valores de exigências nutricionais são variáveis e foram estabelecidos há muitos anos. Assim, o objetivou-se com este trabalho reavaliar as respostas de frangos de corte de alto potencial genético de 1 a 7, 1 a 14 e 1 a 21 dias de idade como também modelar as respostas por diferentes funções matemáticas. Foram conduzidos dois experimentos sob dois níveis de metionina na dieta, tendo como base o método dose-resposta. Para os ensaios, foram utilizados 2.160 pintos de corte machos da linhagem Cobb 500 (1.080 em cada experimento), distribuídos em um delineamento inteiramente casualizado, com seis tratamentos, seis repetições e 30 aves por unidade experimental. Os tratamentos consistiram em rações com níveis de 390, 715, 1.040, 1.365, 1.690 e 2.015 mg/kg de colina. As rações experimentais foram formuladas utilizando-se a técnica da suplementação. Foi formulada uma ração basal à base de milho, farelo de soja, concentrado proteico de soja, açúcar e amido contendo 390 mg/kg de colina total e 0,596% de metionina digestível. O cloreto de colina (62,5%) comercial foi suplementado para obter os demais níveis. O segundo experimento consistiu nos mesmos tratamentos do primeiro, porém com atendimento de 0,440% da metionina digestível. As variáveis mensuradas em ambos os ensaios foram: consumo de ração, consumo de colina, conversão alimentar e ganho de peso. Para modelar as respostas considerou-se o consumo de colina como variável independente e ganho de peso como variável dependente. As respostas foram modeladas por cinco modelos matemáticos: broken line, broken line com ascendência quadrática, cinética de saturação, logístico e monomolecular e o melhor modelo foi selecionado pelo critério de Akaike. As análises estatísticas foram realizadas com auxílio do programa computacional SAS® utilizando o procedimento PROC GLM para análise das pressuposições e análise de variância e PROC NLIN para o ajuste dos modelos. Os modelos que melhor se ajustaram foram: Broken line com ascendência quadrática (1,26 mg/g de GP ensaio 1) e Logístico (1,26 mg/g de GP ensaio 2) para o período de 1 a 7 dias. No período de 1 a 14 dias os modelos selecionados para o experimento com metionina normal e reduzida foram Logístico (1,16 mg/g de GP) e Broken line com ascendência quadrática (1,28 mg/g de GP), respectivamente. Já para o período de 1 a 21 foram o cinética de saturação, estimando uma exigência de 1,35 mg/g de GP para o experimento com metionina normal, e Broken line com ascendência quadrática estimando uma exigência de 1,21 mg de colina/g de GP para o experimento com metionina reduzida. / Choline is an essential nutrient in the diets of chicks. Supplementation levels in feeds is variable and requirements have been established many years ago. Thus, the objective of this study was to reassess the responses of high genetic potential chicks from 1-7, 1-14 and 1 to 21 days as well as modeling the responses of different mathematical functions. Two trials were conducted using two dietary levels of methionine, based on the dose-response method. For the trials were used 2,160 Cobb 500 male broiler chicks (1,080 birds in each experiment), distributed in a completely randomized design with six treatments and six replicates and 30 birds per experimental unit. The treatments consisted of diets with levels of 390, 715, 1,040, 1,365, 1,690 and 2,015 mg/kg of choline. The diets were formulated using the supplementation method. The basal diet was formulated based on corn, soybean meal, soy protein concentrate, sugar and starch, and contained 390 mg/kg of choline and 0.596% of digestible methionine. Commercial choline chloride (62.5%) was supplemented to obtain the other levels. The second trial consisted of the same treatments of the first, but with 0.440% of digestible methionine. The variables measured in both trials were: feed intake (FI), intake of choline (IC), feed conversion (FC) and weight gain (WG). To model the responses, the variables used were the intake of choline as an independent variable and weight gain as the dependent variable. The responses were modeled by five mathematical models: broken line (ascending linear and quadratic segments), saturation kinetic, 4-parameter logistics and monomolecular models and the best model was selected by Akaike information criterion. Statistical analyzes were performed using the computer program SAS® using PROC GLM for analysis of the assumptions and analysis of variance and PROC NLIN to fit the models. The models that best fit were: broken line quadratic segments (1.26 mg/g of WG - trial 1) and Logistics (1.26 mg/g of WG - trial 2) for the period 1-7 days. In the period 1-14 days, the models selected for the experiment with normal and deficient methionine were Logistic (1.16 mg/g WG) and broken line quadratic segments (1.28 mg/g WG), respectively. For the period 1-21days, the models were the saturation kinetics, estimating a requirement of 1.35 mg/g of WG for the experiment with normal methionine, and broken line quadratic segments estimating a requirement of 1.21 mg choline/g of WG for the experiment with deficient methionine.