Influence of dietary bile acids on formation of bile acids in rat

Various taurine-conjugated bile acids were fed to rats at the 1%-level in the diet for 3 or 7 days and the effect on several hydroxylations involved in the biosynthesis and metabolism of bile acids was studied. The hydroxylations studied were all catalyzed by the microsomal fraction of liver homogenate fortified with NADPH. The 7α-hydroxylation of cholesterol was inhibited by feeding taurocholic acid, taurocheno-deoxycholic acid and taurodeoxycholic acid for 3 as well as 7 days. No marked inhibition was obtained with taurohyodeoxycholic acid or taurolithocholic acid. The 12α-hydroxylation of 7α-hydroxy-4-cholesten-3-one was inhibited after 3 as well as 7 days by all bile acids except taurohyodeoxycholic acid. With this acid a marked stimulation of 12α-hydroxylation was observed. The effects of the different bile acids on the 7α-hydroxylation of taurodeoxycholic acid were not very marked. The 6β-hydroxylation of lithocholie acid and taurochenodeoxycholic acid was stimulated by taurocholic acid and taurodeoxycholic acid. The reaction was inhibited by taurochenodeoxycholic acid, at least after 7 days. Taurohyodeoxycholic acid inhibited the 6β-hydroxylation slightly and taurolithocholic acid had no effect. The results were discussed in the light of present knowledge concerning mechanisms of regulation of formation and metabolism of bile acids and it was suggested that the mechanisms may be more complex than previously thought.     

AMINO ACID TRANSPORT IN PERIPHERAL NERVE: ENERGY AND SODIUM DEPENDENCE

Abstract— The energy and sodium dependence of the several carrier-mediated mechanisms for amino acid uptake have been studied in frog sciatic nerve. The different transport mechanisms are found to be variable in their dependence on sodium and metabolic energy. Saturable uptakes of lysine, phenylalanine and valine are relatively independent of the presence or absence of sodium in the incubation medium, indicating that uptakes by those mechanisms subserving basic, large neutral amino acids, and those amino acids containing aromatic or heterocyclic ring structures are largely sodium independent. Saturable uptakes of glutamic acid, proline, glycine and β-alanine are considerably reduced in the absence of sodium; thus carrier mechanisms for uptake of acidic, small neutral amino acids, β-alanine and proline are highly sodium dependent. The efficacies of several cations in substituting for sodium is variable; greatest inhibitions are found when potassium is used to replace sodium.
With the exception of proline, those mechanisms found to be sodium dependent are also found to be energy dependent, since they are inhibited by both DNP and lowered temperature. Although proline uptake is sodium dependent, proline uptake is stimulated by DNP and relatively insensitive to lowered temperature.     

Omega-3 fatty acids and neuropsychiatric disorders

Epidemiological evidence suggests that dietary consumption of the long chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), commonly found in fish or fish oil, may modify the risk for certain neuropsychiatric disorders. As evidence, decreased blood levels of omega-3 fatty acids have been associated with several neuropsychiatric conditions, including Attention Deficit (Hyperactivity) Disorder, Alzheimer's Disease, Schizophrenia and Depression. Supplementation studies, using individual or combination omega-3 fatty acids, suggest the possibility for decreased symptoms associated with some of these conditions. Thus far, however, the benefits of supplementation, in terms of decreasing disease risk and/or aiding in symptom management, are not clear and more research is needed. The reasons for blood fatty acid alterations in these disorders are not known, nor are the potential mechanisms by which omega-3 fatty acids may function in normal neuronal activity and neuropsychiatric disease prevention and/or treatment. It is clear, however, that DHA is the predominant n-3 fatty acid found in the brain and that EPA plays an important role as an anti-inflammatory precursor. Both DHA and EPA can be linked with many aspects of neural function, including neurotransmission, membrane fluidity, ion channel and enzyme regulation and gene expression. This review summarizes the knowledge in terms of dietary omega-3 fatty acid intake and metabolism, as well as evidence pointing to potential mechanisms of omega-3 fatty acids in normal brain functioning, development of neuropsychiatric disorders and efficacy of omega-3 fatty acid supplementation in terms of symptom management.     

Bile acids and signal transduction: role in glucose homeostasis

Bile acids are mainly recognized for their role in dietary lipid absorption and cholesterol homeostasis. However, recent progress in bile acid research suggests that bile acids are important signaling molecules that play a role in glucose homeostasis. Among the various supporting evidence, several reports have demonstrated an improvement of the glycemic index of type 2 diabetic patients treated with diverse bile acid binding resins. Herein, we review the diverse interactions of bile acids with various signaling/response pathways, including calcium mobilization and protein kinase activation, membrane receptor-mediated responses, and nuclear receptor responses. Some of the effects of the bile acids are direct through the activation of specific receptors, i.e., TGR5, CAR, VDR, and FXR, while others imply modulation of the hormonal, growth factor and/or neuromediator responses, i.e., glucagon, EGF, and acetylcholine. We also discuss recent evidence implicating the interaction of bile acids with glucose homeostasis mechanisms, with the integration of our understanding of how the signaling mechanisms modulated by bile acid could regulate glucose metabolism.     

植物氨基酸转运子研究进展

氨基酸是高等植物氮素同化产物长距离运输及在组织间分配的主要形式,通过跨膜转运的方式在植物体内进行运输。氨基酸转运子是位于生物膜上吸收及转运氨基酸的蛋白家族,对植物氮素营养具有重要贡献。本文对植物氨基酸转运子的表达、调控及其与氮素利用效率、植物产量与品质形成、抗逆性及适应性等方面的研究进展进行了综述。     

Reviewing the low efficiency of protein utilization in heavy preruminant calves--a reductionist approach

The efficiency of protein utilization for growth in preruminant calves is decreasing with increasing body weight. In contrast to calves weighing less than 100 kg of body weight, heavy preruminant calves do not respond in protein retention to an increased intake of indispensable amino acids in dose-response studies. The marginal efficiency of protein utilization is low compared with pigs and milk-fed lambs at a similar stage of maturity. A reductionist approach was taken to perceive the potential mechanisms for the low protein utilization in preruminant calves. Neither an imbalance in the dietary protein to energy ratio nor a single limiting indispensable amino acid was responsible for the low efficiency. Also, amino acids were not specifically used to detoxify ammonia. Alternative hypotheses to explain the low efficiency are discussed and result in (i) a reduced post-absorptive supply of amino acids: e.g. by fermentation of milk in the (premature) rumen or preferential amino acid utilization by specific tissues; or (ii) a reduced post-absorptive amino acid utilization: e.g. by decreased insulin sensitivity, utilization of amino acids for gluconeogenesis or an asynchronous nutrient supply. In conclusion, several mechanisms for the low efficiency of protein utilization in heavy preruminant calves were excluded. Other physiological processes which are potentially involved remain to be studied, because the large potential for improving protein utilization in heavy preruminant calves asks for further exploration of their amino acid metabolism.     

Mass spectra of alpha-amino acid oxazolidinones

2-Bis-(chlorodifluoromethyl)-4-substituted-1,3-oxazolidin-5-ones, a new type of alpha-amino acid derivative for gas chromatographic separation, have been studied by low resolution mass spectrometry. These derivatives are obtained by reacting alpha-amino acids with dichlorotetrafluoroacetone. Their structure has been established or confirmed for most protein amino acids and several non-protein alpha-amino acids. The mechanisms responsible for the mass spectral pattern have been rationalized. An interesting feature of this derivatization procedure is that it distinguishes aspartic and glutamic acid from the respective amides. The structure of asparagine and glutamine derivatives has been established. A survey of oxazolidinone mass spectra has provided a list of diagnostically useful ions. Each amino acid can be identified by one or two of its most abundant fragments.     

Oxylipins: Structurally diverse metabolites from fatty acid oxidation

Oxylipins are lipophilic signaling molecules derived from the oxidation of polyunsaturated fatty acids. Initial fatty acid oxidation occurs mainly by the enzymatic or chemical formation of fatty acid hydroperoxides. An array of alternative reactions further converting fatty acid hydroperoxides gives rise to a multitude of oxylipin classes, many with reported signaling functions in plants. Oxylipins include the phytohormone, jasmonic acid, and a number of other molecules including hydroxy-, oxo- or keto-fatty acids or volatile aldehydes that may perform various biological roles as second messengers, messengers in inter-organismic signaling, or even as bactericidal agents. The structural diversity of oxylipins is further increased by esterification of the compounds in plastidial glycolipids, for instance the Arabidopsides, or by conjugation of oxylipins to amino acids or other metabolites. The enzymes involved in oxylipin metabolism are diverse and comprise a multitude of examples with interesting and unusual catalytic properties. In addition, the interplay of different subcellular compartments during oxylipin biosynthesis suggests complex mechanisms of regulation that are not well understood. This review aims at giving an overview of plant oxylipins and the multitude of enzymes responsible for their biosynthesis.     

A TRPA1-dependent mechanism for the pungent sensation of weak acids

Acetic acid produces an irritating sensation that can be attributed to activation of nociceptors within the trigeminal ganglion that innervate the nasal or oral cavities. These sensory neurons sense a diverse array of noxious agents in the environment, allowing animals to actively avoid tissue damage. Although receptor mechanisms have been identified for many noxious chemicals, the mechanisms by which animals detect weak acids, such as acetic acid, are less well understood. Weak acids are only partially dissociated at neutral pH and, as such, some can cross the cell membrane, acidifying the cell cytosol. The nociceptor ion channel TRPA1 is activated by CO(2), through gating of the channel by intracellular protons, making it a candidate to more generally mediate sensory responses to weak acids. To test this possibility, we measured responses to weak acids from heterologously expressed TRPA1 channels and trigeminal neurons with patch clamp recording and Ca(2+) microfluorometry. Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids. Notably, the degree of channel activation was predicted by the degree of intracellular acidification produced by each acid, suggesting that intracellular protons are the proximate stimulus that gates the channel. Responses to weak acids produced a Ca(2+)-independent inactivation that precluded further activation by weak acids or reactive chemicals, whereas preactivation by reactive electrophiles sensitized TRPA1 channels to weak acids. Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice. We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.     

有机酸在植物解铝毒中的作用及生理机制

酸性土壤上铝毒是限制作物产量的一个重要障碍因子,具有螯合能力的有机酸在植物铝的外部排斥机制和内部耐受机制均具有重要作用,在铝的外部排斥解毒过程中,植物通过根系分泌有机酸进入根际,如柠檬酸,草酸,苹果酸等与铝形成稳定的复合体,阻止铝进入共质体,从而达到植物体外解除铝毒害效应的目的,且分泌的有机酸对铝的胁迫诱导表现出高度的专一性,分泌的关键点位于根尖,不同的物种间分泌的有机酸种类,分泌的模式及生理机理存在差异,在铝积累型植物的内部解毒过程中,有机酸与铝形成稳定的化合物,降低植物体内铝离子的生理活性,从而降低细胞内铝离子的毒害效应,如绣球花中铝与柠檬酸形成1：1的复合体,荞麦内铝与草酸形成1：3的复合体,本文就有机酸在植物忍耐和积累铝中的作用及生理机制作一简要综述。     

Separation of phytanic and pristanic acid by high-pressure liquid chromatography: application of the method

The synthesis of pristanic acid from phytanic acid, and a simple reversed-phase high-pressure liquid chromatographic (HPLC) method for the separation and purification of these acids, is described. A base-line separation of [U-3H]phytanic and [U-3H]pristanic acid is achieved with a graphitized carbon column. The isoprenoid metabolites formed after incubation of cultured fibroblasts with phytanic or pristanic acids are extracted with a Sep-Pak C18 cartridge and separated from the substrates by the same reversed-phase HPLC used for substrate purification. The methods are suitable for studies on the mechanisms for degradation of phytanic acid. Recently, different inborn errors with accumulation of phytanic acid have been defined. The present method will be a useful tool in our efforts to define these metabolic defects and their subcellular localization.     

Superspecificity of aminoacyl-tRNA-synthases

The correct aminoacylation of tRNA with the proper aminoacid by aminoacyl-tRNA synthetase is one of the key reactions which determines the overall high fidelity of protein biosynthesis. The initial selection of the amino acid is achieved in the active centre of the synthetase at the activation step due to differences in the side chains binding energies of specific substrate and the competing amino acids present in cell. If, nevertheless, the activation of amino acids structurally similar to the cognate one does proceed, additional mechanisms of correction which are based on the decomposition of unstable noncognate (intermediate or final) product of the tRNA aminoacylation reaction, by synthetase are switched on. In this review the literature on the specificity of aminoacyl-tRNA synthetases at amino acid activation step is analyzed along with the proofreading mechanisms which allow the elimination of the errors, leading to so called superspecifity of aminoacyl-tRNA synthetases.     

Improving amino acid nutrition to prevent intrauterine growth restriction in mammals

Intrauterine growth restriction (IUGR) is one of the most common concerns in human obstetrics and domestic animal production. It is usually caused by placental insufficiency, which decreases fetal uptake of nutrients (especially amino acids) from the placenta. Amino acids are not only building blocks for protein but also key regulators of metabolic pathways in fetoplacental development. The enhanced demands of amino acids by the developing conceptus must be met via active transport systems across the placenta as normal pregnancy advances. Growing evidence indicates that IUGR is associated with a reduction in placental amino acid transport capacity and metabolic pathways within the embryonic/fetal development. The positive relationships between amino acid concentrations in circulating maternal blood and placental amino acid transport into fetus encourage designing new therapies to prevent or treat IUGR by enhancing amino acid availability in maternal diets or maternal circulation. Despite the positive effects of available dietary interventions, nutritional therapy for IUGR is still in its infancy. Based on understanding of the underlying mechanisms whereby amino acids promote fetal growth and of their dietary requirements by IUGR, supplementation with functional amino acids (e.g., arginine and glutamine) hold great promise for preventing fetal growth restriction and improving health and growth of IUGR offspring.     

The role of marine omega-3 (n-3) fatty acids in inflammatory processes, atherosclerosis and plaque stability

Atherosclerosis has an important inflammatory component and acute cardiovascular events can be initiated by inflammatory processes occurring in advanced plaques. Fatty acids influence inflammation through a variety of mechanisms; many of these are mediated by, or associated with, the fatty acid composition of cell membranes. Human inflammatory cells are typically rich in the n-6 fatty acid arachidonic acid, but the contents of arachidonic acid and of the marine n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) can be altered through oral administration of EPA and DHA. Eicosanoids produced from arachidonic acid have roles in inflammation. EPA also gives rise to eicosanoids and these are usually biologically weak. EPA and DHA give rise to resolvins which are anti-inflammatory and inflammation resolving. EPA and DHA also affect production of peptide mediators of inflammation (adhesion molecules, cytokines, etc.). Thus, the fatty acid composition of human inflammatory cells influences their function; the contents of arachidonic acid, EPA and DHA appear to be especially important. The anti-inflammatory effects of marine n-3 polyunsaturated fatty acids (PUFAs) may contribute to their protective actions towards atherosclerosis and plaque rupture.     

Lipotoxicity: when tissues overeat

PURPOSE OF REVIEW: This review will provide the reader with an update on our understanding of the adverse effects of fatty acid accumulation in non-adipose tissues, a phenomenon known as lipotoxicity. Recent studies will be reviewed. Cellular mechanisms involved in the lipotoxic response will be discussed. Physiologic responses to lipid overload and therapeutic approaches to decreasing lipid accumulation will be discussed, as they add to our understanding of important pathophysiologic mechanisms. RECENT FINDINGS: Excess lipid accumulation in non-adipose tissues may arise in the setting of high plasma free fatty acids or triglycerides. Alternatively, lipid overload results from mismatch between free fatty acid import and utilization. Evidence from human studies and animal models suggests that lipid accumulation in the heart, skeletal muscle, pancreas, liver, and kidney play an important role in the pathogenesis of heart failure, obesity and diabetes. Excess free fatty acids may impair normal cell signaling, causing cellular dysfunction. In some circumstances, excess free fatty acids induce apoptotic cell death. SUMMARY: Recent studies provide clues regarding the cellular mechanisms that determine whether excess lipid accumulation is well tolerated or cytotoxic. Critical in this process are physiologic mechanisms for directing excess free fatty acids to specific tissues as well as cellular mechanisms for channeling excess fatty acid to particular metabolic fates. Insight into these mechanisms may contribute to the development of more effective therapies for common human disorders in which lipotoxicity contributes to pathogenesis.     

Postnatal amino acid uptake by the rat small intestine. Energetics of membrane transport systems for amino acids in the developing jejunum

The energetics of amino acid uptake by the developing small intestine was investigated in vitro. L-valine, L-leucine, L-phenylalanine, L-methionine, L-lysine and L-arginine were all actively transported by the newborn rat jejunum. Metabolic inhibitors (e.g. 2,4-dinitrophenol) significantly reduced uptake of all amino acids but uptake against a concentration gradient was not totally abolished. Uptake of all amino acids was reduced at low[Na+]. Inhibition of transport of neutral amino acids by reduced luminal [Na+] was greater than that of basic amino acids, and the tissue was barely able to concentrate the neutral amino acids. [Na+] affected the Michaelis constant (Km) of neutral transport systems for their substrates; for the basic amino acids Km values were unaffected by the presence or absence of Na+. Ouabain significantly inhibited neutral amino acid uptake but had no effect on L-lysine or L-arginine uptake. These results are discussed in terms of the Na+ gradient hypothesis for amino acid transport, and the site of energy input to active transport. The role of glycolysis in providing energy for intestinal transport in the neonatal rat and the efficiency of Na+ dependent and independent transport mechanisms are considered. It is concluded that the energetics of amino acid transport systems in neonatal and adult rats are essentially similar.     

Modulation of substrate specificity within the amino acid editing site of leucyl-tRNA synthetase

The aminoacyl-tRNA synthetases covalently link transfer RNAs to their cognate amino acids. Some of the tRNA synthetases have evolved editing mechanisms to ensure fidelity in this first step of protein synthesis. The amino acid editing site for leucyl- (LeuRS) and isoleucyl- (IleRS) tRNA synthetases reside within homologous CP1 domains. In each case, a threonine-rich peptide and a second conserved GTG region that are separated by about 100 amino acids comprise parts of the hydrolytic editing site. While a number of sites are conserved between these two enzymes and likely confer a commonality to the mechanisms, some positions are idiosyncratic to LeuRS or IleRS. Herein, we provide evidence that a conserved arginine and threonine at respective sites in LeuRS and IleRS diverged to confer amino acid substrate recognition. This site complements other sites in the amino acid binding pocket of the editing active site of Escherichia coli LeuRS, including Thr252 and Val338, which collectively fine-tune amino acid specificity to confer fidelity.     

Intracellular sensing of amino acids in Xenopus laevis oocytes stimulates p70 S6 kinase in a target of rapamycin-dependent manner.

Amino acids exert modulatory effects on proteins involved in control of mRNA translation in animal cells through the target of rapamycin (TOR) signaling pathway. Here we use oocytes of Xenopus laevis to investigate mechanisms by which amino acids are "sensed" in animal cells. Small ( approximately 48%) but physiologically relevant increases in intracellular but not extracellular total amino acid concentration (or Leu or Trp but not Ala, Glu, or Gln alone) resulted in increased phosphorylation of p70(S6K) and its substrate ribosomal protein S6. This response was inhibited by rapamycin, demonstrating that the effects require the TOR pathway. Alcohols of active amino acids substituted for amino acids with lower efficiency. Oocytes were refractory to changes in external amino acid concentration unless surface permeability of the cell to amino acids was increased by overexpression of the System L amino acid transporter. Amino acid-induced, rapamycin-sensitive activation of p70(S6K) was conferred when System L-expressing oocytes were incubated in extracellular amino acids, supporting intracellular localization of the putative amino acid sensor. In contrast to lower eukaryotes such as yeast, which possess an extracellular amino acid sensor, our findings provide the first direct evidence for an intracellular location for the putative amino acid sensor in animal cells that signals increased amino acid availability to TOR/p70(S6K).