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Vogel KR, Arning E, Wasek BL, Bottiglieri T, Gibson KM. Non-physiological amino acid (NPAA) therapy targeting brain phenylalanine reduction: pilot studies in PAHENU2 mice. J Inherit Metab Dis 2013; 36:513-23. [PMID: 22976763 PMCID: PMC3654543 DOI: 10.1007/s10545-012-9524-8] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/05/2012] [Revised: 07/11/2012] [Accepted: 07/19/2012] [Indexed: 11/26/2022]
Abstract
Transport of large neutral amino acids (LNAA) across the blood brain barrier (BBB) is facilitated by the L-type amino acid transporter, LAT1. Peripheral accumulation of one LNAA (e.g., phenylalanine (phe) in PKU) is predicted to increase uptake of the offending amino acid to the detriment of others, resulting in disruption of brain amino acid homeostasis. We hypothesized that selected non-physiological amino acids (NPAAs) such as DL-norleucine (NL), 2-aminonorbornane (NB; 2-aminobicyclo-(2,1,1)-heptane-2-carboxylic acid), 2-aminoisobutyrate (AIB), and N-methyl-aminoisobutyrate (MAIB), acting as competitive inhibitors of various brain amino acid transporters, could reduce brain phe in Pah (enu2) mice, a relevant murine model of PKU. Oral feeding of 5 % NL, 5 % AIB, 0.5 % NB and 3 % MAIB reduced brain phe by 56 % (p < 0.01), -1 % (p = NS), 27 % (p < 0.05) and 14 % (p < 0.01), respectively, compared to untreated subjects. Significant effects on other LNAAs (tyrosine, methionine, branched chain amino acids) were also observed, however, with MAIB displaying the mildest effects. Of interest, MAIB represents an inhibitor of the system A (alanine) transporter that primarily traffics small amino acids and not LNAAs. Our studies represent the first in vivo use of these NPAAs in Pah (enu2) mice, and provide proof-of-principle for their further preclinical development, with the long-term objective of identifying NPAA combinations and concentrations that selectively restrict brain phe transport while minimally impacting other LNAAs and downstream intermediates.
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Affiliation(s)
- Kara R. Vogel
- Section of Clinical Pharmacology, College of Pharmacy, Washington State University, Spokane, WA USA
| | - Erland Arning
- Institute of Metabolic Disease, Baylor Research Institute, Baylor University Medical Center, Dallas, TX USA
| | - Brandi L. Wasek
- Institute of Metabolic Disease, Baylor Research Institute, Baylor University Medical Center, Dallas, TX USA
| | - Teodoro Bottiglieri
- Institute of Metabolic Disease, Baylor Research Institute, Baylor University Medical Center, Dallas, TX USA
| | - K. Michael Gibson
- Section of Clinical Pharmacology, College of Pharmacy, Washington State University, Spokane, WA USA
- Correspondence: Section of Clinical Pharmacology, College of Pharmacy, Washington State University, 313 Wegner Hall, PO Box 646510, Pullman WA 99164-6510; phone 509-335-4754; fax 509-335-5902;
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Abstract
The parasitic protozoa belonging to the kinetoplastids can use both sugars and amino acids as carbon and energy sources. In this review, Benno ter Kuile discusses nutrient acquisition and utilization and how the metabolic strategies reflect the environment encountered in host and vector. Recent genetic and physiological evidence suggests that facilitated diffusion may be the primary uptake mechanism for glucose, and possibly for proline as well, even though there is biochemical and genetic evidence suggesting that active transport occurs, if not across the plasma membrane, then across the membranes of organelles. Trypanosoma brucei seems to have a metabolic strategy that strives for maximum energy efficiency, making no storage materials and thereby limiting the control over its internal conditions. On the other hand, Leishmania donovani does create a storage buffer, entrapping glucose in the cell. In this manner, it maintains constant internal conditions at the expense of energy, enabling it to survive more adverse conditions in the macrophage and in its vector.
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Affiliation(s)
- B H ter Kulle
- Rockefeller University, 1230 York Avenue, New York, NY 10021, USA
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BLUM JJOSEPH, BALBER ANDREWE. Osmotic and Metabolic-Induced Changes in Light Scattering of Leishmania donovani as Measured by Flow Cytometry. J Eukaryot Microbiol 1996. [DOI: 10.1111/j.1550-7408.1996.tb01393.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Blum JJ. Oxidation of alanine, acetate, glutamate, and succinate by digitonin-permeabilized Leishmania major promastigotes. J Eukaryot Microbiol 1996; 43:144-50. [PMID: 8720944 DOI: 10.1111/j.1550-7408.1996.tb04495.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Leishmania major promastigotes were treated with digitonin and the rates at which [1-14C]acetate, [1,4-14C]succinate, [1-14C]glutamate, and [U-14C]alanine are oxidized were measured in the presence of suitable cofactors. Acetate was oxidized at the lowest rate of the four substrates examined, even in the presence of added NAD, CoA, ADP and acetyl-CoA synthase. Its rate of oxidation was negligible if the permeabilized cells were washed before the cofactors were added, indicating the requirement for an as yet unknown factor. Succinate was oxidized at a rate much higher than the very slow rate at which it is oxidized by intact cells. Its rate of oxidation was strongly inhibited by antimycin A, but that of glutamate was scarcely affected. Fumarate inhibited the rate of oxidation of acetate, glutamate, and succinate, but increased that of alanine. Ca++ inhibited the rates of oxidation of alanine and succinate, but not of acetate or glutamate. Increasing the osmolality by addition of mannitol partially inhibited the rate of oxidation of alanine but had little effect on that of glutamate. These results show that appreciable transaminase activity remains in the permeabilized cells and support earlier data indicating the presence of a branched NAD-to-cytochrome oxidase system. These results also provide preliminary information on the sensitivity of the two branches to Ca++, hyperosmolality, and Krebs cycle intermediates.
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Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA
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Kandpal M, Fouce RB, Pal A, Guru PY, Tekwani BL. Kinetics and molecular characteristics of arginine transport by Leishmania donovani promastigotes. Mol Biochem Parasitol 1995; 71:193-201. [PMID: 7477101 DOI: 10.1016/0166-6851(95)00042-y] [Citation(s) in RCA: 47] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Characteristics of transport of L-arginine were studied in Leishmania donovani promastigotes grown in vitro in a defined medium. The promastigotes exhibited a time-dependent, temperature-sensitive, pH-dependent and saturable uptake of arginine. Metabolic inhibitors caused 81-92% inhibition, indicating that arginine influx in promastigotes is an energy requiring process. The presence of Na+ ions was necessary for full activity. Considerable inhibition was also noticed with valinomycin, gramicidin and amiloride. The transporter seems to involve an -SH group at the active site. The most distinctive feature of the leishmanial transporter was that lysine and ornithine did not show significant competition with arginine transport. Other neutral and acidic amino acids, as well as polyamines were also ineffective. The arginine analogues, viz., nitro-L-arginine methyl ester, N-nitro-L-arginine, aminoguanidine, agmatine and D-arginine were not recognised by the transporter, while N-methyl-L-arginine acetate and phospho-L-arginine showed competition, indicating stereo-specificity of the transporter and recognition of both the guanidino group, as well as the arginine side chain by the transporter. No exchange of intracellular [14C]arginine taken up by the promastigotes was noticed during incubation with 2 or 5 mM arginine in the extracellular medium. Eighty percent of the arginine taken up remained in the trichloroacetic acid-soluble fraction. Pentamidine caused competitive inhibition of arginine transport, exhibiting an IC50 value of 40 microM. Results indicate the presence of a novel distinct arginine transporter in Leishmania promastigotes.
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Affiliation(s)
- M Kandpal
- Division of Biochemistry, Central Drug Research Institute, Lucknow, India
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Blum JJ. Effects of hypoxia, acute osmotic stress, and protein kinase inhibitors on the intermediary metabolism of Leishmania. J Eukaryot Microbiol 1994; 41:1-8. [PMID: 8124263 DOI: 10.1111/j.1550-7408.1994.tb05925.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
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Zilberstein D. Transport of nutrients and ions across membranes of trypanosomatid parasites. ADVANCES IN PARASITOLOGY 1993; 32:261-91. [PMID: 8237616 DOI: 10.1016/s0065-308x(08)60209-2] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- D Zilberstein
- Department of Biology, Technion-Israel Institute of Technology, Haifa
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Abstract
Leishmania donovani promastigotes were grown to late log phase, washed and resuspended in iso-osmotic buffer containing L-arginine, and the rate of urea formation was then measured under various conditions. Addition of glucose or mannose activated urea formation, whereas 2-deoxyglucose inhibited and 6-deoxyglucose had no effect. Addition of alanine or of alpha-aminoisobutyrate inhibited urea formation, alanine causing a greater inhibition than alpha-aminoisobutyrate. Addition of leucine, proline, glycine, or lysine had no effect on urea formation. The presence of glutamate also increased the rate of urea formation from arginine, but to a lesser extent than did glucose. The presence of both glucose and alanine caused no net change in urea formation, whereas the inhibitory effect of alanine exceeded the activating effect of glutamate, so that a small inhibition in the rate of urea formation occurred in the presence of both alanine and glutamate. Cells grown to 3-day stationary phase had a markedly reduced rate of arginine catabolism to urea, but the activating effect of glucose and the inhibitory effect of alanine were qualitatively similar to their effects on late log phase cells. Addition of water to cells suspended in buffer also inhibited urea formation, but this appeared to be due primarily to the release of alanine caused by the hypo-osmotic stress. Addition of mannitol to cells suspended in buffer caused a small inhibition of arginine catabolism. Addition of dibutyrylcyclic AMP, 3',5'-cyclic GMP, phorbol myristic acid, or A23187 had no effect on the rate of urea formation from arginine.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
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Abstract
Promastigotes from late-log phase cultures of Leishmania donovani were washed and resuspended in Hanks' Balanced Salt Solution without glucose or phenyl red but with 20 mM (N-[2-hydroxyethyl] piperazine-N'-[2-ethanesulfonic acid]) (HEPES) (HBSS-, 305 mOsm/kg). They were then added to a solution containing 86Rb such that the final osmolality and ionic composition was as desired. Samples were taken at known times and the amount of intracellular 86Rb was measured. Similarly, experiments were performed in which 86Rb was added to the cultures about 18 hr before collection, and the amount of 86Rb released from the washed cells was measured. Under iso-osmotic conditions only about 1.3% of the intracellular 86Rb was released in 900 sec. This increased about 4-fold if the osmolality was reduced from 305-153 mOsm/kg. This is much slower than the very rapid release of alanine in response to hypo-osmotic stress, indicating that alanine release is not via a non-specific pore. Reducing the temperature from 26 degrees C to 3-4 degrees C completely inhibits 86Rb release under iso-osmotic conditions and largely inhibits it under hypo-osmotic conditions. The rate of 86Rb release was not sensitive to K+ concentration and was not altered if chloride was replaced by sulfamate. Ouabain had no effect on either 86Rb uptake or release, but carbonylcyanide P-trifluoromethoxyphenylhydrazone (FCCP) reduced the rate of 86Rb release and, after about a 300 sec exposure, completely inhibited 86Rb uptake. Amiloride partially inhibited 86Rb release, but had no effect on uptake. A decrease in pH from 7.1-5.9 had little effect on 86Rb release under iso-osmotic conditions and slightly increased the rate of release under hypo-osmotic conditions, but it decreased the rate of uptake under both iso-osmotic and hypo-osmotic conditions. Cells taken from 3-day stationary phase cultures released 86Rb more slowly under iso-osmotic conditions than cells from late log phase cultures, but were more responsive to hypo-osmotic stress than were log phase cells. These data appear to rule out an [Na-K-Cl] transporter or a [K-Cl] cotransporter as the means of K+ release, but are consistent with the possibility that a K+/H+ exchanger is present. The possibility that other carrier systems may be present is also discussed.
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Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
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Glaser TA, Mukkada AJ. Proline transport in Leishmania donovani amastigotes: dependence on pH gradients and membrane potential. Mol Biochem Parasitol 1992; 51:1-8. [PMID: 1533014 DOI: 10.1016/0166-6851(92)90194-o] [Citation(s) in RCA: 30] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Amastigotes of Leishmania donovani develop and multiply within the acidic phagolysosomes of mammalian macrophages. Isolated amastigotes are acidophilic; they catabolize substrates and synthesize macromolecules optimally at pH 5.5. Substrate transport in amastigotes has not been characterized. Here we show that amastigotes exhibit an uphill transport of proline (active transport) with an acid pH optimum (pH 5.5). It is dependent upon metabolic energy and is driven by proton motive force. Agents which selectively disturb the component forces of proton motive force, such as carbonyl cyanide chlorophenylhydrazone, nigericin and valinomycin, inhibit proline transport. Transport is sensitive to dicyclohexylcarbodiimide and insensitive to ouabain, demonstrating the involvement of a proton ATPase in the maintenance of proton motive force. It is suggested that the plasma membrane pH gradient probably makes the greatest contribution to proton motive force that drives substrate transport in the amastigote stage.
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Affiliation(s)
- T A Glaser
- Department of Biological Sciences, University of Cincinnati, OH 45221
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Affiliation(s)
- P Schneider
- Institute of Biochemistry, University of Lausanne, Epalinges, Switzerland
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Abstract
The metabolism of leucine by Leishmania donovani was investigated. Washed promastigotes were incubated with [1-14C]- or [U-14C]leucine or [1-14C]alpha-ketoisocaproate (KIC) and 14CO2 release was measured. The amount of KIC-derived acetyl-CoA oxidized in the citric acid cycle was computed. Promastigotes from mid-stationary phase cultures oxidized each of these labeled substrates less rapidly than cells from late log phase cultures, and significantly less acetyl-CoA derived from KIC oxidation was oxidized in the citric acid cycle. Glucose was a stronger inhibitor than was acetate of CO2 formation in the citric acid cycle in log phase promastigotes, but the reverse was observed in cells from mid-stationary phase. Alanine also inhibited leucine catabolism, but glutamate had little effect. Acute hypo-osmotic stress did not affect leucine catabolism, but hyper-osmotic stress caused appreciable inhibition of leucine oxidation. Cells grown under hypo- or hyper-osmotic conditions showed no changes in the effects of hypo- or hyper-osmotic stress on leucine catabolism, i.e. L. donovani is not an osmoconformer with respect to leucine metabolism. Leucine utilization in L. donovani was insensitive to a number of drugs that affect leucine metabolism in mammalian cells, indicating that the leucine pathway in L. donovani is not regulated in the same manner as in mammalian cells.
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Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
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Blum JJ. Effects of osmotic pressure on the oxidative metabolism of Leishmania major promastigotes. THE JOURNAL OF PROTOZOOLOGY 1991; 38:229-33. [PMID: 1679134 DOI: 10.1111/j.1550-7408.1991.tb04434.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Leishmania major promastigotes were washed and resuspended in an iso-osmotic buffer. The rate of oxidation of 14C-labeled substrates was then measured as a function of osmolality. An acute decrease in osmolality (achieved by adding H2O to the cell suspension) caused an increase in the rates of 14CO2 production from [6-14C]glucose and, to a lesser extent, from [1,(3)-14C]glycerol. An acute increase in osmolality (achieved by adding NaCl, KCl, or mannitol) strongly inhibited the rates of 14CO2 production from [1-14C]alanine,[1-14C]glutamate, and [1,(3)-14C]glycerol. The rates of 14CO2 formation from [1-14C]laurate,[1-14C]acetate, and [2-14C]glucose (all of which form [1-14C]acetyl CoA prior to oxidation) were also inhibited, but less strongly, by increasing osmolality. These data suggest that with increasing osmolality there is an inhibition of mitochondrial oxidative capacity, which could facilitate the increase in alanine pool size that occurs in response to hyper-osmotic stress. Similarly, an increase in oxidative capacity would help prevent a rebuild up of the alanine pool after its rapid loss to the medium in response to hypo-osmotic stress.
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Affiliation(s)
- J J Blum
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27706
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Darling TN, Burrows CM, Blum JJ. Rapid shape change and release of ninhydrin-positive substances by Leishmania major promastigotes in response to hypo-osmotic stress. THE JOURNAL OF PROTOZOOLOGY 1990; 37:493-9. [PMID: 2086781 DOI: 10.1111/j.1550-7408.1990.tb01254.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Leishmania major promastigotes were grown to late-log phase and washed and resuspended in an isosmotic buffer. When osmolality was suddenly decreased by 50%, the cells rapidly became shorter and increased in width. Cell volume, calculated assuming a prolate-ellipsoidal shape, increased 1.4 times after 1 min. Over the next several minutes, the average length and width returned to control values while the volume returned to baseline, indicating the ability to regulate volume. Concomitantly with the swelling, large amounts of alanine and other ninhydrin-positive substances were released. All of the alanine pool was released within 1 min after reduction of the osmolality by 66%. Cells pre-loaded with [14C]-aminoisobutyric acid also released it very rapidly upon hypo-osmotic stress. Release of ninhydrin-positive substances resulted from decreased osmolality rather than changes in ionic composition. The same results were obtained if osmolality was decreased by reducing only the NaCl content of the buffer instead of diluting it with water, and mannitol could substitute for the NaCl. Promastigotes were able to grow well over several days in media as low as 154 mOsm/kg. The nature of the signalling mechanisms(s) that initiates the rapid shape change and efflux of ninhydrin-positive substances in response to hypo-osmotic stress is at present unknown.
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Affiliation(s)
- T N Darling
- Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710
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Grain J. The cytoskeleton in protists: nature, structure, and functions. INTERNATIONAL REVIEW OF CYTOLOGY 1986; 104:153-249. [PMID: 3531064 DOI: 10.1016/s0074-7696(08)61926-9] [Citation(s) in RCA: 67] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
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Protonmotive force-driven active transport of D-glucose and L-proline in the protozoan parasite Leishmania donovani. Proc Natl Acad Sci U S A 1985; 82:1716-20. [PMID: 2984665 PMCID: PMC397343 DOI: 10.1073/pnas.82.6.1716] [Citation(s) in RCA: 102] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
Midlogarithmic phase Leishmania donovani promastigotes accumulate 2-deoxy-D-glucose (2-dGlc) and L-proline, maintaining concentration gradient factors across the surface membrane of 78.7 and 60, respectively. Cyanide (1 mM) and iodoacetate (0.5 mM) inhibited the transport of both substrates. L-proline uptake was also inhibited by 2-dGlc (10 mM). Transport of neither substrate was affected by Na+, phlorizin, or ouabain, indicating the sodium-independent transport of both systems. However, N',N'-dicyclohexylcarbodiimide (DCCD; 20 microM) significantly inhibited the transport of both 2-dGlc and L-proline (70% and 90%, respectively). The ionophores valinomycin (1 microM) and nigericin (5 microM) each partially inhibited the uptake of both substrates. In parallel experiments, nigericin and valinomycin were added concomitantly to promastigotes, each at a concentration that individually inhibited the transport of 2-dGlc and L-proline by less than 30%. Under such conditions, the transport of 2-dGlc and L-proline was inhibited by 69% and 78%, respectively. However, these ionophores had no significant effect on the promastigotes cellular ATP level. Carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP; 1 microM) inhibited 2-dGlc (79%) and L-proline (85%) transport, whereas ATP levels of such cells were diminished by only 20%. Symport of D-glucose/H+ and L-proline/H+ was measured directly in cells pretreated with KCN and DCCD. Upon addition of D-glucose to such cells, a rapid movement of protons into the organisms occurred and was reversed upon addition of FCCP. Conversely, no proton movement was observed when L-glucose was added to such cells. L-proline, as D-glucose, caused a rapid influx of protons into the promastigotes, indicating that both substrates were cotransported with protons. We conclude that transport of D-glucose and L-proline in L. donovani promastigotes is protonmotive force-driven and is coupled to both delta pH and delta psi.
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Lerner J. Effectors of amino acid transport processes in animal cell membranes. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY. A, COMPARATIVE PHYSIOLOGY 1985; 81:713-39. [PMID: 2863064 DOI: 10.1016/0300-9629(85)90903-x] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)
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