title: Futile cycling of lactate through the plasma membrane of C6 glioma cells as detected by (13C, 2H) NMR.
abstract: We report a novel ((13)C, (2)H) nuclear magnetic resonance (NMR) procedure to investigate lactate recycling through the monocarboxylate transporter of the plasma membrane of cells in culture. C6 glioma cells were incubated with [3-(13)C]lactate in Krebs-Henseleit Buffer containing 50% (2)H(2)O (vol/vol) for up to 30 hr. (13)C NMR analysis of aliquots progressively taken from the medium, showed: (1) a linearly decreasing singlet at approximately 20.85 parts per million (ppm; -0.119 micromol/mg protein/hr) derived from the methyl carbon of [3-(13)C]lactate; and (2) an exponentially increasing shifted singlet at approximately 20.74 ppm (0.227 micromol/ mg protein/hr) from the methyl carbon of [3-(13)C, 2-(2)H]lactate. The shifted singlet appears because during its transit through the cytosol, [3-(13)C]lactate generates [3-(13)C, 2-(2)H]lactate in the lactate dehydrogenase (LDH) equilibrium, which may return to the incubation medium through the reversible monocarboxylate carrier. The methyl group of [3-(13)C, 2-(2)H]lactate is shifted -0.11 ppm with respect to that of [3-(13)C]lactate, making it possible to distinguish between both molecules by (13)C NMR. During incubations with 2.5 mM [1-(13)C]glucose and 3.98 mM [U-(13)C(3)]lactate or with 2.5 mM [1-(13)C]glucose and 3.93 mM [2-(13)C]pyruvate, C2-deuterated lactate was produced only from [1-(13)C]glucose or [U-(13)C(3)]lactate, revealing that this deuteration process is redox sensitive. When [1-(13)C]glucose and [U-(13)C(3)]lactate were used as substrates, no significant [3-(13)C]lactate production from [1-(13)C]glucose was detected, suggesting that glycolytic lactate production may be stopped under the high lactate concentrations prevailing under mild hypoxic or ischemic episodes or during cerebral activation.
authors.affiliation: Laboratory for Imaging and Spectroscopy by Magnetic Resonance, Instituto de Investigaciones Biomédicas Alberto Sols/CSIC/UAM, c/Arturo Duperier 4, E-28029 Madrid, Spain.
authors.forename: Tiago B
authors.initials: TB
authors.lastname: Rodrigues
authors.forename: Heather L
authors.initials: HL
authors.lastname: Gray
authors.forename: Marina
authors.initials: M
authors.lastname: Benito
authors.forename: Susana
authors.initials: S
authors.lastname: Garrido
authors.forename: Alejandra
authors.initials: A
authors.lastname: Sierra
authors.forename: Carlos F
authors.initials: CF
authors.lastname: Geraldes
authors.forename: Paloma
authors.initials: P
authors.lastname: Ballesteros
authors.forename: Sebastián
authors.initials: S
authors.lastname: Cerdán
mesh_terms: D000818:Animals; D002247:Carbon Isotopes / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism*; D002952:Citric Acid Cycle / Q000502:physiology; D005910:Glioma / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D007554:Isotopes / Q000378:metabolism; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D010427:Pentose Phosphate Pathway / Q000502:physiology*; D019289:Pyruvic Acid / Q000378:metabolism; D015219:Substrate Cycling; D013997:Time Factors; D014325:Tromethamine / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D007554:Isotopes; C074097:Krebs-Henseleit solution; D014325:Tromethamine; D019344:Lactic Acid; D019289:Pyruvic Acid; D007770:L-Lactate Dehydrogenase; D005947:Glucose