Inhibition by excitatory sulphur amino acids of the high-affinityl-glutamate transporter in synaptosomes and in primary cultures of cortical astrocytes and cerebellar neurons |
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Authors: | Roger Griffiths Angus Grieve John Dunlop Inge Damgaard Hanne Fosmark Arne Schousboe |
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Institution: | (1) Department of Biochemistry and Microbiology, University of St. Andrews, KY16 9AL St. Andrews, Fife, Scotland, UK;(2) Pharmabiotec Research Center, The Neurobiology Unit, Department of Biochemistry A, Panum Institute, University of Copenhagen, DK-2200 Copenhagen, Denmark |
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Abstract: | A detailed kinetic study of the inhibitory effects ofl- andd-enantiomers of cysteate, cysteine sulphinate, homocysteine sulphinate, homocysteate, and S-sulpho-cysteine on the neuronal, astroglial and synaptosomal high-affinity glutamate transport system was undertaken.d-3H] Aspartate was used as the transport substrate. Kinetic characterisation of uptake in the absence of sulphur compounds confirmed the high-affinity nature of the transport systems, the Michaelis constant (K
m) ford-aspartate uptake being 6 M, 21 M and 84 M, respectively, in rat brain cortical synaptosomes and primary cultures of mouse cerebellar granule cells and cortical astrocytes. In those cases where significant effects could be demonstrated, the nature of the inhibition was competitive irrespective of the neuronal versus glial systems. The rank order of inhibition was essentially similar in synaptosomes, neurons and astrocytes. Potent inhibition (K
iK
m) of transport in each system was exhibited byl-cysteate, andl- andd-cysteine sulphinate whereas substantially weaker inhibitory effects (K
i>10–1000 times the appropriateK
m value) were exhibited by the remaining sulphur amino acids. In general, inhibition: (i) was markedly stereospecific in favor of thel-enantiomers (except for cysteine sulphinate) and (ii) was found to decrease with increasing chain length. Computer-assisted molecular modelling studies, in which volume contour maps of the sulphur compounds were superimposed on those ofd-aspartate andl-glutamate, demonstrated an order of inhibitory potency which was, qualitatively, in agreement with that obtained quantitatively by in vitro kinetic studies.Special issue dedicated to Dr. Elling Kvamme |
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Keywords: | Glutamate aspartate excitatory sulphur amino acids transport synaptosomes cortical astrocytes granule cells |
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