Synaptosomal plasma membrane transport of excitatory sulphur amino acid transmitter candidates: kinetic characterisation and analysis of carrier specificity.
The transport kinetics of the excitatory sulphur-containing amino acid (SAA) transmitter candidates, L-cysteine sulphinate (L-CSA), L-cysteate (L-CA), L-homocysteine sulphinate (L-HCSA), and L-homocysteate (L-HCA), together with their plasma membrane carrier specificity, was studied in cerebrocortical synaptosome fractions by a sensitive high performance liquid chromatographic assay. A high affinity uptake system could be demonstrated for L-CSA (Km = 57 +/- 6 microM; Vmax = 1.2 +/- 0.1 nmol/min/mg protein) and L-CA (Km = 23 +/- 3 microM; Vmax = 3.6 +/- 0.1 nmol/min/mg protein), whereas L-HCSA (Km = 502 +/- 152 microM; Vmax = 6.1 +/- 1.3 nmol/min/mg protein) and L-HCA (Km = 1550 +/- 169 microM; Vmax = 10.3 +/- 1.1 nmol/min/mg protein) exhibited much lower affinity as transport substrates. In all cases, only a single, saturable Na(+)-dependent component of uptake could be identified, co-existing with a non-saturable, Na(+)-independent influx component. Plasma membrane carrier specificity of the SAAs was established following comparison with other high-affinity neurotransmitter systems. High-affinity L-CSA and L-CA transport and low-affinity L-HCSA and L-HCA transport demonstrate strong positive correlations in inhibition profiles when compared against each other or individually against the high-affinity transport of L-[3H]glutamate, L-[3H]aspartate, or D-[3H]aspartate. Moreover, the transport systems for the excitatory SAAs exhibited a negative correlation when compared in inhibition profiles with the high affinity transport of both [3H] gamma-aminobutyric acid (GABA) and [3H]taurine.(ABSTRACT TRUNCATED AT 250 WORDS)
Department of Biochemistry and Microbiology, University of St. Andrews, Fife, Scotland, United Kingdom.
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A
Grieve
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Butcher
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Griffiths
D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D007700:Kinetics; D018377:Neurotransmitter Agents / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000494:pharmacology; D013379:Substrate Specificity; D013455:Sulfur / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism*
D000596:Amino Acids; D018377:Neurotransmitter Agents; D013455:Sulfur; D012964:Sodium