Synaptosomal plasma membrane transport of excitatory sulphur amino acid transmitter candidates: Kinetic characterisation and analysis of carrier specificity

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Abstract
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 μM; Vmax =; 1.2 ± 0.1 nmol/min/mg protein) and L‐CA (Km =; 23 ± 3 μM; Vmax =; 3.6 ± 0.1 nmol/min/mg protein), whereas L‐HCSA (Km =; 502 ± 152 μM; Vmax =; 6.1 ± 1.3 nmol/min/mg protein) and L‐HCA (Km =; 1550 ± 169 μM; 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] γ‐aminobutyric acid (GABA) and [3H]taurine. Taken together, these results strongly suggest that L‐CSA, L‐CA, L‐HCSA, and L‐HCA share a common synaptosomal plasma membrane transport system with L‐glutamate, L‐aspartate, and D‐aspartate.

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