Transport of Cysteate by Synaptosomes Isolated from Rat Brain: Evidence that It Utilizes the Same Transporter as Aspartate, Glutamate, and Cysteine Sulfinate

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Abstract
Synaptosomes isolated from rat brain accumulated cysteic acid by a high‐affinity transport system (Km= 12.3 ±2.1 μM; Vmax= 2.5 nmol mgprotein‐1 min‐1). This uptake was competitively inhibited by aspartate (Ki= 13.3 ± 1.8 μM) and cysteine sulfinate (Ki= 13.3 ± 2.3 μM). Addition of extrasynaptosomal cysteate, aspartate, or cysteine sulfinate to synaptosomes loaded with [35S]cysteate induced rapid efflux of the cysteate. This efflux occurred via stoichiometric exchange of amino acids with half‐maximal rates at 5.0 ± 1.1 μM aspartate or 8.0 ± 1.3 μM a cysteine sulfinate. Conversely, added extrasynaptosomal cysteate exchanged for endogenous aspartate and glutamate with half‐maximal rates at 5.0 ± 0.4 μM acysteate. In the steady state after maximal accumulation of cysteate, the intrasynaptosomal cysteate concentrations exceeded the extrasynaptosomal concentrations by up to 10,000‐fold. The measured concentration ratios were the same, within experimental error, as those for aspartate and glutamate. Depolarization, with either high [K+] or veratridine, of the plasma membranes of synaptosomes loaded with cysteate caused parallel release of cysteate, aspartate, and glutamate. It is concluded that neurons transport cysteate, cysteine sulfinate, aspartate, and glutamate with the same transport system. This transport system catalyzes homoexchange and heteroex‐change as well as net uptake and release of all these amino acids.

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