Use of cystine to distinguish glutamate binding from glutamate sequestration.

Cystine is shown to be a high-affinity substrate for the chloride-dependent glutamate exchange system in brain membranes, which mediates what has previously been considered chloride-dependent glutamate 'binding'. Similarities in the pharmacological profile and in kinetic properties suggest that this transport system may be a high-affinity adaptation of the transport system xc- present in somatic cells. Since cystine selectively inhibits glutamate sequestration and does not interact with the major glutamate binding sites, it can be used to assess the contribution of sequestration to overall 'binding' of glutamate or glutamate-analogs or to suppress sequestration in receptor binding assays. As an example, it is shown that more than 90% of 'bound' [3H]aminophosphonobutyrate [( 3H]APB) is displaced by cystine and should be interpreted as APB sequestration.