The Ca²⁺-Dependent Dissociation of [³H]Dopamine Bound to an Acidic Glycoprotein Present in the Synaptosomal Cytoplasm of Rat Brain

Abstract

The interaction of [³H]dopamine with acidic proteins present in the synaptosomal cytoplasm of rat brain was studied from the point of view of Ca²⁺-dependent release of dopamine as a neurotransmitter. Some of the proteins were glycoproteins and showed various affinities for [³H]dopamine. [³H]Dopamine bound to an acidic protein with a molecular weight of about 45, 000 daltons became reversibly unbound in the presence of Ca²⁺, while that bound to other proteins did not. The amount of [³H]dopamine bound to the 45, 000-dalton glycoprotein was 48.7 pmol/mg protein, and the Ca²⁺-concentration required to release half of the bound dopamine was found to be 0.2 to 0.25mM under the experimental conditions. However, Cal²⁺ did not bind to any component of the acidic proteins and it is likely that the cation dissociates the bound dopamine by forming a complex with it. No calmodulin (-like protein) was detectable among the acidic proteins. Possible roles of the glycoproteins are discussed in relation to a Ca²⁺-dependent mechanism of neuro-transmitter release.