Further Characterization of the Aggregation and Fusion of Chromaffin Granules by Synexin as a Model for Compound Exocytosis

Synexin was isolated from bovine liver and found to aggregate adrenal chromaffin granules in the same Ca²⁺‐dependent manner as previously described for adrenal synexin. The chromaffin granule aggregating activity of liver synexin was blocked in vitro by the addition of an antibody prepared to the 47,000 molecular weight band extracted from an SDS gel of an adrenal medullary synexin preparation. Chromaffin granules aggregated by synexin fused when exposed to cis‐unsaturated fatty acids at concentrations comparable to those released from phospholipids by stimulated secretory cells. The synexin‐induced aggregation reaction was blocked by Erythrosin B, a common food coloring, and by the phenothiazine antipsychotics trifluoperazine and promethazine. The aggregation and fusion of chromaffin granules thus appears to be a useful model system for studying synexin from diverse tissues and for testing pharmacologically or texicologically active substances for effects on secretory systems.