Thiophosphorylation prevents catecholamine secretion by chemically skinned chromaffin cells

Skinned cells treated with the adenosine triphosphate analog, adenosine-5'-0-(3-thiotriphosphate) showed calcium-dependent thiophosphorylation of cellular constituents. Catecholamine secretion was inhibited when the analog was used as the substrate to promote secretion. The attenuation of secretion was proportional to the percentage of the analog in mixtures with adenosine triphosphate. Moreover, cells treated with the analog were subsequently unable to secrete when presented with MgATP, their normal substrate, indicating that the secretory systems was locked in the thiophosphorylated state. We hypothesize that phosphorylation is the calcium-dependent step required to prime the secretory system for secretion while dephosphorylation is the event required for exocytosis.