The formation of large aggregates by ionic interactions between acidic glucosaminoglycans and cationic secretory proteins has been proposed as one of the critical steps in the concentration process in the condensing vacuoles of secretory cells. In this paper, this hypothesis was tested by studies on the interactions between bovine chymotrypsinogen A and chondroitin sulfate as a simplified model. Small amounts of chondroitin sulfate were found able to induce chymotrypsinogen precipitation. Like zymogen granules, the resulting aggregates were moderately sensitive to ionic strength and insensitive to osmolality. Moreover, their pH dependence was similar to that of isolated zymogen granules. When sulfated glucosaminoglycans isolated from the zymogen granules of the guinea pig pancreas were used instead of chondroitin sulfate, the same kind of interactions with chymotrypsinogen were obtained. Our data support the hypothesis that the strong ionic interactions between those sulfated glucosaminoglycans and cationic proteins could be responsible for the concentration process.

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