Sulfate incorporation into the guinea pig pancreas was investigated by light (LM) and electron microscope (EM) autoradiography using a system of minilobules incubated in vitro for 60 min in Krebs-Ringer bicarbonate medium (KRB) containing 35SO4 z-. In acinar cells, examined by EM autoradiography, the label was found concentrated over Golgi elements (including condensing vacuoles) and zymogen granules.35SO42-was also incorporated by the epithelial cells of the entire pancreatic duct system, the incorporation being surprisingly high in the epithelium of the major ducts. In all ductal epithelia, autoradiographic grains appeared over the Golgi complex and the plasmalemma.Since a contribution of the duct epithelium to the sulfated compounds found in the discharged secretion could not be ruled out, a purified zymogen granule fraction was used as a source material for the isolation of sulfated compounds of acinar origin. The presence of 35S-radioactivity in the zymogen granules and condensing vacuoles of this fraction was ascertained by autoradiography (of sectioned pellets). From a lysate of this zymogen granule fraction, a soluble sulfated compound of low isoelectric point and high molecular weight was isolated by gel filtration under conditions that allowed its satisfactory separation from the bulk of the secretory proteins.

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 inteFactions between those sulfated glucosaminoglycans and cationic proteins could be responsible for the concentration process.KEY WORDS chymotrypsinogen A 9 chondroitin sulfate glucosaminoglycans molecular aggregation zymogen granule condensation turbidity A sulfated macromolecular fraction has recently been isolated from the zymogen granules of the guinea pig pancreas (17, 18). Preliminary results indicate that this fraction is composed of acidic glucosaminoglycans, principally heparan sulfate and chondroitin sulfate (18). Such compounds have been implicated in the process by which pancreatic acinar cells concentrate their secretory proteins in the condensing vacuoles of the Golgi complex (23,16,9). The formation of large aggregates by ionic interactions between those polyanions and cationic secretory proteins could reduce the osmotic activity within the vacuoles and lead to concentration of their content by water loss. In this paper, this hypothesis was tested by studies on the interactions between bovine chymotrypsinogen A (ChTg) and chondroitin sulfate (Ch-SO4) as a simplified model. The results indicate that the ionic interactions of these compounds are strong enough to induce coprecipitation. Sulfated polyanions isolated from the zymogen granules of the guinea pig pancreas were found to interact with bovine ChTg in a manner comparable to that of Ch-SO4. MATERIALS AND METHODSBovine chymotrypsinogen A (Worthington Biochemical Corp., Freehold, N.J.) dissolved (1 mg/ml) in 2 mM Tris maleate buffer, pH 6, was mixed with solutions of J. CELL BIOLOGY 9 The Rockefeller University Press -0021-9525/78/0901-095151.00 951 on