ISOLATION AND PROPERTIES OF SECRETORY GRANULES FROM RAT ISLETS OF LANGERHANS III. Studies of the Stability of the Isolated Beta Granules

A partially purified secretory granule fraction, isolated from rat islets of Langerhans by differential centrifugation, was used for investigating the stability of the beta granules during incubation in various conditions . Effects of pH, temperature, and time were studied ; the granules possessed optimal stability at 4° and pH 6 .0, and could be solubilized at pH 4.0 or 8 .5, or in the presence of sodium deoxycholate, but not by phospholipase c, ouabain, or alloxan . Incubation with glucose or some of its metabolites, or with tolbutamide, ATP, or cyclic 3' , 5'-AMP did not alter the stability of the beta granules Exogenous insulin131 1 was not bound by the isolated granules under the conditions used ; no specific insulindegrading activity could be detected in subcellular fractions of the islets . These findings indicate that intracellular solubilization of the granules with subsequent diffusion of the insulin into the extracellular space is not a likely mode of insulin secretion in vivo, and suggest that a crystalline zinc-insulin complex may exist in the matrix of the beta granules .