Secretion granules of the rabbit parotid gland. Isolation, subfractionation, and characterization of the membrane and content subfractions

A fraction of secretion granules has been isolated from rabbit parotid by a procedure which was found to be especially effective in reducing contamination resulting from aggregation and/or cosedimentation of granules with other cell particulates. The fraction, representing 15 percent (on the average) of the total tissue amylase activity, was homogeneous as judged by electron microscopy and contaminated to exceedingly low levels by other cellular organelles as judged by marker enzymatic and chemical assays. Lysis of the granules was achieved by their gradual exposure to hypotonic NaHCO3, containing 0.5 mM EDTA. The content and the membranes separated by centrifugation of the granule lysate were characterized primarily by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis which indicated that the content was composed of a limited number of molecular weight classes of polypeptides of which three bands (having approximate mol wt 58,000, 33, 000, and 12,000) could be considered major components. The gel profile of the membrane subfraction was characterized by 20-30 Coomassie brilliant blue-staining bands of which a single species of mol wt 40,000 was the conspicuous major polypeptide. Two types of experiments employing gel electrophoretic analysis were carried out for identifying and assessing the extent of residual secretory protein adsorbed to purified granule membranes: (a) examination of staining and radioactivity profiles after mixing of radioactive secretion granule extract with nonradioactively labeled granule membranes and (b) comparison of gel profiles of secretion granule extract and granule membranes with those of unlysed secretion granules and secretory protein dischraged from lobules in vitro or collected by cannulation of parotid ducts, the last two samples being considered physiologic secretory standards. The results indicated that the membranes were contaminated to a substantial degree by residual, poorly extractable secretory protein even though assays of membrane fractions for a typical secretory enzyme activity (amylase) indicated quite through separation of membranes and content. Hence, detailed examination of membrane subfractions for residual content species by gel electrophoresis points to the general unity and sensitivity of this technique as a means for accurately detecting a defined set of polypeptides occurring as contaminants in cellular fractions or organelle subfractions.