Pancreatic secretory-protein pulse-chase experiment

In 1967, James D. Jamieson and George E. Palade incubated guinea pig pancreas slices, following a three-minute pulse of tritium-labeled leucine with excess unlabeled leucine. This pulse-chase analysis marked newly made proteins while reducing subsequent incorporation. Fixed, Epon-embedded thin sections were mounted on coated grids, covered with Ilford L-4 photographic emulsion, developed and stained for electron microscopy. Autoradiography located photographic grains relative to pancreatic acinar cell structures.^[ref]

Pulse-end samples provided a temporal reference: label lay mainly over rough endoplasmic reticulum; later samples shifted toward peripheral Golgi apparatus elements, condensing vacuoles and secretory granules. As a contamination check, isolated granule pellets were also examined by autoradiography: early pellet labeling chiefly marked condensing vacuoles. Grids from the compared pellet samples were coated together to control photographic processing. This supported transport of a previously labeled cohort rather than separate synthesis at each location. Fixed samples did not track individual proteins; localization was finite, and labeling was not absolutely exclusive to secretory cargo. The result concerns this preparation, not every protein in every biological cell.^[ref]