Autoradiographic Location of Protein Synthesized by Liver Slices
The location, migration, and secretory pathways followed by proteins, newly synthesized by the liver, were determined by electron microscopic autoradiography. Small (3x4x0.5 mm) slices of liver from fasted rats were incubated for 2 minutes in medium (0.1 ml per slice)containing H3-4,5-L-leucine and then transferred to medium with unlabeled leucine for further incubation. Total incubation times were 2,4,10,25,40 and 75 minutes. Control slices were incubated in medium containing labeled leucine and puromycin. Slices were fixed in 4% formaldehyde (prepared from paraformaldehyde), post-fixed in osmium tetroxide and embedded on end in Epon. Autoradiographs were prepared using Ilford L-4 and Kodak NTE emulsions. Exposure times varied from 3 to 30 weeks. Parallel experiments were performed in which the total counts per minute per milligram of liver slice at each time were determined by standard direct counting techniques. Electron micrographs (magnification approximately 5,000 X) covering a minimum area of 2,000 μ2 of liver were taken of each specimen. The number of background grains in an adjacent area of equal size was determined for each specimen.