Amino-terminal radiosequencing of double-mutant procathepsin G in RBL cells. RBL/CatG/Gly₂₀₁/▵Gly₁₉Glu₂₀ cells were pulse-labeled with ³H-isoleucine for 30 minutes as described in the Materials and Methods. After pulse-labeling, 100 × 10⁶ cells were subjected to solubilization, immunoprecipitation, SDS-PAGE, and transfer to a PVDF membrane by Western blotting. Radioactive bands containing pulse-labeled 32.5-kD CatG/Gly₂₀₁/▵Gly₁₉Glu₂₀ were excised and subjected to amino acid degradation. The amount of radioactivity in the initial 9 cycles of each sequence analysis is shown (dpm, disintegrations per minute).