An immunocytochemical approach to the demonstration of intracellular processing of mast cell carboxypeptidase

Newly synthesized secretory proteins are transported from the rough endoplasmic reticulum to the Golgi complex where they can undergo posttranslational modification and are then pack aged for secretion by concentration within membrane-bound very small progranules that fuse to form large immature granules. The contents of these vesicles are thought to be then processed, forming mature secretory granules. After acquiring their mature appearance, the secretory granules reside in the cytoplasm until they,re secreted. In this study, we raised antibodies against the first 15 N-terminal amino acids of mast cell pro-carboxypeptidase and the last 14 C-terminal amino acids of mast cell carboxypeptidase. Immunohistochemical localization of the two peptides was carried out in human breast tissue and rat tissue tear, skin, peritoneum, and tongue). In all cases, both epitopes were demonstrated only in mast cell secretory granules. However, mast cells from 3-week-old rats were more positive for the pro-enzyme compared to 3-month-old rats. Human mast cells in breast tissues were mostly negative for the pro-enzyme and positive for the carboxypeptidase. On the basis of these observations, it seems that posttransitional modification of the pro-enzyme to form the active enzyme occurs in the mast cell secretory granules.