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

Newly synthesized secretory proteins are transported from the rough endopla smic reticulum to the Golgi complex where they can undergo posttranslationa l modification and are then pack aged for secretion by concentration within membrane-bound very small progranules that fuse to form large immature gra nules. The contents of these vesicles are thought to be then processed, for ming mature secretory granules. After acquiring their mature appearance, th e secretory granules reside in the cytoplasm until they,re secreted. In thi s study, we raised antibodies against the first 15 N-terminal amino acids o f mast cell pro-carboxypeptidase and the last 14 C-terminal amino acids of mast cell carboxypeptidase. Immunohistochemical localization of the two pep tides was carried out in human breast tissue and rat tissue tear, skin, per itoneum, and tongue). In all cases, both epitopes were demonstrated only in mast cell secretory granules. However, mast cells from 3-week-old rats wer e 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 positi ve for the carboxypeptidase. On the basis of these observations, it seems t hat posttransitional modification of the pro-enzyme to form the active enzy me occurs in the mast cell secretory granules.