Ultrastructural localization of basic proteins in cytoplasmic granules of rat eosinophils and mast cells

The postformalin ammoniacal silver (AS) and the ethanolic phosphotungstic acid (EPTA) methods were used to localize basic protein at the ultrastructural level in cytoplasmic granules of rat eosinophils and mast cells isolated using a Metrizamide gradient. Intense reaction was seen in the granules of EPTA-treated eosinophils. Following incubation of the cells for 2 hr in EPTA alone, the matrix was stained. After longer incubation (10 hr), however, both the matrix and core were stained. Cytoplasmic granules of the mast cell show a slight or negative reaction with EPTA. With the AS technique, a large number of silver particles were seen in the nucleus of both eosinophils and mast cells. The mast cell cytoplasmic granules showed intense reaction, while those from eosinophils showed no clear reaction. Acetylation of the cells under conditions sufficient to block most free amino groups prio to EPTA or AS treatment greatly reduced (EPTA) or abolished (AS) the reaction. The results indicate 1) that eosinophil granules contain basic proteins both in the matrix and the core, 2) that the mast cell granules contain a basic protein (probably the alpha-chymotrypsin-like enzyme), which reacts strongly with AS, and 3) that the AS and EPTA methods have different specificities.