Eosinophil peroxidase accounts for most if not all of the peroxidase activity associated with isolated rat peritoneal mast cells.
Endogenous peroxidase has been reported in rat peritoneal mast cells and granules. Mast cell granules have also been shown to avidly bind exogenous eosinophil peroxidase. To examine the possibility that contaminating eosinophil peroxidase contributes to the reported rat mast cell peroxidase activity, mast cells were increasingly purified over sequential Percoll gradients. Such repeated centrifugations did not affect the histamine content of the cells or the secretory activity of cells, but the small increases in mast cell purity significantly reduced the specific activity of peroxidase; the remaining peroxidase activity of the mast cell fraction was in a range that could easily be accounted for by a small extent of contamination with eosinophils. An upper limit of 0.3 ng peroxidase/10(6) mast cells was determined from these measurements, ten times less than the values previously reported. When isolated mast cells were deliberately contaminated with soluble eosinophil peroxidase followed by granule isolation, the granules showed increased peroxidase activity, confirming the ability of mast cell granules to bind exogenous peroxidase.