Effect of proteases on the β-thromboglobulin radioimmunoassay

Rat peritoneal mast cells and mast cell granules were evaluated by radioimmunoassay for the presence of β-thromboglobulin and platelet factor 4. The initial assays indicated that a β-thromboglobin cross reacting material was released from mast cells by compound 48/80 in a similar dose-dependent manner as histamine release. The material was also found to be associated with purified granules. However, the use of protease inhibitors in the buffers completely abolished the positive assays. Further evaluation of the effects of various proteases on the β-thromboglobulin assay indicated that elastase would also generate a false positive assay which could then be neutralized by the use of α 1-antitrypsin as a protease inhibitor. There was no protease effect on the platelet factor 4 radioimmunoassay which always showed no detectable amounts with mast cells, granules or proteases. These results clearly indicate the artifactual positive assays which can arise when using certain radioimmunoassay tests in the presence of cell proteases. The use of protease inhibitors is a necessary control when applying a radioimmunoassay to a system with potentially active proteases.