On the nature of the presumed receptor for IgE on mast cells. IV. Inhibition of the PCA-blocking activity of cell-free particulate preparations and intact rat peritoneal mast cells by inhibitors of proteases.
The incubation of IgE-containing solutions from rat serum with particulate preparations from rat peritoneal mast cells results in the disappearance of some of the PCA-reactive IgE in the solution. This PCA blocking assay was used to measure the 'binding' of IgE to intact rat mast cells or to particulate preparations derived from mast cells. The PCA-blocking activity at pH 4.8 was up to 100-fold greater than that seen at a physiologic pH of 6.6. PCA-blodking activity was inhibited at both these pH conditions by high concentrations of several trypsin inhibitors. The inhibitors were generally more active at the more acid pH. Among the active inhibitors were soybean and limabean trypsin inhibitors, chymostatin, and p-nitrophenyl-p'-guanidinobenzoate. Inhibitors of acid proteases, such as pepstatin and diazaacetylnorleucine methyl ester were inactive. The results support the proposition that under certain conditions IgE degradation by a specific proteolytic enzyme which is located uniquely on the plasma membrane of mast cells can account for a major portion of the PCA-blocking activity of these cells.