Binding of active components of compound 48/80 to rat peritoneal mast cells

The binding characteristics of compound 48/80 were examined using rat mast cells and fractionated 14C-labeled compound 48/80 components at 4 degrees C in vitro where no degranulation of the cells occurred. The binding potencies of these components in the presence of Ca2+ generally paralleled their histamine releasing activities, except in the case of fractions G (decamer) and H (nonamer), both Ca2+-independent releasers, for the binding of which Ca2+ was inhibitory. Scatchard analyses and displacement studies indicated that the mast cells had two types of binding sites with high and low affinities for fractions D (tridecamer, Ca2+-dependent releaser, Kd = 3.41 X 10(-8) M and 3.35 X 10(-6) M) and H (Ca2+-independent releaser, Kd = 1.11 X 10(-7) M and 9.11 X 10(-6) M), respectively. These sites partially overlapped each other, and also the fraction D site partially overlapped the IgE site and the fraction H site overlapped the neurotensin or substance P site.