PROPERTIES OF ACHETAKININ BINDING-SITES ON MALPIGHIAN TUBULE MEMBRANES FROM THE HOUSE CRICKET, ACHETA-DOMESTICUS

A biologically active I-125-labeled analogue of AK-II (3'-hydroxyphenyl propionic-Gly-Gly-Gly-Phe-Ser-Pro-Trp-Gly-NH2) was used to investigate the properties of achetakinin binding sites on plasma membranes from Malpighian tubules of Acheta domesticus. With optimized conditions, binding was rapid, reversible, and specific, and saturation studies revealed a single class of binding sites with K-d 0.55 nM and B-max 39.9 fmol/mg membrane protein. The affinities of achetakinins for binding sites on tubule membranes ranked AK-V > AK III > AK-II > AK-I greater than or equal to AK-IV, in general agreement with their potencies in functional assays. However, IC50 values were several orders of magnitude higher than corresponding values for EC(50) which suggests a considerable receptor reserve.