Specific binding of a 125I-labeled substance P analog to rat submaxillary gland

The substance P (SP) analog [tyrosine1, norleucine11]-substance P [( Tyr1, Nle11]-SP) was iodinated by the chloramine-T method to yield the mono iodinated derivative; both peptides were biologically active on the guinea-pig ileum SP receptor. Saturable, reversible binding of [125l-Tyr1, Nle11]-SP to a single class of noninteracting sites on the rat submaxillary gland homogenate was demonstrated with an affinity of 9.26 +/- 0.8 nM and a maximum binding of 15.7 +/- 1.25 or 250 +/- 20 fmol/mg of protein. The relative potencies of SP and its fragments SP(2-11) and SP(4-11), as measured by their IC50, are in agreement with their rank order in the salivation assay, whereas physalaemin was found apparently weaker than expected. However, their rank order in displacing [125I-Tyr1, Nle11]-SP was identical to their rank order in displacing [125I-Bolton-Hunter]-SP bound to mouse mesencephalic cells in primary culture.