Characterization of a Fluorescent Substance P Analog

We describe the development and characterization of substance P labeled at Lys3 with fluorescein ([fluorescein Lys3]SP) as a fluorescent probe for the neurokinin 1 (NK1) receptor. [fluorescein Lys3]SP is an agonist at the human NK1 receptor, with an affinity for both the high-affinity and low-affinity binding states of the receptor approximately 6-fold lower than that of substance P. Binding of the probe to the human NK1 receptor expressed in Sf9 insect cells was observed directly by monitoring either a decrease in fluorescence intensity or an increase in anisotropy of the [fluorescein Lys3]SP. Detection by anisotropy gave the larger signal and thus was used to characterize the interaction of [fluorescein Lys3]SP with the receptor. The anisotropy of the bound ligand was 0.17, compared to 0.04 for the free ligand. The fluorescence was quenched by about 15% upon binding to the receptor. Bound [fluorescein Lys3]SP was displaced by unlabeled SP and by the quinuclidine antagonist L-703,606. As expected for an agonist, binding was also reduced by the addition of the nonhydrolyzable guanine nucleotide analog GppNHp. [fluorescein Lys3]SP should provide a useful structural and kinetic probe for the NK1 receptor.