Synthetic probes for studying tuftsin-receptor interactions on human polymorphonuclear leukocytes

Tuftsin is an immunopotentiating tetrapeptide of the sequence L-Thr-L-Lys-L-Pro-L-Arg with anti-microbial and anti-tumor enhancing capabilities. These enhancing functions are manifested through the host's granulocytes and monocytes. In delineating tuftsin's mechanism of action, both radiolabeled and fluorescent probes were synthesized. The radiolabeled probe of tuftsin, [L-proly-3,4-3H(N)]-tuftsin, was obtained through the synthesis and subsequent catalytic hydrogenation of [L-3,4-dehydroproly]3-tuftsin using tritium gas. This procedure yielded a probe with a specific activity of 44.9 Ci/mmole. This radiolabeled probe of tuftsin was used in competitive inhibition studies with tuftsin, the tuftsin analogues Lys-Pro-Arg, Thr-Lys-Pro-Arg(NO2) and [Δ3-pro3]-tuftsin as well as with the chemotactic peptide f-Mewt-Leu-Phe. The studies show that tuftsin initially binds to diffusely distributed receptors on the surface of human granulocytes. The tuftsin-receptor complexes then rapidly redistribute to form patches which are then internalized. Whether redistribution and internalization of tuftsin-receptor complexes is crucial in effecting a biological response is still not clear. This process, however, may provide the target cell with an early time point in modulating the biological effects of tuftsin through down-regulation of cell surface receptor sites