Binding of enkephalin/dextran conjugates to opioid receptors

[ala2]Enkephalin molecules were connected to dextran, and the affinities of the enkephalin/dextran conjugates for opioid receptors were studied. Two kinds of enkephalin derivatives, YaGFLGK-NH2 and YaGFLGS'S'PS'S'PS'KP-OMe (S' represents a Sar residue) were prepared. They retained the high affinity of the enkephalin unit toward opioid receptors. On the other hand, receptor affinity of the enkephalin derivative/dextran conjugates became lower than that of the enkephalin derivatives. Fluorescence from the Tyr residue of the conjugates in a buffer solution was less quenched by succinimide than that of the enkephalin derivatives. Therefore, in these conjugates, the binding of the enkephalin moieties to receptors should be hindered by a steric effect of the dextran matrix. However, the receptor affinity (as defined on the basis of an enkephalin unit) increased on increasing the amount of enkephalin units connected to the dextran matrix, especially in the case of the connection through a shorter spacer arm, suggesting simultaneous binding to a few receptors by the conjugate.