Competitive binding of dynorphin-(1-13) and β-endorphin to cerebroside sulfate in solution

Circular dichroism was used as a probe for competitive binding of two opioid peptides, dynorphin-(1-13) and β-endorphin, with cerebroside sulfate, a membrane lipid thought to be part of the morphine receptor complex. The rationale was that bound β-endorphine is patially helical but bound dynorphin-(1-13) remains unordered, thus making it possible to detect the degree of binding of β-endorphin. The addition of dynorphin-(1-13) to a cerebroside sulfate solution of β-endorphin invariably displaced β-endorphin from the peptide-lipid complex, but the addition of β-endorphin had little effect of dynorphin-(1-13) bound to the lipid. Simlar results were obtained for competitive binding of the two peptides with two other amphiphiles, sodium dodecyl and decyl sulfate. The maximum number of binding sites on dynorphin-(1-13) and β-endorphin was between five and six, which coincides with the five positively charged side chains plus an αNH₃⁺ group at the NH₂ terminus on both peptide molecules. The results support our working hypothesis that dynorphin-(1-13) may displace β-endorphin bound to the receptor, which in turn can account for the inhibition of β-endorphin-induced analgesia by dynorphin-(1-13).