The specifically 13C-labeled (90% 13C-enriched) peptide hormone derivatives [l-hemi[2-l3C]cystine]oxytocin, [l-hemi[l-13C]cystine]oxytocin, and [2-[2-13C]tyrosine]-oxytocin and the analogue [3-[2-13C]leucine]oxytocin were prepared by total synthesis and used to study the interactions of the neurohypophyseal hormones with the bovine neurophysins as a function of pH and temperature. Under all conditions, whether high or low pH, the chemical shifts of the labeled carbon atoms of the bound hormones are the same, but they are shifted significantly from their positions in the free hormone. These results indicate that interactions of the side chain and disulfide moieties of the hormone with the neurophysins do not change as a function of pH. At neutral pH and 20-35 °C, the labeled atoms of the hormone are in slow exchange (1-5 s-1) with the neurophysins for the above hormone derivatives, but at low pH they are in intermediate or fast exchange depending upon the pH and temperature. At low pH, the dissociation rate constant (Koff) is about 100-fold greater than the value at neutral pH, and this increase appears to be due exclusively to the breaking of the salt bridge involving the N-terminal amino group of oxytocin and a side-chain carboxyl group of neurophysin. Since the dissociation constant (Kd) also increases by about 100-fold in going from neutral to low pH, the association rate constant is deduced to be the same at neutral and low pH. In contrast to the low pH results, an increase in pH (from 6.6 to 10.5) leads to a continual decrease in the binding constant but to no apparent change in the dissociation rate constant. The bound hormone is always in slow exchange at high pH, even when the binding constant has been reduced by 2 or 3 orders of magnitude. At high pH, the decrease in binding affinity is due solely to the deprotonation of the α-amino group of the free hormone. Thus, at high pH the apparent association rate constant decreases, while the dissociation rate constant remains unchanged.
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