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rate equation (6-1) reveals that the plot of dB /dt (initial) as a
sp
function of S. is a line possessing slope B k In fact, the observed
curvature of such a plot (a decrease in slope at high values of SL) has
even been offered as possible evidence that the glucocorticoid receptor
binding process may involve multiple steps and unobserved transient
intermediate states (Kaine et al., 1975; Pratt et al., 1975). (The most
probable explanation of this anomaly is that in some experiments the
constant incubation time was too long to provide a reliable estimate of
the initial dB^/dt at the high ligand concentrations; e.g., Yeakley et
al., 1980.) Aspects of the two different experimental designs that may
be used for the determination of kfl have been combined in figure 6-4, in
which the initial rate of increase of specific binding, dBSp/dt
(initial), has been plotted (after normalization by the receptor
concentration Bq) as a function of total ligand concentration (S^) for
the six experimental determinations of the dexamethasone association
rate constant (k,). The ordinates were determined in two different
ways: the initial slopes of the individual temporal binding curves
(derived from the first three points of each curve) were taken as the
estimates of dB^/dt (initial) to generate one set of points ( ); the
other set of points ( #) was generated by calculating the initial
slopes of the binding curves (i.e., dB^/dt = kaS^BQ at t=0, from equa
tion 6-1) from the values of k derived from the complete individual
ci
experiments as described above in "Methods". A line passing through the
origin was fit to each set of points by least-squares regression.
Although there was considerable scatter in the data, each set of points
was at least consistent with the same straight line passing through the
origin, indicating that the result of the inter-experiment "initial