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reduce the concentration of free labeled steroid to a negligible level,
and the addition of high concentrations of unlabeled steroids to block
the further association of [ Hjsteroid with the receptors; in some
experiments high concentrations of unlabeled progesterone were added
both to accelerate the dissociation of the remaining [ Hjdexamethasone-
receptor complexes and to block further association of [ Hjdexamethasone
with the glucocorticoid binding sites. Thus, when the association of
3
[ Hjsteroid is blocked, the rate equation (2-1) becomes
dBsp/dt = -Vsp (6-3>
Integration yields the familiar first order dissociation relations
1n[Bsp/B(0)] = -kdt (6-4)
and finally
Bsp(t) = B(0)exp(-kdt), (6-5)
where B(0) is the initial concentration of specifically-bound ligand at
the beginning of the dissociation period. The rate constant kd is the
slope of the plot of ln[B(t)/B(0)] as a function of the dissociation
sp
time t. The dissociation rate is frequently also reported as the half-
life (t.) of the bound complex:
2
t, = (ln2)/kd .
(6-6)